Node.js v14.4.0 Documentation

Table of Contents

About this Documentation#

Welcome to the official API reference documentation for Node.js!

Node.js is a JavaScript runtime built on the V8 JavaScript engine.

Contributing#

Report errors in this documentation in the issue tracker. See the contributing guide for directions on how to submit pull requests.

Stability Index#

Throughout the documentation are indications of a section's stability. Some APIs are so proven and so relied upon that they are unlikely to ever change at all. Others are brand new and experimental, or known to be hazardous.

The stability indices are as follows:

Stability: 0 - Deprecated. The feature may emit warnings. Backward compatibility is not guaranteed.

Stability: 1 - Experimental. The feature is not subject to Semantic Versioning rules. Non-backward compatible changes or removal may occur in any future release. Use of the feature is not recommended in production environments.

Stability: 2 - Stable. Compatibility with the npm ecosystem is a high priority.

Use caution when making use of Experimental features, particularly within modules. End users may not be aware that experimental features are being used. Bugs or behavior changes may surprise end users when Experimental API modifications occur. To avoid surprises, use of an Experimental feature may need a command-line flag. Experimental features may also emit a warning.

JSON Output#

Added in: v0.6.12

Every .html document has a corresponding .json document. This is for IDEs and other utilities that consume the documentation.

System calls and man pages#

Node.js functions which wrap a system call will document that. The docs link to the corresponding man pages which describe how the system call works.

Most Unix system calls have Windows analogues. Still, behavior differences may be unavoidable.

Usage & Example#

Usage#

node [options] [V8 options] [script.js | -e "script" | - ] [arguments]

Please see the Command Line Options document for more information.

Example#

An example of a web server written with Node.js which responds with 'Hello, World!':

Commands in this document start with $ or > to replicate how they would appear in a user's terminal. Do not include the $ and > characters. They are there to show the start of each command.

Lines that don’t start with $ or > character show the output of the previous command.

First, make sure to have downloaded and installed Node.js. See this guide for further install information.

Now, create an empty project folder called projects, then navigate into it.

Linux and Mac:

$ mkdir ~/projects
$ cd ~/projects

Windows CMD:

> mkdir %USERPROFILE%\projects
> cd %USERPROFILE%\projects

Windows PowerShell:

> mkdir $env:USERPROFILE\projects
> cd $env:USERPROFILE\projects

Next, create a new source file in the projects folder and call it hello-world.js.

Open hello-world.js in any preferred text editor and paste in the following content:

const http = require('http');

const hostname = '127.0.0.1';
const port = 3000;

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello, World!\n');
});

server.listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);
});

Save the file, go back to the terminal window, and enter the following command:

$ node hello-world.js

Output like this should appear in the terminal:

Server running at http://127.0.0.1:3000/

Now, open any preferred web browser and visit http://127.0.0.1:3000.

If the browser displays the string Hello, World!, that indicates the server is working.

Assert#

Stability: 2 - Stable

The assert module provides a set of assertion functions for verifying invariants.

Strict assertion mode#

History
VersionChanges
v13.9.0, v12.16.2

Changed "strict mode" to "strict assertion mode" and "legacy mode" to "legacy assertion mode" to avoid confusion with the more usual meaining of "strict mode".

v9.9.0

Added error diffs to the strict assertion mode.

v9.9.0

Added strict assertion mode to the assert module.

v9.9.0

Added in: v9.9.0

In strict assertion mode, non-strict methods behave like their corresponding strict methods. For example, assert.deepEqual() will behave like assert.deepStrictEqual().

In strict assertion mode, error messages for objects display a diff. In legacy assertion mode, error messages for objects display the objects, often truncated.

To use strict assertion mode:

const assert = require('assert').strict;

Example error diff:

const assert = require('assert').strict;

assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected ... Lines skipped
//
//   [
//     [
// ...
//       2,
// +     3
// -     '3'
//     ],
// ...
//     5
//   ]

To deactivate the colors, use the NO_COLOR or NODE_DISABLE_COLORS environment variables. This will also deactivate the colors in the REPL. For more on color support in terminal environments, read the tty getColorDepth() documentation.

Legacy assertion mode#

Legacy assertion mode uses the Abstract Equality Comparison in:

To use legacy assertion mode:

const assert = require('assert');

Whenever possible, use the strict assertion mode instead. Otherwise, the Abstract Equality Comparison may cause surprising results. This is especially true for assert.deepEqual(), where the comparison rules are lax:

// WARNING: This does not throw an AssertionError!
assert.deepEqual(/a/gi, new Date());

Class: assert.AssertionError[src]#

Indicates the failure of an assertion. All errors thrown by the assert module will be instances of the AssertionError class.

new assert.AssertionError(options)#

Added in: v0.1.21

  • options <Object>

    • message <string> If provided, the error message is set to this value.
    • actual <any> The actual property on the error instance.
    • expected <any> The expected property on the error instance.
    • operator <string> The operator property on the error instance.
    • stackStartFn <Function> If provided, the generated stack trace omits frames before this function.

A subclass of Error that indicates the failure of an assertion.

All instances contain the built-in Error properties (message and name) and:

  • actual <any> Set to the actual argument for methods such as assert.strictEqual().
  • expected <any> Set to the expected value for methods such as assert.strictEqual().
  • generatedMessage <boolean> Indicates if the message was auto-generated (true) or not.
  • code <string> Value is always ERR_ASSERTION to show that the error is an assertion error.
  • operator <string> Set to the passed in operator value.
const assert = require('assert');

// Generate an AssertionError to compare the error message later:
const { message } = new assert.AssertionError({
  actual: 1,
  expected: 2,
  operator: 'strictEqual'
});

// Verify error output:
try {
  assert.strictEqual(1, 2);
} catch (err) {
  assert(err instanceof assert.AssertionError);
  assert.strictEqual(err.message, message);
  assert.strictEqual(err.name, 'AssertionError');
  assert.strictEqual(err.actual, 1);
  assert.strictEqual(err.expected, 2);
  assert.strictEqual(err.code, 'ERR_ASSERTION');
  assert.strictEqual(err.operator, 'strictEqual');
  assert.strictEqual(err.generatedMessage, true);
}

Class: assert.CallTracker#

Added in: v14.2.0

Stability: 1 - Experimental

This feature is currently experimental and behavior might still change.

### new assert.CallTracker()

Added in: v14.2.0

Creates a new CallTracker object which can be used to track if functions were called a specific number of times. The tracker.verify() must be called for the verification to take place. The usual pattern would be to call it in a process.on('exit') handler.

const assert = require('assert');

const tracker = new assert.CallTracker();

function func() {}

// callsfunc() must be called exactly 1 time before tracker.verify().
const callsfunc = tracker.calls(func, 1);

callsfunc();

// Calls tracker.verify() and verifies if all tracker.calls() functions have
// been called exact times.
process.on('exit', () => {
  tracker.verify();
});

tracker.calls([fn][, exact])#

Added in: v14.2.0

The wrapper function is expected to be called exactly exact times. If the function has not been called exactly exact times when tracker.verify() is called, then tracker.verify() will throw an error.

const assert = require('assert');

// Creates call tracker.
const tracker = new assert.CallTracker();

function func() {}

// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func);

tracker.report()#

Added in: v14.2.0
  • Returns: <Array> of objects containing information about the wrapper functions returned by tracker.calls().

  • Object <Object>

    • message <string>
    • actual <number> The actual number of times the function was called.
    • expected <number> The number of times the function was expected to be called.
    • operator <string> The name of the function that is wrapped.
    • stack <Object> A stack trace of the function.

The arrays contains information about the expected and actual number of calls of the functions that have not been called the expected number of times.

const assert = require('assert');

// Creates call tracker.
const tracker = new assert.CallTracker();

function func() {}

function foo() {}

// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func, 2);

// Returns an array containing information on callsfunc()
tracker.report();
// [
//  {
//    message: 'Expected the func function to be executed 2 time(s) but was
//    executed 0 time(s).',
//    actual: 0,
//    expected: 2,
//    operator: 'func',
//    stack: stack trace
//  }
// ]

tracker.verify()#

Added in: v14.2.0

Iterates through the list of functions passed to tracker.calls() and will throw an error for functions that have not been called the expected number of times.

const assert = require('assert');

// Creates call tracker.
const tracker = new assert.CallTracker();

function func() {}

// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func, 2);

callsfunc();

// Will throw an error since callsfunc() was only called once.
tracker.verify();

assert(value[, message])#

Added in: v0.5.9

An alias of assert.ok().

assert.deepEqual(actual, expected[, message])#

History
VersionChanges
v14.0.0

NaN is now treated as being identical in case both sides are NaN.

v12.0.0

The type tags are now properly compared and there are a couple minor comparison adjustments to make the check less surprising.

v9.0.0

The Error names and messages are now properly compared

v8.0.0

The Set and Map content is also compared

v6.4.0, v4.7.1

Typed array slices are handled correctly now.

v6.1.0, v4.5.0

Objects with circular references can be used as inputs now.

v5.10.1, v4.4.3

Handle non-Uint8Array typed arrays correctly.

v0.1.21

Added in: v0.1.21

Strict assertion mode

An alias of assert.deepStrictEqual().

Legacy assertion mode

Stability: 0 - Deprecated: Use assert.deepStrictEqual() instead.

Tests for deep equality between the actual and expected parameters. Consider using assert.deepStrictEqual() instead. assert.deepEqual() can have surprising results.

Deep equality means that the enumerable "own" properties of child objects are also recursively evaluated by the following rules.

Comparison details#

  • Primitive values are compared with the Abstract Equality Comparison ( == ) with the exception of NaN. It is treated as being identical in case both sides are NaN.
  • Type tags of objects should be the same.
  • Only enumerable "own" properties are considered.
  • Error names and messages are always compared, even if these are not enumerable properties.
  • Object wrappers are compared both as objects and unwrapped values.
  • Object properties are compared unordered.
  • Map keys and Set items are compared unordered.
  • Recursion stops when both sides differ or both sides encounter a circular reference.
  • Implementation does not test the [[Prototype]] of objects.
  • Symbol properties are not compared.
  • WeakMap and WeakSet comparison does not rely on their values.

The following example does not throw an AssertionError because the primitives are considered equal by the Abstract Equality Comparison ( == ).

// WARNING: This does not throw an AssertionError!
assert.deepEqual('+00000000', false);

"Deep" equality means that the enumerable "own" properties of child objects are evaluated also:

const assert = require('assert');

const obj1 = {
  a: {
    b: 1
  }
};
const obj2 = {
  a: {
    b: 2
  }
};
const obj3 = {
  a: {
    b: 1
  }
};
const obj4 = Object.create(obj1);

assert.deepEqual(obj1, obj1);
// OK

// Values of b are different:
assert.deepEqual(obj1, obj2);
// AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }

assert.deepEqual(obj1, obj3);
// OK

// Prototypes are ignored:
assert.deepEqual(obj1, obj4);
// AssertionError: { a: { b: 1 } } deepEqual {}

If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.deepStrictEqual(actual, expected[, message])#

History
VersionChanges
v9.0.0

Enumerable symbol properties are now compared.

v9.0.0

The NaN is now compared using the SameValueZero comparison.

v8.5.0

The Error names and messages are now properly compared

v8.0.0

The Set and Map content is also compared

v6.4.0, v4.7.1

Typed array slices are handled correctly now.

v6.1.0

Objects with circular references can be used as inputs now.

v5.10.1, v4.4.3

Handle non-Uint8Array typed arrays correctly.

v1.2.0

Added in: v1.2.0

Tests for deep equality between the actual and expected parameters. "Deep" equality means that the enumerable "own" properties of child objects are recursively evaluated also by the following rules.

Comparison details#

const assert = require('assert').strict;

// This fails because 1 !== '1'.
assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
//   {
// +   a: 1
// -   a: '1'
//   }

// The following objects don't have own properties
const date = new Date();
const object = {};
const fakeDate = {};
Object.setPrototypeOf(fakeDate, Date.prototype);

// Different [[Prototype]]:
assert.deepStrictEqual(object, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + {}
// - Date {}

// Different type tags:
assert.deepStrictEqual(date, fakeDate);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 2018-04-26T00:49:08.604Z
// - Date {}

assert.deepStrictEqual(NaN, NaN);
// OK, because of the SameValue comparison

// Different unwrapped numbers:
assert.deepStrictEqual(new Number(1), new Number(2));
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + [Number: 1]
// - [Number: 2]

assert.deepStrictEqual(new String('foo'), Object('foo'));
// OK because the object and the string are identical when unwrapped.

assert.deepStrictEqual(-0, -0);
// OK

// Different zeros using the SameValue Comparison:
assert.deepStrictEqual(0, -0);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
// + 0
// - -0

const symbol1 = Symbol();
const symbol2 = Symbol();
assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 });
// OK, because it is the same symbol on both objects.

assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 });
// AssertionError [ERR_ASSERTION]: Inputs identical but not reference equal:
//
// {
//   [Symbol()]: 1
// }

const weakMap1 = new WeakMap();
const weakMap2 = new WeakMap([[{}, {}]]);
const weakMap3 = new WeakMap();
weakMap3.unequal = true;

assert.deepStrictEqual(weakMap1, weakMap2);
// OK, because it is impossible to compare the entries

// Fails because weakMap3 has a property that weakMap1 does not contain:
assert.deepStrictEqual(weakMap1, weakMap3);
// AssertionError: Expected inputs to be strictly deep-equal:
// + actual - expected
//
//   WeakMap {
// +   [items unknown]
// -   [items unknown],
// -   unequal: true
//   }

If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.doesNotMatch(string, regexp[, message])#

Added in: v13.6.0, v12.16.0

Stability: 1 - Experimental

Expects the string input not to match the regular expression.

This feature is currently experimental and the name might change or it might be completely removed again.

const assert = require('assert').strict;

assert.doesNotMatch('I will fail', /fail/);
// AssertionError [ERR_ASSERTION]: The input was expected to not match the ...

assert.doesNotMatch(123, /pass/);
// AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

assert.doesNotMatch('I will pass', /different/);
// OK

If the values do match, or if the string argument is of another type than string, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.doesNotReject(asyncFn[, error][, message])#

Added in: v10.0.0

Awaits the asyncFn promise or, if asyncFn is a function, immediately calls the function and awaits the returned promise to complete. It will then check that the promise is not rejected.

If asyncFn is a function and it throws an error synchronously, assert.doesNotReject() will return a rejected Promise with that error. If the function does not return a promise, assert.doesNotReject() will return a rejected Promise with an ERR_INVALID_RETURN_VALUE error. In both cases the error handler is skipped.

Using assert.doesNotReject() is actually not useful because there is little benefit in catching a rejection and then rejecting it again. Instead, consider adding a comment next to the specific code path that should not reject and keep error messages as expressive as possible.

If specified, error can be a Class, RegExp or a validation function. See assert.throws() for more details.

Besides the async nature to await the completion behaves identically to assert.doesNotThrow().

(async () => {
  await assert.doesNotReject(
    async () => {
      throw new TypeError('Wrong value');
    },
    SyntaxError
  );
})();

assert.doesNotReject(Promise.reject(new TypeError('Wrong value')))
  .then(() => {
    // ...
  });

assert.doesNotThrow(fn[, error][, message])#

History
VersionChanges
v5.11.0, v4.4.5

The message parameter is respected now.

v4.2.0

The error parameter can now be an arrow function.

v0.1.21

Added in: v0.1.21

Asserts that the function fn does not throw an error.

Using assert.doesNotThrow() is actually not useful because there is no benefit in catching an error and then rethrowing it. Instead, consider adding a comment next to the specific code path that should not throw and keep error messages as expressive as possible.

When assert.doesNotThrow() is called, it will immediately call the fn function.

If an error is thrown and it is the same type as that specified by the error parameter, then an AssertionError is thrown. If the error is of a different type, or if the error parameter is undefined, the error is propagated back to the caller.

If specified, error can be a Class, RegExp or a validation function. See assert.throws() for more details.

The following, for instance, will throw the TypeError because there is no matching error type in the assertion:

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  SyntaxError
);

However, the following will result in an AssertionError with the message 'Got unwanted exception...':

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  TypeError
);

If an AssertionError is thrown and a value is provided for the message parameter, the value of message will be appended to the AssertionError message:

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  /Wrong value/,
  'Whoops'
);
// Throws: AssertionError: Got unwanted exception: Whoops

assert.equal(actual, expected[, message])#

History
VersionChanges
v14.0.0

NaN is now treated as being identical in case both sides are NaN.

v0.1.21

Added in: v0.1.21

Strict assertion mode

An alias of assert.strictEqual().

Legacy assertion mode

Stability: 0 - Deprecated: Use assert.strictEqual() instead.

Tests shallow, coercive equality between the actual and expected parameters using the Abstract Equality Comparison ( == ). NaN is special handled and treated as being identical in case both sides are NaN.

const assert = require('assert');

assert.equal(1, 1);
// OK, 1 == 1
assert.equal(1, '1');
// OK, 1 == '1'
assert.equal(NaN, NaN);
// OK

assert.equal(1, 2);
// AssertionError: 1 == 2
assert.equal({ a: { b: 1 } }, { a: { b: 1 } });
// AssertionError: { a: { b: 1 } } == { a: { b: 1 } }

If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.fail([message])#

Added in: v0.1.21

Throws an AssertionError with the provided error message or a default error message. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

const assert = require('assert').strict;

assert.fail();
// AssertionError [ERR_ASSERTION]: Failed

assert.fail('boom');
// AssertionError [ERR_ASSERTION]: boom

assert.fail(new TypeError('need array'));
// TypeError: need array

Using assert.fail() with more than two arguments is possible but deprecated. See below for further details.

assert.fail(actual, expected[, message[, operator[, stackStartFn]]])#

History
VersionChanges
v10.0.0

Calling assert.fail() with more than one argument is deprecated and emits a warning.

v0.1.21

Added in: v0.1.21

Stability: 0 - Deprecated: Use assert.fail([message]) or other assert functions instead.

If message is falsy, the error message is set as the values of actual and expected separated by the provided operator. If just the two actual and expected arguments are provided, operator will default to '!='. If message is provided as third argument it will be used as the error message and the other arguments will be stored as properties on the thrown object. If stackStartFn is provided, all stack frames above that function will be removed from stacktrace (see Error.captureStackTrace). If no arguments are given, the default message Failed will be used.

const assert = require('assert').strict;

assert.fail('a', 'b');
// AssertionError [ERR_ASSERTION]: 'a' != 'b'

assert.fail(1, 2, undefined, '>');
// AssertionError [ERR_ASSERTION]: 1 > 2

assert.fail(1, 2, 'fail');
// AssertionError [ERR_ASSERTION]: fail

assert.fail(1, 2, 'whoops', '>');
// AssertionError [ERR_ASSERTION]: whoops

assert.fail(1, 2, new TypeError('need array'));
// TypeError: need array

In the last three cases actual, expected, and operator have no influence on the error message.

Example use of stackStartFn for truncating the exception's stacktrace:

function suppressFrame() {
  assert.fail('a', 'b', undefined, '!==', suppressFrame);
}
suppressFrame();
// AssertionError [ERR_ASSERTION]: 'a' !== 'b'
//     at repl:1:1
//     at ContextifyScript.Script.runInThisContext (vm.js:44:33)
//     ...

assert.ifError(value)#

History
VersionChanges
v10.0.0

Instead of throwing the original error it is now wrapped into an [AssertionError][] that contains the full stack trace.

v10.0.0

Value may now only be undefined or null. Before all falsy values were handled the same as null and did not throw.

v0.1.97

Added in: v0.1.97

Throws value if value is not undefined or null. This is useful when testing the error argument in callbacks. The stack trace contains all frames from the error passed to ifError() including the potential new frames for ifError() itself.

const assert = require('assert').strict;

assert.ifError(null);
// OK
assert.ifError(0);
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 0
assert.ifError('error');
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 'error'
assert.ifError(new Error());
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: Error

// Create some random error frames.
let err;
(function errorFrame() {
  err = new Error('test error');
})();

(function ifErrorFrame() {
  assert.ifError(err);
})();
// AssertionError [ERR_ASSERTION]: ifError got unwanted exception: test error
//     at ifErrorFrame
//     at errorFrame

assert.match(string, regexp[, message])#

Added in: v13.6.0, v12.16.0

Stability: 1 - Experimental

Expects the string input to match the regular expression.

This feature is currently experimental and the name might change or it might be completely removed again.

const assert = require('assert').strict;

assert.match('I will fail', /pass/);
// AssertionError [ERR_ASSERTION]: The input did not match the regular ...

assert.match(123, /pass/);
// AssertionError [ERR_ASSERTION]: The "string" argument must be of type string.

assert.match('I will pass', /pass/);
// OK

If the values do not match, or if the string argument is of another type than string, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.notDeepEqual(actual, expected[, message])#

History
VersionChanges
v14.0.0

NaN is now treated as being identical in case both sides are NaN.

v9.0.0

The Error names and messages are now properly compared

v8.0.0

The Set and Map content is also compared

v6.4.0, v4.7.1

Typed array slices are handled correctly now.

v6.1.0, v4.5.0

Objects with circular references can be used as inputs now.

v5.10.1, v4.4.3

Handle non-Uint8Array typed arrays correctly.

v0.1.21

Added in: v0.1.21

Strict assertion mode

An alias of assert.notDeepStrictEqual().

Legacy assertion mode

Stability: 0 - Deprecated: Use assert.notDeepStrictEqual() instead.

Tests for any deep inequality. Opposite of assert.deepEqual().

const assert = require('assert');

const obj1 = {
  a: {
    b: 1
  }
};
const obj2 = {
  a: {
    b: 2
  }
};
const obj3 = {
  a: {
    b: 1
  }
};
const obj4 = Object.create(obj1);

assert.notDeepEqual(obj1, obj1);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj2);
// OK

assert.notDeepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj4);
// OK

If the values are deeply equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.notDeepStrictEqual(actual, expected[, message])#

History
VersionChanges
v9.0.0

The -0 and +0 are not considered equal anymore.

v9.0.0

The NaN is now compared using the SameValueZero comparison.

v9.0.0

The Error names and messages are now properly compared

v8.0.0

The Set and Map content is also compared

v6.4.0, v4.7.1

Typed array slices are handled correctly now.

v6.1.0

Objects with circular references can be used as inputs now.

v5.10.1, v4.4.3

Handle non-Uint8Array typed arrays correctly.

v1.2.0

Added in: v1.2.0

Tests for deep strict inequality. Opposite of assert.deepStrictEqual().

const assert = require('assert').strict;

assert.notDeepStrictEqual({ a: 1 }, { a: '1' });
// OK

If the values are deeply and strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.notEqual(actual, expected[, message])#

History
VersionChanges
v14.0.0

NaN is now treated as being identical in case both sides are NaN.

v0.1.21

Added in: v0.1.21

Strict assertion mode

An alias of assert.notStrictEqual().

Legacy assertion mode

Stability: 0 - Deprecated: Use assert.notStrictEqual() instead.

Tests shallow, coercive inequality with the Abstract Equality Comparison (!= ). NaN is special handled and treated as being identical in case both sides are NaN.

const assert = require('assert');

assert.notEqual(1, 2);
// OK

assert.notEqual(1, 1);
// AssertionError: 1 != 1

assert.notEqual(1, '1');
// AssertionError: 1 != '1'

If the values are equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.notStrictEqual(actual, expected[, message])#

History
VersionChanges
v10.0.0

Used comparison changed from Strict Equality to Object.is()

v0.1.21

Added in: v0.1.21

Tests strict inequality between the actual and expected parameters as determined by the SameValue Comparison.

const assert = require('assert').strict;

assert.notStrictEqual(1, 2);
// OK

assert.notStrictEqual(1, 1);
// AssertionError [ERR_ASSERTION]: Expected "actual" to be strictly unequal to:
//
// 1

assert.notStrictEqual(1, '1');
// OK

If the values are strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.ok(value[, message])#

History
VersionChanges
v10.0.0

The assert.ok() (no arguments) will now use a predefined error message.

v0.1.21

Added in: v0.1.21

Tests if value is truthy. It is equivalent to assert.equal(!!value, true, message).

If value is not truthy, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError. If no arguments are passed in at all message will be set to the string: 'No value argument passed to `assert.ok()`'.

Be aware that in the repl the error message will be different to the one thrown in a file! See below for further details.

const assert = require('assert').strict;

assert.ok(true);
// OK
assert.ok(1);
// OK

assert.ok();
// AssertionError: No value argument passed to `assert.ok()`

assert.ok(false, 'it\'s false');
// AssertionError: it's false

// In the repl:
assert.ok(typeof 123 === 'string');
// AssertionError: false == true

// In a file (e.g. test.js):
assert.ok(typeof 123 === 'string');
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(typeof 123 === 'string')

assert.ok(false);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(false)

assert.ok(0);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert.ok(0)

// Using `assert()` works the same:
assert(0);
// AssertionError: The expression evaluated to a falsy value:
//
//   assert(0)

assert.rejects(asyncFn[, error][, message])#

Added in: v10.0.0

Awaits the asyncFn promise or, if asyncFn is a function, immediately calls the function and awaits the returned promise to complete. It will then check that the promise is rejected.

If asyncFn is a function and it throws an error synchronously, assert.rejects() will return a rejected Promise with that error. If the function does not return a promise, assert.rejects() will return a rejected Promise with an ERR_INVALID_RETURN_VALUE error. In both cases the error handler is skipped.

Besides the async nature to await the completion behaves identically to assert.throws().

If specified, error can be a Class, RegExp, a validation function, an object where each property will be tested for, or an instance of error where each property will be tested for including the non-enumerable message and name properties.

If specified, message will be the message provided by the AssertionError if the asyncFn fails to reject.

(async () => {
  await assert.rejects(
    async () => {
      throw new TypeError('Wrong value');
    },
    {
      name: 'TypeError',
      message: 'Wrong value'
    }
  );
})();

(async () => {
  await assert.rejects(
    async () => {
      throw new TypeError('Wrong value');
    },
    (err) => {
      assert.strictEqual(err.name, 'TypeError');
      assert.strictEqual(err.message, 'Wrong value');
      return true;
    }
  );
})();

assert.rejects(
  Promise.reject(new Error('Wrong value')),
  Error
).then(() => {
  // ...
});

error cannot be a string. If a string is provided as the second argument, then error is assumed to be omitted and the string will be used for message instead. This can lead to easy-to-miss mistakes. Please read the example in assert.throws() carefully if using a string as the second argument gets considered.

assert.strictEqual(actual, expected[, message])#

History
VersionChanges
v10.0.0

Used comparison changed from Strict Equality to Object.is()

v0.1.21

Added in: v0.1.21

Tests strict equality between the actual and expected parameters as determined by the SameValue Comparison.

const assert = require('assert').strict;

assert.strictEqual(1, 2);
// AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
//
// 1 !== 2

assert.strictEqual(1, 1);
// OK

assert.strictEqual('Hello foobar', 'Hello World!');
// AssertionError [ERR_ASSERTION]: Expected inputs to be strictly equal:
// + actual - expected
//
// + 'Hello foobar'
// - 'Hello World!'
//          ^

const apples = 1;
const oranges = 2;
assert.strictEqual(apples, oranges, `apples ${apples} !== oranges ${oranges}`);
// AssertionError [ERR_ASSERTION]: apples 1 !== oranges 2

assert.strictEqual(1, '1', new TypeError('Inputs are not identical'));
// TypeError: Inputs are not identical

If the values are not strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError.

assert.throws(fn[, error][, message])#

History
VersionChanges
v10.2.0

The error parameter can be an object containing regular expressions now.

v9.9.0

The error parameter can now be an object as well.

v4.2.0

The error parameter can now be an arrow function.

v0.1.21

Added in: v0.1.21

Expects the function fn to throw an error.

If specified, error can be a Class, RegExp, a validation function, a validation object where each property will be tested for strict deep equality, or an instance of error where each property will be tested for strict deep equality including the non-enumerable message and name properties. When using an object, it is also possible to use a regular expression, when validating against a string property. See below for examples.

If specified, message will be appended to the message provided by the AssertionError if the fn call fails to throw or in case the error validation fails.

Custom validation object/error instance:

const err = new TypeError('Wrong value');
err.code = 404;
err.foo = 'bar';
err.info = {
  nested: true,
  baz: 'text'
};
err.reg = /abc/i;

assert.throws(
  () => {
    throw err;
  },
  {
    name: 'TypeError',
    message: 'Wrong value',
    info: {
      nested: true,
      baz: 'text'
    }
    // Only properties on the validation object will be tested for.
    // Using nested objects requires all properties to be present. Otherwise
    // the validation is going to fail.
  }
);

// Using regular expressions to validate error properties:
assert.throws(
  () => {
    throw err;
  },
  {
    // The `name` and `message` properties are strings and using regular
    // expressions on those will match against the string. If they fail, an
    // error is thrown.
    name: /^TypeError$/,
    message: /Wrong/,
    foo: 'bar',
    info: {
      nested: true,
      // It is not possible to use regular expressions for nested properties!
      baz: 'text'
    },
    // The `reg` property contains a regular expression and only if the
    // validation object contains an identical regular expression, it is going
    // to pass.
    reg: /abc/i
  }
);

// Fails due to the different `message` and `name` properties:
assert.throws(
  () => {
    const otherErr = new Error('Not found');
    // Copy all enumerable properties from `err` to `otherErr`.
    for (const [key, value] of Object.entries(err)) {
      otherErr[key] = value;
    }
    throw otherErr;
  },
  // The error's `message` and `name` properties will also be checked when using
  // an error as validation object.
  err
);

Validate instanceof using constructor:

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  Error
);

Validate error message using RegExp:

Using a regular expression runs .toString on the error object, and will therefore also include the error name.

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  /^Error: Wrong value$/
);

Custom error validation:

The function must return true to indicate all internal validations passed. It will otherwise fail with an AssertionError.

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  (err) => {
    assert(err instanceof Error);
    assert(/value/.test(err));
    // Avoid returning anything from validation functions besides `true`.
    // Otherwise, it's not clear what part of the validation failed. Instead,
    // throw an error about the specific validation that failed (as done in this
    // example) and add as much helpful debugging information to that error as
    // possible.
    return true;
  },
  'unexpected error'
);

error cannot be a string. If a string is provided as the second argument, then error is assumed to be omitted and the string will be used for message instead. This can lead to easy-to-miss mistakes. Using the same message as the thrown error message is going to result in an ERR_AMBIGUOUS_ARGUMENT error. Please read the example below carefully if using a string as the second argument gets considered:

function throwingFirst() {
  throw new Error('First');
}

function throwingSecond() {
  throw new Error('Second');
}

function notThrowing() {}

// The second argument is a string and the input function threw an Error.
// The first case will not throw as it does not match for the error message
// thrown by the input function!
assert.throws(throwingFirst, 'Second');
// In the next example the message has no benefit over the message from the
// error and since it is not clear if the user intended to actually match
// against the error message, Node.js throws an `ERR_AMBIGUOUS_ARGUMENT` error.
assert.throws(throwingSecond, 'Second');
// TypeError [ERR_AMBIGUOUS_ARGUMENT]

// The string is only used (as message) in case the function does not throw:
assert.throws(notThrowing, 'Second');
// AssertionError [ERR_ASSERTION]: Missing expected exception: Second

// If it was intended to match for the error message do this instead:
// It does not throw because the error messages match.
assert.throws(throwingSecond, /Second$/);

// If the error message does not match, an AssertionError is thrown.
assert.throws(throwingFirst, /Second$/);
// AssertionError [ERR_ASSERTION]

Due to the confusing error-prone notation, avoid a string as the second argument.

Async Hooks#

Stability: 1 - Experimental

The async_hooks module provides an API to track asynchronous resources. It can be accessed using:

const async_hooks = require('async_hooks');

Terminology#

An asynchronous resource represents an object with an associated callback. This callback may be called multiple times, for example, the 'connection' event in net.createServer(), or just a single time like in fs.open(). A resource can also be closed before the callback is called. AsyncHook does not explicitly distinguish between these different cases but will represent them as the abstract concept that is a resource.

If Workers are used, each thread has an independent async_hooks interface, and each thread will use a new set of async IDs.

Public API#

Overview#

Following is a simple overview of the public API.

const async_hooks = require('async_hooks');

// Return the ID of the current execution context.
const eid = async_hooks.executionAsyncId();

// Return the ID of the handle responsible for triggering the callback of the
// current execution scope to call.
const tid = async_hooks.triggerAsyncId();

// Create a new AsyncHook instance. All of these callbacks are optional.
const asyncHook =
    async_hooks.createHook({ init, before, after, destroy, promiseResolve });

// Allow callbacks of this AsyncHook instance to call. This is not an implicit
// action after running the constructor, and must be explicitly run to begin
// executing callbacks.
asyncHook.enable();

// Disable listening for new asynchronous events.
asyncHook.disable();

//
// The following are the callbacks that can be passed to createHook().
//

// init is called during object construction. The resource may not have
// completed construction when this callback runs, therefore all fields of the
// resource referenced by "asyncId" may not have been populated.
function init(asyncId, type, triggerAsyncId, resource) { }

// Before is called just before the resource's callback is called. It can be
// called 0-N times for handles (e.g. TCPWrap), and will be called exactly 1
// time for requests (e.g. FSReqCallback).
function before(asyncId) { }

// After is called just after the resource's callback has finished.
function after(asyncId) { }

// Destroy is called when the resource is destroyed.
function destroy(asyncId) { }

// promiseResolve is called only for promise resources, when the
// `resolve` function passed to the `Promise` constructor is invoked
// (either directly or through other means of resolving a promise).
function promiseResolve(asyncId) { }

async_hooks.createHook(callbacks)#

Added in: v8.1.0

Registers functions to be called for different lifetime events of each async operation.

The callbacks init()/before()/after()/destroy() are called for the respective asynchronous event during a resource's lifetime.

All callbacks are optional. For example, if only resource cleanup needs to be tracked, then only the destroy callback needs to be passed. The specifics of all functions that can be passed to callbacks is in the Hook Callbacks section.

const async_hooks = require('async_hooks');

const asyncHook = async_hooks.createHook({
  init(asyncId, type, triggerAsyncId, resource) { },
  destroy(asyncId) { }
});

The callbacks will be inherited via the prototype chain:

class MyAsyncCallbacks {
  init(asyncId, type, triggerAsyncId, resource) { }
  destroy(asyncId) {}
}

class MyAddedCallbacks extends MyAsyncCallbacks {
  before(asyncId) { }
  after(asyncId) { }
}

const asyncHook = async_hooks.createHook(new MyAddedCallbacks());
Error Handling#

If any AsyncHook callbacks throw, the application will print the stack trace and exit. The exit path does follow that of an uncaught exception, but all 'uncaughtException' listeners are removed, thus forcing the process to exit. The 'exit' callbacks will still be called unless the application is run with --abort-on-uncaught-exception, in which case a stack trace will be printed and the application exits, leaving a core file.

The reason for this error handling behavior is that these callbacks are running at potentially volatile points in an object's lifetime, for example during class construction and destruction. Because of this, it is deemed necessary to bring down the process quickly in order to prevent an unintentional abort in the future. This is subject to change in the future if a comprehensive analysis is performed to ensure an exception can follow the normal control flow without unintentional side effects.

Printing in AsyncHooks callbacks#

Because printing to the console is an asynchronous operation, console.log() will cause the AsyncHooks callbacks to be called. Using console.log() or similar asynchronous operations inside an AsyncHooks callback function will thus cause an infinite recursion. An easy solution to this when debugging is to use a synchronous logging operation such as fs.writeFileSync(file, msg, flag). This will print to the file and will not invoke AsyncHooks recursively because it is synchronous.

const fs = require('fs');
const util = require('util');

function debug(...args) {
  // Use a function like this one when debugging inside an AsyncHooks callback
  fs.writeFileSync('log.out', `${util.format(...args)}\n`, { flag: 'a' });
}

If an asynchronous operation is needed for logging, it is possible to keep track of what caused the asynchronous operation using the information provided by AsyncHooks itself. The logging should then be skipped when it was the logging itself that caused AsyncHooks callback to call. By doing this the otherwise infinite recursion is broken.

Class: AsyncHook#

The class AsyncHook exposes an interface for tracking lifetime events of asynchronous operations.

asyncHook.enable()#

Enable the callbacks for a given AsyncHook instance. If no callbacks are provided enabling is a noop.

The AsyncHook instance is disabled by default. If the AsyncHook instance should be enabled immediately after creation, the following pattern can be used.

const async_hooks = require('async_hooks');

const hook = async_hooks.createHook(callbacks).enable();

asyncHook.disable()#

Disable the callbacks for a given AsyncHook instance from the global pool of AsyncHook callbacks to be executed. Once a hook has been disabled it will not be called again until enabled.

For API consistency disable() also returns the AsyncHook instance.

Hook Callbacks#

Key events in the lifetime of asynchronous events have been categorized into four areas: instantiation, before/after the callback is called, and when the instance is destroyed.

init(asyncId, type, triggerAsyncId, resource)#
  • asyncId <number> A unique ID for the async resource.
  • type <string> The type of the async resource.
  • triggerAsyncId <number> The unique ID of the async resource in whose execution context this async resource was created.
  • resource <Object> Reference to the resource representing the async operation, needs to be released during destroy.

Called when a class is constructed that has the possibility to emit an asynchronous event. This does not mean the instance must call before/after before destroy is called, only that the possibility exists.

This behavior can be observed by doing something like opening a resource then closing it before the resource can be used. The following snippet demonstrates this.

require('net').createServer().listen(function() { this.close(); });
// OR
clearTimeout(setTimeout(() => {}, 10));

Every new resource is assigned an ID that is unique within the scope of the current Node.js instance.

type#

The type is a string identifying the type of resource that caused init to be called. Generally, it will correspond to the name of the resource's constructor.

FSEVENTWRAP, FSREQCALLBACK, GETADDRINFOREQWRAP, GETNAMEINFOREQWRAP, HTTPINCOMINGMESSAGE,
HTTPCLIENTREQUEST, JSSTREAM, PIPECONNECTWRAP, PIPEWRAP, PROCESSWRAP, QUERYWRAP,
SHUTDOWNWRAP, SIGNALWRAP, STATWATCHER, TCPCONNECTWRAP, TCPSERVERWRAP, TCPWRAP,
TTYWRAP, UDPSENDWRAP, UDPWRAP, WRITEWRAP, ZLIB, SSLCONNECTION, PBKDF2REQUEST,
RANDOMBYTESREQUEST, TLSWRAP, Microtask, Timeout, Immediate, TickObject

There is also the PROMISE resource type, which is used to track Promise instances and asynchronous work scheduled by them.

Users are able to define their own type when using the public embedder API.

It is possible to have type name collisions. Embedders are encouraged to use unique prefixes, such as the npm package name, to prevent collisions when listening to the hooks.

triggerAsyncId#

triggerAsyncId is the asyncId of the resource that caused (or "triggered") the new resource to initialize and that caused init to call. This is different from async_hooks.executionAsyncId() that only shows when a resource was created, while triggerAsyncId shows why a resource was created.

The following is a simple demonstration of triggerAsyncId:

async_hooks.createHook({
  init(asyncId, type, triggerAsyncId) {
    const eid = async_hooks.executionAsyncId();
    fs.writeSync(
      1, `${type}(${asyncId}): trigger: ${triggerAsyncId} execution: ${eid}\n`);
  }
}).enable();

require('net').createServer((conn) => {}).listen(8080);

Output when hitting the server with nc localhost 8080:

TCPSERVERWRAP(5): trigger: 1 execution: 1
TCPWRAP(7): trigger: 5 execution: 0

The TCPSERVERWRAP is the server which receives the connections.

The TCPWRAP is the new connection from the client. When a new connection is made, the TCPWrap instance is immediately constructed. This happens outside of any JavaScript stack. (An executionAsyncId() of 0 means that it is being executed from C++ with no JavaScript stack above it.) With only that information, it would be impossible to link resources together in terms of what caused them to be created, so triggerAsyncId is given the task of propagating what resource is responsible for the new resource's existence.

resource#

resource is an object that represents the actual async resource that has been initialized. This can contain useful information that can vary based on the value of type. For instance, for the GETADDRINFOREQWRAP resource type, resource provides the host name used when looking up the IP address for the host in net.Server.listen(). The API for accessing this information is currently not considered public, but using the Embedder API, users can provide and document their own resource objects. For example, such a resource object could contain the SQL query being executed.

In some cases the resource object is reused for performance reasons, it is thus not safe to use it as a key in a WeakMap or add properties to it.

Asynchronous context example#

The following is an example with additional information about the calls to init between the before and after calls, specifically what the callback to listen() will look like. The output formatting is slightly more elaborate to make calling context easier to see.

let indent = 0;
async_hooks.createHook({
  init(asyncId, type, triggerAsyncId) {
    const eid = async_hooks.executionAsyncId();
    const indentStr = ' '.repeat(indent);
    fs.writeSync(
      1,
      `${indentStr}${type}(${asyncId}):` +
      ` trigger: ${triggerAsyncId} execution: ${eid}\n`);
  },
  before(asyncId) {
    const indentStr = ' '.repeat(indent);
    fs.writeFileSync('log.out',
                     `${indentStr}before:  ${asyncId}\n`, { flag: 'a' });
    indent += 2;
  },
  after(asyncId) {
    indent -= 2;
    const indentStr = ' '.repeat(indent);
    fs.writeFileSync('log.out',
                     `${indentStr}after:  ${asyncId}\n`, { flag: 'a' });
  },
  destroy(asyncId) {
    const indentStr = ' '.repeat(indent);
    fs.writeFileSync('log.out',
                     `${indentStr}destroy:  ${asyncId}\n`, { flag: 'a' });
  },
}).enable();

require('net').createServer(() => {}).listen(8080, () => {
  // Let's wait 10ms before logging the server started.
  setTimeout(() => {
    console.log('>>>', async_hooks.executionAsyncId());
  }, 10);
});

Output from only starting the server:

TCPSERVERWRAP(5): trigger: 1 execution: 1
TickObject(6): trigger: 5 execution: 1
before:  6
  Timeout(7): trigger: 6 execution: 6
after:   6
destroy: 6
before:  7
>>> 7
  TickObject(8): trigger: 7 execution: 7
after:   7
before:  8
after:   8

As illustrated in the example, executionAsyncId() and execution each specify the value of the current execution context; which is delineated by calls to before and after.

Only using execution to graph resource allocation results in the following:

Timeout(7) -> TickObject(6) -> root(1)

The TCPSERVERWRAP is not part of this graph, even though it was the reason for console.log() being called. This is because binding to a port without a host name is a synchronous operation, but to maintain a completely asynchronous API the user's callback is placed in a process.nextTick().

The graph only shows when a resource was created, not why, so to track the why use triggerAsyncId.

before(asyncId)#

When an asynchronous operation is initiated (such as a TCP server receiving a new connection) or completes (such as writing data to disk) a callback is called to notify the user. The before callback is called just before said callback is executed. asyncId is the unique identifier assigned to the resource about to execute the callback.

The before callback will be called 0 to N times. The before callback will typically be called 0 times if the asynchronous operation was cancelled or, for example, if no connections are received by a TCP server. Persistent asynchronous resources like a TCP server will typically call the before callback multiple times, while other operations like fs.open() will call it only once.

after(asyncId)#

Called immediately after the callback specified in before is completed.

If an uncaught exception occurs during execution of the callback, then after will run after the 'uncaughtException' event is emitted or a domain's handler runs.

destroy(asyncId)#

Called after the resource corresponding to asyncId is destroyed. It is also called asynchronously from the embedder API emitDestroy().

Some resources depend on garbage collection for cleanup, so if a reference is made to the resource object passed to init it is possible that destroy will never be called, causing a memory leak in the application. If the resource does not depend on garbage collection, then this will not be an issue.

promiseResolve(asyncId)#
Added in: v8.6.0

Called when the resolve function passed to the Promise constructor is invoked (either directly or through other means of resolving a promise).

resolve() does not do any observable synchronous work.

The Promise is not necessarily fulfilled or rejected at this point if the Promise was resolved by assuming the state of another Promise.

new Promise((resolve) => resolve(true)).then((a) => {});

calls the following callbacks:

init for PROMISE with id 5, trigger id: 1
  promise resolve 5      # corresponds to resolve(true)
init for PROMISE with id 6, trigger id: 5  # the Promise returned by then()
  before 6               # the then() callback is entered
  promise resolve 6      # the then() callback resolves the promise by returning
  after 6

async_hooks.executionAsyncResource()#

Added in: v13.9.0
  • Returns: <Object> The resource representing the current execution. Useful to store data within the resource.

Resource objects returned by executionAsyncResource() are most often internal Node.js handle objects with undocumented APIs. Using any functions or properties on the object is likely to crash your application and should be avoided.

Using executionAsyncResource() in the top-level execution context will return an empty object as there is no handle or request object to use, but having an object representing the top-level can be helpful.

const { open } = require('fs');
const { executionAsyncId, executionAsyncResource } = require('async_hooks');

console.log(executionAsyncId(), executionAsyncResource());  // 1 {}
open(__filename, 'r', (err, fd) => {
  console.log(executionAsyncId(), executionAsyncResource());  // 7 FSReqWrap
});

This can be used to implement continuation local storage without the use of a tracking Map to store the metadata:

const { createServer } = require('http');
const {
  executionAsyncId,
  executionAsyncResource,
  createHook
} = require('async_hooks');
const sym = Symbol('state'); // Private symbol to avoid pollution

createHook({
  init(asyncId, type, triggerAsyncId, resource) {
    const cr = executionAsyncResource();
    if (cr) {
      resource[sym] = cr[sym];
    }
  }
}).enable();

const server = createServer(function(req, res) {
  executionAsyncResource()[sym] = { state: req.url };
  setTimeout(function() {
    res.end(JSON.stringify(executionAsyncResource()[sym]));
  }, 100);
}).listen(3000);

async_hooks.executionAsyncId()#

History
VersionChanges
v8.2.0

Renamed from currentId

v8.1.0

Added in: v8.1.0

  • Returns: <number> The asyncId of the current execution context. Useful to track when something calls.
const async_hooks = require('async_hooks');

console.log(async_hooks.executionAsyncId());  // 1 - bootstrap
fs.open(path, 'r', (err, fd) => {
  console.log(async_hooks.executionAsyncId());  // 6 - open()
});

The ID returned from executionAsyncId() is related to execution timing, not causality (which is covered by triggerAsyncId()):

const server = net.createServer((conn) => {
  // Returns the ID of the server, not of the new connection, because the
  // callback runs in the execution scope of the server's MakeCallback().
  async_hooks.executionAsyncId();

}).listen(port, () => {
  // Returns the ID of a TickObject (i.e. process.nextTick()) because all
  // callbacks passed to .listen() are wrapped in a nextTick().
  async_hooks.executionAsyncId();
});

Promise contexts may not get precise executionAsyncIds by default. See the section on promise execution tracking.

async_hooks.triggerAsyncId()#

  • Returns: <number> The ID of the resource responsible for calling the callback that is currently being executed.
const server = net.createServer((conn) => {
  // The resource that caused (or triggered) this callback to be called
  // was that of the new connection. Thus the return value of triggerAsyncId()
  // is the asyncId of "conn".
  async_hooks.triggerAsyncId();

}).listen(port, () => {
  // Even though all callbacks passed to .listen() are wrapped in a nextTick()
  // the callback itself exists because the call to the server's .listen()
  // was made. So the return value would be the ID of the server.
  async_hooks.triggerAsyncId();
});

Promise contexts may not get valid triggerAsyncIds by default. See the section on promise execution tracking.

Promise execution tracking#

By default, promise executions are not assigned asyncIds due to the relatively expensive nature of the promise introspection API provided by V8. This means that programs using promises or async/await will not get correct execution and trigger ids for promise callback contexts by default.

const ah = require('async_hooks');
Promise.resolve(1729).then(() => {
  console.log(`eid ${ah.executionAsyncId()} tid ${ah.triggerAsyncId()}`);
});
// produces:
// eid 1 tid 0

Observe that the then() callback claims to have executed in the context of the outer scope even though there was an asynchronous hop involved. Also, the triggerAsyncId value is 0, which means that we are missing context about the resource that caused (triggered) the then() callback to be executed.

Installing async hooks via async_hooks.createHook enables promise execution tracking:

const ah = require('async_hooks');
ah.createHook({ init() {} }).enable(); // forces PromiseHooks to be enabled.
Promise.resolve(1729).then(() => {
  console.log(`eid ${ah.executionAsyncId()} tid ${ah.triggerAsyncId()}`);
});
// produces:
// eid 7 tid 6

In this example, adding any actual hook function enabled the tracking of promises. There are two promises in the example above; the promise created by Promise.resolve() and the promise returned by the call to then(). In the example above, the first promise got the asyncId 6 and the latter got asyncId 7. During the execution of the then() callback, we are executing in the context of promise with asyncId 7. This promise was triggered by async resource 6.

Another subtlety with promises is that before and after callbacks are run only on chained promises. That means promises not created by then()/catch() will not have the before and after callbacks fired on them. For more details see the details of the V8 PromiseHooks API.

JavaScript Embedder API#

Library developers that handle their own asynchronous resources performing tasks like I/O, connection pooling, or managing callback queues may use the AsyncResource JavaScript API so that all the appropriate callbacks are called.

Class: AsyncResource#

The class AsyncResource is designed to be extended by the embedder's async resources. Using this, users can easily trigger the lifetime events of their own resources.

The init hook will trigger when an AsyncResource is instantiated.

The following is an overview of the AsyncResource API.

const { AsyncResource, executionAsyncId } = require('async_hooks');

// AsyncResource() is meant to be extended. Instantiating a
// new AsyncResource() also triggers init. If triggerAsyncId is omitted then
// async_hook.executionAsyncId() is used.
const asyncResource = new AsyncResource(
  type, { triggerAsyncId: executionAsyncId(), requireManualDestroy: false }
);

// Run a function in the execution context of the resource. This will
// * establish the context of the resource
// * trigger the AsyncHooks before callbacks
// * call the provided function `fn` with the supplied arguments
// * trigger the AsyncHooks after callbacks
// * restore the original execution context
asyncResource.runInAsyncScope(fn, thisArg, ...args);

// Call AsyncHooks destroy callbacks.
asyncResource.emitDestroy();

// Return the unique ID assigned to the AsyncResource instance.
asyncResource.asyncId();

// Return the trigger ID for the AsyncResource instance.
asyncResource.triggerAsyncId();

new AsyncResource(type[, options])#

  • type <string> The type of async event.

  • options <Object>

    • triggerAsyncId <number> The ID of the execution context that created this async event. Default: executionAsyncId().
    • requireManualDestroy <boolean> If set to true, disables emitDestroy when the object is garbage collected. This usually does not need to be set (even if emitDestroy is called manually), unless the resource's asyncId is retrieved and the sensitive API's emitDestroy is called with it. When set to false, the emitDestroy call on garbage collection will only take place if there is at least one active destroy hook. Default: false.

Example usage:

class DBQuery extends AsyncResource {
  constructor(db) {
    super('DBQuery');
    this.db = db;
  }

  getInfo(query, callback) {
    this.db.get(query, (err, data) => {
      this.runInAsyncScope(callback, null, err, data);
    });
  }

  close() {
    this.db = null;
    this.emitDestroy();
  }
}

asyncResource.runInAsyncScope(fn[, thisArg, ...args])#

Added in: v9.6.0
  • fn <Function> The function to call in the execution context of this async resource.
  • thisArg <any> The receiver to be used for the function call.
  • ...args <any> Optional arguments to pass to the function.

Call the provided function with the provided arguments in the execution context of the async resource. This will establish the context, trigger the AsyncHooks before callbacks, call the function, trigger the AsyncHooks after callbacks, and then restore the original execution context.

asyncResource.emitDestroy()#

Call all destroy hooks. This should only ever be called once. An error will be thrown if it is called more than once. This must be manually called. If the resource is left to be collected by the GC then the destroy hooks will never be called.

asyncResource.asyncId()#

  • Returns: <number> The unique asyncId assigned to the resource.

asyncResource.triggerAsyncId()#

  • Returns: <number> The same triggerAsyncId that is passed to the AsyncResource constructor.

Using AsyncResource for a Worker thread pool#

The following example shows how to use the AsyncResource class to properly provide async tracking for a Worker pool. Other resource pools, such as database connection pools, can follow a similar model.

Assuming that the task is adding two numbers, using a file named task_processor.js with the following content:

const { parentPort } = require('worker_threads');
parentPort.on('message', (task) => {
  parentPort.postMessage(task.a + task.b);
});

a Worker pool around it could use the following structure:

const { AsyncResource } = require('async_hooks');
const { EventEmitter } = require('events');
const path = require('path');
const { Worker } = require('worker_threads');

const kTaskInfo = Symbol('kTaskInfo');
const kWorkerFreedEvent = Symbol('kWorkerFreedEvent');

class WorkerPoolTaskInfo extends AsyncResource {
  constructor(callback) {
    super('WorkerPoolTaskInfo');
    this.callback = callback;
  }

  done(err, result) {
    this.runInAsyncScope(this.callback, null, err, result);
    this.emitDestroy();  // `TaskInfo`s are used only once.
  }
}

class WorkerPool extends EventEmitter {
  constructor(numThreads) {
    super();
    this.numThreads = numThreads;
    this.workers = [];
    this.freeWorkers = [];

    for (let i = 0; i < numThreads; i++)
      this.addNewWorker();
  }

  addNewWorker() {
    const worker = new Worker(path.resolve(__dirname, 'task_processor.js'));
    worker.on('message', (result) => {
      // In case of success: Call the callback that was passed to `runTask`,
      // remove the `TaskInfo` associated with the Worker, and mark it as free
      // again.
      worker[kTaskInfo].done(null, result);
      worker[kTaskInfo] = null;
      this.freeWorkers.push(worker);
      this.emit(kWorkerFreedEvent);
    });
    worker.on('error', (err) => {
      // In case of an uncaught exception: Call the callback that was passed to
      // `runTask` with the error.
      if (worker[kTaskInfo])
        worker[kTaskInfo].done(err, null);
      else
        this.emit('error', err);
      // Remove the worker from the list and start a new Worker to replace the
      // current one.
      this.workers.splice(this.workers.indexOf(worker), 1);
      this.addNewWorker();
    });
    this.workers.push(worker);
    this.freeWorkers.push(worker);
    this.emit(kWorkerFreedEvent);
  }

  runTask(task, callback) {
    if (this.freeWorkers.length === 0) {
      // No free threads, wait until a worker thread becomes free.
      this.once(kWorkerFreedEvent, () => this.runTask(task, callback));
      return;
    }

    const worker = this.freeWorkers.pop();
    worker[kTaskInfo] = new WorkerPoolTaskInfo(callback);
    worker.postMessage(task);
  }

  close() {
    for (const worker of this.workers) worker.terminate();
  }
}

module.exports = WorkerPool;

Without the explicit tracking added by the WorkerPoolTaskInfo objects, it would appear that the callbacks are associated with the individual Worker objects. However, the creation of the Workers is not associated with the creation of the tasks and does not provide information about when tasks were scheduled.

This pool could be used as follows:

const WorkerPool = require('./worker_pool.js');
const os = require('os');

const pool = new WorkerPool(os.cpus().length);

let finished = 0;
for (let i = 0; i < 10; i++) {
  pool.runTask({ a: 42, b: 100 }, (err, result) => {
    console.log(i, err, result);
    if (++finished === 10)
      pool.close();
  });
}

Class: AsyncLocalStorage#

Added in: v13.10.0

This class is used to create asynchronous state within callbacks and promise chains. It allows storing data throughout the lifetime of a web request or any other asynchronous duration. It is similar to thread-local storage in other languages.

The following example uses AsyncLocalStorage to build a simple logger that assigns IDs to incoming HTTP requests and includes them in messages logged within each request.

const http = require('http');
const { AsyncLocalStorage } = require('async_hooks');

const asyncLocalStorage = new AsyncLocalStorage();

function logWithId(msg) {
  const id = asyncLocalStorage.getStore();
  console.log(`${id !== undefined ? id : '-'}:`, msg);
}

let idSeq = 0;
http.createServer((req, res) => {
  asyncLocalStorage.run(idSeq++, () => {
    logWithId('start');
    // Imagine any chain of async operations here
    setImmediate(() => {
      logWithId('finish');
      res.end();
    });
  });
}).listen(8080);

http.get('http://localhost:8080');
http.get('http://localhost:8080');
// Prints:
//   0: start
//   1: start
//   0: finish
//   1: finish

When having multiple instances of AsyncLocalStorage, they are independent from each other. It is safe to instantiate this class multiple times.

new AsyncLocalStorage()#

Added in: v13.10.0

Creates a new instance of AsyncLocalStorage. Store is only provided within a run method call.

asyncLocalStorage.disable()#

Added in: v13.10.0

This method disables the instance of AsyncLocalStorage. All subsequent calls to asyncLocalStorage.getStore() will return undefined until asyncLocalStorage.run() is called again.

When calling asyncLocalStorage.disable(), all current contexts linked to the instance will be exited.

Calling asyncLocalStorage.disable() is required before the asyncLocalStorage can be garbage collected. This does not apply to stores provided by the asyncLocalStorage, as those objects are garbage collected along with the corresponding async resources.

This method is to be used when the asyncLocalStorage is not in use anymore in the current process.

asyncLocalStorage.getStore()#

Added in: v13.10.0

This method returns the current store. If this method is called outside of an asynchronous context initialized by calling asyncLocalStorage.run, it will return undefined.

asyncLocalStorage.enterWith(store)#

Added in: v13.11.0

Calling asyncLocalStorage.enterWith(store) will transition into the context for the remainder of the current synchronous execution and will persist through any following asynchronous calls.

Example:

const store = { id: 1 };
asyncLocalStorage.enterWith(store);
asyncLocalStorage.getStore(); // Returns the store object
someAsyncOperation(() => {
  asyncLocalStorage.getStore(); // Returns the same object
});

This transition will continue for the entire synchronous execution. This means that if, for example, the context is entered within an event handler subsequent event handlers will also run within that context unless specifically bound to another context with an AsyncResource.

const store = { id: 1 };

emitter.on('my-event', () => {
  asyncLocalStorage.enterWith(store);
});
emitter.on('my-event', () => {
  asyncLocalStorage.getStore(); // Returns the same object
});

asyncLocalStorage.getStore(); // Returns undefined
emitter.emit('my-event');
asyncLocalStorage.getStore(); // Returns the same object

asyncLocalStorage.run(store, callback[, ...args])#

Added in: v13.10.0

This methods runs a function synchronously within a context and return its return value. The store is not accessible outside of the callback function or the asynchronous operations created within the callback.

Optionally, arguments can be passed to the function. They will be passed to the callback function.

If the callback function throws an error, it will be thrown by run too. The stacktrace will not be impacted by this call and the context will be exited.

Example:

const store = { id: 2 };
try {
  asyncLocalStorage.run(store, () => {
    asyncLocalStorage.getStore(); // Returns the store object
    throw new Error();
  });
} catch (e) {
  asyncLocalStorage.getStore(); // Returns undefined
  // The error will be caught here
}

asyncLocalStorage.exit(callback[, ...args])#

Added in: v13.10.0

This methods runs a function synchronously outside of a context and return its return value. The store is not accessible within the callback function or the asynchronous operations created within the callback.

Optionally, arguments can be passed to the function. They will be passed to the callback function.

If the callback function throws an error, it will be thrown by exit too. The stacktrace will not be impacted by this call and the context will be re-entered.

Example:

// Within a call to run
try {
  asyncLocalStorage.getStore(); // Returns the store object or value
  asyncLocalStorage.exit(() => {
    asyncLocalStorage.getStore(); // Returns undefined
    throw new Error();
  });
} catch (e) {
  asyncLocalStorage.getStore(); // Returns the same object or value
  // The error will be caught here
}

Usage with async/await#

If, within an async function, only one await call is to run within a context, the following pattern should be used:

async function fn() {
  await asyncLocalStorage.run(new Map(), () => {
    asyncLocalStorage.getStore().set('key', value);
    return foo(); // The return value of foo will be awaited
  });
}

In this example, the store is only available in the callback function and the functions called by foo. Outside of run, calling getStore will return undefined.

Troubleshooting#

In most cases your application or library code should have no issues with AsyncLocalStorage. But in rare cases you may face situations when the current store is lost in one of asynchronous operations. Then you should consider the following options.

If your code is callback-based, it is enough to promisify it with util.promisify(), so it starts working with native promises.

If you need to keep using callback-based API, or your code assumes a custom thenable implementation, you should use AsyncResource class to associate the asynchronous operation with the correct execution context.

Buffer#

Stability: 2 - Stable

In Node.js, Buffer objects are used to represent binary data in the form of a sequence of bytes. Many Node.js APIs, for example streams and file system operations, support Buffers, as interactions with the operating system or other processes generally always happen in terms of binary data.

The Buffer class is a subclass of the Uint8Array class that is built into the JavaScript language. A number of additional methods are supported that cover additional use cases. Node.js APIs accept plain Uint8Arrays wherever Buffers are supported as well.

Instances of the Buffer class, and Uint8Arrays in general, are similar to arrays of integers from 0 to 255, but correspond to fixed-sized blocks of memory and cannot contain any other values. The size of a Buffer is established when it is created and cannot be changed.

The Buffer class is within the global scope, making it unlikely that one would need to ever use require('buffer').Buffer.

// Creates a zero-filled Buffer of length 10.
const buf1 = Buffer.alloc(10);

// Creates a Buffer of length 10,
// filled with bytes which all have the value `1`.
const buf2 = Buffer.alloc(10, 1);

// Creates an uninitialized buffer of length 10.
// This is faster than calling Buffer.alloc() but the returned
// Buffer instance might contain old data that needs to be
// overwritten using fill(), write(), or other functions that fill the Buffer's
// contents.
const buf3 = Buffer.allocUnsafe(10);

// Creates a Buffer containing the bytes [1, 2, 3].
const buf4 = Buffer.from([1, 2, 3]);

// Creates a Buffer containing the bytes [1, 1, 1, 1] – the entries
// are all truncated using `(value & 255)` to fit into the range 0–255.
const buf5 = Buffer.from([257, 257.5, -255, '1']);

// Creates a Buffer containing the UTF-8-encoded bytes for the string 'tést':
// [0x74, 0xc3, 0xa9, 0x73, 0x74] (in hexadecimal notation)
// [116, 195, 169, 115, 116] (in decimal notation)
const buf6 = Buffer.from('tést');

// Creates a Buffer containing the Latin-1 bytes [0x74, 0xe9, 0x73, 0x74].
const buf7 = Buffer.from('tést', 'latin1');

Buffers and Character Encodings#

History
VersionChanges
v6.4.0

Introduced latin1 as an alias for binary.

v5.0.0

Removed the deprecated raw and raws encodings.

When converting between Buffers and strings, a character encoding may be specified. If no character encoding is specified, UTF-8 will be used as the default.

const buf = Buffer.from('hello world', 'utf8');

console.log(buf.toString('hex'));
// Prints: 68656c6c6f20776f726c64
console.log(buf.toString('base64'));
// Prints: aGVsbG8gd29ybGQ=

console.log(Buffer.from('fhqwhgads', 'utf8'));
// Prints: <Buffer 66 68 71 77 68 67 61 64 73>
console.log(Buffer.from('fhqwhgads', 'utf16le'));
// Prints: <Buffer 66 00 68 00 71 00 77 00 68 00 67 00 61 00 64 00 73 00>

The character encodings currently supported by Node.js are the following:

  • 'utf8': Multi-byte encoded Unicode characters. Many web pages and other document formats use UTF-8. This is the default character encoding. When decoding a Buffer into a string that does not exclusively contain valid UTF-8 data, the Unicode replacement character U+FFFD � will be used to represent those errors.

  • 'utf16le': Multi-byte encoded Unicode characters. Unlike 'utf8', each character in the string will be encoded using either 2 or 4 bytes. Node.js only supports the little-endian variant of UTF-16.

  • 'latin1': Latin-1 stands for ISO-8859-1. This character encoding only supports the Unicode characters from U+0000 to U+00FF. Each character is encoded using a single byte. Characters that do not fit into that range are truncated and will be mapped to characters in that range.

Converting a Buffer into a string using one of the above is referred to as decoding, and converting a string into a Buffer is referred to as encoding.

Node.js also supports the following two binary-to-text encodings. For binary-to-text encodings, the naming convention is reversed: Converting a Buffer into a string is typically referred to as encoding, and converting a string into a Buffer as decoding.

  • 'base64': Base64 encoding. When creating a Buffer from a string, this encoding will also correctly accept "URL and Filename Safe Alphabet" as specified in RFC 4648, Section 5.

  • 'hex': Encode each byte as two hexadecimal characters. Data truncation may occur when decoding string that do exclusively contain valid hexadecimal characters. See below for an example.

The following legacy character encodings are also supported:

  • 'ascii': For 7-bit ASCII data only. When encoding a string into a Buffer, this is equivalent to using 'latin1'. When decoding a Buffer into a string, using encoding this will additionally unset the highest bit of each byte before decoding as 'latin1'. Generally, there should be no reason to use this encoding, as 'utf8' (or, if the data is known to always be ASCII-only, 'latin1') will be a better choice when encoding or decoding ASCII-only text. It is only provided for legacy compatibility.

  • 'binary': Alias for 'latin1'. See binary strings for more background on this topic. The name of this encoding can be very misleading, as all of the encodings listed here convert between strings and binary data. For converting between strings and Buffers, typically 'utf-8' is the right choice.

  • 'ucs2': Alias of 'utf16le'. UCS-2 used to refer to a variant of UTF-16 that did not support characters that had code points larger than U+FFFF. In Node.js, these code points are always supported.

Buffer.from('1ag', 'hex');
// Prints <Buffer 1a>, data truncated when first non-hexadecimal value
// ('g') encountered.

Buffer.from('1a7g', 'hex');
// Prints <Buffer 1a>, data truncated when data ends in single digit ('7').

Buffer.from('1634', 'hex');
// Prints <Buffer 16 34>, all data represented.

Modern Web browsers follow the WHATWG Encoding Standard which aliases both 'latin1' and 'ISO-8859-1' to 'win-1252'. This means that while doing something like http.get(), if the returned charset is one of those listed in the WHATWG specification it is possible that the server actually returned 'win-1252'-encoded data, and using 'latin1' encoding may incorrectly decode the characters.

Buffers and TypedArrays#

History
VersionChanges
v3.0.0

The Buffers class now inherits from Uint8Array.

Buffer instances are also Uint8Array instances, which is the language’s built-in class for working with binary data. Uint8Array in turn is a subclass of TypedArray. Therefore, all TypedArray methods are also available on Buffers. However, there are subtle incompatibilities between the Buffer API and the TypedArray API.

In particular:

There are two ways to create new TypedArray instances from a Buffer.

When passing a Buffer to a TypedArray constructor, the Buffer’s elements will be copied, interpreted as an array of integers, and not as a byte array of the target type. For example, new Uint32Array(Buffer.from([1, 2, 3, 4])) creates a 4-element Uint32Array with elements [1, 2, 3, 4], rather than a Uint32Array with a single element [0x1020304] or [0x4030201].

In order to create a TypedArray that shares its memory with the Buffer, the underlying ArrayBuffer can be passed to the TypedArray constructor instead:

const buf = Buffer.from('hello', 'utf16le');
const uint16arr = new Uint16Array(
  buf.buffer, buf.byteOffset, buf.length / Uint16Array.BYTES_PER_ELEMENT);

It is also possible to create a new Buffer that shares the same allocated memory as a TypedArray instance by using the TypedArray object’s .buffer property in the same way. Buffer.from() behaves like new Uint8Array() in this context.

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Copies the contents of `arr`.
const buf1 = Buffer.from(arr);
// Shares memory with `arr`.
const buf2 = Buffer.from(arr.buffer);

console.log(buf1);
// Prints: <Buffer 88 a0>
console.log(buf2);
// Prints: <Buffer 88 13 a0 0f>

arr[1] = 6000;

console.log(buf1);
// Prints: <Buffer 88 a0>
console.log(buf2);
// Prints: <Buffer 88 13 70 17>

When creating a Buffer using a TypedArray's .buffer, it is possible to use only a portion of the underlying ArrayBuffer by passing in byteOffset and length parameters.

const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16);

console.log(buf.length);
// Prints: 16

The Buffer.from() and TypedArray.from() have different signatures and implementations. Specifically, the TypedArray variants accept a second argument that is a mapping function that is invoked on every element of the typed array:

  • TypedArray.from(source[, mapFn[, thisArg]])

The Buffer.from() method, however, does not support the use of a mapping function:

Buffers and iteration#

Buffer instances can be iterated over using for..of syntax:

const buf = Buffer.from([1, 2, 3]);

for (const b of buf) {
  console.log(b);
}
// Prints:
//   1
//   2
//   3

Additionally, the buf.values(), buf.keys(), and buf.entries() methods can be used to create iterators.

Class: Buffer#

The Buffer class is a global type for dealing with binary data directly. It can be constructed in a variety of ways.

Class Method: Buffer.alloc(size[, fill[, encoding]])#

History
VersionChanges
v10.0.0

Attempting to fill a non-zero length buffer with a zero length buffer triggers a thrown exception.

v10.0.0

Specifying an invalid string for fill triggers a thrown exception.

v8.9.3

Specifying an invalid string for fill now results in a zero-filled buffer.

v5.10.0

Added in: v5.10.0

Allocates a new Buffer of size bytes. If fill is undefined, the Buffer will be zero-filled.

const buf = Buffer.alloc(5);

console.log(buf);
// Prints: <Buffer 00 00 00 00 00>

If size is larger than buffer.constants.MAX_LENGTH or smaller than 0, ERR_INVALID_OPT_VALUE is thrown.

If fill is specified, the allocated Buffer will be initialized by calling buf.fill(fill).

const buf = Buffer.alloc(5, 'a');

console.log(buf);
// Prints: <Buffer 61 61 61 61 61>

If both fill and encoding are specified, the allocated Buffer will be initialized by calling buf.fill(fill, encoding).

const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64');

console.log(buf);
// Prints: <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64>

Calling Buffer.alloc() can be measurably slower than the alternative Buffer.allocUnsafe() but ensures that the newly created Buffer instance contents will never contain sensitive data from previous allocations, including data that might not have been allocated for Buffers.

A TypeError will be thrown if size is not a number.

Class Method: Buffer.allocUnsafe(size)#

History
VersionChanges
v7.0.0

Passing a negative size will now throw an error.

v5.10.0

Added in: v5.10.0

  • size <integer> The desired length of the new Buffer.

Allocates a new Buffer of size bytes. If size is larger than buffer.constants.MAX_LENGTH or smaller than 0, ERR_INVALID_OPT_VALUE is thrown.

The underlying memory for Buffer instances created in this way is not initialized. The contents of the newly created Buffer are unknown and may contain sensitive data. Use Buffer.alloc() instead to initialize Buffer instances with zeroes.

const buf = Buffer.allocUnsafe(10);

console.log(buf);
// Prints (contents may vary): <Buffer a0 8b 28 3f 01 00 00 00 50 32>

buf.fill(0);

console.log(buf);
// Prints: <Buffer 00 00 00 00 00 00 00 00 00 00>

A TypeError will be thrown if size is not a number.

The Buffer module pre-allocates an internal Buffer instance of size Buffer.poolSize that is used as a pool for the fast allocation of new Buffer instances created using Buffer.allocUnsafe(), Buffer.from(array), and the deprecated new Buffer(size) constructor only when size is less than or equal to Buffer.poolSize >> 1 (floor of Buffer.poolSize divided by two).

Use of this pre-allocated internal memory pool is a key difference between calling Buffer.alloc(size, fill) vs. Buffer.allocUnsafe(size).fill(fill). Specifically, Buffer.alloc(size, fill) will never use the internal Buffer pool, while Buffer.allocUnsafe(size).fill(fill) will use the internal Buffer pool if size is less than or equal to half Buffer.poolSize. The difference is subtle but can be important when an application requires the additional performance that Buffer.allocUnsafe() provides.

Class Method: Buffer.allocUnsafeSlow(size)#

Added in: v5.12.0
  • size <integer> The desired length of the new Buffer.

Allocates a new Buffer of size bytes. If size is larger than buffer.constants.MAX_LENGTH or smaller than 0, ERR_INVALID_OPT_VALUE is thrown. A zero-length Buffer is created if size is 0.

The underlying memory for Buffer instances created in this way is not initialized. The contents of the newly created Buffer are unknown and may contain sensitive data. Use buf.fill(0) to initialize such Buffer instances with zeroes.

When using Buffer.allocUnsafe() to allocate new Buffer instances, allocations under 4KB are sliced from a single pre-allocated Buffer. This allows applications to avoid the garbage collection overhead of creating many individually allocated Buffer instances. This approach improves both performance and memory usage by eliminating the need to track and clean up as many individual ArrayBuffer objects.

However, in the case where a developer may need to retain a small chunk of memory from a pool for an indeterminate amount of time, it may be appropriate to create an un-pooled Buffer instance using Buffer.allocUnsafeSlow() and then copying out the relevant bits.

// Need to keep around a few small chunks of memory.
const store = [];

socket.on('readable', () => {
  let data;
  while (null !== (data = readable.read())) {
    // Allocate for retained data.
    const sb = Buffer.allocUnsafeSlow(10);

    // Copy the data into the new allocation.
    data.copy(sb, 0, 0, 10);

    store.push(sb);
  }
});

A TypeError will be thrown if size is not a number.

Class Method: Buffer.byteLength(string[, encoding])#

History
VersionChanges
v7.0.0

Passing invalid input will now throw an error.

v5.10.0

The string parameter can now be any TypedArray, DataView or ArrayBuffer.

v0.1.90

Added in: v0.1.90

Returns the byte length of a string when encoded using encoding. This is not the same as String.prototype.length, which does not account for the encoding that is used to convert the string into bytes.

For 'base64' and 'hex', this function assumes valid input. For strings that contain non-base64/hex-encoded data (e.g. whitespace), the return value might be greater than the length of a Buffer created from the string.

const str = '\u00bd + \u00bc = \u00be';

console.log(`${str}: ${str.length} characters, ` +
            `${Buffer.byteLength(str, 'utf8')} bytes`);
// Prints: ½ + ¼ = ¾: 9 characters, 12 bytes

When string is a Buffer/DataView/TypedArray/ArrayBuffer/ SharedArrayBuffer, the byte length as reported by .byteLength is returned.

Class Method: Buffer.compare(buf1, buf2)#

History
VersionChanges
v8.0.0

The arguments can now be Uint8Arrays.

v0.11.13

Added in: v0.11.13

Compares buf1 to buf2, typically for the purpose of sorting arrays of Buffer instances. This is equivalent to calling buf1.compare(buf2).

const buf1 = Buffer.from('1234');
const buf2 = Buffer.from('0123');
const arr = [buf1, buf2];

console.log(arr.sort(Buffer.compare));
// Prints: [ <Buffer 30 31 32 33>, <Buffer 31 32 33 34> ]
// (This result is equal to: [buf2, buf1].)

Class Method: Buffer.concat(list[, totalLength])#

History
VersionChanges
v8.0.0

The elements of list can now be Uint8Arrays.

v0.7.11

Added in: v0.7.11

Returns a new Buffer which is the result of concatenating all the Buffer instances in the list together.

If the list has no items, or if the totalLength is 0, then a new zero-length Buffer is returned.

If totalLength is not provided, it is calculated from the Buffer instances in list by adding their lengths.

If totalLength is provided, it is coerced to an unsigned integer. If the combined length of the Buffers in list exceeds totalLength, the result is truncated to totalLength.

// Create a single `Buffer` from a list of three `Buffer` instances.

const buf1 = Buffer.alloc(10);
const buf2 = Buffer.alloc(14);
const buf3 = Buffer.alloc(18);
const totalLength = buf1.length + buf2.length + buf3.length;

console.log(totalLength);
// Prints: 42

const bufA = Buffer.concat([buf1, buf2, buf3], totalLength);

console.log(bufA);
// Prints: <Buffer 00 00 00 00 ...>
console.log(bufA.length);
// Prints: 42

Class Method: Buffer.from(array)#

Added in: v5.10.0

Allocates a new Buffer using an array of bytes in the range 0 – 255. Array entries outside that range will be truncated to fit into it.

// Creates a new Buffer containing the UTF-8 bytes of the string 'buffer'.
const buf = Buffer.from([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);

A TypeError will be thrown if array is not an Array or another type appropriate for Buffer.from() variants.

Buffer.from(array) and Buffer.from(string) may also use the internal Buffer pool like Buffer.allocUnsafe() does.

Class Method: Buffer.from(arrayBuffer[, byteOffset[, length]])#

Added in: v5.10.0

This creates a view of the ArrayBuffer without copying the underlying memory. For example, when passed a reference to the .buffer property of a TypedArray instance, the newly created Buffer will share the same allocated memory as the TypedArray.

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Shares memory with `arr`.
const buf = Buffer.from(arr.buffer);

console.log(buf);
// Prints: <Buffer 88 13 a0 0f>

// Changing the original Uint16Array changes the Buffer also.
arr[1] = 6000;

console.log(buf);
// Prints: <Buffer 88 13 70 17>

The optional byteOffset and length arguments specify a memory range within the arrayBuffer that will be shared by the Buffer.

const ab = new ArrayBuffer(10);
const buf = Buffer.from(ab, 0, 2);

console.log(buf.length);
// Prints: 2

A TypeError will be thrown if arrayBuffer is not an ArrayBuffer or a SharedArrayBuffer or another type appropriate for Buffer.from() variants.

Class Method: Buffer.from(buffer)#

Added in: v5.10.0

Copies the passed buffer data onto a new Buffer instance.

const buf1 = Buffer.from('buffer');
const buf2 = Buffer.from(buf1);

buf1[0] = 0x61;

console.log(buf1.toString());
// Prints: auffer
console.log(buf2.toString());
// Prints: buffer

A TypeError will be thrown if buffer is not a Buffer or another type appropriate for Buffer.from() variants.

Class Method: Buffer.from(object[, offsetOrEncoding[, length]])#

Added in: v8.2.0
  • object <Object> An object supporting Symbol.toPrimitive or valueOf().
  • offsetOrEncoding <integer> | <string> A byte-offset or encoding, depending on the value returned either by object.valueOf() or object[Symbol.toPrimitive]().
  • length <integer> A length, depending on the value returned either by object.valueOf() or object[Symbol.toPrimitive]().

For objects whose valueOf() function returns a value not strictly equal to object, returns Buffer.from(object.valueOf(), offsetOrEncoding, length).

const buf = Buffer.from(new String('this is a test'));
// Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>

For objects that support Symbol.toPrimitive, returns Buffer.from(object[Symbol.toPrimitive](), offsetOrEncoding, length).

class Foo {
  [Symbol.toPrimitive]() {
    return 'this is a test';
  }
}

const buf = Buffer.from(new Foo(), 'utf8');
// Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>

A TypeError will be thrown if object does not have the mentioned methods or is not of another type appropriate for Buffer.from() variants.

Class Method: Buffer.from(string[, encoding])#

Added in: v5.10.0
  • string <string> A string to encode.
  • encoding <string> The encoding of string. Default: 'utf8'.

Creates a new Buffer containing string. The encoding parameter identifies the character encoding to be used when converting string into bytes.

const buf1 = Buffer.from('this is a tést');
const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex');

console.log(buf1.toString());
// Prints: this is a tést
console.log(buf2.toString());
// Prints: this is a tést
console.log(buf1.toString('latin1'));
// Prints: this is a tést

A TypeError will be thrown if string is not a string or another type appropriate for Buffer.from() variants.

Class Method: Buffer.isBuffer(obj)#

Added in: v0.1.101

Returns true if obj is a Buffer, false otherwise.

Class Method: Buffer.isEncoding(encoding)#

Added in: v0.9.1

Returns true if encoding is the name of a supported character encoding, or false otherwise.

console.log(Buffer.isEncoding('utf-8'));
// Prints: true

console.log(Buffer.isEncoding('hex'));
// Prints: true

console.log(Buffer.isEncoding('utf/8'));
// Prints: false

console.log(Buffer.isEncoding(''));
// Prints: false

Class Property: Buffer.poolSize#

Added in: v0.11.3

This is the size (in bytes) of pre-allocated internal Buffer instances used for pooling. This value may be modified.

buf[index]#

The index operator [index] can be used to get and set the octet at position index in buf. The values refer to individual bytes, so the legal value range is between 0x00 and 0xFF (hex) or 0 and 255 (decimal).

This operator is inherited from Uint8Array, so its behavior on out-of-bounds access is the same as Uint8Array. In other words, buf[index] returns undefined when index is negative or >= buf.length, and buf[index] = value does not modify the buffer if index is negative or >= buf.length.

// Copy an ASCII string into a `Buffer` one byte at a time.
// (This only works for ASCII-only strings. In general, one should use
// `Buffer.from()` to perform this conversion.)

const str = 'Node.js';
const buf = Buffer.allocUnsafe(str.length);

for (let i = 0; i < str.length; i++) {
  buf[i] = str.charCodeAt(i);
}

console.log(buf.toString('utf8'));
// Prints: Node.js

buf.buffer#

  • <ArrayBuffer> The underlying ArrayBuffer object based on which this Buffer object is created.

This ArrayBuffer is not guaranteed to correspond exactly to the original Buffer. See the notes on buf.byteOffset for details.

const arrayBuffer = new ArrayBuffer(16);
const buffer = Buffer.from(arrayBuffer);

console.log(buffer.buffer === arrayBuffer);
// Prints: true

buf.byteOffset#

  • <integer> The byteOffset on the underlying ArrayBuffer object based on which this Buffer object is created.

When setting byteOffset in Buffer.from(ArrayBuffer, byteOffset, length), or sometimes when allocating a buffer smaller than Buffer.poolSize, the buffer doesn't start from a zero offset on the underlying ArrayBuffer.

This can cause problems when accessing the underlying ArrayBuffer directly using buf.buffer, as other parts of the ArrayBuffer may be unrelated to the buf object itself.

A common issue when creating a TypedArray object that shares its memory with a Buffer is that in this case one needs to specify the byteOffset correctly:

// Create a buffer smaller than `Buffer.poolSize`.
const nodeBuffer = new Buffer.from([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);

// When casting the Node.js Buffer to an Int8Array, use the byteOffset
// to refer only to the part of `nodeBuffer.buffer` that contains the memory
// for `nodeBuffer`.
new Int8Array(nodeBuffer.buffer, nodeBuffer.byteOffset, nodeBuffer.length);

buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]])#

History
VersionChanges
v8.0.0

The target parameter can now be a Uint8Array.

v5.11.0

Additional parameters for specifying offsets are supported now.

v0.11.13

Added in: v0.11.13

  • target <Buffer> | <Uint8Array> A Buffer or Uint8Array with which to compare buf.
  • targetStart <integer> The offset within target at which to begin comparison. Default: 0.
  • targetEnd <integer> The offset within target at which to end comparison (not inclusive). Default: target.length.
  • sourceStart <integer> The offset within buf at which to begin comparison. Default: 0.
  • sourceEnd <integer> The offset within buf at which to end comparison (not inclusive). Default: buf.length.
  • Returns: <integer>

Compares buf with target and returns a number indicating whether buf comes before, after, or is the same as target in sort order. Comparison is based on the actual sequence of bytes in each Buffer.

  • 0 is returned if target is the same as buf
  • 1 is returned if target should come before buf when sorted.
  • -1 is returned if target should come after buf when sorted.
const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('BCD');
const buf3 = Buffer.from('ABCD');

console.log(buf1.compare(buf1));
// Prints: 0
console.log(buf1.compare(buf2));
// Prints: -1
console.log(buf1.compare(buf3));
// Prints: -1
console.log(buf2.compare(buf1));
// Prints: 1
console.log(buf2.compare(buf3));
// Prints: 1
console.log([buf1, buf2, buf3].sort(Buffer.compare));
// Prints: [ <Buffer 41 42 43>, <Buffer 41 42 43 44>, <Buffer 42 43 44> ]
// (This result is equal to: [buf1, buf3, buf2].)

The optional targetStart, targetEnd, sourceStart, and sourceEnd arguments can be used to limit the comparison to specific ranges within target and buf respectively.

const buf1 = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8, 9]);
const buf2 = Buffer.from([5, 6, 7, 8, 9, 1, 2, 3, 4]);

console.log(buf1.compare(buf2, 5, 9, 0, 4));
// Prints: 0
console.log(buf1.compare(buf2, 0, 6, 4));
// Prints: -1
console.log(buf1.compare(buf2, 5, 6, 5));
// Prints: 1

ERR_OUT_OF_RANGE is thrown if targetStart < 0, sourceStart < 0, targetEnd > target.byteLength, or sourceEnd > source.byteLength.

buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]])#

Added in: v0.1.90
  • target <Buffer> | <Uint8Array> A Buffer or Uint8Array to copy into.
  • targetStart <integer> The offset within target at which to begin writing. Default: 0.
  • sourceStart <integer> The offset within buf from which to begin copying. Default: 0.
  • sourceEnd <integer> The offset within buf at which to stop copying (not inclusive). Default: buf.length.
  • Returns: <integer> The number of bytes copied.

Copies data from a region of buf to a region in target, even if the target memory region overlaps with buf.

TypedArray#set() performs the same operation, and is available for all TypedArrays, including Node.js Buffers, although it takes different function arguments.

// Create two `Buffer` instances.
const buf1 = Buffer.allocUnsafe(26);
const buf2 = Buffer.allocUnsafe(26).fill('!');

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf1[i] = i + 97;
}

// Copy `buf1` bytes 16 through 19 into `buf2` starting at byte 8 of `buf2`.
buf1.copy(buf2, 8, 16, 20);
// This is equivalent to:
// buf2.set(buf1.subarray(16, 20), 8);

console.log(buf2.toString('ascii', 0, 25));
// Prints: !!!!!!!!qrst!!!!!!!!!!!!!

// Create a `Buffer` and copy data from one region to an overlapping region
// within the same `Buffer`.

const buf = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf[i] = i + 97;
}

buf.copy(buf, 0, 4, 10);

console.log(buf.toString());
// Prints: efghijghijklmnopqrstuvwxyz

buf.entries()#

Added in: v1.1.0

Creates and returns an iterator of [index, byte] pairs from the contents of buf.

// Log the entire contents of a `Buffer`.

const buf = Buffer.from('buffer');

for (const pair of buf.entries()) {
  console.log(pair);
}
// Prints:
//   [0, 98]
//   [1, 117]
//   [2, 102]
//   [3, 102]
//   [4, 101]
//   [5, 114]

buf.equals(otherBuffer)#

History
VersionChanges
v8.0.0

The arguments can now be Uint8Arrays.

v0.11.13

Added in: v0.11.13

Returns true if both buf and otherBuffer have exactly the same bytes, false otherwise. Equivalent to buf.compare(otherBuffer) === 0.

const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('414243', 'hex');
const buf3 = Buffer.from('ABCD');

console.log(buf1.equals(buf2));
// Prints: true
console.log(buf1.equals(buf3));
// Prints: false

buf.fill(value[, offset[, end]][, encoding])#

History
VersionChanges
v11.0.0

Throws ERR_OUT_OF_RANGE instead of ERR_INDEX_OUT_OF_RANGE.

v10.0.0

Negative end values throw an ERR_INDEX_OUT_OF_RANGE error.

v10.0.0

Attempting to fill a non-zero length buffer with a zero length buffer triggers a thrown exception.

v10.0.0

Specifying an invalid string for value triggers a thrown exception.

v5.7.0

The encoding parameter is supported now.

v0.5.0

Added in: v0.5.0

Fills buf with the specified value. If the offset and end are not given, the entire buf will be filled:

// Fill a `Buffer` with the ASCII character 'h'.

const b = Buffer.allocUnsafe(50).fill('h');

console.log(b.toString());
// Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

value is coerced to a uint32 value if it is not a string, Buffer, or integer. If the resulting integer is greater than 255 (decimal), buf will be filled with value & 255.

If the final write of a fill() operation falls on a multi-byte character, then only the bytes of that character that fit into buf are written:

// Fill a `Buffer` with character that takes up two bytes in UTF-8.

console.log(Buffer.allocUnsafe(5).fill('\u0222'));
// Prints: <Buffer c8 a2 c8 a2 c8>

If value contains invalid characters, it is truncated; if no valid fill data remains, an exception is thrown:

const buf = Buffer.allocUnsafe(5);

console.log(buf.fill('a'));
// Prints: <Buffer 61 61 61 61 61>
console.log(buf.fill('aazz', 'hex'));
// Prints: <Buffer aa aa aa aa aa>
console.log(buf.fill('zz', 'hex'));
// Throws an exception.

buf.includes(value[, byteOffset][, encoding])#

Added in: v5.3.0
  • value <string> | <Buffer> | <Uint8Array> | <integer> What to search for.
  • byteOffset <integer> Where to begin searching in buf. If negative, then offset is calculated from the end of buf. Default: 0.
  • encoding <string> If value is a string, this is its encoding. Default: 'utf8'.
  • Returns: <boolean> true if value was found in buf, false otherwise.

Equivalent to buf.indexOf() !== -1.

const buf = Buffer.from('this is a buffer');

console.log(buf.includes('this'));
// Prints: true
console.log(buf.includes('is'));
// Prints: true
console.log(buf.includes(Buffer.from('a buffer')));
// Prints: true
console.log(buf.includes(97));
// Prints: true (97 is the decimal ASCII value for 'a')
console.log(buf.includes(Buffer.from('a buffer example')));
// Prints: false
console.log(buf.includes(Buffer.from('a buffer example').slice(0, 8)));
// Prints: true
console.log(buf.includes('this', 4));
// Prints: false

buf.indexOf(value[, byteOffset][, encoding])#

History
VersionChanges
v8.0.0

The value can now be a Uint8Array.

v5.7.0, v4.4.0

When encoding is being passed, the byteOffset parameter is no longer required.

v1.5.0

Added in: v1.5.0

  • value <string> | <Buffer> | <Uint8Array> | <integer> What to search for.
  • byteOffset <integer> Where to begin searching in buf. If negative, then offset is calculated from the end of buf. Default: 0.
  • encoding <string> If value is a string, this is the encoding used to determine the binary representation of the string that will be searched for in buf. Default: 'utf8'.
  • Returns: <integer> The index of the first occurrence of value in buf, or -1 if buf does not contain value.

If value is:

  • a string, value is interpreted according to the character encoding in encoding.
  • a Buffer or Uint8Array, value will be used in its entirety. To compare a partial Buffer, use buf.slice().
  • a number, value will be interpreted as an unsigned 8-bit integer value between 0 and 255.
const buf = Buffer.from('this is a buffer');

console.log(buf.indexOf('this'));
// Prints: 0
console.log(buf.indexOf('is'));
// Prints: 2
console.log(buf.indexOf(Buffer.from('a buffer')));
// Prints: 8
console.log(buf.indexOf(97));
// Prints: 8 (97 is the decimal ASCII value for 'a')
console.log(buf.indexOf(Buffer.from('a buffer example')));
// Prints: -1
console.log(buf.indexOf(Buffer.from('a buffer example').slice(0, 8)));
// Prints: 8

const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');

console.log(utf16Buffer.indexOf('\u03a3', 0, 'utf16le'));
// Prints: 4
console.log(utf16Buffer.indexOf('\u03a3', -4, 'utf16le'));
// Prints: 6

If value is not a string, number, or Buffer, this method will throw a TypeError. If value is a number, it will be coerced to a valid byte value, an integer between 0 and 255.

If byteOffset is not a number, it will be coerced to a number. If the result of coercion is NaN or 0, then the entire buffer will be searched. This behavior matches String#indexOf().

const b = Buffer.from('abcdef');

// Passing a value that's a number, but not a valid byte.
// Prints: 2, equivalent to searching for 99 or 'c'.
console.log(b.indexOf(99.9));
console.log(b.indexOf(256 + 99));

// Passing a byteOffset that coerces to NaN or 0.
// Prints: 1, searching the whole buffer.
console.log(b.indexOf('b', undefined));
console.log(b.indexOf('b', {}));
console.log(b.indexOf('b', null));
console.log(b.indexOf('b', []));

If value is an empty string or empty Buffer and byteOffset is less than buf.length, byteOffset will be returned. If value is empty and byteOffset is at least buf.length, buf.length will be returned.

buf.keys()#

Added in: v1.1.0

Creates and returns an iterator of buf keys (indices).

const buf = Buffer.from('buffer');

for (const key of buf.keys()) {
  console.log(key);
}
// Prints:
//   0
//   1
//   2
//   3
//   4
//   5

buf.lastIndexOf(value[, byteOffset][, encoding])#

History
VersionChanges
v8.0.0

The value can now be a Uint8Array.

v6.0.0

Added in: v6.0.0

  • value <string> | <Buffer> | <Uint8Array> | <integer> What to search for.
  • byteOffset <integer> Where to begin searching in buf. If negative, then offset is calculated from the end of buf. Default: buf.length - 1.
  • encoding <string> If value is a string, this is the encoding used to determine the binary representation of the string that will be searched for in buf. Default: 'utf8'.
  • Returns: <integer> The index of the last occurrence of value in buf, or -1 if buf does not contain value.

Identical to buf.indexOf(), except the last occurrence of value is found rather than the first occurrence.

const buf = Buffer.from('this buffer is a buffer');

console.log(buf.lastIndexOf('this'));
// Prints: 0
console.log(buf.lastIndexOf('buffer'));
// Prints: 17
console.log(buf.lastIndexOf(Buffer.from('buffer')));
// Prints: 17
console.log(buf.lastIndexOf(97));
// Prints: 15 (97 is the decimal ASCII value for 'a')
console.log(buf.lastIndexOf(Buffer.from('yolo')));
// Prints: -1
console.log(buf.lastIndexOf('buffer', 5));
// Prints: 5
console.log(buf.lastIndexOf('buffer', 4));
// Prints: -1

const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le');

console.log(utf16Buffer.lastIndexOf('\u03a3', undefined, 'utf16le'));
// Prints: 6
console.log(utf16Buffer.lastIndexOf('\u03a3', -5, 'utf16le'));
// Prints: 4

If value is not a string, number, or Buffer, this method will throw a TypeError. If value is a number, it will be coerced to a valid byte value, an integer between 0 and 255.

If byteOffset is not a number, it will be coerced to a number. Any arguments that coerce to NaN, like {} or undefined, will search the whole buffer. This behavior matches String#lastIndexOf().

const b = Buffer.from('abcdef');

// Passing a value that's a number, but not a valid byte.
// Prints: 2, equivalent to searching for 99 or 'c'.
console.log(b.lastIndexOf(99.9));
console.log(b.lastIndexOf(256 + 99));

// Passing a byteOffset that coerces to NaN.
// Prints: 1, searching the whole buffer.
console.log(b.lastIndexOf('b', undefined));
console.log(b.lastIndexOf('b', {}));

// Passing a byteOffset that coerces to 0.
// Prints: -1, equivalent to passing 0.
console.log(b.lastIndexOf('b', null));
console.log(b.lastIndexOf('b', []));

If value is an empty string or empty Buffer, byteOffset will be returned.

buf.length#

Added in: v0.1.90

Returns the number of bytes in buf.

// Create a `Buffer` and write a shorter string to it using UTF-8.

const buf = Buffer.alloc(1234);

console.log(buf.length);
// Prints: 1234

buf.write('some string', 0, 'utf8');

console.log(buf.length);
// Prints: 1234

buf.parent#

Deprecated since: v8.0.0

Stability: 0 - Deprecated: Use buf.buffer instead.

The buf.parent property is a deprecated alias for buf.buffer.

buf.readBigInt64BE([offset])#

buf.readBigInt64LE([offset])#

Added in: v12.0.0, v10.20.0
  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <bigint>

Reads a signed 64-bit integer from buf at the specified offset with the specified endianness (readBigInt64BE() reads as big endian, readBigInt64LE() reads as little endian).

Integers read from a Buffer are interpreted as two's complement signed values.

buf.readBigUInt64BE([offset])#

buf.readBigUInt64LE([offset])#

Added in: v12.0.0, v10.20.0
  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <bigint>

Reads an unsigned 64-bit integer from buf at the specified offset with the specified endianness (readBigUInt64BE() reads as big endian, readBigUInt64LE() reads as little endian).

const buf = Buffer.from([0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff]);

console.log(buf.readBigUInt64BE(0));
// Prints: 4294967295n

console.log(buf.readBigUInt64LE(0));
// Prints: 18446744069414584320n

buf.readDoubleBE([offset])#

buf.readDoubleLE([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <number>

Reads a 64-bit double from buf at the specified offset with the specified endianness (readDoubleBE() reads as big endian, readDoubleLE() reads as little endian).

const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);

console.log(buf.readDoubleBE(0));
// Prints: 8.20788039913184e-304
console.log(buf.readDoubleLE(0));
// Prints: 5.447603722011605e-270
console.log(buf.readDoubleLE(1));
// Throws ERR_OUT_OF_RANGE.

buf.readFloatBE([offset])#

buf.readFloatLE([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <number>

Reads a 32-bit float from buf at the specified offset with the specified endianness (readFloatBE() reads as big endian, readFloatLE() reads as little endian).

const buf = Buffer.from([1, 2, 3, 4]);

console.log(buf.readFloatBE(0));
// Prints: 2.387939260590663e-38
console.log(buf.readFloatLE(0));
// Prints: 1.539989614439558e-36
console.log(buf.readFloatLE(1));
// Throws ERR_OUT_OF_RANGE.

buf.readInt8([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.0

Added in: v0.5.0

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 1. Default: 0.
  • Returns: <integer>

Reads a signed 8-bit integer from buf at the specified offset.

Integers read from a Buffer are interpreted as two's complement signed values.

const buf = Buffer.from([-1, 5]);

console.log(buf.readInt8(0));
// Prints: -1
console.log(buf.readInt8(1));
// Prints: 5
console.log(buf.readInt8(2));
// Throws ERR_OUT_OF_RANGE.

buf.readInt16BE([offset])#

buf.readInt16LE([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
  • Returns: <integer>

Reads a signed 16-bit integer from buf at the specified offset with the specified endianness (readInt16BE() reads as big endian, readInt16LE() reads as little endian).

Integers read from a Buffer are interpreted as two's complement signed values.

const buf = Buffer.from([0, 5]);

console.log(buf.readInt16BE(0));
// Prints: 5
console.log(buf.readInt16LE(0));
// Prints: 1280
console.log(buf.readInt16LE(1));
// Throws ERR_OUT_OF_RANGE.

buf.readInt32BE([offset])#

buf.readInt32LE([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer>

Reads a signed 32-bit integer from buf at the specified offset with the specified endianness (readInt32BE() reads as big endian, readInt32LE() reads as little endian).

Integers read from a Buffer are interpreted as two's complement signed values.

const buf = Buffer.from([0, 0, 0, 5]);

console.log(buf.readInt32BE(0));
// Prints: 5
console.log(buf.readInt32LE(0));
// Prints: 83886080
console.log(buf.readInt32LE(1));
// Throws ERR_OUT_OF_RANGE.

buf.readIntBE(offset, byteLength)#

buf.readIntLE(offset, byteLength)#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset and byteLength to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to read. Must satisfy 0 < byteLength <= 6.
  • Returns: <integer>

Reads byteLength number of bytes from buf at the specified offset and interprets the result as a two's complement signed value. Supports up to 48 bits of accuracy.

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

console.log(buf.readIntLE(0, 6).toString(16));
// Prints: -546f87a9cbee
console.log(buf.readIntBE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readIntBE(1, 6).toString(16));
// Throws ERR_OUT_OF_RANGE.
console.log(buf.readIntBE(1, 0).toString(16));
// Throws ERR_OUT_OF_RANGE.

buf.readUInt8([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.0

Added in: v0.5.0

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 1. Default: 0.
  • Returns: <integer>

Reads an unsigned 8-bit integer from buf at the specified offset.

const buf = Buffer.from([1, -2]);

console.log(buf.readUInt8(0));
// Prints: 1
console.log(buf.readUInt8(1));
// Prints: 254
console.log(buf.readUInt8(2));
// Throws ERR_OUT_OF_RANGE.

buf.readUInt16BE([offset])#

buf.readUInt16LE([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
  • Returns: <integer>

Reads an unsigned 16-bit integer from buf at the specified offset with the specified endianness (readUInt16BE() reads as big endian, readUInt16LE() reads as little endian).

const buf = Buffer.from([0x12, 0x34, 0x56]);

console.log(buf.readUInt16BE(0).toString(16));
// Prints: 1234
console.log(buf.readUInt16LE(0).toString(16));
// Prints: 3412
console.log(buf.readUInt16BE(1).toString(16));
// Prints: 3456
console.log(buf.readUInt16LE(1).toString(16));
// Prints: 5634
console.log(buf.readUInt16LE(2).toString(16));
// Throws ERR_OUT_OF_RANGE.

buf.readUInt32BE([offset])#

buf.readUInt32LE([offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer>

Reads an unsigned 32-bit integer from buf at the specified offset with the specified endianness (readUInt32BE() reads as big endian, readUInt32LE() reads as little endian).

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);

console.log(buf.readUInt32BE(0).toString(16));
// Prints: 12345678
console.log(buf.readUInt32LE(0).toString(16));
// Prints: 78563412
console.log(buf.readUInt32LE(1).toString(16));
// Throws ERR_OUT_OF_RANGE.

buf.readUIntBE(offset, byteLength)#

buf.readUIntLE(offset, byteLength)#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset and byteLength to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to read. Must satisfy 0 < byteLength <= 6.
  • Returns: <integer>

Reads byteLength number of bytes from buf at the specified offset and interprets the result as an unsigned integer. Supports up to 48 bits of accuracy.

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

console.log(buf.readUIntBE(0, 6).toString(16));
// Prints: 1234567890ab
console.log(buf.readUIntLE(0, 6).toString(16));
// Prints: ab9078563412
console.log(buf.readUIntBE(1, 6).toString(16));
// Throws ERR_OUT_OF_RANGE.

buf.subarray([start[, end]])#

Added in: v3.0.0

Returns a new Buffer that references the same memory as the original, but offset and cropped by the start and end indices.

Specifying end greater than buf.length will return the same result as that of end equal to buf.length.

This method is inherited from TypedArray#subarray().

Modifying the new Buffer slice will modify the memory in the original Buffer because the allocated memory of the two objects overlap.

// Create a `Buffer` with the ASCII alphabet, take a slice, and modify one byte
// from the original `Buffer`.

const buf1 = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf1[i] = i + 97;
}

const buf2 = buf1.subarray(0, 3);

console.log(buf2.toString('ascii', 0, buf2.length));
// Prints: abc

buf1[0] = 33;

console.log(buf2.toString('ascii', 0, buf2.length));
// Prints: !bc

Specifying negative indexes causes the slice to be generated relative to the end of buf rather than the beginning.

const buf = Buffer.from('buffer');

console.log(buf.subarray(-6, -1).toString());
// Prints: buffe
// (Equivalent to buf.subarray(0, 5).)

console.log(buf.subarray(-6, -2).toString());
// Prints: buff
// (Equivalent to buf.subarray(0, 4).)

console.log(buf.subarray(-5, -2).toString());
// Prints: uff
// (Equivalent to buf.subarray(1, 4).)

buf.slice([start[, end]])#

History
VersionChanges
v7.1.0, v6.9.2

Coercing the offsets to integers now handles values outside the 32-bit integer range properly.

v7.0.0

All offsets are now coerced to integers before doing any calculations with them.

v0.3.0

Added in: v0.3.0

Returns a new Buffer that references the same memory as the original, but offset and cropped by the start and end indices.

This is the same behavior as buf.subarray().

This method is not compatible with the Uint8Array.prototype.slice(), which is a superclass of Buffer. To copy the slice, use Uint8Array.prototype.slice().

const buf = Buffer.from('buffer');

const copiedBuf = Uint8Array.prototype.slice.call(buf);
copiedBuf[0]++;
console.log(copiedBuf.toString());
// Prints: cuffer

console.log(buf.toString());
// Prints: buffer

buf.swap16()#

Added in: v5.10.0

Interprets buf as an array of unsigned 16-bit integers and swaps the byte order in-place. Throws ERR_INVALID_BUFFER_SIZE if buf.length is not a multiple of 2.

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf1.swap16();

console.log(buf1);
// Prints: <Buffer 02 01 04 03 06 05 08 07>

const buf2 = Buffer.from([0x1, 0x2, 0x3]);

buf2.swap16();
// Throws ERR_INVALID_BUFFER_SIZE.

One convenient use of buf.swap16() is to perform a fast in-place conversion between UTF-16 little-endian and UTF-16 big-endian:

const buf = Buffer.from('This is little-endian UTF-16', 'utf16le');
buf.swap16(); // Convert to big-endian UTF-16 text.

buf.swap32()#

Added in: v5.10.0

Interprets buf as an array of unsigned 32-bit integers and swaps the byte order in-place. Throws ERR_INVALID_BUFFER_SIZE if buf.length is not a multiple of 4.

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf1.swap32();

console.log(buf1);
// Prints: <Buffer 04 03 02 01 08 07 06 05>

const buf2 = Buffer.from([0x1, 0x2, 0x3]);

buf2.swap32();
// Throws ERR_INVALID_BUFFER_SIZE.

buf.swap64()#

Added in: v6.3.0

Interprets buf as an array of 64-bit numbers and swaps byte order in-place. Throws ERR_INVALID_BUFFER_SIZE if buf.length is not a multiple of 8.

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

console.log(buf1);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf1.swap64();

console.log(buf1);
// Prints: <Buffer 08 07 06 05 04 03 02 01>

const buf2 = Buffer.from([0x1, 0x2, 0x3]);

buf2.swap64();
// Throws ERR_INVALID_BUFFER_SIZE.

buf.toJSON()#

Added in: v0.9.2

Returns a JSON representation of buf. JSON.stringify() implicitly calls this function when stringifying a Buffer instance.

Buffer.from() accepts objects in the format returned from this method. In particular, Buffer.from(buf.toJSON()) works like Buffer.from(buf).

const buf = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5]);
const json = JSON.stringify(buf);

console.log(json);
// Prints: {"type":"Buffer","data":[1,2,3,4,5]}

const copy = JSON.parse(json, (key, value) => {
  return value && value.type === 'Buffer' ?
    Buffer.from(value) :
    value;
});

console.log(copy);
// Prints: <Buffer 01 02 03 04 05>

buf.toString([encoding[, start[, end]]])#

Added in: v0.1.90
  • encoding <string> The character encoding to use. Default: 'utf8'.
  • start <integer> The byte offset to start decoding at. Default: 0.
  • end <integer> The byte offset to stop decoding at (not inclusive). Default: buf.length.
  • Returns: <string>

Decodes buf to a string according to the specified character encoding in encoding. start and end may be passed to decode only a subset of buf.

If encoding is 'utf8' and a byte sequence in the input is not valid UTF-8, then each invalid byte is replaced with the replacement character U+FFFD.

The maximum length of a string instance (in UTF-16 code units) is available as buffer.constants.MAX_STRING_LENGTH.

const buf1 = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'.
  buf1[i] = i + 97;
}

console.log(buf1.toString('utf8'));
// Prints: abcdefghijklmnopqrstuvwxyz
console.log(buf1.toString('utf8', 0, 5));
// Prints: abcde

const buf2 = Buffer.from('tést');

console.log(buf2.toString('hex'));
// Prints: 74c3a97374
console.log(buf2.toString('utf8', 0, 3));
// Prints: té
console.log(buf2.toString(undefined, 0, 3));
// Prints: té

buf.values()#

Added in: v1.1.0

Creates and returns an iterator for buf values (bytes). This function is called automatically when a Buffer is used in a for..of statement.

const buf = Buffer.from('buffer');

for (const value of buf.values()) {
  console.log(value);
}
// Prints:
//   98
//   117
//   102
//   102
//   101
//   114

for (const value of buf) {
  console.log(value);
}
// Prints:
//   98
//   117
//   102
//   102
//   101
//   114

buf.write(string[, offset[, length]][, encoding])#

Added in: v0.1.90
  • string <string> String to write to buf.
  • offset <integer> Number of bytes to skip before starting to write string. Default: 0.
  • length <integer> Maximum number of bytes to write (written bytes will not exceed buf.length - offset). Default: buf.length - offset.
  • encoding <string> The character encoding of string. Default: 'utf8'.
  • Returns: <integer> Number of bytes written.

Writes string to buf at offset according to the character encoding in encoding. The length parameter is the number of bytes to write. If buf did not contain enough space to fit the entire string, only part of string will be written. However, partially encoded characters will not be written.

const buf = Buffer.alloc(256);

const len = buf.write('\u00bd + \u00bc = \u00be', 0);

console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);
// Prints: 12 bytes: ½ + ¼ = ¾

const buffer = Buffer.alloc(10);

const length = buffer.write('abcd', 8);

console.log(`${length} bytes: ${buffer.toString('utf8', 8, 10)}`);
// Prints: 2 bytes : ab

buf.writeBigInt64BE(value[, offset])#

buf.writeBigInt64LE(value[, offset])#

Added in: v12.0.0, v10.20.0
  • value <bigint> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with the specified endianness (writeBigInt64BE() writes as big endian, writeBigInt64LE() writes as little endian).

value is interpreted and written as a two's complement signed integer.

const buf = Buffer.allocUnsafe(8);

buf.writeBigInt64BE(0x0102030405060708n, 0);

console.log(buf);
// Prints: <Buffer 01 02 03 04 05 06 07 08>

buf.writeBigUInt64BE(value[, offset])#

buf.writeBigUInt64LE(value[, offset])#

Added in: v12.0.0, v10.20.0
  • value <bigint> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endianness (writeBigUInt64BE() writes as big endian, writeBigUInt64LE() writes as little endian).

const buf = Buffer.allocUnsafe(8);

buf.writeBigUInt64LE(0xdecafafecacefaden, 0);

console.log(buf);
// Prints: <Buffer de fa ce ca fe fa ca de>

buf.writeDoubleBE(value[, offset])#

buf.writeDoubleLE(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • value <number> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 8. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with the specified endianness (writeDoubleBE() writes as big endian, writeDoubleLE() writes as little endian). value must be a JavaScript number. Behavior is undefined when value is anything other than a JavaScript number.

const buf = Buffer.allocUnsafe(8);

buf.writeDoubleBE(123.456, 0);

console.log(buf);
// Prints: <Buffer 40 5e dd 2f 1a 9f be 77>

buf.writeDoubleLE(123.456, 0);

console.log(buf);
// Prints: <Buffer 77 be 9f 1a 2f dd 5e 40>

buf.writeFloatBE(value[, offset])#

buf.writeFloatLE(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • value <number> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endianness (writeFloatBE() writes as big endian, writeFloatLE() writes as little endian). value must be a JavaScript number. Behavior is undefined when value is anything other than a JavaScript number.

const buf = Buffer.allocUnsafe(4);

buf.writeFloatBE(0xcafebabe, 0);

console.log(buf);
// Prints: <Buffer 4f 4a fe bb>

buf.writeFloatLE(0xcafebabe, 0);

console.log(buf);
// Prints: <Buffer bb fe 4a 4f>

buf.writeInt8(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.0

Added in: v0.5.0

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 1. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset. value must be a valid signed 8-bit integer. Behavior is undefined when value is anything other than a signed 8-bit integer.

value is interpreted and written as a two's complement signed integer.

const buf = Buffer.allocUnsafe(2);

buf.writeInt8(2, 0);
buf.writeInt8(-2, 1);

console.log(buf);
// Prints: <Buffer 02 fe>

buf.writeInt16BE(value[, offset])#

buf.writeInt16LE(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with the specified endianness (writeInt16BE() writes as big endian, writeInt16LE() writes as little endian). value must be a valid signed 16-bit integer. Behavior is undefined when value is anything other than a signed 16-bit integer.

value is interpreted and written as a two's complement signed integer.

const buf = Buffer.allocUnsafe(4);

buf.writeInt16BE(0x0102, 0);
buf.writeInt16LE(0x0304, 2);

console.log(buf);
// Prints: <Buffer 01 02 04 03>

buf.writeInt32BE(value[, offset])#

buf.writeInt32LE(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with the specified endianness (writeInt32BE() writes aS big endian, writeInt32LE() writes as little endian). value must be a valid signed 32-bit integer. Behavior is undefined when value is anything other than a signed 32-bit integer.

value is interpreted and written as a two's complement signed integer.

const buf = Buffer.allocUnsafe(8);

buf.writeInt32BE(0x01020304, 0);
buf.writeInt32LE(0x05060708, 4);

console.log(buf);
// Prints: <Buffer 01 02 03 04 08 07 06 05>

buf.writeIntBE(value, offset, byteLength)#

buf.writeIntLE(value, offset, byteLength)#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset and byteLength to uint32 anymore.

v0.11.15

Added in: v0.11.15

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to write. Must satisfy 0 < byteLength <= 6.
  • Returns: <integer> offset plus the number of bytes written.

Writes byteLength bytes of value to buf at the specified offset. Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than a signed integer.

const buf = Buffer.allocUnsafe(6);

buf.writeIntBE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer 12 34 56 78 90 ab>

buf.writeIntLE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer ab 90 78 56 34 12>

buf.writeUInt8(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.0

Added in: v0.5.0

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 1. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset. value must be a valid unsigned 8-bit integer. Behavior is undefined when value is anything other than an unsigned 8-bit integer.

const buf = Buffer.allocUnsafe(4);

buf.writeUInt8(0x3, 0);
buf.writeUInt8(0x4, 1);
buf.writeUInt8(0x23, 2);
buf.writeUInt8(0x42, 3);

console.log(buf);
// Prints: <Buffer 03 04 23 42>

buf.writeUInt16BE(value[, offset])#

buf.writeUInt16LE(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 2. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with the specified endianness (writeUInt16BE() writes as big endian, writeUInt16LE() writes as little endian). value must be a valid unsigned 16-bit integer. Behavior is undefined when value is anything other than an unsigned 16-bit integer.

const buf = Buffer.allocUnsafe(4);

buf.writeUInt16BE(0xdead, 0);
buf.writeUInt16BE(0xbeef, 2);

console.log(buf);
// Prints: <Buffer de ad be ef>

buf.writeUInt16LE(0xdead, 0);
buf.writeUInt16LE(0xbeef, 2);

console.log(buf);
// Prints: <Buffer ad de ef be>

buf.writeUInt32BE(value[, offset])#

buf.writeUInt32LE(value[, offset])#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4. Default: 0.
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with the specified endianness (writeUInt32BE() writes as big endian, writeUInt32LE() writes as little endian). value must be a valid unsigned 32-bit integer. Behavior is undefined when value is anything other than an unsigned 32-bit integer.

const buf = Buffer.allocUnsafe(4);

buf.writeUInt32BE(0xfeedface, 0);

console.log(buf);
// Prints: <Buffer fe ed fa ce>

buf.writeUInt32LE(0xfeedface, 0);

console.log(buf);
// Prints: <Buffer ce fa ed fe>

buf.writeUIntBE(value, offset, byteLength)#

buf.writeUIntLE(value, offset, byteLength)#

History
VersionChanges
v10.0.0

Removed noAssert and no implicit coercion of the offset and byteLength to uint32 anymore.

v0.5.5

Added in: v0.5.5

  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to write. Must satisfy 0 < byteLength <= 6.
  • Returns: <integer> offset plus the number of bytes written.

Writes byteLength bytes of value to buf at the specified offset. Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than an unsigned integer.

const buf = Buffer.allocUnsafe(6);

buf.writeUIntBE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer 12 34 56 78 90 ab>

buf.writeUIntLE(0x1234567890ab, 0, 6);

console.log(buf);
// Prints: <Buffer ab 90 78 56 34 12>

new Buffer(array)#

History
VersionChanges
v10.0.0

Calling this constructor emits a deprecation warning when run from code outside the node_modules directory.

v7.2.1

Calling this constructor no longer emits a deprecation warning.

v7.0.0

Calling this constructor emits a deprecation warning now.

v6.0.0

Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.from(array) instead.

See Buffer.from(array).

new Buffer(arrayBuffer[, byteOffset[, length]])#

History
VersionChanges
v10.0.0

Calling this constructor emits a deprecation warning when run from code outside the node_modules directory.

v7.2.1

Calling this constructor no longer emits a deprecation warning.

v7.0.0

Calling this constructor emits a deprecation warning now.

v6.0.0

The byteOffset and length parameters are supported now.

v6.0.0

Deprecated since: v6.0.0

v3.0.0

Added in: v3.0.0

Stability: 0 - Deprecated: Use Buffer.from(arrayBuffer[, byteOffset[, length]]) instead.

See Buffer.from(arrayBuffer[, byteOffset[, length]]).

new Buffer(buffer)#

History
VersionChanges
v10.0.0

Calling this constructor emits a deprecation warning when run from code outside the node_modules directory.

v7.2.1

Calling this constructor no longer emits a deprecation warning.

v7.0.0

Calling this constructor emits a deprecation warning now.

v6.0.0

Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.from(buffer) instead.

See Buffer.from(buffer).

new Buffer(size)#

History
VersionChanges
v10.0.0

Calling this constructor emits a deprecation warning when run from code outside the node_modules directory.

v8.0.0

The new Buffer(size) will return zero-filled memory by default.

v7.2.1

Calling this constructor no longer emits a deprecation warning.

v7.0.0

Calling this constructor emits a deprecation warning now.

v6.0.0

Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.alloc() instead (also see Buffer.allocUnsafe()).

  • size <integer> The desired length of the new Buffer.

See Buffer.alloc() and Buffer.allocUnsafe(). This variant of the constructor is equivalent to Buffer.alloc().

new Buffer(string[, encoding])#

History
VersionChanges
v10.0.0

Calling this constructor emits a deprecation warning when run from code outside the node_modules directory.

v7.2.1

Calling this constructor no longer emits a deprecation warning.

v7.0.0

Calling this constructor emits a deprecation warning now.

v6.0.0

Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.from(string[, encoding]) instead.

  • string <string> String to encode.
  • encoding <string> The encoding of string. Default: 'utf8'.

See Buffer.from(string[, encoding]).

buffer.INSPECT_MAX_BYTES#

Added in: v0.5.4

Returns the maximum number of bytes that will be returned when buf.inspect() is called. This can be overridden by user modules. See util.inspect() for more details on buf.inspect() behavior.

This is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

buffer.kMaxLength#

Added in: v3.0.0
  • <integer> The largest size allowed for a single Buffer instance.

An alias for buffer.constants.MAX_LENGTH.

This is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

buffer.transcode(source, fromEnc, toEnc)#

History
VersionChanges
v8.0.0

The source parameter can now be a Uint8Array.

v7.1.0

Added in: v7.1.0

Re-encodes the given Buffer or Uint8Array instance from one character encoding to another. Returns a new Buffer instance.

Throws if the fromEnc or toEnc specify invalid character encodings or if conversion from fromEnc to toEnc is not permitted.

Encodings supported by buffer.transcode() are: 'ascii', 'utf8', 'utf16le', 'ucs2', 'latin1', and 'binary'.

The transcoding process will use substitution characters if a given byte sequence cannot be adequately represented in the target encoding. For instance:

const buffer = require('buffer');

const newBuf = buffer.transcode(Buffer.from('€'), 'utf8', 'ascii');
console.log(newBuf.toString('ascii'));
// Prints: '?'

Because the Euro (€) sign is not representable in US-ASCII, it is replaced with ? in the transcoded Buffer.

This is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

Class: SlowBuffer#

Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.allocUnsafeSlow() instead.

See Buffer.allocUnsafeSlow(). This was never a class in the sense that the constructor always returned a Buffer instance, rather than a SlowBuffer instance.

new SlowBuffer(size)#

Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Buffer.allocUnsafeSlow() instead.

  • size <integer> The desired length of the new SlowBuffer.

See Buffer.allocUnsafeSlow().

Buffer Constants#

Added in: v8.2.0

buffer.constants is a property on the buffer module returned by require('buffer'), not on the Buffer global or a Buffer instance.

buffer.constants.MAX_LENGTH#

Added in: v8.2.0
  • <integer> The largest size allowed for a single Buffer instance.

On 32-bit architectures, this value currently is (2^30)-1 (~1GB). On 64-bit architectures, this value currently is (2^31)-1 (~2GB).

This value is also available as buffer.kMaxLength.

buffer.constants.MAX_STRING_LENGTH#

Added in: v8.2.0
  • <integer> The largest length allowed for a single string instance.

Represents the largest length that a string primitive can have, counted in UTF-16 code units.

This value may depend on the JS engine that is being used.

Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe()#

In versions of Node.js prior to 6.0.0, Buffer instances were created using the Buffer constructor function, which allocates the returned Buffer differently based on what arguments are provided:

  • Passing a number as the first argument to Buffer() (e.g. new Buffer(10)) allocates a new Buffer object of the specified size. Prior to Node.js 8.0.0, the memory allocated for such Buffer instances is not initialized and can contain sensitive data. Such Buffer instances must be subsequently initialized by using either buf.fill(0) or by writing to the entire Buffer before reading data from the Buffer. While this behavior is intentional to improve performance, development experience has demonstrated that a more explicit distinction is required between creating a fast-but-uninitialized Buffer versus creating a slower-but-safer Buffer. Since Node.js 8.0.0, Buffer(num) and new Buffer(num) return a Buffer with initialized memory.
  • Passing a string, array, or Buffer as the first argument copies the passed object's data into the Buffer.
  • Passing an ArrayBuffer or a SharedArrayBuffer returns a Buffer that shares allocated memory with the given array buffer.

Because the behavior of new Buffer() is different depending on the type of the first argument, security and reliability issues can be inadvertently introduced into applications when argument validation or Buffer initialization is not performed.

For example, if an attacker can cause an application to receive a number where a string is expected, the application may call new Buffer(100) instead of new Buffer("100"), leading it to allocate a 100 byte buffer instead of allocating a 3 byte buffer with content "100". This is commonly possible using JSON API calls. Since JSON distinguishes between numeric and string types, it allows injection of numbers where a naively written application that does not validate its input sufficiently might expect to always receive a string. Before Node.js 8.0.0, the 100 byte buffer might contain arbitrary pre-existing in-memory data, so may be used to expose in-memory secrets to a remote attacker. Since Node.js 8.0.0, exposure of memory cannot occur because the data is zero-filled. However, other attacks are still possible, such as causing very large buffers to be allocated by the server, leading to performance degradation or crashing on memory exhaustion.

To make the creation of Buffer instances more reliable and less error-prone, the various forms of the new Buffer() constructor have been deprecated and replaced by separate Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe() methods.

Developers should migrate all existing uses of the new Buffer() constructors to one of these new APIs.

Buffer instances returned by Buffer.allocUnsafe() and Buffer.from(array) may be allocated off a shared internal memory pool if size is less than or equal to half Buffer.poolSize. Instances returned by Buffer.allocUnsafeSlow() never use the shared internal memory pool.

The --zero-fill-buffers command line option#

Added in: v5.10.0

Node.js can be started using the --zero-fill-buffers command line option to cause all newly-allocated Buffer instances to be zero-filled upon creation by default. Without the option, buffers created with Buffer.allocUnsafe(), Buffer.allocUnsafeSlow(), and new SlowBuffer(size) are not zero-filled. Use of this flag can have a measurable negative impact on performance. Use the --zero-fill-buffers option only when necessary to enforce that newly allocated Buffer instances cannot contain old data that is potentially sensitive.

$ node --zero-fill-buffers
> Buffer.allocUnsafe(5);
<Buffer 00 00 00 00 00>

What makes Buffer.allocUnsafe() and Buffer.allocUnsafeSlow() "unsafe"?#

When calling Buffer.allocUnsafe() and Buffer.allocUnsafeSlow(), the segment of allocated memory is uninitialized (it is not zeroed-out). While this design makes the allocation of memory quite fast, the allocated segment of memory might contain old data that is potentially sensitive. Using a Buffer created by Buffer.allocUnsafe() without completely overwriting the memory can allow this old data to be leaked when the Buffer memory is read.

While there are clear performance advantages to using Buffer.allocUnsafe(), extra care must be taken in order to avoid introducing security vulnerabilities into an application.

C++ Addons#

Addons are dynamically-linked shared objects written in C++. The require() function can load Addons as ordinary Node.js modules. Addons provide an interface between JavaScript and C/C++ libraries.

There are three options for implementing Addons: N-API, nan, or direct use of internal V8, libuv and Node.js libraries. Unless there is a need for direct access to functionality which is not exposed by N-API, use N-API. Refer to C/C++ Addons with N-API for more information on N-API.

When not using N-API, implementing Addons is complicated, involving knowledge of several components and APIs:

  • V8: the C++ library Node.js currently uses to provide the JavaScript implementation. V8 provides the mechanisms for creating objects, calling functions, etc. V8's API is documented mostly in the v8.h header file (deps/v8/include/v8.h in the Node.js source tree), which is also available online.

  • libuv: The C library that implements the Node.js event loop, its worker threads and all of the asynchronous behaviors of the platform. It also serves as a cross-platform abstraction library, giving easy, POSIX-like access across all major operating systems to many common system tasks, such as interacting with the filesystem, sockets, timers, and system events. libuv also provides a pthreads-like threading abstraction that may be used to power more sophisticated asynchronous Addons that need to move beyond the standard event loop. Addon authors are encouraged to think about how to avoid blocking the event loop with I/O or other time-intensive tasks by off-loading work via libuv to non-blocking system operations, worker threads or a custom use of libuv's threads.

  • Internal Node.js libraries. Node.js itself exports C++ APIs that Addons can use, the most important of which is the node::ObjectWrap class.

  • Node.js includes other statically linked libraries including OpenSSL. These other libraries are located in the deps/ directory in the Node.js source tree. Only the libuv, OpenSSL, V8 and zlib symbols are purposefully re-exported by Node.js and may be used to various extents by Addons. See Linking to libraries included with Node.js for additional information.

All of the following examples are available for download and may be used as the starting-point for an Addon.

Hello world#

This "Hello world" example is a simple Addon, written in C++, that is the equivalent of the following JavaScript code:

module.exports.hello = () => 'world';

First, create the file hello.cc:

// hello.cc
#include <node.h>

namespace demo {

using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::NewStringType;
using v8::Object;
using v8::String;
using v8::Value;

void Method(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  args.GetReturnValue().Set(String::NewFromUtf8(
      isolate, "world", NewStringType::kNormal).ToLocalChecked());
}

void Initialize(Local<Object> exports) {
  NODE_SET_METHOD(exports, "hello", Method);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Initialize)

}  // namespace demo

All Node.js Addons must export an initialization function following the pattern:

void Initialize(Local<Object> exports);
NODE_MODULE(NODE_GYP_MODULE_NAME, Initialize)

There is no semi-colon after NODE_MODULE as it's not a function (see node.h).

The module_name must match the filename of the final binary (excluding the .node suffix).

In the hello.cc example, then, the initialization function is Initialize and the addon module name is addon.

When building addons with node-gyp, using the macro NODE_GYP_MODULE_NAME as the first parameter of NODE_MODULE() will ensure that the name of the final binary will be passed to NODE_MODULE().

Context-aware addons#

There are environments in which Node.js addons may need to be loaded multiple times in multiple contexts. For example, the Electron runtime runs multiple instances of Node.js in a single process. Each instance will have its own require() cache, and thus each instance will need a native addon to behave correctly when loaded via require(). From the addon's perspective, this means that it must support multiple initializations.

A context-aware addon can be constructed by using the macro NODE_MODULE_INITIALIZER, which expands to the name of a function which Node.js will expect to find when it loads an addon. An addon can thus be initialized as in the following example:

using namespace v8;

extern "C" NODE_MODULE_EXPORT void
NODE_MODULE_INITIALIZER(Local<Object> exports,
                        Local<Value> module,
                        Local<Context> context) {
  /* Perform addon initialization steps here. */
}

Another option is to use the macro NODE_MODULE_INIT(), which will also construct a context-aware addon. Unlike NODE_MODULE(), which is used to construct an addon around a given addon initializer function, NODE_MODULE_INIT() serves as the declaration of such an initializer to be followed by a function body.

The following three variables may be used inside the function body following an invocation of NODE_MODULE_INIT():

  • Local<Object> exports,
  • Local<Value> module, and
  • Local<Context> context

The choice to build a context-aware addon carries with it the responsibility of carefully managing global static data. Since the addon may be loaded multiple times, potentially even from different threads, any global static data stored in the addon must be properly protected, and must not contain any persistent references to JavaScript objects. The reason for this is that JavaScript objects are only valid in one context, and will likely cause a crash when accessed from the wrong context or from a different thread than the one on which they were created.

The context-aware addon can be structured to avoid global static data by performing the following steps:

  • Define a class which will hold per-addon-instance data and which has a static member of the form

    ```C++
static void DeleteInstance(void* data) {
  // Cast `data` to an instance of the class and delete it.
}
```
  • Heap-allocate an instance of this class in the addon initializer. This can be accomplished using the new keyword.
  • Call node::AddEnvironmentCleanupHook(), passing it the above-created instance and a pointer to DeleteInstance(). This will ensure the instance is deleted when the environment is torn down.
  • Store the instance of the class in a v8::External, and
  • Pass the v8::External to all methods exposed to JavaScript by passing it to v8::FunctionTemplate::New() or v8::Function::New() which creates the native-backed JavaScript functions. The third parameter of v8::FunctionTemplate::New() or v8::Function::New() accepts the v8::External and makes it available in the native callback using the v8::FunctionCallbackInfo::Data() method.

This will ensure that the per-addon-instance data reaches each binding that can be called from JavaScript. The per-addon-instance data must also be passed into any asynchronous callbacks the addon may create.

The following example illustrates the implementation of a context-aware addon:

#include <node.h>

using namespace v8;

class AddonData {
 public:
  explicit AddonData(Isolate* isolate):
      call_count(0) {
    // Ensure this per-addon-instance data is deleted at environment cleanup.
    node::AddEnvironmentCleanupHook(isolate, DeleteInstance, this);
  }

  // Per-addon data.
  int call_count;

  static void DeleteInstance(void* data) {
    delete static_cast<AddonData*>(data);
  }
};

static void Method(const v8::FunctionCallbackInfo<v8::Value>& info) {
  // Retrieve the per-addon-instance data.
  AddonData* data =
      reinterpret_cast<AddonData*>(info.Data().As<External>()->Value());
  data->call_count++;
  info.GetReturnValue().Set((double)data->call_count);
}

// Initialize this addon to be context-aware.
NODE_MODULE_INIT(/* exports, module, context */) {
  Isolate* isolate = context->GetIsolate();

  // Create a new instance of `AddonData` for this instance of the addon and
  // tie its life cycle to that of the Node.js environment.
  AddonData* data = new AddonData(isolate);

  // Wrap the data in a `v8::External` so we can pass it to the method we
  // expose.
  Local<External> external = External::New(isolate, data);

  // Expose the method `Method` to JavaScript, and make sure it receives the
  // per-addon-instance data we created above by passing `external` as the
  // third parameter to the `FunctionTemplate` constructor.
  exports->Set(context,
               String::NewFromUtf8(isolate, "method", NewStringType::kNormal)
                  .ToLocalChecked(),
               FunctionTemplate::New(isolate, Method, external)
                  ->GetFunction(context).ToLocalChecked()).FromJust();
}

Worker support#

In order to be loaded from multiple Node.js environments, such as a main thread and a Worker thread, an add-on needs to either:

  • Be an N-API addon, or
  • Be declared as context-aware using NODE_MODULE_INIT() as described above

In order to support Worker threads, addons need to clean up any resources they may have allocated when such a thread exists. This can be achieved through the usage of the AddEnvironmentCleanupHook() function:

void AddEnvironmentCleanupHook(v8::Isolate* isolate,
                               void (*fun)(void* arg),
                               void* arg);

This function adds a hook that will run before a given Node.js instance shuts down. If necessary, such hooks can be removed using RemoveEnvironmentCleanupHook() before they are run, which has the same signature. Callbacks are run in last-in first-out order.

The following addon.cc uses AddEnvironmentCleanupHook:

// addon.cc
#include <assert.h>
#include <stdlib.h>
#include <node.h>

using node::AddEnvironmentCleanupHook;
using v8::HandleScope;
using v8::Isolate;
using v8::Local;
using v8::Object;

// Note: In a real-world application, do not rely on static/global data.
static char cookie[] = "yum yum";
static int cleanup_cb1_called = 0;
static int cleanup_cb2_called = 0;

static void cleanup_cb1(void* arg) {
  Isolate* isolate = static_cast<Isolate*>(arg);
  HandleScope scope(isolate);
  Local<Object> obj = Object::New(isolate);
  assert(!obj.IsEmpty());  // assert VM is still alive
  assert(obj->IsObject());
  cleanup_cb1_called++;
}

static void cleanup_cb2(void* arg) {
  assert(arg == static_cast<void*>(cookie));
  cleanup_cb2_called++;
}

static void sanity_check(void*) {
  assert(cleanup_cb1_called == 1);
  assert(cleanup_cb2_called == 1);
}

// Initialize this addon to be context-aware.
NODE_MODULE_INIT(/* exports, module, context */) {
  Isolate* isolate = context->GetIsolate();

  AddEnvironmentCleanupHook(isolate, sanity_check, nullptr);
  AddEnvironmentCleanupHook(isolate, cleanup_cb2, cookie);
  AddEnvironmentCleanupHook(isolate, cleanup_cb1, isolate);
}

Test in JavaScript by running:

// test.js
require('./build/Release/addon');

Building#

Once the source code has been written, it must be compiled into the binary addon.node file. To do so, create a file called binding.gyp in the top-level of the project describing the build configuration of the module using a JSON-like format. This file is used by node-gyp, a tool written specifically to compile Node.js Addons.

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [ "hello.cc" ]
    }
  ]
}

A version of the node-gyp utility is bundled and distributed with Node.js as part of npm. This version is not made directly available for developers to use and is intended only to support the ability to use the npm install command to compile and install Addons. Developers who wish to use node-gyp directly can install it using the command npm install -g node-gyp. See the node-gyp installation instructions for more information, including platform-specific requirements.

Once the binding.gyp file has been created, use node-gyp configure to generate the appropriate project build files for the current platform. This will generate either a Makefile (on Unix platforms) or a vcxproj file (on Windows) in the build/ directory.

Next, invoke the node-gyp build command to generate the compiled addon.node file. This will be put into the build/Release/ directory.

When using npm install to install a Node.js Addon, npm uses its own bundled version of node-gyp to perform this same set of actions, generating a compiled version of the Addon for the user's platform on demand.

Once built, the binary Addon can be used from within Node.js by pointing require() to the built addon.node module:

// hello.js
const addon = require('./build/Release/addon');

console.log(addon.hello());
// Prints: 'world'

Because the exact path to the compiled Addon binary can vary depending on how it is compiled (i.e. sometimes it may be in ./build/Debug/), Addons can use the bindings package to load the compiled module.

While the bindings package implementation is more sophisticated in how it locates Addon modules, it is essentially using a try…catch pattern similar to:

try {
  return require('./build/Release/addon.node');
} catch (err) {
  return require('./build/Debug/addon.node');
}

Linking to libraries included with Node.js#

Node.js uses statically linked libraries such as V8, libuv and OpenSSL. All Addons are required to link to V8 and may link to any of the other dependencies as well. Typically, this is as simple as including the appropriate #include <...> statements (e.g. #include <v8.h>) and node-gyp will locate the appropriate headers automatically. However, there are a few caveats to be aware of:

  • When node-gyp runs, it will detect the specific release version of Node.js and download either the full source tarball or just the headers. If the full source is downloaded, Addons will have complete access to the full set of Node.js dependencies. However, if only the Node.js headers are downloaded, then only the symbols exported by Node.js will be available.

  • node-gyp can be run using the --nodedir flag pointing at a local Node.js source image. Using this option, the Addon will have access to the full set of dependencies.

Loading Addons using require()#

The filename extension of the compiled Addon binary is .node (as opposed to .dll or .so). The require() function is written to look for files with the .node file extension and initialize those as dynamically-linked libraries.

When calling require(), the .node extension can usually be omitted and Node.js will still find and initialize the Addon. One caveat, however, is that Node.js will first attempt to locate and load modules or JavaScript files that happen to share the same base name. For instance, if there is a file addon.js in the same directory as the binary addon.node, then require('addon') will give precedence to the addon.js file and load it instead.

Native Abstractions for Node.js#

Each of the examples illustrated in this document make direct use of the Node.js and V8 APIs for implementing Addons. The V8 API can, and has, changed dramatically from one V8 release to the next (and one major Node.js release to the next). With each change, Addons may need to be updated and recompiled in order to continue functioning. The Node.js release schedule is designed to minimize the frequency and impact of such changes but there is little that Node.js can do currently to ensure stability of the V8 APIs.

The Native Abstractions for Node.js (or nan) provide a set of tools that Addon developers are recommended to use to keep compatibility between past and future releases of V8 and Node.js. See the nan examples for an illustration of how it can be used.

N-API#

Stability: 2 - Stable

N-API is an API for building native Addons. It is independent from the underlying JavaScript runtime (e.g. V8) and is maintained as part of Node.js itself. This API will be Application Binary Interface (ABI) stable across versions of Node.js. It is intended to insulate Addons from changes in the underlying JavaScript engine and allow modules compiled for one version to run on later versions of Node.js without recompilation. Addons are built/packaged with the same approach/tools outlined in this document (node-gyp, etc.). The only difference is the set of APIs that are used by the native code. Instead of using the V8 or Native Abstractions for Node.js APIs, the functions available in the N-API are used.

Creating and maintaining an addon that benefits from the ABI stability provided by N-API carries with it certain implementation considerations.

To use N-API in the above "Hello world" example, replace the content of hello.cc with the following. All other instructions remain the same.

// hello.cc using N-API
#include <node_api.h>

namespace demo {

napi_value Method(napi_env env, napi_callback_info args) {
  napi_value greeting;
  napi_status status;

  status = napi_create_string_utf8(env, "world", NAPI_AUTO_LENGTH, &greeting);
  if (status != napi_ok) return nullptr;
  return greeting;
}

napi_value init(napi_env env, napi_value exports) {
  napi_status status;
  napi_value fn;

  status = napi_create_function(env, nullptr, 0, Method, nullptr, &fn);
  if (status != napi_ok) return nullptr;

  status = napi_set_named_property(env, exports, "hello", fn);
  if (status != napi_ok) return nullptr;
  return exports;
}

NAPI_MODULE(NODE_GYP_MODULE_NAME, init)

}  // namespace demo

The functions available and how to use them are documented in C/C++ Addons with N-API.

Addon examples#

Following are some example Addons intended to help developers get started. The examples make use of the V8 APIs. Refer to the online V8 reference for help with the various V8 calls, and V8's Embedder's Guide for an explanation of several concepts used such as handles, scopes, function templates, etc.

Each of these examples using the following binding.gyp file:

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [ "addon.cc" ]
    }
  ]
}

In cases where there is more than one .cc file, simply add the additional filename to the sources array:

"sources": ["addon.cc", "myexample.cc"]

Once the binding.gyp file is ready, the example Addons can be configured and built using node-gyp:

$ node-gyp configure build

Function arguments#

Addons will typically expose objects and functions that can be accessed from JavaScript running within Node.js. When functions are invoked from JavaScript, the input arguments and return value must be mapped to and from the C/C++ code.

The following example illustrates how to read function arguments passed from JavaScript and how to return a result:

// addon.cc
#include <node.h>

namespace demo {

using v8::Exception;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::NewStringType;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;

// This is the implementation of the "add" method
// Input arguments are passed using the
// const FunctionCallbackInfo<Value>& args struct
void Add(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  // Check the number of arguments passed.
  if (args.Length() < 2) {
    // Throw an Error that is passed back to JavaScript
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate,
                            "Wrong number of arguments",
                            NewStringType::kNormal).ToLocalChecked()));
    return;
  }

  // Check the argument types
  if (!args[0]->IsNumber() || !args[1]->IsNumber()) {
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate,
                            "Wrong arguments",
                            NewStringType::kNormal).ToLocalChecked()));
    return;
  }

  // Perform the operation
  double value =
      args[0].As<Number>()->Value() + args[1].As<Number>()->Value();
  Local<Number> num = Number::New(isolate, value);

  // Set the return value (using the passed in
  // FunctionCallbackInfo<Value>&)
  args.GetReturnValue().Set(num);
}

void Init(Local<Object> exports) {
  NODE_SET_METHOD(exports, "add", Add);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

}  // namespace demo

Once compiled, the example Addon can be required and used from within Node.js:

// test.js
const addon = require('./build/Release/addon');

console.log('This should be eight:', addon.add(3, 5));

Callbacks#

It is common practice within Addons to pass JavaScript functions to a C++ function and execute them from there. The following example illustrates how to invoke such callbacks:

// addon.cc
#include <node.h>

namespace demo {

using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::NewStringType;
using v8::Null;
using v8::Object;
using v8::String;
using v8::Value;

void RunCallback(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();
  Local<Function> cb = Local<Function>::Cast(args[0]);
  const unsigned argc = 1;
  Local<Value> argv[argc] = {
      String::NewFromUtf8(isolate,
                          "hello world",
                          NewStringType::kNormal).ToLocalChecked() };
  cb->Call(context, Null(isolate), argc, argv).ToLocalChecked();
}

void Init(Local<Object> exports, Local<Object> module) {
  NODE_SET_METHOD(module, "exports", RunCallback);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

}  // namespace demo

This example uses a two-argument form of Init() that receives the full module object as the second argument. This allows the Addon to completely overwrite exports with a single function instead of adding the function as a property of exports.

To test it, run the following JavaScript:

// test.js
const addon = require('./build/Release/addon');

addon((msg) => {
  console.log(msg);
// Prints: 'hello world'
});

In this example, the callback function is invoked synchronously.

Object factory#

Addons can create and return new objects from within a C++ function as illustrated in the following example. An object is created and returned with a property msg that echoes the string passed to createObject():

// addon.cc
#include <node.h>

namespace demo {

using v8::Context;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::NewStringType;
using v8::Object;
using v8::String;
using v8::Value;

void CreateObject(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();

  Local<Object> obj = Object::New(isolate);
  obj->Set(context,
           String::NewFromUtf8(isolate,
                               "msg",
                               NewStringType::kNormal).ToLocalChecked(),
                               args[0]->ToString(context).ToLocalChecked())
           .FromJust();

  args.GetReturnValue().Set(obj);
}

void Init(Local<Object> exports, Local<Object> module) {
  NODE_SET_METHOD(module, "exports", CreateObject);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

}  // namespace demo

To test it in JavaScript:

// test.js
const addon = require('./build/Release/addon');

const obj1 = addon('hello');
const obj2 = addon('world');
console.log(obj1.msg, obj2.msg);
// Prints: 'hello world'

Function factory#

Another common scenario is creating JavaScript functions that wrap C++ functions and returning those back to JavaScript:

// addon.cc
#include <node.h>

namespace demo {

using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::NewStringType;
using v8::Object;
using v8::String;
using v8::Value;

void MyFunction(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  args.GetReturnValue().Set(String::NewFromUtf8(
      isolate, "hello world", NewStringType::kNormal).ToLocalChecked());
}

void CreateFunction(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  Local<Context> context = isolate->GetCurrentContext();
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, MyFunction);
  Local<Function> fn = tpl->GetFunction(context).ToLocalChecked();

  // omit this to make it anonymous
  fn->SetName(String::NewFromUtf8(
      isolate, "theFunction", NewStringType::kNormal).ToLocalChecked());

  args.GetReturnValue().Set(fn);
}

void Init(Local<Object> exports, Local<Object> module) {
  NODE_SET_METHOD(module, "exports", CreateFunction);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, Init)

}  // namespace demo

To test:

// test.js
const addon = require('./build/Release/addon');

const fn = addon();
console.log(fn());
// Prints: 'hello world'

Wrapping C++ objects#

It is also possible to wrap C++ objects/classes in a way that allows new instances to be created using the JavaScript new operator:

// addon.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using v8::Local;
using v8::Object;

void InitAll(Local<Object> exports) {
  MyObject::Init(exports);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll)

}  // namespace demo

Then, in myobject.h, the wrapper class inherits from node::ObjectWrap:

// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H

#include <node.h>
#include <node_object_wrap.h>

namespace demo {

class MyObject : public node::ObjectWrap {
 public:
  static void Init(v8::Local<v8::Object> exports);

 private:
  explicit MyObject(double value = 0);
  ~MyObject();

  static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
  static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);

  double value_;
};

}  // namespace demo

#endif

In myobject.cc, implement the various methods that are to be exposed. Below, the method plusOne() is exposed by adding it to the constructor's prototype:

// myobject.cc
#include "myobject.h"

namespace demo {

using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::NewStringType;
using v8::Number;
using v8::Object;
using v8::ObjectTemplate;
using v8::String;
using v8::Value;

MyObject::MyObject(double value) : value_(value) {
}

MyObject::~MyObject() {
}

void MyObject::Init(Local<Object> exports) {
  Isolate* isolate = exports->GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();

  Local<ObjectTemplate> addon_data_tpl = ObjectTemplate::New(isolate);
  addon_data_tpl->SetInternalFieldCount(1);  // 1 field for the MyObject::New()
  Local<Object> addon_data =
      addon_data_tpl->NewInstance(context).ToLocalChecked();

  // Prepare constructor template
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New, addon_data);
  tpl->SetClassName(String::NewFromUtf8(
      isolate, "MyObject", NewStringType::kNormal).ToLocalChecked());
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  // Prototype
  NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);

  Local<Function> constructor = tpl->GetFunction(context).ToLocalChecked();
  addon_data->SetInternalField(0, constructor);
  exports->Set(context, String::NewFromUtf8(
      isolate, "MyObject", NewStringType::kNormal).ToLocalChecked(),
               constructor).FromJust();
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ?
        0 : args[0]->NumberValue(context).FromMaybe(0);
    MyObject* obj = new MyObject(value);
    obj->Wrap(args.This());
    args.GetReturnValue().Set(args.This());
  } else {
    // Invoked as plain function `MyObject(...)`, turn into construct call.
    const int argc = 1;
    Local<Value> argv[argc] = { args[0] };
    Local<Function> cons =
        args.Data().As<Object>()->GetInternalField(0).As<Function>();
    Local<Object> result =
        cons->NewInstance(context, argc, argv).ToLocalChecked();
    args.GetReturnValue().Set(result);
  }
}

void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder());
  obj->value_ += 1;

  args.GetReturnValue().Set(Number::New(isolate, obj->value_));
}

}  // namespace demo

To build this example, the myobject.cc file must be added to the binding.gyp:

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [
        "addon.cc",
        "myobject.cc"
      ]
    }
  ]
}

Test it with:

// test.js
const addon = require('./build/Release/addon');

const obj = new addon.MyObject(10);
console.log(obj.plusOne());
// Prints: 11
console.log(obj.plusOne());
// Prints: 12
console.log(obj.plusOne());
// Prints: 13

The destructor for a wrapper object will run when the object is garbage-collected. For destructor testing, there are command-line flags that can be used to make it possible to force garbage collection. These flags are provided by the underlying V8 JavaScript engine. They are subject to change or removal at any time. They are not documented by Node.js or V8, and they should never be used outside of testing.

Factory of wrapped objects#

Alternatively, it is possible to use a factory pattern to avoid explicitly creating object instances using the JavaScript new operator:

const obj = addon.createObject();
// instead of:
// const obj = new addon.Object();

First, the createObject() method is implemented in addon.cc:

// addon.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;

void CreateObject(const FunctionCallbackInfo<Value>& args) {
  MyObject::NewInstance(args);
}

void InitAll(Local<Object> exports, Local<Object> module) {
  MyObject::Init(exports->GetIsolate());

  NODE_SET_METHOD(module, "exports", CreateObject);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll)

}  // namespace demo

In myobject.h, the static method NewInstance() is added to handle instantiating the object. This method takes the place of using new in JavaScript:

// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H

#include <node.h>
#include <node_object_wrap.h>

namespace demo {

class MyObject : public node::ObjectWrap {
 public:
  static void Init(v8::Isolate* isolate);
  static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);

 private:
  explicit MyObject(double value = 0);
  ~MyObject();

  static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
  static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);
  static v8::Global<v8::Function> constructor;
  double value_;
};

}  // namespace demo

#endif

The implementation in myobject.cc is similar to the previous example:

// myobject.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using node::AddEnvironmentCleanupHook;
using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Global;
using v8::Isolate;
using v8::Local;
using v8::NewStringType;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;

// Warning! This is not thread-safe, this addon cannot be used for worker
// threads.
Global<Function> MyObject::constructor;

MyObject::MyObject(double value) : value_(value) {
}

MyObject::~MyObject() {
}

void MyObject::Init(Isolate* isolate) {
  // Prepare constructor template
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
  tpl->SetClassName(String::NewFromUtf8(
      isolate, "MyObject", NewStringType::kNormal).ToLocalChecked());
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  // Prototype
  NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);

  Local<Context> context = isolate->GetCurrentContext();
  constructor.Reset(isolate, tpl->GetFunction(context).ToLocalChecked());

  AddEnvironmentCleanupHook(isolate, [](void*) {
    constructor.Reset();
  }, nullptr);
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ?
        0 : args[0]->NumberValue(context).FromMaybe(0);
    MyObject* obj = new MyObject(value);
    obj->Wrap(args.This());
    args.GetReturnValue().Set(args.This());
  } else {
    // Invoked as plain function `MyObject(...)`, turn into construct call.
    const int argc = 1;
    Local<Value> argv[argc] = { args[0] };
    Local<Function> cons = Local<Function>::New(isolate, constructor);
    Local<Object> instance =
        cons->NewInstance(context, argc, argv).ToLocalChecked();
    args.GetReturnValue().Set(instance);
  }
}

void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  const unsigned argc = 1;
  Local<Value> argv[argc] = { args[0] };
  Local<Function> cons = Local<Function>::New(isolate, constructor);
  Local<Context> context = isolate->GetCurrentContext();
  Local<Object> instance =
      cons->NewInstance(context, argc, argv).ToLocalChecked();

  args.GetReturnValue().Set(instance);
}

void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder());
  obj->value_ += 1;

  args.GetReturnValue().Set(Number::New(isolate, obj->value_));
}

}  // namespace demo

Once again, to build this example, the myobject.cc file must be added to the binding.gyp:

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [
        "addon.cc",
        "myobject.cc"
      ]
    }
  ]
}

Test it with:

// test.js
const createObject = require('./build/Release/addon');

const obj = createObject(10);
console.log(obj.plusOne());
// Prints: 11
console.log(obj.plusOne());
// Prints: 12
console.log(obj.plusOne());
// Prints: 13

const obj2 = createObject(20);
console.log(obj2.plusOne());
// Prints: 21
console.log(obj2.plusOne());
// Prints: 22
console.log(obj2.plusOne());
// Prints: 23

Passing wrapped objects around#

In addition to wrapping and returning C++ objects, it is possible to pass wrapped objects around by unwrapping them with the Node.js helper function node::ObjectWrap::Unwrap. The following examples shows a function add() that can take two MyObject objects as input arguments:

// addon.cc
#include <node.h>
#include <node_object_wrap.h>
#include "myobject.h"

namespace demo {

using v8::Context;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;

void CreateObject(const FunctionCallbackInfo<Value>& args) {
  MyObject::NewInstance(args);
}

void Add(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();

  MyObject* obj1 = node::ObjectWrap::Unwrap<MyObject>(
      args[0]->ToObject(context).ToLocalChecked());
  MyObject* obj2 = node::ObjectWrap::Unwrap<MyObject>(
      args[1]->ToObject(context).ToLocalChecked());

  double sum = obj1->value() + obj2->value();
  args.GetReturnValue().Set(Number::New(isolate, sum));
}

void InitAll(Local<Object> exports) {
  MyObject::Init(exports->GetIsolate());

  NODE_SET_METHOD(exports, "createObject", CreateObject);
  NODE_SET_METHOD(exports, "add", Add);
}

NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll)

}  // namespace demo

In myobject.h, a new public method is added to allow access to private values after unwrapping the object.

// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H

#include <node.h>
#include <node_object_wrap.h>

namespace demo {

class MyObject : public node::ObjectWrap {
 public:
  static void Init(v8::Isolate* isolate);
  static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);
  inline double value() const { return value_; }

 private:
  explicit MyObject(double value = 0);
  ~MyObject();

  static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
  static v8::Global<v8::Function> constructor;
  double value_;
};

}  // namespace demo

#endif

The implementation of myobject.cc is similar to before:

// myobject.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using node::AddEnvironmentCleanupHook;
using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Global;
using v8::Isolate;
using v8::Local;
using v8::NewStringType;
using v8::Object;
using v8::String;
using v8::Value;

// Warning! This is not thread-safe, this addon cannot be used for worker
// threads.
Global<Function> MyObject::constructor;

MyObject::MyObject(double value) : value_(value) {
}

MyObject::~MyObject() {
}

void MyObject::Init(Isolate* isolate) {
  // Prepare constructor template
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
  tpl->SetClassName(String::NewFromUtf8(
      isolate, "MyObject", NewStringType::kNormal).ToLocalChecked());
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  Local<Context> context = isolate->GetCurrentContext();
  constructor.Reset(isolate, tpl->GetFunction(context).ToLocalChecked());

  AddEnvironmentCleanupHook(isolate, [](void*) {
    constructor.Reset();
  }, nullptr);
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Context> context = isolate->GetCurrentContext();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ?
        0 : args[0]->NumberValue(context).FromMaybe(0);
    MyObject* obj = new MyObject(value);
    obj->Wrap(args.This());
    args.GetReturnValue().Set(args.This());
  } else {
    // Invoked as plain function `MyObject(...)`, turn into construct call.
    const int argc = 1;
    Local<Value> argv[argc] = { args[0] };
    Local<Function> cons = Local<Function>::New(isolate, constructor);
    Local<Object> instance =
        cons->NewInstance(context, argc, argv).ToLocalChecked();
    args.GetReturnValue().Set(instance);
  }
}

void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  const unsigned argc = 1;
  Local<Value> argv[argc] = { args[0] };
  Local<Function> cons = Local<Function>::New(isolate, constructor);
  Local<Context> context = isolate->GetCurrentContext();
  Local<Object> instance =
      cons->NewInstance(context, argc, argv).ToLocalChecked();

  args.GetReturnValue().Set(instance);
}

}  // namespace demo

Test it with:

// test.js
const addon = require('./build/Release/addon');

const obj1 = addon.createObject(10);
const obj2 = addon.createObject(20);
const result = addon.add(obj1, obj2);

console.log(result);
// Prints: 30

N-API#

Stability: 2 - Stable

N-API (pronounced N as in the letter, followed by API) is an API for building native Addons. It is independent from the underlying JavaScript runtime (for example, V8) and is maintained as part of Node.js itself. This API will be Application Binary Interface (ABI) stable across versions of Node.js. It is intended to insulate Addons from changes in the underlying JavaScript engine and allow modules compiled for one major version to run on later major versions of Node.js without recompilation. The ABI Stability guide provides a more in-depth explanation.

Addons are built/packaged with the same approach/tools outlined in the section titled C++ Addons. The only difference is the set of APIs that are used by the native code. Instead of using the V8 or Native Abstractions for Node.js APIs, the functions available in the N-API are used.

APIs exposed by N-API are generally used to create and manipulate JavaScript values. Concepts and operations generally map to ideas specified in the ECMA-262 Language Specification. The APIs have the following properties:

  • All N-API calls return a status code of type napi_status. This status indicates whether the API call succeeded or failed.
  • The API's return value is passed via an out parameter.
  • All JavaScript values are abstracted behind an opaque type named napi_value.
  • In case of an error status code, additional information can be obtained using napi_get_last_error_info. More information can be found in the error handling section Error Handling.

The N-API is a C API that ensures ABI stability across Node.js versions and different compiler levels. A C++ API can be easier to use. To support using C++, the project maintains a C++ wrapper module called node-addon-api. This wrapper provides an inlineable C++ API. Binaries built with node-addon-api will depend on the symbols for the N-API C-based functions exported by Node.js. node-addon-api is a more efficient way to write code that calls N-API. Take, for example, the following node-addon-api code. The first section shows the node-addon-api code and the second section shows what actually gets used in the addon.

Object obj = Object::New(env);
obj["foo"] = String::New(env, "bar");

napi_status status;
napi_value object, string;
status = napi_create_object(env, &object);
if (status != napi_ok) {
  napi_throw_error(env, ...);
  return;
}

status = napi_create_string_utf8(env, "bar", NAPI_AUTO_LENGTH, &string);
if (status != napi_ok) {
  napi_throw_error(env, ...);
  return;
}

status = napi_set_named_property(env, object, "foo", string);
if (status != napi_ok) {
  napi_throw_error(env, ...);
  return;
}

The end result is that the addon only uses the exported C APIs. As a result, it still gets the benefits of the ABI stability provided by the C API.

When using node-addon-api instead of the C APIs, start with the API docs for node-addon-api.

Implications of ABI Stability#

Although N-API provides an ABI stability guarantee, other parts of Node.js do not, and any external libraries used from the addon may not. In particular, none of the following APIs provide an ABI stability guarantee across major versions:

  • the Node.js C++ APIs available via any of

    #include <node.h>
#include <node_buffer.h>
#include <node_version.h>
#include <node_object_wrap.h>

  • the libuv APIs which are also included with Node.js and available via

    #include <uv.h>

  • the V8 API available via

    #include <v8.h>

Thus, for an addon to remain ABI-compatible across Node.js major versions, it must make use exclusively of N-API by restricting itself to using

#include <node_api.h>

and by checking, for all external libraries that it uses, that the external library makes ABI stability guarantees similar to N-API.

Building#

Unlike modules written in JavaScript, developing and deploying Node.js native addons using N-API requires an additional set of tools. Besides the basic tools required to develop for Node.js, the native addon developer requires a toolchain that can compile C and C++ code into a binary. In addition, depending upon how the native addon is deployed, the user of the native addon will also need to have a C/C++ toolchain installed.

For Linux developers, the necessary C/C++ toolchain packages are readily available. GCC is widely used in the Node.js community to build and test across a variety of plarforms. For many developers, the LLVM compiler infrastructure is also a good choice.

For Mac developers, Xcode offers all the required compiler tools. However, it is not necessary to install the entire Xcode IDE. The following command installs the necessary toolchain:

xcode-select --install

For Windows developers, Visual Studio offers all the required compiler tools. However, it is not necessary to install the entire Visual Studio IDE. The following command installs the necessary toolchain:

npm install --global --production windows-build-tools

The sections below describe the additional tools available for developing and deploying Node.js native addons.

Build tools#

Both the tools listed here require that users of the native addon have a C/C++ toolchain installed in order to successfully install the native addon.

node-gyp#

node-gyp is a build system based on Google's GYP tool and comes bundled with npm. GYP, and therefore node-gyp, requires that Python be installed.

Historically, node-gyp has been the tool of choice for building native addons. It has widespread adoption and documentation. However, some developers have run into limitations in node-gyp.

CMake.js#

CMake.js is an alternative build system based on CMake.

CMake.js is a good choice for projects that already use CMake or for developers affected by limitations in node-gyp.

Uploading precompiled binaries#

The three tools listed here permit native addon developers and maintainers to create and upload binaries to public or private servers. These tools are typically integrated with CI/CD build systems like Travis CI and AppVeyor to build and upload binaries for a variety of platforms and architectures. These binaries are then available for download by users who do not need to have a C/C++ toolchain installed.

node-pre-gyp#

node-pre-gyp is a tool based on node-gyp that adds the ability to upload binaries to a server of the developer's choice. node-pre-gyp has particularly good support for uploading binaries to Amazon S3.

prebuild#

prebuild is a tool that supports builds using either node-gyp or CMake.js. Unlike node-pre-gyp which supports a variety of servers, prebuild uploads binaries only to GitHub releases. prebuild is a good choice for GitHub projects using CMake.js.

prebuildify#

prebuildify is tool based on node-gyp. The advantage of prebuildify is that the built binaries are bundled with the native module when it's uploaded to npm. The binaries are downloaded from npm and are immediately available to the module user when the native module is installed.

Usage#

In order to use the N-API functions, include the file node_api.h which is located in the src directory in the node development tree:

#include <node_api.h>

This will opt into the default NAPI_VERSION for the given release of Node.js. In order to ensure compatibility with specific versions of N-API, the version can be specified explicitly when including the header:

#define NAPI_VERSION 3
#include <node_api.h>

This restricts the N-API surface to just the functionality that was available in the specified (and earlier) versions.

Some of the N-API surface is considered experimental and requires explicit opt-in to access those APIs:

#define NAPI_EXPERIMENTAL
#include <node_api.h>

In this case the entire API surface, including any experimental APIs, will be available to the module code.

N-API Version Matrix#

N-API versions are additive and versioned independently from Node.js. Version 4 is an extension to version 3 in that it has all of the APIs from version 3 with some additions. This means that it is not necessary to recompile for new versions of Node.js which are listed as supporting a later version.

123456
v6.xv6.14.2*
v8.xv8.0.0*v8.10.0*v8.11.2v8.16.0
v9.xv9.0.0*v9.3.0*v9.11.0*
v10.xv10.0.0v10.0.0v10.0.0v10.16.0v10.17.0v10.20.0
v11.xv11.0.0v11.0.0v11.0.0v11.8.0
v12.xv12.0.0v12.0.0v12.0.0v12.0.0v12.11.0
v13.xv13.0.0v13.0.0v13.0.0v13.0.0v13.0.0
v14.xv14.0.0v14.0.0v14.0.0v14.0.0v14.0.0v14.0.0

* Indicates that the N-API version was released as experimental

The N-APIs associated strictly with accessing ECMAScript features from native code can be found separately in js_native_api.h and js_native_api_types.h. The APIs defined in these headers are included in node_api.h and node_api_types.h. The headers are structured in this way in order to allow implementations of N-API outside of Node.js. For those implementations the Node.js specific APIs may not be applicable.

The Node.js-specific parts of an addon can be separated from the code that exposes the actual functionality to the JavaScript environment so that the latter may be used with multiple implementations of N-API. In the example below, addon.c and addon.h refer only to js_native_api.h. This ensures that addon.c can be reused to compile against either the Node.js implementation of N-API or any implementation of N-API outside of Node.js.

addon_node.c is a separate file that contains the Node.js specific entry point to the addon and which instantiates the addon by calling into addon.c when the addon is loaded into a Node.js environment.

// addon.h
#ifndef _ADDON_H_
#define _ADDON_H_
#include <js_native_api.h>
napi_value create_addon(napi_env env);
#endif  // _ADDON_H_

// addon.c
#include "addon.h"

#define NAPI_CALL(env, call)                                      \
  do {                                                            \
    napi_status status = (call);                                  \
    if (status != napi_ok) {                                      \
      const napi_extended_error_info* error_info = NULL;          \
      napi_get_last_error_info((env), &error_info);               \
      bool is_pending;                                            \
      napi_is_exception_pending((env), &is_pending);              \
      if (!is_pending) {                                          \
        const char* message = (error_info->error_message == NULL) \
            ? "empty error message"                               \
            : error_info->error_message;                          \
        napi_throw_error((env), NULL, message);                   \
        return NULL;                                              \
      }                                                           \
    }                                                             \
  } while(0)

static napi_value
DoSomethingUseful(napi_env env, napi_callback_info info) {
  // Do something useful.
  return NULL;
}

napi_value create_addon(napi_env env) {
  napi_value result;
  NAPI_CALL(env, napi_create_object(env, &result));

  napi_value exported_function;
  NAPI_CALL(env, napi_create_function(env,
                                      "doSomethingUseful",
                                      NAPI_AUTO_LENGTH,
                                      DoSomethingUseful,
                                      NULL,
                                      &exported_function));

  NAPI_CALL(env, napi_set_named_property(env,
                                         result,
                                         "doSomethingUseful",
                                         exported_function));

  return result;
}

// addon_node.c
#include <node_api.h>
#include "addon.h"

NAPI_MODULE_INIT() {
  // This function body is expected to return a `napi_value`.
  // The variables `napi_env env` and `napi_value exports` may be used within
  // the body, as they are provided by the definition of `NAPI_MODULE_INIT()`.
  return create_addon(env);
}

Environment Life Cycle APIs#

Stability: 1 - Experimental

Section 8.7 of the ECMAScript Language Specification defines the concept of an "Agent" as a self-contained environment in which JavaScript code runs. Multiple such Agents may be started and terminated either concurrently or in sequence by the process.

A Node.js environment corresponds to an ECMAScript Agent. In the main process, an environment is created at startup, and additional environments can be created on separate threads to serve as worker threads. When Node.js is embedded in another application, the main thread of the application may also construct and destroy a Node.js environment multiple times during the life cycle of the application process such that each Node.js environment created by the application may, in turn, during its life cycle create and destroy additional environments as worker threads.

From the perspective of a native addon this means that the bindings it provides may be called multiple times, from multiple contexts, and even concurrently from multiple threads.

Native addons may need to allocate global state of which they make use during their entire life cycle such that the state must be unique to each instance of the addon.

To this end, N-API provides a way to allocate data such that its life cycle is tied to the life cycle of the Agent.

napi_set_instance_data#

Added in: v12.8.0, v10.20.0 N-API version: 6 
napi_status napi_set_instance_data(napi_env env,
                                   void* data,
                                   napi_finalize finalize_cb,
                                   void* finalize_hint);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] data: The data item to make available to bindings of this instance.
  • [in] finalize_cb: The function to call when the environment is being torn down. The function receives data so that it might free it.
  • [in] finalize_hint: Optional hint to pass to the finalize callback during collection.

Returns napi_ok if the API succeeded.

This API associates data with the currently running Agent. data can later be retrieved using napi_get_instance_data(). Any existing data associated with the currently running Agent which was set by means of a previous call to napi_set_instance_data() will be overwritten. If a finalize_cb was provided by the previous call, it will not be called.

napi_get_instance_data#

Added in: v12.8.0, v10.20.0 N-API version: 6 
napi_status napi_get_instance_data(napi_env env,
                                   void** data);
  • [in] env: The environment that the N-API call is invoked under.
  • [out] data: The data item that was previously associated with the currently running Agent by a call to napi_set_instance_data().

Returns napi_ok if the API succeeded.

This API retrieves data that was previously associated with the currently running Agent via napi_set_instance_data(). If no data is set, the call will succeed and data will be set to NULL.

Basic N-API Data Types#

N-API exposes the following fundamental datatypes as abstractions that are consumed by the various APIs. These APIs should be treated as opaque, introspectable only with other N-API calls.

napi_status#

Added in: v8.0.0 N-API version: 1

Integral status code indicating the success or failure of a N-API call. Currently, the following status codes are supported.

typedef enum {
  napi_ok,
  napi_invalid_arg,
  napi_object_expected,
  napi_string_expected,
  napi_name_expected,
  napi_function_expected,
  napi_number_expected,
  napi_boolean_expected,
  napi_array_expected,
  napi_generic_failure,
  napi_pending_exception,
  napi_cancelled,
  napi_escape_called_twice,
  napi_handle_scope_mismatch,
  napi_callback_scope_mismatch,
  napi_queue_full,
  napi_closing,
  napi_bigint_expected,
  napi_date_expected,
  napi_arraybuffer_expected,
  napi_detachable_arraybuffer_expected,
  napi_would_deadlock,
} napi_status;

If additional information is required upon an API returning a failed status, it can be obtained by calling napi_get_last_error_info.

napi_extended_error_info#

Added in: v8.0.0 N-API version: 1 
typedef struct {
  const char* error_message;
  void* engine_reserved;
  uint32_t engine_error_code;
  napi_status error_code;
} napi_extended_error_info;
  • error_message: UTF8-encoded string containing a VM-neutral description of the error.
  • engine_reserved: Reserved for VM-specific error details. This is currently not implemented for any VM.
  • engine_error_code: VM-specific error code. This is currently not implemented for any VM.
  • error_code: The N-API status code that originated with the last error.

See the Error Handling section for additional information.

napi_env#

napi_env is used to represent a context that the underlying N-API implementation can use to persist VM-specific state. This structure is passed to native functions when they're invoked, and it must be passed back when making N-API calls. Specifically, the same napi_env that was passed in when the initial native function was called must be passed to any subsequent nested N-API calls. Caching the napi_env for the purpose of general reuse, and passing the napi_env between instances of the same addon running on different Worker threads is not allowed. The napi_env becomes invalid when an instance of a native addon is unloaded. Notification of this event is delivered through the callbacks given to napi_add_env_cleanup_hook and napi_set_instance_data.

napi_value#

This is an opaque pointer that is used to represent a JavaScript value.

napi_threadsafe_function#

Added in: v10.6.0 N-API version: 4

This is an opaque pointer that represents a JavaScript function which can be called asynchronously from multiple threads via napi_call_threadsafe_function().

napi_threadsafe_function_release_mode#

Added in: v10.6.0 N-API version: 4

A value to be given to napi_release_threadsafe_function() to indicate whether the thread-safe function is to be closed immediately (napi_tsfn_abort) or merely released (napi_tsfn_release) and thus available for subsequent use via napi_acquire_threadsafe_function() and napi_call_threadsafe_function().

typedef enum {
  napi_tsfn_release,
  napi_tsfn_abort
} napi_threadsafe_function_release_mode;

napi_threadsafe_function_call_mode#

Added in: v10.6.0 N-API version: 4

A value to be given to napi_call_threadsafe_function() to indicate whether the call should block whenever the queue associated with the thread-safe function is full.

typedef enum {
  napi_tsfn_nonblocking,
  napi_tsfn_blocking
} napi_threadsafe_function_call_mode;

N-API Memory Management types#

napi_handle_scope#

This is an abstraction used to control and modify the lifetime of objects created within a particular scope. In general, N-API values are created within the context of a handle scope. When a native method is called from JavaScript, a default handle scope will exist. If the user does not explicitly create a new handle scope, N-API values will be created in the default handle scope. For any invocations of code outside the execution of a native method (for instance, during a libuv callback invocation), the module is required to create a scope before invoking any functions that can result in the creation of JavaScript values.

Handle scopes are created using napi_open_handle_scope and are destroyed using napi_close_handle_scope. Closing the scope can indicate to the GC that all napi_values created during the lifetime of the handle scope are no longer referenced from the current stack frame.

For more details, review the Object Lifetime Management.

napi_escapable_handle_scope#

Added in: v8.0.0 N-API version: 1

Escapable handle scopes are a special type of handle scope to return values created within a particular handle scope to a parent scope.

napi_ref#

Added in: v8.0.0 N-API version: 1

This is the abstraction to use to reference a napi_value. This allows for users to manage the lifetimes of JavaScript values, including defining their minimum lifetimes explicitly.

For more details, review the Object Lifetime Management.

N-API Callback types#

napi_callback_info#

Added in: v8.0.0 N-API version: 1

Opaque datatype that is passed to a callback function. It can be used for getting additional information about the context in which the callback was invoked.

napi_callback#

Added in: v8.0.0 N-API version: 1

Function pointer type for user-provided native functions which are to be exposed to JavaScript via N-API. Callback functions should satisfy the following signature:

typedef napi_value (*napi_callback)(napi_env, napi_callback_info);

napi_finalize#

Added in: v8.0.0 N-API version: 1

Function pointer type for add-on provided functions that allow the user to be notified when externally-owned data is ready to be cleaned up because the object with which it was associated with, has been garbage-collected. The user must provide a function satisfying the following signature which would get called upon the object's collection. Currently, napi_finalize can be used for finding out when objects that have external data are collected.

typedef void (*napi_finalize)(napi_env env,
                              void* finalize_data,
                              void* finalize_hint);

napi_async_execute_callback#

Added in: v8.0.0 N-API version: 1

Function pointer used with functions that support asynchronous operations. Callback functions must satisfy the following signature:

typedef void (*napi_async_execute_callback)(napi_env env, void* data);

Implementations of this function must avoid making N-API calls that execute JavaScript or interact with JavaScript objects. N-API calls should be in the napi_async_complete_callback instead. Do not use the napi_env parameter as it will likely result in execution of JavaScript.

napi_async_complete_callback#

Added in: v8.0.0 N-API version: 1

Function pointer used with functions that support asynchronous operations. Callback functions must satisfy the following signature:

typedef void (*napi_async_complete_callback)(napi_env env,
                                             napi_status status,
                                             void* data);

napi_threadsafe_function_call_js#

Added in: v10.6.0 N-API version: 4

Function pointer used with asynchronous thread-safe function calls. The callback will be called on the main thread. Its purpose is to use a data item arriving via the queue from one of the secondary threads to construct the parameters necessary for a call into JavaScript, usually via napi_call_function, and then make the call into JavaScript.

The data arriving from the secondary thread via the queue is given in the data parameter and the JavaScript function to call is given in the js_callback parameter.

N-API sets up the environment prior to calling this callback, so it is sufficient to call the JavaScript function via napi_call_function rather than via napi_make_callback.

Callback functions must satisfy the following signature:

typedef void (*napi_threadsafe_function_call_js)(napi_env env,
                                                 napi_value js_callback,
                                                 void* context,
                                                 void* data);
  • [in] env: The environment to use for API calls, or NULL if the thread-safe function is being torn down and data may need to be freed.
  • [in] js_callback: The JavaScript function to call, or NULL if the thread-safe function is being torn down and data may need to be freed. It may also be NULL if the thread-safe function was created without js_callback.
  • [in] context: The optional data with which the thread-safe function was created.
  • [in] data: Data created by the secondary thread. It is the responsibility of the callback to convert this native data to JavaScript values (with N-API functions) that can be passed as parameters when js_callback is invoked. This pointer is managed entirely by the threads and this callback. Thus this callback should free the data.

Error Handling#

N-API uses both return values and JavaScript exceptions for error handling. The following sections explain the approach for each case.

Return values#

All of the N-API functions share the same error handling pattern. The return type of all API functions is napi_status.

The return value will be napi_ok if the request was successful and no uncaught JavaScript exception was thrown. If an error occurred AND an exception was thrown, the napi_status value for the error will be returned. If an exception was thrown, and no error occurred, napi_pending_exception will be returned.

In cases where a return value other than napi_ok or napi_pending_exception is returned, napi_is_exception_pending must be called to check if an exception is pending. See the section on exceptions for more details.

The full set of possible napi_status values is defined in napi_api_types.h.

The napi_status return value provides a VM-independent representation of the error which occurred. In some cases it is useful to be able to get more detailed information, including a string representing the error as well as VM (engine)-specific information.

In order to retrieve this information napi_get_last_error_info is provided which returns a napi_extended_error_info structure. The format of the napi_extended_error_info structure is as follows:

Added in: v8.0.0 N-API version: 1 
typedef struct napi_extended_error_info {
  const char* error_message;
  void* engine_reserved;
  uint32_t engine_error_code;
  napi_status error_code;
};
  • error_message: Textual representation of the error that occurred.
  • engine_reserved: Opaque handle reserved for engine use only.
  • engine_error_code: VM specific error code.
  • error_code: n-api status code for the last error.

napi_get_last_error_info returns the information for the last N-API call that was made.

Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes.

napi_get_last_error_info#

Added in: v8.0.0 N-API version: 1 
napi_status
napi_get_last_error_info(napi_env env,
                         const napi_extended_error_info** result);
  • [in] env: The environment that the API is invoked under.
  • [out] result: The napi_extended_error_info structure with more information about the error.

Returns napi_ok if the API succeeded.

This API retrieves a napi_extended_error_info structure with information about the last error that occurred.

The content of the napi_extended_error_info returned is only valid up until an n-api function is called on the same env.

Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes.

This API can be called even if there is a pending JavaScript exception.

Exceptions#

Any N-API function call may result in a pending JavaScript exception. This is the case for any of the API functions, even those that may not cause the execution of JavaScript.

If the napi_status returned by a function is napi_ok then no exception is pending and no additional action is required. If the napi_status returned is anything other than napi_ok or napi_pending_exception, in order to try to recover and continue instead of simply returning immediately, napi_is_exception_pending must be called in order to determine if an exception is pending or not.

In many cases when an N-API function is called and an exception is already pending, the function will return immediately with a napi_status of napi_pending_exception. However, this is not the case for all functions. N-API allows a subset of the functions to be called to allow for some minimal cleanup before returning to JavaScript. In that case, napi_status will reflect the status for the function. It will not reflect previous pending exceptions. To avoid confusion, check the error status after every function call.

When an exception is pending one of two approaches can be employed.

The first approach is to do any appropriate cleanup and then return so that execution will return to JavaScript. As part of the transition back to JavaScript, the exception will be thrown at the point in the JavaScript code where the native method was invoked. The behavior of most N-API calls is unspecified while an exception is pending, and many will simply return napi_pending_exception, so do as little as possible and then return to JavaScript where the exception can be handled.

The second approach is to try to handle the exception. There will be cases where the native code can catch the exception, take the appropriate action, and then continue. This is only recommended in specific cases where it is known that the exception can be safely handled. In these cases napi_get_and_clear_last_exception can be used to get and clear the exception. On success, result will contain the handle to the last JavaScript Object thrown. If it is determined, after retrieving the exception, the exception cannot be handled after all it can be re-thrown it with napi_throw where error is the JavaScript Error object to be thrown.

The following utility functions are also available in case native code needs to throw an exception or determine if a napi_value is an instance of a JavaScript Error object: napi_throw_error, napi_throw_type_error, napi_throw_range_error and napi_is_error.

The following utility functions are also available in case native code needs to create an Error object: napi_create_error, napi_create_type_error, and napi_create_range_error, where result is the napi_value that refers to the newly created JavaScript Error object.

The Node.js project is adding error codes to all of the errors generated internally. The goal is for applications to use these error codes for all error checking. The associated error messages will remain, but will only be meant to be used for logging and display with the expectation that the message can change without SemVer applying. In order to support this model with N-API, both in internal functionality and for module specific functionality (as its good practice), the throw_ and create_ functions take an optional code parameter which is the string for the code to be added to the error object. If the optional parameter is NULL then no code will be associated with the error. If a code is provided, the name associated with the error is also updated to be:

originalName [code]

where originalName is the original name associated with the error and code is the code that was provided. For example, if the code is 'ERR_ERROR_1' and a TypeError is being created the name will be:

TypeError [ERR_ERROR_1]

napi_throw#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_throw(napi_env env, napi_value error);
  • [in] env: The environment that the API is invoked under.
  • [in] error: The JavaScript value to be thrown.

Returns napi_ok if the API succeeded.

This API throws the JavaScript value provided.

napi_throw_error#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_throw_error(napi_env env,
                                         const char* code,
                                         const char* msg);
  • [in] env: The environment that the API is invoked under.
  • [in] code: Optional error code to be set on the error.
  • [in] msg: C string representing the text to be associated with the error.

Returns napi_ok if the API succeeded.

This API throws a JavaScript Error with the text provided.

napi_throw_type_error#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_throw_type_error(napi_env env,
                                              const char* code,
                                              const char* msg);
  • [in] env: The environment that the API is invoked under.
  • [in] code: Optional error code to be set on the error.
  • [in] msg: C string representing the text to be associated with the error.

Returns napi_ok if the API succeeded.

This API throws a JavaScript TypeError with the text provided.

napi_throw_range_error#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_throw_range_error(napi_env env,
                                               const char* code,
                                               const char* msg);
  • [in] env: The environment that the API is invoked under.
  • [in] code: Optional error code to be set on the error.
  • [in] msg: C string representing the text to be associated with the error.

Returns napi_ok if the API succeeded.

This API throws a JavaScript RangeError with the text provided.

napi_is_error#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_is_error(napi_env env,
                                      napi_value value,
                                      bool* result);
  • [in] env: The environment that the API is invoked under.
  • [in] value: The napi_value to be checked.
  • [out] result: Boolean value that is set to true if napi_value represents an error, false otherwise.

Returns napi_ok if the API succeeded.

This API queries a napi_value to check if it represents an error object.

napi_create_error#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_create_error(napi_env env,
                                          napi_value code,
                                          napi_value msg,
                                          napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] code: Optional napi_value with the string for the error code to be associated with the error.
  • [in] msg: napi_value that references a JavaScript String to be used as the message for the Error.
  • [out] result: napi_value representing the error created.

Returns napi_ok if the API succeeded.

This API returns a JavaScript Error with the text provided.

napi_create_type_error#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_create_type_error(napi_env env,
                                               napi_value code,
                                               napi_value msg,
                                               napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] code: Optional napi_value with the string for the error code to be associated with the error.
  • [in] msg: napi_value that references a JavaScript String to be used as the message for the Error.
  • [out] result: napi_value representing the error created.

Returns napi_ok if the API succeeded.

This API returns a JavaScript TypeError with the text provided.

napi_create_range_error#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_create_range_error(napi_env env,
                                                napi_value code,
                                                napi_value msg,
                                                napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] code: Optional napi_value with the string for the error code to be associated with the error.
  • [in] msg: napi_value that references a JavaScript String to be used as the message for the Error.
  • [out] result: napi_value representing the error created.

Returns napi_ok if the API succeeded.

This API returns a JavaScript RangeError with the text provided.

napi_get_and_clear_last_exception#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_and_clear_last_exception(napi_env env,
                                              napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [out] result: The exception if one is pending, NULL otherwise.

Returns napi_ok if the API succeeded.

This API can be called even if there is a pending JavaScript exception.

napi_is_exception_pending#

Added in: v8.0.0 N-API version: 1 
napi_status napi_is_exception_pending(napi_env env, bool* result);
  • [in] env: The environment that the API is invoked under.
  • [out] result: Boolean value that is set to true if an exception is pending.

Returns napi_ok if the API succeeded.

This API can be called even if there is a pending JavaScript exception.

napi_fatal_exception#

Added in: v9.10.0 N-API version: 3 
napi_status napi_fatal_exception(napi_env env, napi_value err);
  • [in] env: The environment that the API is invoked under.
  • [in] err: The error that is passed to 'uncaughtException'.

Trigger an 'uncaughtException' in JavaScript. Useful if an async callback throws an exception with no way to recover.

Fatal Errors#

In the event of an unrecoverable error in a native module, a fatal error can be thrown to immediately terminate the process.

napi_fatal_error#

Added in: v8.2.0 N-API version: 1 
NAPI_NO_RETURN void napi_fatal_error(const char* location,
                                                 size_t location_len,
                                                 const char* message,
                                                 size_t message_len);
  • [in] location: Optional location at which the error occurred.
  • [in] location_len: The length of the location in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
  • [in] message: The message associated with the error.
  • [in] message_len: The length of the message in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.

The function call does not return, the process will be terminated.

This API can be called even if there is a pending JavaScript exception.

Object Lifetime management#

As N-API calls are made, handles to objects in the heap for the underlying VM may be returned as napi_values. These handles must hold the objects 'live' until they are no longer required by the native code, otherwise the objects could be collected before the native code was finished using them.

As object handles are returned they are associated with a 'scope'. The lifespan for the default scope is tied to the lifespan of the native method call. The result is that, by default, handles remain valid and the objects associated with these handles will be held live for the lifespan of the native method call.

In many cases, however, it is necessary that the handles remain valid for either a shorter or longer lifespan than that of the native method. The sections which follow describe the N-API functions that can be used to change the handle lifespan from the default.

Making handle lifespan shorter than that of the native method#

It is often necessary to make the lifespan of handles shorter than the lifespan of a native method. For example, consider a native method that has a loop which iterates through the elements in a large array:

for (int i = 0; i < 1000000; i++) {
  napi_value result;
  napi_status status = napi_get_element(env, object, i, &result);
  if (status != napi_ok) {
    break;
  }
  // do something with element
}

This would result in a large number of handles being created, consuming substantial resources. In addition, even though the native code could only use the most recent handle, all of the associated objects would also be kept alive since they all share the same scope.

To handle this case, N-API provides the ability to establish a new 'scope' to which newly created handles will be associated. Once those handles are no longer required, the scope can be 'closed' and any handles associated with the scope are invalidated. The methods available to open/close scopes are napi_open_handle_scope and napi_close_handle_scope.

N-API only supports a single nested hierarchy of scopes. There is only one active scope at any time, and all new handles will be associated with that scope while it is active. Scopes must be closed in the reverse order from which they are opened. In addition, all scopes created within a native method must be closed before returning from that method.

Taking the earlier example, adding calls to napi_open_handle_scope and napi_close_handle_scope would ensure that at most a single handle is valid throughout the execution of the loop:

for (int i = 0; i < 1000000; i++) {
  napi_handle_scope scope;
  napi_status status = napi_open_handle_scope(env, &scope);
  if (status != napi_ok) {
    break;
  }
  napi_value result;
  status = napi_get_element(env, object, i, &result);
  if (status != napi_ok) {
    break;
  }
  // do something with element
  status = napi_close_handle_scope(env, scope);
  if (status != napi_ok) {
    break;
  }
}

When nesting scopes, there are cases where a handle from an inner scope needs to live beyond the lifespan of that scope. N-API supports an 'escapable scope' in order to support this case. An escapable scope allows one handle to be 'promoted' so that it 'escapes' the current scope and the lifespan of the handle changes from the current scope to that of the outer scope.

The methods available to open/close escapable scopes are napi_open_escapable_handle_scope and napi_close_escapable_handle_scope.

The request to promote a handle is made through napi_escape_handle which can only be called once.

napi_open_handle_scope#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_open_handle_scope(napi_env env,
                                               napi_handle_scope* result);
  • [in] env: The environment that the API is invoked under.
  • [out] result: napi_value representing the new scope.

Returns napi_ok if the API succeeded.

This API open a new scope.

napi_close_handle_scope#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_close_handle_scope(napi_env env,
                                                napi_handle_scope scope);
  • [in] env: The environment that the API is invoked under.
  • [in] scope: napi_value representing the scope to be closed.

Returns napi_ok if the API succeeded.

This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created.

This API can be called even if there is a pending JavaScript exception.

napi_open_escapable_handle_scope#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status
    napi_open_escapable_handle_scope(napi_env env,
                                     napi_handle_scope* result);
  • [in] env: The environment that the API is invoked under.
  • [out] result: napi_value representing the new scope.

Returns napi_ok if the API succeeded.

This API open a new scope from which one object can be promoted to the outer scope.

napi_close_escapable_handle_scope#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status
    napi_close_escapable_handle_scope(napi_env env,
                                      napi_handle_scope scope);
  • [in] env: The environment that the API is invoked under.
  • [in] scope: napi_value representing the scope to be closed.

Returns napi_ok if the API succeeded.

This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created.

This API can be called even if there is a pending JavaScript exception.

napi_escape_handle#

Added in: v8.0.0 N-API version: 1 
napi_status napi_escape_handle(napi_env env,
                               napi_escapable_handle_scope scope,
                               napi_value escapee,
                               napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] scope: napi_value representing the current scope.
  • [in] escapee: napi_value representing the JavaScript Object to be escaped.
  • [out] result: napi_value representing the handle to the escaped Object in the outer scope.

Returns napi_ok if the API succeeded.

This API promotes the handle to the JavaScript object so that it is valid for the lifetime of the outer scope. It can only be called once per scope. If it is called more than once an error will be returned.

This API can be called even if there is a pending JavaScript exception.

References to objects with a lifespan longer than that of the native method#

In some cases an addon will need to be able to create and reference objects with a lifespan longer than that of a single native method invocation. For example, to create a constructor and later use that constructor in a request to creates instances, it must be possible to reference the constructor object across many different instance creation requests. This would not be possible with a normal handle returned as a napi_value as described in the earlier section. The lifespan of a normal handle is managed by scopes and all scopes must be closed before the end of a native method.

N-API provides methods to create persistent references to an object. Each persistent reference has an associated count with a value of 0 or higher. The count determines if the reference will keep the corresponding object live. References with a count of 0 do not prevent the object from being collected and are often called 'weak' references. Any count greater than 0 will prevent the object from being collected.

References can be created with an initial reference count. The count can then be modified through napi_reference_ref and napi_reference_unref. If an object is collected while the count for a reference is 0, all subsequent calls to get the object associated with the reference napi_get_reference_value will return NULL for the returned napi_value. An attempt to call napi_reference_ref for a reference whose object has been collected will result in an error.

References must be deleted once they are no longer required by the addon. When a reference is deleted it will no longer prevent the corresponding object from being collected. Failure to delete a persistent reference will result in a 'memory leak' with both the native memory for the persistent reference and the corresponding object on the heap being retained forever.

There can be multiple persistent references created which refer to the same object, each of which will either keep the object live or not based on its individual count.

napi_create_reference#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_create_reference(napi_env env,
                                              napi_value value,
                                              uint32_t initial_refcount,
                                              napi_ref* result);
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing the Object to which we want a reference.
  • [in] initial_refcount: Initial reference count for the new reference.
  • [out] result: napi_ref pointing to the new reference.

Returns napi_ok if the API succeeded.

This API create a new reference with the specified reference count to the Object passed in.

napi_delete_reference#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_delete_reference(napi_env env, napi_ref ref);
  • [in] env: The environment that the API is invoked under.
  • [in] ref: napi_ref to be deleted.

Returns napi_ok if the API succeeded.

This API deletes the reference passed in.

This API can be called even if there is a pending JavaScript exception.

napi_reference_ref#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_reference_ref(napi_env env,
                                           napi_ref ref,
                                           uint32_t* result);
  • [in] env: The environment that the API is invoked under.
  • [in] ref: napi_ref for which the reference count will be incremented.
  • [out] result: The new reference count.

Returns napi_ok if the API succeeded.

This API increments the reference count for the reference passed in and returns the resulting reference count.

napi_reference_unref#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_reference_unref(napi_env env,
                                             napi_ref ref,
                                             uint32_t* result);
  • [in] env: The environment that the API is invoked under.
  • [in] ref: napi_ref for which the reference count will be decremented.
  • [out] result: The new reference count.

Returns napi_ok if the API succeeded.

This API decrements the reference count for the reference passed in and returns the resulting reference count.

napi_get_reference_value#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_get_reference_value(napi_env env,
                                                 napi_ref ref,
                                                 napi_value* result);

the napi_value passed in or out of these methods is a handle to the object to which the reference is related.

  • [in] env: The environment that the API is invoked under.
  • [in] ref: napi_ref for which we requesting the corresponding Object.
  • [out] result: The napi_value for the Object referenced by the napi_ref.

Returns napi_ok if the API succeeded.

If still valid, this API returns the napi_value representing the JavaScript Object associated with the napi_ref. Otherwise, result will be NULL.

Cleanup on exit of the current Node.js instance#

While a Node.js process typically releases all its resources when exiting, embedders of Node.js, or future Worker support, may require addons to register clean-up hooks that will be run once the current Node.js instance exits.

N-API provides functions for registering and un-registering such callbacks. When those callbacks are run, all resources that are being held by the addon should be freed up.

napi_add_env_cleanup_hook#

Added in: v10.2.0 N-API version: 3 
NODE_EXTERN napi_status napi_add_env_cleanup_hook(napi_env env,
                                                  void (*fun)(void* arg),
                                                  void* arg);

Registers fun as a function to be run with the arg parameter once the current Node.js environment exits.

A function can safely be specified multiple times with different arg values. In that case, it will be called multiple times as well. Providing the same fun and arg values multiple times is not allowed and will lead the process to abort.

The hooks will be called in reverse order, i.e. the most recently added one will be called first.

Removing this hook can be done by using napi_remove_env_cleanup_hook. Typically, that happens when the resource for which this hook was added is being torn down anyway.

napi_remove_env_cleanup_hook#

Added in: v10.2.0 N-API version: 3 
NAPI_EXTERN napi_status napi_remove_env_cleanup_hook(napi_env env,
                                                     void (*fun)(void* arg),
                                                     void* arg);

Unregisters fun as a function to be run with the arg parameter once the current Node.js environment exits. Both the argument and the function value need to be exact matches.

The function must have originally been registered with napi_add_env_cleanup_hook, otherwise the process will abort.

Module registration#

N-API modules are registered in a manner similar to other modules except that instead of using the NODE_MODULE macro the following is used:

NAPI_MODULE(NODE_GYP_MODULE_NAME, Init)

The next difference is the signature for the Init method. For a N-API module it is as follows:

napi_value Init(napi_env env, napi_value exports);

The return value from Init is treated as the exports object for the module. The Init method is passed an empty object via the exports parameter as a convenience. If Init returns NULL, the parameter passed as exports is exported by the module. N-API modules cannot modify the module object but can specify anything as the exports property of the module.

To add the method hello as a function so that it can be called as a method provided by the addon:

napi_value Init(napi_env env, napi_value exports) {
  napi_status status;
  napi_property_descriptor desc =
    {"hello", NULL, Method, NULL, NULL, NULL, napi_default, NULL};
  status = napi_define_properties(env, exports, 1, &desc);
  if (status != napi_ok) return NULL;
  return exports;
}

To set a function to be returned by the require() for the addon:

napi_value Init(napi_env env, napi_value exports) {
  napi_value method;
  napi_status status;
  status = napi_create_function(env, "exports", NAPI_AUTO_LENGTH, Method, NULL, &method);
  if (status != napi_ok) return NULL;
  return method;
}

To define a class so that new instances can be created (often used with Object Wrap):

// NOTE: partial example, not all referenced code is included
napi_value Init(napi_env env, napi_value exports) {
  napi_status status;
  napi_property_descriptor properties[] = {
    { "value", NULL, NULL, GetValue, SetValue, NULL, napi_default, NULL },
    DECLARE_NAPI_METHOD("plusOne", PlusOne),
    DECLARE_NAPI_METHOD("multiply", Multiply),
  };

  napi_value cons;
  status =
      napi_define_class(env, "MyObject", New, NULL, 3, properties, &cons);
  if (status != napi_ok) return NULL;

  status = napi_create_reference(env, cons, 1, &constructor);
  if (status != napi_ok) return NULL;

  status = napi_set_named_property(env, exports, "MyObject", cons);
  if (status != napi_ok) return NULL;

  return exports;
}

If the module will be loaded multiple times during the lifetime of the Node.js process, use the NAPI_MODULE_INIT macro to initialize the module:

NAPI_MODULE_INIT() {
  napi_value answer;
  napi_status result;

  status = napi_create_int64(env, 42, &answer);
  if (status != napi_ok) return NULL;

  status = napi_set_named_property(env, exports, "answer", answer);
  if (status != napi_ok) return NULL;

  return exports;
}

This macro includes NAPI_MODULE, and declares an Init function with a special name and with visibility beyond the addon. This will allow Node.js to initialize the module even if it is loaded multiple times.

There are a few design considerations when declaring a module that may be loaded multiple times. The documentation of context-aware addons provides more details.

The variables env and exports will be available inside the function body following the macro invocation.

For more details on setting properties on objects, see the section on Working with JavaScript Properties.

For more details on building addon modules in general, refer to the existing API.

Working with JavaScript Values#

N-API exposes a set of APIs to create all types of JavaScript values. Some of these types are documented under Section 6 of the ECMAScript Language Specification.

Fundamentally, these APIs are used to do one of the following:

  1. Create a new JavaScript object
  2. Convert from a primitive C type to an N-API value
  3. Convert from N-API value to a primitive C type
  4. Get global instances including undefined and null

N-API values are represented by the type napi_value. Any N-API call that requires a JavaScript value takes in a napi_value. In some cases, the API does check the type of the napi_value up-front. However, for better performance, it's better for the caller to make sure that the napi_value in question is of the JavaScript type expected by the API.

Enum types#

napi_key_collection_mode#

Added in: v13.7.0, v10.20.0 N-API version: 6 
typedef enum {
  napi_key_include_prototypes,
  napi_key_own_only
} napi_key_collection_mode;

Describes the Keys/Properties filter enums:

napi_key_collection_mode limits the range of collected properties.

napi_key_own_only limits the collected properties to the given object only. napi_key_include_prototypes will include all keys of the objects's prototype chain as well.

napi_key_filter#

Added in: v13.7.0, v10.20.0 N-API version: 6 
typedef enum {
  napi_key_all_properties = 0,
  napi_key_writable = 1,
  napi_key_enumerable = 1 << 1,
  napi_key_configurable = 1 << 2,
  napi_key_skip_strings = 1 << 3,
  napi_key_skip_symbols = 1 << 4
} napi_key_filter;

Property filter bits. They can be or'ed to build a composite filter.

napi_key_conversion#

Added in: v13.7.0, v10.20.0 N-API version: 6 
typedef enum {
  napi_key_keep_numbers,
  napi_key_numbers_to_strings
} napi_key_conversion;

napi_key_numbers_to_strings will convert integer indices to strings. napi_key_keep_numbers will return numbers for integer indices.

napi_valuetype#

typedef enum {
  // ES6 types (corresponds to typeof)
  napi_undefined,
  napi_null,
  napi_boolean,
  napi_number,
  napi_string,
  napi_symbol,
  napi_object,
  napi_function,
  napi_external,
  napi_bigint,
} napi_valuetype;

Describes the type of a napi_value. This generally corresponds to the types described in Section 6.1 of the ECMAScript Language Specification. In addition to types in that section, napi_valuetype can also represent Functions and Objects with external data.

A JavaScript value of type napi_external appears in JavaScript as a plain object such that no properties can be set on it, and no prototype.

napi_typedarray_type#

typedef enum {
  napi_int8_array,
  napi_uint8_array,
  napi_uint8_clamped_array,
  napi_int16_array,
  napi_uint16_array,
  napi_int32_array,
  napi_uint32_array,
  napi_float32_array,
  napi_float64_array,
  napi_bigint64_array,
  napi_biguint64_array,
} napi_typedarray_type;

This represents the underlying binary scalar datatype of the TypedArray. Elements of this enum correspond to Section 22.2 of the ECMAScript Language Specification.

Object Creation Functions#

napi_create_array#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_array(napi_env env, napi_value* result)
  • [in] env: The environment that the N-API call is invoked under.
  • [out] result: A napi_value representing a JavaScript Array.

Returns napi_ok if the API succeeded.

This API returns an N-API value corresponding to a JavaScript Array type. JavaScript arrays are described in Section 22.1 of the ECMAScript Language Specification.

napi_create_array_with_length#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_array_with_length(napi_env env,
                                          size_t length,
                                          napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] length: The initial length of the Array.
  • [out] result: A napi_value representing a JavaScript Array.

Returns napi_ok if the API succeeded.

This API returns an N-API value corresponding to a JavaScript Array type. The Array's length property is set to the passed-in length parameter. However, the underlying buffer is not guaranteed to be pre-allocated by the VM when the array is created. That behavior is left to the underlying VM implementation. If the buffer must be a contiguous block of memory that can be directly read and/or written via C, consider using napi_create_external_arraybuffer.

JavaScript arrays are described in Section 22.1 of the ECMAScript Language Specification.

napi_create_arraybuffer#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_arraybuffer(napi_env env,
                                    size_t byte_length,
                                    void** data,
                                    napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] length: The length in bytes of the array buffer to create.
  • [out] data: Pointer to the underlying byte buffer of the ArrayBuffer.
  • [out] result: A napi_value representing a JavaScript ArrayBuffer.

Returns napi_ok if the API succeeded.

This API returns an N-API value corresponding to a JavaScript ArrayBuffer. ArrayBuffers are used to represent fixed-length binary data buffers. They are normally used as a backing-buffer for TypedArray objects. The ArrayBuffer allocated will have an underlying byte buffer whose size is determined by the length parameter that's passed in. The underlying buffer is optionally returned back to the caller in case the caller wants to directly manipulate the buffer. This buffer can only be written to directly from native code. To write to this buffer from JavaScript, a typed array or DataView object would need to be created.

JavaScript ArrayBuffer objects are described in Section 24.1 of the ECMAScript Language Specification.

napi_create_buffer#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_buffer(napi_env env,
                               size_t size,
                               void** data,
                               napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] size: Size in bytes of the underlying buffer.
  • [out] data: Raw pointer to the underlying buffer.
  • [out] result: A napi_value representing a node::Buffer.

Returns napi_ok if the API succeeded.

This API allocates a node::Buffer object. While this is still a fully-supported data structure, in most cases using a TypedArray will suffice.

napi_create_buffer_copy#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_buffer_copy(napi_env env,
                                    size_t length,
                                    const void* data,
                                    void** result_data,
                                    napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] size: Size in bytes of the input buffer (should be the same as the size of the new buffer).
  • [in] data: Raw pointer to the underlying buffer to copy from.
  • [out] result_data: Pointer to the new Buffer's underlying data buffer.
  • [out] result: A napi_value representing a node::Buffer.

Returns napi_ok if the API succeeded.

This API allocates a node::Buffer object and initializes it with data copied from the passed-in buffer. While this is still a fully-supported data structure, in most cases using a TypedArray will suffice.

napi_create_date#

Added in: v11.11.0, v10.17.0 N-API version: 5 
napi_status napi_create_date(napi_env env,
                             double time,
                             napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] time: ECMAScript time value in milliseconds since 01 January, 1970 UTC.
  • [out] result: A napi_value representing a JavaScript Date.

Returns napi_ok if the API succeeded.

This API does not observe leap seconds; they are ignored, as ECMAScript aligns with POSIX time specification.

This API allocates a JavaScript Date object.

JavaScript Date objects are described in Section 20.3 of the ECMAScript Language Specification.

napi_create_external#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_external(napi_env env,
                                 void* data,
                                 napi_finalize finalize_cb,
                                 void* finalize_hint,
                                 napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] data: Raw pointer to the external data.
  • [in] finalize_cb: Optional callback to call when the external value is being collected.
  • [in] finalize_hint: Optional hint to pass to the finalize callback during collection.
  • [out] result: A napi_value representing an external value.

Returns napi_ok if the API succeeded.

This API allocates a JavaScript value with external data attached to it. This is used to pass external data through JavaScript code, so it can be retrieved later by native code using napi_get_value_external.

The API adds a napi_finalize callback which will be called when the JavaScript object just created is ready for garbage collection. It is similar to napi_wrap() except that:

  • the native data cannot be retrieved later using napi_unwrap(),
  • nor can it be removed later using napi_remove_wrap(), and
  • the object created by the API can be used with napi_wrap().

The created value is not an object, and therefore does not support additional properties. It is considered a distinct value type: calling napi_typeof() with an external value yields napi_external.

napi_create_external_arraybuffer#

Added in: v8.0.0 N-API version: 1 
napi_status
napi_create_external_arraybuffer(napi_env env,
                                 void* external_data,
                                 size_t byte_length,
                                 napi_finalize finalize_cb,
                                 void* finalize_hint,
                                 napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] external_data: Pointer to the underlying byte buffer of the ArrayBuffer.
  • [in] byte_length: The length in bytes of the underlying buffer.
  • [in] finalize_cb: Optional callback to call when the ArrayBuffer is being collected.
  • [in] finalize_hint: Optional hint to pass to the finalize callback during collection.
  • [out] result: A napi_value representing a JavaScript ArrayBuffer.

Returns napi_ok if the API succeeded.

This API returns an N-API value corresponding to a JavaScript ArrayBuffer. The underlying byte buffer of the ArrayBuffer is externally allocated and managed. The caller must ensure that the byte buffer remains valid until the finalize callback is called.

The API adds a napi_finalize callback which will be called when the JavaScript object just created is ready for garbage collection. It is similar to napi_wrap() except that:

  • the native data cannot be retrieved later using napi_unwrap(),
  • nor can it be removed later using napi_remove_wrap(), and
  • the object created by the API can be used with napi_wrap().

JavaScript ArrayBuffers are described in Section 24.1 of the ECMAScript Language Specification.

napi_create_external_buffer#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_external_buffer(napi_env env,
                                        size_t length,
                                        void* data,
                                        napi_finalize finalize_cb,
                                        void* finalize_hint,
                                        napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] length: Size in bytes of the input buffer (should be the same as the size of the new buffer).
  • [in] data: Raw pointer to the underlying buffer to copy from.
  • [in] finalize_cb: Optional callback to call when the ArrayBuffer is being collected.
  • [in] finalize_hint: Optional hint to pass to the finalize callback during collection.
  • [out] result: A napi_value representing a node::Buffer.

Returns napi_ok if the API succeeded.

This API allocates a node::Buffer object and initializes it with data backed by the passed in buffer. While this is still a fully-supported data structure, in most cases using a TypedArray will suffice.

The API adds a napi_finalize callback which will be called when the JavaScript object just created is ready for garbage collection. It is similar to napi_wrap() except that:

  • the native data cannot be retrieved later using napi_unwrap(),
  • nor can it be removed later using napi_remove_wrap(), and
  • the object created by the API can be used with napi_wrap().

For Node.js >=4 Buffers are Uint8Arrays.

napi_create_object#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_object(napi_env env, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [out] result: A napi_value representing a JavaScript Object.

Returns napi_ok if the API succeeded.

This API allocates a default JavaScript Object. It is the equivalent of doing new Object() in JavaScript.

The JavaScript Object type is described in Section 6.1.7 of the ECMAScript Language Specification.

napi_create_symbol#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_symbol(napi_env env,
                               napi_value description,
                               napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] description: Optional napi_value which refers to a JavaScript String to be set as the description for the symbol.
  • [out] result: A napi_value representing a JavaScript Symbol.

Returns napi_ok if the API succeeded.

This API creates a JavaScript Symbol object from a UTF8-encoded C string.

The JavaScript Symbol type is described in Section 19.4 of the ECMAScript Language Specification.

napi_create_typedarray#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_typedarray(napi_env env,
                                   napi_typedarray_type type,
                                   size_t length,
                                   napi_value arraybuffer,
                                   size_t byte_offset,
                                   napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] type: Scalar datatype of the elements within the TypedArray.
  • [in] length: Number of elements in the TypedArray.
  • [in] arraybuffer: ArrayBuffer underlying the typed array.
  • [in] byte_offset: The byte offset within the ArrayBuffer from which to start projecting the TypedArray.
  • [out] result: A napi_value representing a JavaScript TypedArray.

Returns napi_ok if the API succeeded.

This API creates a JavaScript TypedArray object over an existing ArrayBuffer. TypedArray objects provide an array-like view over an underlying data buffer where each element has the same underlying binary scalar datatype.

It's required that (length * size_of_element) + byte_offset should be <= the size in bytes of the array passed in. If not, a RangeError exception is raised.

JavaScript TypedArray objects are described in Section 22.2 of the ECMAScript Language Specification.

napi_create_dataview#

Added in: v8.3.0 N-API version: 1 
napi_status napi_create_dataview(napi_env env,
                                 size_t byte_length,
                                 napi_value arraybuffer,
                                 size_t byte_offset,
                                 napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] length: Number of elements in the DataView.
  • [in] arraybuffer: ArrayBuffer underlying the DataView.
  • [in] byte_offset: The byte offset within the ArrayBuffer from which to start projecting the DataView.
  • [out] result: A napi_value representing a JavaScript DataView.

Returns napi_ok if the API succeeded.

This API creates a JavaScript DataView object over an existing ArrayBuffer. DataView objects provide an array-like view over an underlying data buffer, but one which allows items of different size and type in the ArrayBuffer.

It is required that byte_length + byte_offset is less than or equal to the size in bytes of the array passed in. If not, a RangeError exception is raised.

JavaScript DataView objects are described in Section 24.3 of the ECMAScript Language Specification.

Functions to convert from C types to N-API#

napi_create_int32#

Added in: v8.4.0 N-API version: 1 
napi_status napi_create_int32(napi_env env, int32_t value, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: Integer value to be represented in JavaScript.
  • [out] result: A napi_value representing a JavaScript Number.

Returns napi_ok if the API succeeded.

This API is used to convert from the C int32_t type to the JavaScript Number type.

The JavaScript Number type is described in Section 6.1.6 of the ECMAScript Language Specification.

napi_create_uint32#

Added in: v8.4.0 N-API version: 1 
napi_status napi_create_uint32(napi_env env, uint32_t value, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: Unsigned integer value to be represented in JavaScript.
  • [out] result: A napi_value representing a JavaScript Number.

Returns napi_ok if the API succeeded.

This API is used to convert from the C uint32_t type to the JavaScript Number type.

The JavaScript Number type is described in Section 6.1.6 of the ECMAScript Language Specification.

napi_create_int64#

Added in: v8.4.0 N-API version: 1 
napi_status napi_create_int64(napi_env env, int64_t value, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: Integer value to be represented in JavaScript.
  • [out] result: A napi_value representing a JavaScript Number.

Returns napi_ok if the API succeeded.

This API is used to convert from the C int64_t type to the JavaScript Number type.

The JavaScript Number type is described in Section 6.1.6 of the ECMAScript Language Specification. Note the complete range of int64_t cannot be represented with full precision in JavaScript. Integer values outside the range of Number.MIN_SAFE_INTEGER -(2^53 - 1) - Number.MAX_SAFE_INTEGER (2^53 - 1) will lose precision.

napi_create_double#

Added in: v8.4.0 N-API version: 1 
napi_status napi_create_double(napi_env env, double value, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: Double-precision value to be represented in JavaScript.
  • [out] result: A napi_value representing a JavaScript Number.

Returns napi_ok if the API succeeded.

This API is used to convert from the C double type to the JavaScript Number type.

The JavaScript Number type is described in Section 6.1.6 of the ECMAScript Language Specification.

napi_create_bigint_int64#

Added in: v10.7.0 N-API version: 6 
napi_status napi_create_bigint_int64(napi_env env,
                                     int64_t value,
                                     napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] value: Integer value to be represented in JavaScript.
  • [out] result: A napi_value representing a JavaScript BigInt.

Returns napi_ok if the API succeeded.

This API converts the C int64_t type to the JavaScript BigInt type.

napi_create_bigint_uint64#

Added in: v10.7.0 N-API version: 6 
napi_status napi_create_bigint_uint64(napi_env env,
                                      uint64_t value,
                                      napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] value: Unsigned integer value to be represented in JavaScript.
  • [out] result: A napi_value representing a JavaScript BigInt.

Returns napi_ok if the API succeeded.

This API converts the C uint64_t type to the JavaScript BigInt type.

napi_create_bigint_words#

Added in: v10.7.0 N-API version: 6 
napi_status napi_create_bigint_words(napi_env env,
                                     int sign_bit,
                                     size_t word_count,
                                     const uint64_t* words,
                                     napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] sign_bit: Determines if the resulting BigInt will be positive or negative.
  • [in] word_count: The length of the words array.
  • [in] words: An array of uint64_t little-endian 64-bit words.
  • [out] result: A napi_value representing a JavaScript BigInt.

Returns napi_ok if the API succeeded.

This API converts an array of unsigned 64-bit words into a single BigInt value.

The resulting BigInt is calculated as: (–1)sign_bit (words[0] × (264)0 + words[1] × (264)1 + …)

napi_create_string_latin1#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_string_latin1(napi_env env,
                                      const char* str,
                                      size_t length,
                                      napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] str: Character buffer representing an ISO-8859-1-encoded string.
  • [in] length: The length of the string in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
  • [out] result: A napi_value representing a JavaScript String.

Returns napi_ok if the API succeeded.

This API creates a JavaScript String object from an ISO-8859-1-encoded C string. The native string is copied.

The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification.

napi_create_string_utf16#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_string_utf16(napi_env env,
                                     const char16_t* str,
                                     size_t length,
                                     napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] str: Character buffer representing a UTF16-LE-encoded string.
  • [in] length: The length of the string in two-byte code units, or NAPI_AUTO_LENGTH if it is null-terminated.
  • [out] result: A napi_value representing a JavaScript String.

Returns napi_ok if the API succeeded.

This API creates a JavaScript String object from a UTF16-LE-encoded C string. The native string is copied.

The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification.

napi_create_string_utf8#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_string_utf8(napi_env env,
                                    const char* str,
                                    size_t length,
                                    napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] str: Character buffer representing a UTF8-encoded string.
  • [in] length: The length of the string in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
  • [out] result: A napi_value representing a JavaScript String.

Returns napi_ok if the API succeeded.

This API creates a JavaScript String object from a UTF8-encoded C string. The native string is copied.

The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification.

Functions to convert from N-API to C types#

napi_get_array_length#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_array_length(napi_env env,
                                  napi_value value,
                                  uint32_t* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing the JavaScript Array whose length is being queried.
  • [out] result: uint32 representing length of the array.

Returns napi_ok if the API succeeded.

This API returns the length of an array.

Array length is described in Section 22.1.4.1 of the ECMAScript Language Specification.

napi_get_arraybuffer_info#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_arraybuffer_info(napi_env env,
                                      napi_value arraybuffer,
                                      void** data,
                                      size_t* byte_length)
  • [in] env: The environment that the API is invoked under.
  • [in] arraybuffer: napi_value representing the ArrayBuffer being queried.
  • [out] data: The underlying data buffer of the ArrayBuffer. If byte_length is 0, this may be NULL or any other pointer value.
  • [out] byte_length: Length in bytes of the underlying data buffer.

Returns napi_ok if the API succeeded.

This API is used to retrieve the underlying data buffer of an ArrayBuffer and its length.

WARNING: Use caution while using this API. The lifetime of the underlying data buffer is managed by the ArrayBuffer even after it's returned. A possible safe way to use this API is in conjunction with napi_create_reference, which can be used to guarantee control over the lifetime of the ArrayBuffer. It's also safe to use the returned data buffer within the same callback as long as there are no calls to other APIs that might trigger a GC.

napi_get_buffer_info#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_buffer_info(napi_env env,
                                 napi_value value,
                                 void** data,
                                 size_t* length)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing the node::Buffer being queried.
  • [out] data: The underlying data buffer of the node::Buffer. If length is 0, this may be NULL or any other pointer value.
  • [out] length: Length in bytes of the underlying data buffer.

Returns napi_ok if the API succeeded.

This API is used to retrieve the underlying data buffer of a node::Buffer and it's length.

Warning: Use caution while using this API since the underlying data buffer's lifetime is not guaranteed if it's managed by the VM.

napi_get_prototype#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_prototype(napi_env env,
                               napi_value object,
                               napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] object: napi_value representing JavaScript Object whose prototype to return. This returns the equivalent of Object.getPrototypeOf (which is not the same as the function's prototype property).
  • [out] result: napi_value representing prototype of the given object.

Returns napi_ok if the API succeeded.

napi_get_typedarray_info#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_typedarray_info(napi_env env,
                                     napi_value typedarray,
                                     napi_typedarray_type* type,
                                     size_t* length,
                                     void** data,
                                     napi_value* arraybuffer,
                                     size_t* byte_offset)
  • [in] env: The environment that the API is invoked under.
  • [in] typedarray: napi_value representing the TypedArray whose properties to query.
  • [out] type: Scalar datatype of the elements within the TypedArray.
  • [out] length: The number of elements in the TypedArray.
  • [out] data: The data buffer underlying the TypedArray adjusted by the byte_offset value so that it points to the first element in the TypedArray. If the length of the array is 0, this may be NULL or any other pointer value.
  • [out] arraybuffer: The ArrayBuffer underlying the TypedArray.
  • [out] byte_offset: The byte offset within the underlying native array at which the first element of the arrays is located. The value for the data parameter has already been adjusted so that data points to the first element in the array. Therefore, the first byte of the native array would be at data - byte_offset.

Returns napi_ok if the API succeeded.

This API returns various properties of a typed array.

Warning: Use caution while using this API since the underlying data buffer is managed by the VM.

napi_get_dataview_info#

Added in: v8.3.0 N-API version: 1 
napi_status napi_get_dataview_info(napi_env env,
                                   napi_value dataview,
                                   size_t* byte_length,
                                   void** data,
                                   napi_value* arraybuffer,
                                   size_t* byte_offset)
  • [in] env: The environment that the API is invoked under.
  • [in] dataview: napi_value representing the DataView whose properties to query.
  • [out] byte_length: Number of bytes in the DataView.
  • [out] data: The data buffer underlying the DataView. If byte_length is 0, this may be NULL or any other pointer value.
  • [out] arraybuffer: ArrayBuffer underlying the DataView.
  • [out] byte_offset: The byte offset within the data buffer from which to start projecting the DataView.

Returns napi_ok if the API succeeded.

This API returns various properties of a DataView.

napi_get_date_value#

Added in: v11.11.0, v10.17.0 N-API version: 5 
napi_status napi_get_date_value(napi_env env,
                                napi_value value,
                                double* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing a JavaScript Date.
  • [out] result: Time value as a double represented as milliseconds since midnight at the beginning of 01 January, 1970 UTC.

This API does not observe leap seconds; they are ignored, as ECMAScript aligns with POSIX time specification.

Returns napi_ok if the API succeeded. If a non-date napi_value is passed in it returns napi_date_expected.

This API returns the C double primitive of time value for the given JavaScript Date.

napi_get_value_bool#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_bool(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript Boolean.
  • [out] result: C boolean primitive equivalent of the given JavaScript Boolean.

Returns napi_ok if the API succeeded. If a non-boolean napi_value is passed in it returns napi_boolean_expected.

This API returns the C boolean primitive equivalent of the given JavaScript Boolean.

napi_get_value_double#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_double(napi_env env,
                                  napi_value value,
                                  double* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript Number.
  • [out] result: C double primitive equivalent of the given JavaScript Number.

Returns napi_ok if the API succeeded. If a non-number napi_value is passed in it returns napi_number_expected.

This API returns the C double primitive equivalent of the given JavaScript Number.

napi_get_value_bigint_int64#

Added in: v10.7.0 N-API version: 6 
napi_status napi_get_value_bigint_int64(napi_env env,
                                        napi_value value,
                                        int64_t* result,
                                        bool* lossless);
  • [in] env: The environment that the API is invoked under
  • [in] value: napi_value representing JavaScript BigInt.
  • [out] result: C int64_t primitive equivalent of the given JavaScript BigInt.
  • [out] lossless: Indicates whether the BigInt value was converted losslessly.

Returns napi_ok if the API succeeded. If a non-BigInt is passed in it returns napi_bigint_expected.

This API returns the C int64_t primitive equivalent of the given JavaScript BigInt. If needed it will truncate the value, setting lossless to false.

napi_get_value_bigint_uint64#

Added in: v10.7.0 N-API version: 6 
napi_status napi_get_value_bigint_uint64(napi_env env,
                                        napi_value value,
                                        uint64_t* result,
                                        bool* lossless);
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript BigInt.
  • [out] result: C uint64_t primitive equivalent of the given JavaScript BigInt.
  • [out] lossless: Indicates whether the BigInt value was converted losslessly.

Returns napi_ok if the API succeeded. If a non-BigInt is passed in it returns napi_bigint_expected.

This API returns the C uint64_t primitive equivalent of the given JavaScript BigInt. If needed it will truncate the value, setting lossless to false.

napi_get_value_bigint_words#

Added in: v10.7.0 N-API version: 6 
napi_status napi_get_value_bigint_words(napi_env env,
                                        napi_value value,
                                        int* sign_bit,
                                        size_t* word_count,
                                        uint64_t* words);
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript BigInt.
  • [out] sign_bit: Integer representing if the JavaScript BigInt is positive or negative.
  • [in/out] word_count: Must be initialized to the length of the words array. Upon return, it will be set to the actual number of words that would be needed to store this BigInt.
  • [out] words: Pointer to a pre-allocated 64-bit word array.

Returns napi_ok if the API succeeded.

This API converts a single BigInt value into a sign bit, 64-bit little-endian array, and the number of elements in the array. sign_bit and words may be both set to NULL, in order to get only word_count.

napi_get_value_external#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_external(napi_env env,
                                    napi_value value,
                                    void** result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript external value.
  • [out] result: Pointer to the data wrapped by the JavaScript external value.

Returns napi_ok if the API succeeded. If a non-external napi_value is passed in it returns napi_invalid_arg.

This API retrieves the external data pointer that was previously passed to napi_create_external().

napi_get_value_int32#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_int32(napi_env env,
                                 napi_value value,
                                 int32_t* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript Number.
  • [out] result: C int32 primitive equivalent of the given JavaScript Number.

Returns napi_ok if the API succeeded. If a non-number napi_value is passed in napi_number_expected.

This API returns the C int32 primitive equivalent of the given JavaScript Number.

If the number exceeds the range of the 32 bit integer, then the result is truncated to the equivalent of the bottom 32 bits. This can result in a large positive number becoming a negative number if the value is > 2^31 -1.

Non-finite number values (NaN, +Infinity, or -Infinity) set the result to zero.

napi_get_value_int64#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_int64(napi_env env,
                                 napi_value value,
                                 int64_t* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript Number.
  • [out] result: C int64 primitive equivalent of the given JavaScript Number.

Returns napi_ok if the API succeeded. If a non-number napi_value is passed in it returns napi_number_expected.

This API returns the C int64 primitive equivalent of the given JavaScript Number.

Number values outside the range of Number.MIN_SAFE_INTEGER -(2^53 - 1) - Number.MAX_SAFE_INTEGER (2^53 - 1) will lose precision.

Non-finite number values (NaN, +Infinity, or -Infinity) set the result to zero.

napi_get_value_string_latin1#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_string_latin1(napi_env env,
                                         napi_value value,
                                         char* buf,
                                         size_t bufsize,
                                         size_t* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript string.
  • [in] buf: Buffer to write the ISO-8859-1-encoded string into. If NULL is passed in, the length of the string (in bytes) is returned.
  • [in] bufsize: Size of the destination buffer. When this value is insufficient, the returned string will be truncated.
  • [out] result: Number of bytes copied into the buffer, excluding the null terminator.

Returns napi_ok if the API succeeded. If a non-String napi_value is passed in it returns napi_string_expected.

This API returns the ISO-8859-1-encoded string corresponding the value passed in.

napi_get_value_string_utf8#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_string_utf8(napi_env env,
                                       napi_value value,
                                       char* buf,
                                       size_t bufsize,
                                       size_t* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript string.
  • [in] buf: Buffer to write the UTF8-encoded string into. If NULL is passed in, the length of the string (in bytes) is returned.
  • [in] bufsize: Size of the destination buffer. When this value is insufficient, the returned string will be truncated.
  • [out] result: Number of bytes copied into the buffer, excluding the null terminator.

Returns napi_ok if the API succeeded. If a non-String napi_value is passed in it returns napi_string_expected.

This API returns the UTF8-encoded string corresponding the value passed in.

napi_get_value_string_utf16#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_string_utf16(napi_env env,
                                        napi_value value,
                                        char16_t* buf,
                                        size_t bufsize,
                                        size_t* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript string.
  • [in] buf: Buffer to write the UTF16-LE-encoded string into. If NULL is passed in, the length of the string (in 2-byte code units) is returned.
  • [in] bufsize: Size of the destination buffer. When this value is insufficient, the returned string will be truncated.
  • [out] result: Number of 2-byte code units copied into the buffer, excluding the null terminator.

Returns napi_ok if the API succeeded. If a non-String napi_value is passed in it returns napi_string_expected.

This API returns the UTF16-encoded string corresponding the value passed in.

napi_get_value_uint32#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_value_uint32(napi_env env,
                                  napi_value value,
                                  uint32_t* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: napi_value representing JavaScript Number.
  • [out] result: C primitive equivalent of the given napi_value as a uint32_t.

Returns napi_ok if the API succeeded. If a non-number napi_value is passed in it returns napi_number_expected.

This API returns the C primitive equivalent of the given napi_value as a uint32_t.

Functions to get global instances#

napi_get_boolean#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_boolean(napi_env env, bool value, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The value of the boolean to retrieve.
  • [out] result: napi_value representing JavaScript Boolean singleton to retrieve.

Returns napi_ok if the API succeeded.

This API is used to return the JavaScript singleton object that is used to represent the given boolean value.

napi_get_global#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_global(napi_env env, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [out] result: napi_value representing JavaScript global object.

Returns napi_ok if the API succeeded.

This API returns the global object.

napi_get_null#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_null(napi_env env, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [out] result: napi_value representing JavaScript null object.

Returns napi_ok if the API succeeded.

This API returns the null object.

napi_get_undefined#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_undefined(napi_env env, napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [out] result: napi_value representing JavaScript Undefined value.

Returns napi_ok if the API succeeded.

This API returns the Undefined object.

Working with JavaScript Values and Abstract Operations#

N-API exposes a set of APIs to perform some abstract operations on JavaScript values. Some of these operations are documented under Section 7 of the ECMAScript Language Specification.

These APIs support doing one of the following:

  1. Coerce JavaScript values to specific JavaScript types (such as Number or String).
  2. Check the type of a JavaScript value.
  3. Check for equality between two JavaScript values.

napi_coerce_to_bool#

Added in: v8.0.0 N-API version: 1 
napi_status napi_coerce_to_bool(napi_env env,
                                napi_value value,
                                napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to coerce.
  • [out] result: napi_value representing the coerced JavaScript Boolean.

Returns napi_ok if the API succeeded.

This API implements the abstract operation ToBoolean() as defined in Section 7.1.2 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.

napi_coerce_to_number#

Added in: v8.0.0 N-API version: 1 
napi_status napi_coerce_to_number(napi_env env,
                                  napi_value value,
                                  napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to coerce.
  • [out] result: napi_value representing the coerced JavaScript Number.

Returns napi_ok if the API succeeded.

This API implements the abstract operation ToNumber() as defined in Section 7.1.3 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.

napi_coerce_to_object#

Added in: v8.0.0 N-API version: 1 
napi_status napi_coerce_to_object(napi_env env,
                                  napi_value value,
                                  napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to coerce.
  • [out] result: napi_value representing the coerced JavaScript Object.

Returns napi_ok if the API succeeded.

This API implements the abstract operation ToObject() as defined in Section 7.1.13 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.

napi_coerce_to_string#

Added in: v8.0.0 N-API version: 1 
napi_status napi_coerce_to_string(napi_env env,
                                  napi_value value,
                                  napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to coerce.
  • [out] result: napi_value representing the coerced JavaScript String.

Returns napi_ok if the API succeeded.

This API implements the abstract operation ToString() as defined in Section 7.1.13 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object.

napi_typeof#

Added in: v8.0.0 N-API version: 1 
napi_status napi_typeof(napi_env env, napi_value value, napi_valuetype* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value whose type to query.
  • [out] result: The type of the JavaScript value.

Returns napi_ok if the API succeeded.

  • napi_invalid_arg if the type of value is not a known ECMAScript type and value is not an External value.

This API represents behavior similar to invoking the typeof Operator on the object as defined in Section 12.5.5 of the ECMAScript Language Specification. However, it has support for detecting an External value. If value has a type that is invalid, an error is returned.

napi_instanceof#

Added in: v8.0.0 N-API version: 1 
napi_status napi_instanceof(napi_env env,
                            napi_value object,
                            napi_value constructor,
                            bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] object: The JavaScript value to check.
  • [in] constructor: The JavaScript function object of the constructor function to check against.
  • [out] result: Boolean that is set to true if object instanceof constructor is true.

Returns napi_ok if the API succeeded.

This API represents invoking the instanceof Operator on the object as defined in Section 12.10.4 of the ECMAScript Language Specification.

napi_is_array#

Added in: v8.0.0 N-API version: 1 
napi_status napi_is_array(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to check.
  • [out] result: Whether the given object is an array.

Returns napi_ok if the API succeeded.

This API represents invoking the IsArray operation on the object as defined in Section 7.2.2 of the ECMAScript Language Specification.

napi_is_arraybuffer#

Added in: v8.0.0 N-API version: 1 
napi_status napi_is_arraybuffer(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to check.
  • [out] result: Whether the given object is an ArrayBuffer.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is an array buffer.

napi_is_buffer#

Added in: v8.0.0 N-API version: 1 
napi_status napi_is_buffer(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to check.
  • [out] result: Whether the given napi_value represents a node::Buffer object.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is a buffer.

napi_is_date#

Added in: v11.11.0, v10.17.0 N-API version: 5 
napi_status napi_is_date(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to check.
  • [out] result: Whether the given napi_value represents a JavaScript Date object.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is a date.

napi_is_error#

Added in: v8.0.0 N-API version: 1 
napi_status napi_is_error(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to check.
  • [out] result: Whether the given napi_value represents an Error object.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is an Error.

napi_is_typedarray#

Added in: v8.0.0 N-API version: 1 
napi_status napi_is_typedarray(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to check.
  • [out] result: Whether the given napi_value represents a TypedArray.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is a typed array.

napi_is_dataview#

Added in: v8.3.0 N-API version: 1 
napi_status napi_is_dataview(napi_env env, napi_value value, bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] value: The JavaScript value to check.
  • [out] result: Whether the given napi_value represents a DataView.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in is a DataView.

napi_strict_equals#

Added in: v8.0.0 N-API version: 1 
napi_status napi_strict_equals(napi_env env,
                               napi_value lhs,
                               napi_value rhs,
                               bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] lhs: The JavaScript value to check.
  • [in] rhs: The JavaScript value to check against.
  • [out] result: Whether the two napi_value objects are equal.

Returns napi_ok if the API succeeded.

This API represents the invocation of the Strict Equality algorithm as defined in Section 7.2.14 of the ECMAScript Language Specification.

napi_detach_arraybuffer#

Added in: v13.0.0, v12.16.0

Stability: 1 - Experimental

napi_status napi_detach_arraybuffer(napi_env env,
                                    napi_value arraybuffer)
  • [in] env: The environment that the API is invoked under.
  • [in] arraybuffer: The JavaScript ArrayBuffer to be detached.

Returns napi_ok if the API succeeded. If a non-detachable ArrayBuffer is passed in it returns napi_detachable_arraybuffer_expected.

Generally, an ArrayBuffer is non-detachable if it has been detached before. The engine may impose additional conditions on whether an ArrayBuffer is detachable. For example, V8 requires that the ArrayBuffer be external, that is, created with napi_create_external_arraybuffer.

This API represents the invocation of the ArrayBuffer detach operation as defined in Section 24.1.1.3 of the ECMAScript Language Specification.

napi_is_detached_arraybuffer#

Added in: v13.3.0, v12.16.0

Stability: 1 - Experimental

napi_status napi_is_detached_arraybuffer(napi_env env,
                                         napi_value arraybuffer,
                                         bool* result)
  • [in] env: The environment that the API is invoked under.
  • [in] arraybuffer: The JavaScript ArrayBuffer to be checked.
  • [out] result: Whether the arraybuffer is detached.

Returns napi_ok if the API succeeded.

The ArrayBuffer is considered detached if its internal data is null.

This API represents the invocation of the ArrayBuffer IsDetachedBuffer operation as defined in Section 24.1.1.2 of the ECMAScript Language Specification.

Working with JavaScript Properties#

N-API exposes a set of APIs to get and set properties on JavaScript objects. Some of these types are documented under Section 7 of the ECMAScript Language Specification.

Properties in JavaScript are represented as a tuple of a key and a value. Fundamentally, all property keys in N-API can be represented in one of the following forms:

  • Named: a simple UTF8-encoded string
  • Integer-Indexed: an index value represented by uint32_t
  • JavaScript value: these are represented in N-API by napi_value. This can be a napi_value representing a String, Number, or Symbol.

N-API values are represented by the type napi_value. Any N-API call that requires a JavaScript value takes in a napi_value. However, it's the caller's responsibility to make sure that the napi_value in question is of the JavaScript type expected by the API.

The APIs documented in this section provide a simple interface to get and set properties on arbitrary JavaScript objects represented by napi_value.

For instance, consider the following JavaScript code snippet:

const obj = {};
obj.myProp = 123;

The equivalent can be done using N-API values with the following snippet:

napi_status status = napi_generic_failure;

// const obj = {}
napi_value obj, value;
status = napi_create_object(env, &obj);
if (status != napi_ok) return status;

// Create a napi_value for 123
status = napi_create_int32(env, 123, &value);
if (status != napi_ok) return status;

// obj.myProp = 123
status = napi_set_named_property(env, obj, "myProp", value);
if (status != napi_ok) return status;

Indexed properties can be set in a similar manner. Consider the following JavaScript snippet:

const arr = [];
arr[123] = 'hello';

The equivalent can be done using N-API values with the following snippet:

napi_status status = napi_generic_failure;

// const arr = [];
napi_value arr, value;
status = napi_create_array(env, &arr);
if (status != napi_ok) return status;

// Create a napi_value for 'hello'
status = napi_create_string_utf8(env, "hello", NAPI_AUTO_LENGTH, &value);
if (status != napi_ok) return status;

// arr[123] = 'hello';
status = napi_set_element(env, arr, 123, value);
if (status != napi_ok) return status;

Properties can be retrieved using the APIs described in this section. Consider the following JavaScript snippet:

const arr = [];
const value = arr[123];

The following is the approximate equivalent of the N-API counterpart:

napi_status status = napi_generic_failure;

// const arr = []
napi_value arr, value;
status = napi_create_array(env, &arr);
if (status != napi_ok) return status;

// const value = arr[123]
status = napi_get_element(env, arr, 123, &value);
if (status != napi_ok) return status;

Finally, multiple properties can also be defined on an object for performance reasons. Consider the following JavaScript:

const obj = {};
Object.defineProperties(obj, {
  'foo': { value: 123, writable: true, configurable: true, enumerable: true },
  'bar': { value: 456, writable: true, configurable: true, enumerable: true }
});

The following is the approximate equivalent of the N-API counterpart:

napi_status status = napi_status_generic_failure;

// const obj = {};
napi_value obj;
status = napi_create_object(env, &obj);
if (status != napi_ok) return status;

// Create napi_values for 123 and 456
napi_value fooValue, barValue;
status = napi_create_int32(env, 123, &fooValue);
if (status != napi_ok) return status;
status = napi_create_int32(env, 456, &barValue);
if (status != napi_ok) return status;

// Set the properties
napi_property_descriptor descriptors[] = {
  { "foo", NULL, NULL, NULL, NULL, fooValue, napi_default, NULL },
  { "bar", NULL, NULL, NULL, NULL, barValue, napi_default, NULL }
}
status = napi_define_properties(env,
                                obj,
                                sizeof(descriptors) / sizeof(descriptors[0]),
                                descriptors);
if (status != napi_ok) return status;

Structures#

napi_property_attributes#

typedef enum {
  napi_default = 0,
  napi_writable = 1 << 0,
  napi_enumerable = 1 << 1,
  napi_configurable = 1 << 2,

  // Used with napi_define_class to distinguish static properties
  // from instance properties. Ignored by napi_define_properties.
  napi_static = 1 << 10,
} napi_property_attributes;

napi_property_attributes are flags used to control the behavior of properties set on a JavaScript object. Other than napi_static they correspond to the attributes listed in Section 6.1.7.1 of the ECMAScript Language Specification. They can be one or more of the following bitflags:

  • napi_default: No explicit attributes are set on the property. By default, a property is read only, not enumerable and not configurable.
  • napi_writable: The property is writable.
  • napi_enumerable: The property is enumerable.
  • napi_configurable: The property is configurable as defined in Section 6.1.7.1 of the ECMAScript Language Specification.
  • napi_static: The property will be defined as a static property on a class as opposed to an instance property, which is the default. This is used only by napi_define_class. It is ignored by napi_define_properties.

napi_property_descriptor#

typedef struct {
  // One of utf8name or name should be NULL.
  const char* utf8name;
  napi_value name;

  napi_callback method;
  napi_callback getter;
  napi_callback setter;
  napi_value value;

  napi_property_attributes attributes;
  void* data;
} napi_property_descriptor;
  • utf8name: Optional String describing the key for the property, encoded as UTF8. One of utf8name or name must be provided for the property.
  • name: Optional napi_value that points to a JavaScript string or symbol to be used as the key for the property. One of utf8name or name must be provided for the property.
  • value: The value that's retrieved by a get access of the property if the property is a data property. If this is passed in, set getter, setter, method and data to NULL (since these members won't be used).
  • getter: A function to call when a get access of the property is performed. If this is passed in, set value and method to NULL (since these members won't be used). The given function is called implicitly by the runtime when the property is accessed from JavaScript code (or if a get on the property is performed using a N-API call).
  • setter: A function to call when a set access of the property is performed. If this is passed in, set value and method to NULL (since these members won't be used). The given function is called implicitly by the runtime when the property is set from JavaScript code (or if a set on the property is performed using a N-API call).
  • method: Set this to make the property descriptor object's value property to be a JavaScript function represented by method. If this is passed in, set value, getter and setter to NULL (since these members won't be used).
  • attributes: The attributes associated with the particular property. See napi_property_attributes.
  • data: The callback data passed into method, getter and setter if this function is invoked.

Functions#

napi_get_property_names#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_property_names(napi_env env,
                                    napi_value object,
                                    napi_value* result);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object from which to retrieve the properties.
  • [out] result: A napi_value representing an array of JavaScript values that represent the property names of the object. The API can be used to iterate over result using napi_get_array_length and napi_get_element.

Returns napi_ok if the API succeeded.

This API returns the names of the enumerable properties of object as an array of strings. The properties of object whose key is a symbol will not be included.

napi_get_all_property_names#

Added in: v13.7.0, v10.20.0 N-API version: 6 
napi_get_all_property_names(napi_env env,
                            napi_value object,
                            napi_key_collection_mode key_mode,
                            napi_key_filter key_filter,
                            napi_key_conversion key_conversion,
                            napi_value* result);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object from which to retrieve the properties.
  • [in] key_mode: Whether to retrieve prototype properties as well.
  • [in] key_filter: Which properties to retrieve (enumerable/readable/writable).
  • [in] key_conversion: Whether to convert numbered property keys to strings.
  • [out] result: A napi_value representing an array of JavaScript values that represent the property names of the object. napi_get_array_length and napi_get_element can be used to iterate over result.

Returns napi_ok if the API succeeded.

This API returns an array containing the names of the available properties of this object.

napi_set_property#

Added in: v8.0.0 N-API version: 1 
napi_status napi_set_property(napi_env env,
                              napi_value object,
                              napi_value key,
                              napi_value value);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object on which to set the property.
  • [in] key: The name of the property to set.
  • [in] value: The property value.

Returns napi_ok if the API succeeded.

This API set a property on the Object passed in.

napi_get_property#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_property(napi_env env,
                              napi_value object,
                              napi_value key,
                              napi_value* result);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object from which to retrieve the property.
  • [in] key: The name of the property to retrieve.
  • [out] result: The value of the property.

Returns napi_ok if the API succeeded.

This API gets the requested property from the Object passed in.

napi_has_property#

Added in: v8.0.0 N-API version: 1 
napi_status napi_has_property(napi_env env,
                              napi_value object,
                              napi_value key,
                              bool* result);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object to query.
  • [in] key: The name of the property whose existence to check.
  • [out] result: Whether the property exists on the object or not.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in has the named property.

napi_delete_property#

Added in: v8.2.0 N-API version: 1 
napi_status napi_delete_property(napi_env env,
                                 napi_value object,
                                 napi_value key,
                                 bool* result);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object to query.
  • [in] key: The name of the property to delete.
  • [out] result: Whether the property deletion succeeded or not. result can optionally be ignored by passing NULL.

Returns napi_ok if the API succeeded.

This API attempts to delete the key own property from object.

napi_has_own_property#

Added in: v8.2.0 N-API version: 1 
napi_status napi_has_own_property(napi_env env,
                                  napi_value object,
                                  napi_value key,
                                  bool* result);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object to query.
  • [in] key: The name of the own property whose existence to check.
  • [out] result: Whether the own property exists on the object or not.

Returns napi_ok if the API succeeded.

This API checks if the Object passed in has the named own property. key must be a string or a Symbol, or an error will be thrown. N-API will not perform any conversion between data types.

napi_set_named_property#

Added in: v8.0.0 N-API version: 1 
napi_status napi_set_named_property(napi_env env,
                                    napi_value object,
                                    const char* utf8Name,
                                    napi_value value);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object on which to set the property.
  • [in] utf8Name: The name of the property to set.
  • [in] value: The property value.

Returns napi_ok if the API succeeded.

This method is equivalent to calling napi_set_property with a napi_value created from the string passed in as utf8Name.

napi_get_named_property#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_named_property(napi_env env,
                                    napi_value object,
                                    const char* utf8Name,
                                    napi_value* result);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object from which to retrieve the property.
  • [in] utf8Name: The name of the property to get.
  • [out] result: The value of the property.

Returns napi_ok if the API succeeded.

This method is equivalent to calling napi_get_property with a napi_value created from the string passed in as utf8Name.

napi_has_named_property#

Added in: v8.0.0 N-API version: 1 
napi_status napi_has_named_property(napi_env env,
                                    napi_value object,
                                    const char* utf8Name,
                                    bool* result);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object to query.
  • [in] utf8Name: The name of the property whose existence to check.
  • [out] result: Whether the property exists on the object or not.

Returns napi_ok if the API succeeded.

This method is equivalent to calling napi_has_property with a napi_value created from the string passed in as utf8Name.

napi_set_element#

Added in: v8.0.0 N-API version: 1 
napi_status napi_set_element(napi_env env,
                             napi_value object,
                             uint32_t index,
                             napi_value value);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object from which to set the properties.
  • [in] index: The index of the property to set.
  • [in] value: The property value.

Returns napi_ok if the API succeeded.

This API sets and element on the Object passed in.

napi_get_element#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_element(napi_env env,
                             napi_value object,
                             uint32_t index,
                             napi_value* result);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object from which to retrieve the property.
  • [in] index: The index of the property to get.
  • [out] result: The value of the property.

Returns napi_ok if the API succeeded.

This API gets the element at the requested index.

napi_has_element#

Added in: v8.0.0 N-API version: 1 
napi_status napi_has_element(napi_env env,
                             napi_value object,
                             uint32_t index,
                             bool* result);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object to query.
  • [in] index: The index of the property whose existence to check.
  • [out] result: Whether the property exists on the object or not.

Returns napi_ok if the API succeeded.

This API returns if the Object passed in has an element at the requested index.

napi_delete_element#

Added in: v8.2.0 N-API version: 1 
napi_status napi_delete_element(napi_env env,
                                napi_value object,
                                uint32_t index,
                                bool* result);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object to query.
  • [in] index: The index of the property to delete.
  • [out] result: Whether the element deletion succeeded or not. result can optionally be ignored by passing NULL.

Returns napi_ok if the API succeeded.

This API attempts to delete the specified index from object.

napi_define_properties#

Added in: v8.0.0 N-API version: 1 
napi_status napi_define_properties(napi_env env,
                                   napi_value object,
                                   size_t property_count,
                                   const napi_property_descriptor* properties);
  • [in] env: The environment that the N-API call is invoked under.
  • [in] object: The object from which to retrieve the properties.
  • [in] property_count: The number of elements in the properties array.
  • [in] properties: The array of property descriptors.

Returns napi_ok if the API succeeded.

This method allows the efficient definition of multiple properties on a given object. The properties are defined using property descriptors (see napi_property_descriptor). Given an array of such property descriptors, this API will set the properties on the object one at a time, as defined by DefineOwnProperty() (described in Section 9.1.6 of the ECMA-262 specification).

Working with JavaScript Functions#

N-API provides a set of APIs that allow JavaScript code to call back into native code. N-API APIs that support calling back into native code take in a callback functions represented by the napi_callback type. When the JavaScript VM calls back to native code, the napi_callback function provided is invoked. The APIs documented in this section allow the callback function to do the following:

  • Get information about the context in which the callback was invoked.
  • Get the arguments passed into the callback.
  • Return a napi_value back from the callback.

Additionally, N-API provides a set of functions which allow calling JavaScript functions from native code. One can either call a function like a regular JavaScript function call, or as a constructor function.

Any non-NULL data which is passed to this API via the data field of the napi_property_descriptor items can be associated with object and freed whenever object is garbage-collected by passing both object and the data to napi_add_finalizer.

napi_call_function#

Added in: v8.0.0 N-API version: 1 
NAPI_EXTERN napi_status napi_call_function(napi_env env,
                                           napi_value recv,
                                           napi_value func,
                                           size_t argc,
                                           const napi_value* argv,
                                           napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] recv: The this object passed to the called function.
  • [in] func: napi_value representing the JavaScript function to be invoked.
  • [in] argc: The count of elements in the argv array.
  • [in] argv: Array of napi_values representing JavaScript values passed in as arguments to the function.
  • [out] result: napi_value representing the JavaScript object returned.

Returns napi_ok if the API succeeded.

This method allows a JavaScript function object to be called from a native add-on. This is the primary mechanism of calling back from the add-on's native code into JavaScript. For the special case of calling into JavaScript after an async operation, see napi_make_callback.

A sample use case might look as follows. Consider the following JavaScript snippet:

function AddTwo(num) {
  return num + 2;
}

Then, the above function can be invoked from a native add-on using the following code:

// Get the function named "AddTwo" on the global object
napi_value global, add_two, arg;
napi_status status = napi_get_global(env, &global);
if (status != napi_ok) return;

status = napi_get_named_property(env, global, "AddTwo", &add_two);
if (status != napi_ok) return;

// const arg = 1337
status = napi_create_int32(env, 1337, &arg);
if (status != napi_ok) return;

napi_value* argv = &arg;
size_t argc = 1;

// AddTwo(arg);
napi_value return_val;
status = napi_call_function(env, global, add_two, argc, argv, &return_val);
if (status != napi_ok) return;

// Convert the result back to a native type
int32_t result;
status = napi_get_value_int32(env, return_val, &result);
if (status != napi_ok) return;

napi_create_function#

Added in: v8.0.0 N-API version: 1 
napi_status napi_create_function(napi_env env,
                                 const char* utf8name,
                                 size_t length,
                                 napi_callback cb,
                                 void* data,
                                 napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] utf8Name: The name of the function encoded as UTF8. This is visible within JavaScript as the new function object's name property.
  • [in] length: The length of the utf8name in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
  • [in] cb: The native function which should be called when this function object is invoked.
  • [in] data: User-provided data context. This will be passed back into the function when invoked later.
  • [out] result: napi_value representing the JavaScript function object for the newly created function.

Returns napi_ok if the API succeeded.

This API allows an add-on author to create a function object in native code. This is the primary mechanism to allow calling into the add-on's native code from JavaScript.

The newly created function is not automatically visible from script after this call. Instead, a property must be explicitly set on any object that is visible to JavaScript, in order for the function to be accessible from script.

In order to expose a function as part of the add-on's module exports, set the newly created function on the exports object. A sample module might look as follows:

napi_value SayHello(napi_env env, napi_callback_info info) {
  printf("Hello\n");
  return NULL;
}

napi_value Init(napi_env env, napi_value exports) {
  napi_status status;

  napi_value fn;
  status = napi_create_function(env, NULL, 0, SayHello, NULL, &fn);
  if (status != napi_ok) return NULL;

  status = napi_set_named_property(env, exports, "sayHello", fn);
  if (status != napi_ok) return NULL;

  return exports;
}

NAPI_MODULE(NODE_GYP_MODULE_NAME, Init)

Given the above code, the add-on can be used from JavaScript as follows:

const myaddon = require('./addon');
myaddon.sayHello();

The string passed to require() is the name of the target in binding.gyp responsible for creating the .node file.

Any non-NULL data which is passed to this API via the data parameter can be associated with the resulting JavaScript function (which is returned in the result parameter) and freed whenever the function is garbage-collected by passing both the JavaScript function and the data to napi_add_finalizer.

JavaScript Functions are described in Section 19.2 of the ECMAScript Language Specification.

napi_get_cb_info#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_cb_info(napi_env env,
                             napi_callback_info cbinfo,
                             size_t* argc,
                             napi_value* argv,
                             napi_value* thisArg,
                             void** data)
  • [in] env: The environment that the API is invoked under.
  • [in] cbinfo: The callback info passed into the callback function.
  • [in-out] argc: Specifies the size of the provided argv array and receives the actual count of arguments.
  • [out] argv: Buffer to which the napi_value representing the arguments are copied. If there are more arguments than the provided count, only the requested number of arguments are copied. If there are fewer arguments provided than claimed, the rest of argv is filled with napi_value values that represent undefined.
  • [out] this: Receives the JavaScript this argument for the call.
  • [out] data: Receives the data pointer for the callback.

Returns napi_ok if the API succeeded.

This method is used within a callback function to retrieve details about the call like the arguments and the this pointer from a given callback info.

napi_get_new_target#

Added in: v8.6.0 N-API version: 1 
napi_status napi_get_new_target(napi_env env,
                                napi_callback_info cbinfo,
                                napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] cbinfo: The callback info passed into the callback function.
  • [out] result: The new.target of the constructor call.

Returns napi_ok if the API succeeded.

This API returns the new.target of the constructor call. If the current callback is not a constructor call, the result is NULL.

napi_new_instance#

Added in: v8.0.0 N-API version: 1 
napi_status napi_new_instance(napi_env env,
                              napi_value cons,
                              size_t argc,
                              napi_value* argv,
                              napi_value* result)
  • [in] env: The environment that the API is invoked under.
  • [in] cons: napi_value representing the JavaScript function to be invoked as a constructor.
  • [in] argc: The count of elements in the argv array.
  • [in] argv: Array of JavaScript values as napi_value representing the arguments to the constructor.
  • [out] result: napi_value representing the JavaScript object returned, which in this case is the constructed object.

This method is used to instantiate a new JavaScript value using a given napi_value that represents the constructor for the object. For example, consider the following snippet:

function MyObject(param) {
  this.param = param;
}

const arg = 'hello';
const value = new MyObject(arg);

The following can be approximated in N-API using the following snippet:

// Get the constructor function MyObject
napi_value global, constructor, arg, value;
napi_status status = napi_get_global(env, &global);
if (status != napi_ok) return;

status = napi_get_named_property(env, global, "MyObject", &constructor);
if (status != napi_ok) return;

// const arg = "hello"
status = napi_create_string_utf8(env, "hello", NAPI_AUTO_LENGTH, &arg);
if (status != napi_ok) return;

napi_value* argv = &arg;
size_t argc = 1;

// const value = new MyObject(arg)
status = napi_new_instance(env, constructor, argc, argv, &value);

Returns napi_ok if the API succeeded.

Object Wrap#

N-API offers a way to "wrap" C++ classes and instances so that the class constructor and methods can be called from JavaScript.

  1. The napi_define_class API defines a JavaScript class with constructor, static properties and methods, and instance properties and methods that correspond to the C++ class.
  2. When JavaScript code invokes the constructor, the constructor callback uses napi_wrap to wrap a new C++ instance in a JavaScript object, then returns the wrapper object.
  3. When JavaScript code invokes a method or property accessor on the class, the corresponding napi_callback C++ function is invoked. For an instance callback, napi_unwrap obtains the C++ instance that is the target of the call.

For wrapped objects it may be difficult to distinguish between a function called on a class prototype and a function called on an instance of a class. A common pattern used to address this problem is to save a persistent reference to the class constructor for later instanceof checks.

napi_value MyClass_constructor = NULL;
status = napi_get_reference_value(env, MyClass::es_constructor, &MyClass_constructor);
assert(napi_ok == status);
bool is_instance = false;
status = napi_instanceof(env, es_this, MyClass_constructor, &is_instance);
assert(napi_ok == status);
if (is_instance) {
  // napi_unwrap() ...
} else {
  // otherwise...
}

The reference must be freed once it is no longer needed.

napi_define_class#

Added in: v8.0.0 N-API version: 1 
napi_status napi_define_class(napi_env env,
                              const char* utf8name,
                              size_t length,
                              napi_callback constructor,
                              void* data,
                              size_t property_count,
                              const napi_property_descriptor* properties,
                              napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] utf8name: Name of the JavaScript constructor function; this is not required to be the same as the C++ class name, though it is recommended for clarity.
  • [in] length: The length of the utf8name in bytes, or NAPI_AUTO_LENGTH if it is null-terminated.
  • [in] constructor: Callback function that handles constructing instances of the class. (This should be a static method on the class, not an actual C++ constructor function.)
  • [in] data: Optional data to be passed to the constructor callback as the data property of the callback info.
  • [in] property_count: Number of items in the properties array argument.
  • [in] properties: Array of property descriptors describing static and instance data properties, accessors, and methods on the class See napi_property_descriptor.
  • [out] result: A napi_value representing the constructor function for the class.

Returns napi_ok if the API succeeded.

Defines a JavaScript class that corresponds to a C++ class, including:

  • A JavaScript constructor function that has the class name and invokes the provided C++ constructor callback.
  • Properties on the constructor function corresponding to static data properties, accessors, and methods of the C++ class (defined by property descriptors with the napi_static attribute).
  • Properties on the constructor function's prototype object corresponding to non-static data properties, accessors, and methods of the C++ class (defined by property descriptors without the napi_static attribute).

The C++ constructor callback should be a static method on the class that calls the actual class constructor, then wraps the new C++ instance in a JavaScript object, and returns the wrapper object. See napi_wrap() for details.

The JavaScript constructor function returned from napi_define_class is often saved and used later, to construct new instances of the class from native code, and/or check whether provided values are instances of the class. In that case, to prevent the function value from being garbage-collected, create a persistent reference to it using napi_create_reference and ensure the reference count is kept >= 1.

Any non-NULL data which is passed to this API via the data parameter or via the data field of the napi_property_descriptor array items can be associated with the resulting JavaScript constructor (which is returned in the result parameter) and freed whenever the class is garbage-collected by passing both the JavaScript function and the data to napi_add_finalizer.

napi_wrap#

Added in: v8.0.0 N-API version: 1 
napi_status napi_wrap(napi_env env,
                      napi_value js_object,
                      void* native_object,
                      napi_finalize finalize_cb,
                      void* finalize_hint,
                      napi_ref* result);
  • [in] env: The environment that the API is invoked under.
  • [in] js_object: The JavaScript object that will be the wrapper for the native object.
  • [in] native_object: The native instance that will be wrapped in the JavaScript object.
  • [in] finalize_cb: Optional native callback that can be used to free the native instance when the JavaScript object is ready for garbage-collection.
  • [in] finalize_hint: Optional contextual hint that is passed to the finalize callback.
  • [out] result: Optional reference to the wrapped object.

Returns napi_ok if the API succeeded.

Wraps a native instance in a JavaScript object. The native instance can be retrieved later using napi_unwrap().

When JavaScript code invokes a constructor for a class that was defined using napi_define_class(), the napi_callback for the constructor is invoked. After constructing an instance of the native class, the callback must then call napi_wrap() to wrap the newly constructed instance in the already-created JavaScript object that is the this argument to the constructor callback. (That this object was created from the constructor function's prototype, so it already has definitions of all the instance properties and methods.)

Typically when wrapping a class instance, a finalize callback should be provided that simply deletes the native instance that is received as the data argument to the finalize callback.

The optional returned reference is initially a weak reference, meaning it has a reference count of 0. Typically this reference count would be incremented temporarily during async operations that require the instance to remain valid.

Caution: The optional returned reference (if obtained) should be deleted via napi_delete_reference ONLY in response to the finalize callback invocation. If it is deleted before then, then the finalize callback may never be invoked. Therefore, when obtaining a reference a finalize callback is also required in order to enable correct disposal of the reference.

Calling napi_wrap() a second time on an object will return an error. To associate another native instance with the object, use napi_remove_wrap() first.

napi_unwrap#

Added in: v8.0.0 N-API version: 1 
napi_status napi_unwrap(napi_env env,
                        napi_value js_object,
                        void** result);
  • [in] env: The environment that the API is invoked under.
  • [in] js_object: The object associated with the native instance.
  • [out] result: Pointer to the wrapped native instance.

Returns napi_ok if the API succeeded.

Retrieves a native instance that was previously wrapped in a JavaScript object using napi_wrap().

When JavaScript code invokes a method or property accessor on the class, the corresponding napi_callback is invoked. If the callback is for an instance method or accessor, then the this argument to the callback is the wrapper object; the wrapped C++ instance that is the target of the call can be obtained then by calling napi_unwrap() on the wrapper object.

napi_remove_wrap#

Added in: v8.5.0 N-API version: 1 
napi_status napi_remove_wrap(napi_env env,
                             napi_value js_object,
                             void** result);
  • [in] env: The environment that the API is invoked under.
  • [in] js_object: The object associated with the native instance.
  • [out] result: Pointer to the wrapped native instance.

Returns napi_ok if the API succeeded.

Retrieves a native instance that was previously wrapped in the JavaScript object js_object using napi_wrap() and removes the wrapping. If a finalize callback was associated with the wrapping, it will no longer be called when the JavaScript object becomes garbage-collected.

napi_add_finalizer#

Added in: v8.0.0 N-API version: 5 
napi_status napi_add_finalizer(napi_env env,
                               napi_value js_object,
                               void* native_object,
                               napi_finalize finalize_cb,
                               void* finalize_hint,
                               napi_ref* result);
  • [in] env: The environment that the API is invoked under.
  • [in] js_object: The JavaScript object to which the native data will be attached.
  • [in] native_object: The native data that will be attached to the JavaScript object.
  • [in] finalize_cb: Native callback that will be used to free the native data when the JavaScript object is ready for garbage-collection.
  • [in] finalize_hint: Optional contextual hint that is passed to the finalize callback.
  • [out] result: Optional reference to the JavaScript object.

Returns napi_ok if the API succeeded.

Adds a napi_finalize callback which will be called when the JavaScript object in js_object is ready for garbage collection. This API is similar to napi_wrap() except that:

  • the native data cannot be retrieved later using napi_unwrap(),
  • nor can it be removed later using napi_remove_wrap(), and
  • the API can be called multiple times with different data items in order to attach each of them to the JavaScript object, and
  • the object manipulated by the API can be used with napi_wrap().

Caution: The optional returned reference (if obtained) should be deleted via napi_delete_reference ONLY in response to the finalize callback invocation. If it is deleted before then, then the finalize callback may never be invoked. Therefore, when obtaining a reference a finalize callback is also required in order to enable correct disposal of the reference.

Simple Asynchronous Operations#

Addon modules often need to leverage async helpers from libuv as part of their implementation. This allows them to schedule work to be executed asynchronously so that their methods can return in advance of the work being completed. This allows them to avoid blocking overall execution of the Node.js application.

N-API provides an ABI-stable interface for these supporting functions which covers the most common asynchronous use cases.

N-API defines the napi_async_work structure which is used to manage asynchronous workers. Instances are created/deleted with napi_create_async_work and napi_delete_async_work.

The execute and complete callbacks are functions that will be invoked when the executor is ready to execute and when it completes its task respectively.

The execute function should avoid making any N-API calls that could result in the execution of JavaScript or interaction with JavaScript objects. Most often, any code that needs to make N-API calls should be made in complete callback instead. Avoid using the napi_env parameter in the execute callback as it will likely execute JavaScript.

These functions implement the following interfaces:

typedef void (*napi_async_execute_callback)(napi_env env,
                                            void* data);
typedef void (*napi_async_complete_callback)(napi_env env,
                                             napi_status status,
                                             void* data);

When these methods are invoked, the data parameter passed will be the addon-provided void* data that was passed into the napi_create_async_work call.

Once created the async worker can be queued for execution using the napi_queue_async_work function:

napi_status napi_queue_async_work(napi_env env,
                                  napi_async_work work);

napi_cancel_async_work can be used if the work needs to be cancelled before the work has started execution.

After calling napi_cancel_async_work, the complete callback will be invoked with a status value of napi_cancelled. The work should not be deleted before the complete callback invocation, even when it was cancelled.

napi_create_async_work#

History
VersionChanges
v8.6.0

Added async_resource and async_resource_name parameters.

v8.0.0

Added in: v8.0.0

N-API version: 1 
napi_status napi_create_async_work(napi_env env,
                                   napi_value async_resource,
                                   napi_value async_resource_name,
                                   napi_async_execute_callback execute,
                                   napi_async_complete_callback complete,
                                   void* data,
                                   napi_async_work* result);
  • [in] env: The environment that the API is invoked under.
  • [in] async_resource: An optional object associated with the async work that will be passed to possible async_hooks init hooks.
  • [in] async_resource_name: Identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API.
  • [in] execute: The native function which should be called to execute the logic asynchronously. The given function is called from a worker pool thread and can execute in parallel with the main event loop thread.
  • [in] complete: The native function which will be called when the asynchronous logic is completed or is cancelled. The given function is called from the main event loop thread.
  • [in] data: User-provided data context. This will be passed back into the execute and complete functions.
  • [out] result: napi_async_work* which is the handle to the newly created async work.

Returns napi_ok if the API succeeded.

This API allocates a work object that is used to execute logic asynchronously. It should be freed using napi_delete_async_work once the work is no longer required.

async_resource_name should be a null-terminated, UTF-8-encoded string.

The async_resource_name identifier is provided by the user and should be representative of the type of async work being performed. It is also recommended to apply namespacing to the identifier, e.g. by including the module name. See the async_hooks documentation for more information.

napi_delete_async_work#

Added in: v8.0.0 N-API version: 1 
napi_status napi_delete_async_work(napi_env env,
                                   napi_async_work work);
  • [in] env: The environment that the API is invoked under.
  • [in] work: The handle returned by the call to napi_create_async_work.

Returns napi_ok if the API succeeded.

This API frees a previously allocated work object.

This API can be called even if there is a pending JavaScript exception.

napi_queue_async_work#

Added in: v8.0.0 N-API version: 1 
napi_status napi_queue_async_work(napi_env env,
                                  napi_async_work work);
  • [in] env: The environment that the API is invoked under.
  • [in] work: The handle returned by the call to napi_create_async_work.

Returns napi_ok if the API succeeded.

This API requests that the previously allocated work be scheduled for execution. Once it returns successfully, this API must not be called again with the same napi_async_work item or the result will be undefined.

napi_cancel_async_work#

Added in: v8.0.0 N-API version: 1 
napi_status napi_cancel_async_work(napi_env env,
                                   napi_async_work work);
  • [in] env: The environment that the API is invoked under.
  • [in] work: The handle returned by the call to napi_create_async_work.

Returns napi_ok if the API succeeded.

This API cancels queued work if it has not yet been started. If it has already started executing, it cannot be cancelled and napi_generic_failure will be returned. If successful, the complete callback will be invoked with a status value of napi_cancelled. The work should not be deleted before the complete callback invocation, even if it has been successfully cancelled.

This API can be called even if there is a pending JavaScript exception.

Custom Asynchronous Operations#

The simple asynchronous work APIs above may not be appropriate for every scenario. When using any other asynchronous mechanism, the following APIs are necessary to ensure an asynchronous operation is properly tracked by the runtime.

napi_async_init#

Added in: v8.6.0 N-API version: 1 
napi_status napi_async_init(napi_env env,
                            napi_value async_resource,
                            napi_value async_resource_name,
                            napi_async_context* result)
  • [in] env: The environment that the API is invoked under.
  • [in] async_resource: Object associated with the async work that will be passed to possible async_hooks init hooks. In order to retain ABI compatibility with previous versions, passing NULL for async_resource will not result in an error, however, this will result incorrect operation of async hooks for the napi_async_context created. Potential issues include loss of async context when using the AsyncLocalStorage API.
  • [in] async_resource_name: Identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API.
  • [out] result: The initialized async context.

Returns napi_ok if the API succeeded.

napi_async_destroy#

Added in: v8.6.0 N-API version: 1 
napi_status napi_async_destroy(napi_env env,
                               napi_async_context async_context);
  • [in] env: The environment that the API is invoked under.
  • [in] async_context: The async context to be destroyed.

Returns napi_ok if the API succeeded.

This API can be called even if there is a pending JavaScript exception.

napi_make_callback#

History
VersionChanges
v8.6.0

Added async_context parameter.

v8.0.0

Added in: v8.0.0

N-API version: 1 
NAPI_EXTERN napi_status napi_make_callback(napi_env env,
                                           napi_async_context async_context,
                                           napi_value recv,
                                           napi_value func,
                                           size_t argc,
                                           const napi_value* argv,
                                           napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] async_context: Context for the async operation that is invoking the callback. This should normally be a value previously obtained from napi_async_init. However NULL is also allowed, which indicates the current async context (if any) is to be used for the callback.
  • [in] recv: The this object passed to the called function.
  • [in] func: napi_value representing the JavaScript function to be invoked.
  • [in] argc: The count of elements in the argv array.
  • [in] argv: Array of JavaScript values as napi_value representing the arguments to the function.
  • [out] result: napi_value representing the JavaScript object returned.

Returns napi_ok if the API succeeded.

This method allows a JavaScript function object to be called from a native add-on. This API is similar to napi_call_function. However, it is used to call from native code back into JavaScript after returning from an async operation (when there is no other script on the stack). It is a fairly simple wrapper around node::MakeCallback.

Note it is not necessary to use napi_make_callback from within a napi_async_complete_callback; in that situation the callback's async context has already been set up, so a direct call to napi_call_function is sufficient and appropriate. Use of the napi_make_callback function may be required when implementing custom async behavior that does not use napi_create_async_work.

napi_open_callback_scope#

Added in: v9.6.0 N-API version: 3 
NAPI_EXTERN napi_status napi_open_callback_scope(napi_env env,
                                                 napi_value resource_object,
                                                 napi_async_context context,
                                                 napi_callback_scope* result)
  • [in] env: The environment that the API is invoked under.
  • [in] resource_object: An object associated with the async work that will be passed to possible async_hooks init hooks.
  • [in] context: Context for the async operation that is invoking the callback. This should be a value previously obtained from napi_async_init.
  • [out] result: The newly created scope.

There are cases (for example, resolving promises) where it is necessary to have the equivalent of the scope associated with a callback in place when making certain N-API calls. If there is no other script on the stack the napi_open_callback_scope and napi_close_callback_scope functions can be used to open/close the required scope.

napi_close_callback_scope#

Added in: v9.6.0 N-API version: 3 
NAPI_EXTERN napi_status napi_close_callback_scope(napi_env env,
                                                  napi_callback_scope scope)
  • [in] env: The environment that the API is invoked under.
  • [in] scope: The scope to be closed.

This API can be called even if there is a pending JavaScript exception.

Version Management#

napi_get_node_version#

Added in: v8.4.0 N-API version: 1 
typedef struct {
  uint32_t major;
  uint32_t minor;
  uint32_t patch;
  const char* release;
} napi_node_version;

napi_status napi_get_node_version(napi_env env,
                                  const napi_node_version** version);
  • [in] env: The environment that the API is invoked under.
  • [out] version: A pointer to version information for Node.js itself.

Returns napi_ok if the API succeeded.

This function fills the version struct with the major, minor, and patch version of Node.js that is currently running, and the release field with the value of process.release.name.

The returned buffer is statically allocated and does not need to be freed.

napi_get_version#

Added in: v8.0.0 N-API version: 1 
napi_status napi_get_version(napi_env env,
                             uint32_t* result);
  • [in] env: The environment that the API is invoked under.
  • [out] result: The highest version of N-API supported.

Returns napi_ok if the API succeeded.

This API returns the highest N-API version supported by the Node.js runtime. N-API is planned to be additive such that newer releases of Node.js may support additional API functions. In order to allow an addon to use a newer function when running with versions of Node.js that support it, while providing fallback behavior when running with Node.js versions that don't support it:

  • Call napi_get_version() to determine if the API is available.
  • If available, dynamically load a pointer to the function using uv_dlsym().
  • Use the dynamically loaded pointer to invoke the function.
  • If the function is not available, provide an alternate implementation that does not use the function.

Memory Management#

napi_adjust_external_memory#

Added in: v8.5.0 N-API version: 1 
NAPI_EXTERN napi_status napi_adjust_external_memory(napi_env env,
                                                    int64_t change_in_bytes,
                                                    int64_t* result);
  • [in] env: The environment that the API is invoked under.
  • [in] change_in_bytes: The change in externally allocated memory that is kept alive by JavaScript objects.
  • [out] result: The adjusted value

Returns napi_ok if the API succeeded.

This function gives V8 an indication of the amount of externally allocated memory that is kept alive by JavaScript objects (i.e. a JavaScript object that points to its own memory allocated by a native module). Registering externally allocated memory will trigger global garbage collections more often than it would otherwise.

Promises#

N-API provides facilities for creating Promise objects as described in Section 25.4 of the ECMA specification. It implements promises as a pair of objects. When a promise is created by napi_create_promise(), a "deferred" object is created and returned alongside the Promise. The deferred object is bound to the created Promise and is the only means to resolve or reject the Promise using napi_resolve_deferred() or napi_reject_deferred(). The deferred object that is created by napi_create_promise() is freed by napi_resolve_deferred() or napi_reject_deferred(). The Promise object may be returned to JavaScript where it can be used in the usual fashion.

For example, to create a promise and pass it to an asynchronous worker:

napi_deferred deferred;
napi_value promise;
napi_status status;

// Create the promise.
status = napi_create_promise(env, &deferred, &promise);
if (status != napi_ok) return NULL;

// Pass the deferred to a function that performs an asynchronous action.
do_something_asynchronous(deferred);

// Return the promise to JS
return promise;

The above function do_something_asynchronous() would perform its asynchronous action and then it would resolve or reject the deferred, thereby concluding the promise and freeing the deferred:

napi_deferred deferred;
napi_value undefined;
napi_status status;

// Create a value with which to conclude the deferred.
status = napi_get_undefined(env, &undefined);
if (status != napi_ok) return NULL;

// Resolve or reject the promise associated with the deferred depending on
// whether the asynchronous action succeeded.
if (asynchronous_action_succeeded) {
  status = napi_resolve_deferred(env, deferred, undefined);
} else {
  status = napi_reject_deferred(env, deferred, undefined);
}
if (status != napi_ok) return NULL;

// At this point the deferred has been freed, so we should assign NULL to it.
deferred = NULL;

napi_create_promise#

Added in: v8.5.0 N-API version: 1 
napi_status napi_create_promise(napi_env env,
                                napi_deferred* deferred,
                                napi_value* promise);
  • [in] env: The environment that the API is invoked under.
  • [out] deferred: A newly created deferred object which can later be passed to napi_resolve_deferred() or napi_reject_deferred() to resolve resp. reject the associated promise.
  • [out] promise: The JavaScript promise associated with the deferred object.

Returns napi_ok if the API succeeded.

This API creates a deferred object and a JavaScript promise.

napi_resolve_deferred#

Added in: v8.5.0 N-API version: 1 
napi_status napi_resolve_deferred(napi_env env,
                                  napi_deferred deferred,
                                  napi_value resolution);
  • [in] env: The environment that the API is invoked under.
  • [in] deferred: The deferred object whose associated promise to resolve.
  • [in] resolution: The value with which to resolve the promise.

This API resolves a JavaScript promise by way of the deferred object with which it is associated. Thus, it can only be used to resolve JavaScript promises for which the corresponding deferred object is available. This effectively means that the promise must have been created using napi_create_promise() and the deferred object returned from that call must have been retained in order to be passed to this API.

The deferred object is freed upon successful completion.

napi_reject_deferred#

Added in: v8.5.0 N-API version: 1 
napi_status napi_reject_deferred(napi_env env,
                                 napi_deferred deferred,
                                 napi_value rejection);
  • [in] env: The environment that the API is invoked under.
  • [in] deferred: The deferred object whose associated promise to resolve.
  • [in] rejection: The value with which to reject the promise.

This API rejects a JavaScript promise by way of the deferred object with which it is associated. Thus, it can only be used to reject JavaScript promises for which the corresponding deferred object is available. This effectively means that the promise must have been created using napi_create_promise() and the deferred object returned from that call must have been retained in order to be passed to this API.

The deferred object is freed upon successful completion.

napi_is_promise#

Added in: v8.5.0 N-API version: 1 
napi_status napi_is_promise(napi_env env,
                            napi_value value,
                            bool* is_promise);
  • [in] env: The environment that the API is invoked under.
  • [in] value: The value to examine
  • [out] is_promise: Flag indicating whether promise is a native promise object (that is, a promise object created by the underlying engine).

Script execution#

N-API provides an API for executing a string containing JavaScript using the underlying JavaScript engine.

napi_run_script#

Added in: v8.5.0 N-API version: 1 
NAPI_EXTERN napi_status napi_run_script(napi_env env,
                                        napi_value script,
                                        napi_value* result);
  • [in] env: The environment that the API is invoked under.
  • [in] script: A JavaScript string containing the script to execute.
  • [out] result: The value resulting from having executed the script.

This function executes a string of JavaScript code and returns its result with the following caveats:

  • Unlike eval, this function does not allow the script to access the current lexical scope, and therefore also does not allow to access the module scope, meaning that pseudo-globals such as require will not be available.
  • The script can access the global scope. Function and var declarations in the script will be added to the global object. Variable declarations made using let and const will be visible globally, but will not be added to the global object.
  • The value of this is global within the script.

libuv event loop#

N-API provides a function for getting the current event loop associated with a specific napi_env.

napi_get_uv_event_loop#

Added in: v8.10.0, v9.3.0 N-API version: 2 
NAPI_EXTERN napi_status napi_get_uv_event_loop(napi_env env,
                                               struct uv_loop_s** loop);
  • [in] env: The environment that the API is invoked under.
  • [out] loop: The current libuv loop instance.

Asynchronous Thread-safe Function Calls#

JavaScript functions can normally only be called from a native addon's main thread. If an addon creates additional threads, then N-API functions that require a napi_env, napi_value, or napi_ref must not be called from those threads.

When an addon has additional threads and JavaScript functions need to be invoked based on the processing completed by those threads, those threads must communicate with the addon's main thread so that the main thread can invoke the JavaScript function on their behalf. The thread-safe function APIs provide an easy way to do this.

These APIs provide the type napi_threadsafe_function as well as APIs to create, destroy, and call objects of this type. napi_create_threadsafe_function() creates a persistent reference to a napi_value that holds a JavaScript function which can be called from multiple threads. The calls happen asynchronously. This means that values with which the JavaScript callback is to be called will be placed in a queue, and, for each value in the queue, a call will eventually be made to the JavaScript function.

Upon creation of a napi_threadsafe_function a napi_finalize callback can be provided. This callback will be invoked on the main thread when the thread-safe function is about to be destroyed. It receives the context and the finalize data given during construction, and provides an opportunity for cleaning up after the threads e.g. by calling uv_thread_join(). Aside from the main loop thread, no threads should be using the thread-safe function after the finalize callback completes.

The context given during the call to napi_create_threadsafe_function() can be retrieved from any thread with a call to napi_get_threadsafe_function_context().

napi_call_threadsafe_function() can then be used for initiating a call into JavaScript. napi_call_threadsafe_function() accepts a parameter which controls whether the API behaves blockingly. If set to napi_tsfn_nonblocking, the API behaves non-blockingly, returning napi_queue_full if the queue was full, preventing data from being successfully added to the queue. If set to napi_tsfn_blocking, the API blocks until space becomes available in the queue. napi_call_threadsafe_function() never blocks if the thread-safe function was created with a maximum queue size of 0.

As a special case, when napi_call_threadsafe_function() is called from a JavaScript thread, it will return napi_would_deadlock if the queue is full and it was called with napi_tsfn_blocking. The reason for this is that the JavaScript thread is responsible for removing items from the queue, thereby reducing their number. Thus, if it waits for room to become available on the queue, then it will deadlock.

napi_call_threadsafe_function() will also return napi_would_deadlock if the thread-safe function created on one JavaScript thread is called from another JavaScript thread. The reason for this is to prevent a deadlock arising from the possibility that the two JavaScript threads end up waiting on one another, thereby both deadlocking.

The actual call into JavaScript is controlled by the callback given via the call_js_cb parameter. call_js_cb is invoked on the main thread once for each value that was placed into the queue by a successful call to napi_call_threadsafe_function(). If such a callback is not given, a default callback will be used, and the resulting JavaScript call will have no arguments. The call_js_cb callback receives the JavaScript function to call as a napi_value in its parameters, as well as the void* context pointer used when creating the napi_threadsafe_function, and the next data pointer that was created by one of the secondary threads. The callback can then use an API such as napi_call_function() to call into JavaScript.

The callback may also be invoked with env and call_js_cb both set to NULL to indicate that calls into JavaScript are no longer possible, while items remain in the queue that may need to be freed. This normally occurs when the Node.js process exits while there is a thread-safe function still active.

It is not necessary to call into JavaScript via napi_make_callback() because N-API runs call_js_cb in a context appropriate for callbacks.

Threads can be added to and removed from a napi_threadsafe_function object during its existence. Thus, in addition to specifying an initial number of threads upon creation, napi_acquire_threadsafe_function can be called to indicate that a new thread will start making use of the thread-safe function. Similarly, napi_release_threadsafe_function can be called to indicate that an existing thread will stop making use of the thread-safe function.

napi_threadsafe_function objects are destroyed when every thread which uses the object has called napi_release_threadsafe_function() or has received a return status of napi_closing in response to a call to napi_call_threadsafe_function. The queue is emptied before the napi_threadsafe_function is destroyed. napi_release_threadsafe_function() should be the last API call made in conjunction with a given napi_threadsafe_function, because after the call completes, there is no guarantee that the napi_threadsafe_function is still allocated. For the same reason, do not make use of a thread-safe function after receiving a return value of napi_closing in response to a call to napi_call_threadsafe_function. Data associated with the napi_threadsafe_function can be freed in its napi_finalize callback which was passed to napi_create_threadsafe_function().

Once the number of threads making use of a napi_threadsafe_function reaches zero, no further threads can start making use of it by calling napi_acquire_threadsafe_function(). In fact, all subsequent API calls associated with it, except napi_release_threadsafe_function(), will return an error value of napi_closing.

The thread-safe function can be "aborted" by giving a value of napi_tsfn_abort to napi_release_threadsafe_function(). This will cause all subsequent APIs associated with the thread-safe function except napi_release_threadsafe_function() to return napi_closing even before its reference count reaches zero. In particular, napi_call_threadsafe_function() will return napi_closing, thus informing the threads that it is no longer possible to make asynchronous calls to the thread-safe function. This can be used as a criterion for terminating the thread. Upon receiving a return value of napi_closing from napi_call_threadsafe_function() a thread must make no further use of the thread-safe function because it is no longer guaranteed to be allocated.

Similarly to libuv handles, thread-safe functions can be "referenced" and "unreferenced". A "referenced" thread-safe function will cause the event loop on the thread on which it is created to remain alive until the thread-safe function is destroyed. In contrast, an "unreferenced" thread-safe function will not prevent the event loop from exiting. The APIs napi_ref_threadsafe_function and napi_unref_threadsafe_function exist for this purpose.

napi_create_threadsafe_function#

History
VersionChanges
v12.6.0, v10.17.0

Made func parameter optional with custom call_js_cb.

v10.6.0

Added in: v10.6.0

N-API version: 4 
NAPI_EXTERN napi_status
napi_create_threadsafe_function(napi_env env,
                                napi_value func,
                                napi_value async_resource,
                                napi_value async_resource_name,
                                size_t max_queue_size,
                                size_t initial_thread_count,
                                void* thread_finalize_data,
                                napi_finalize thread_finalize_cb,
                                void* context,
                                napi_threadsafe_function_call_js call_js_cb,
                                napi_threadsafe_function* result);
  • [in] env: The environment that the API is invoked under.
  • [in] func: An optional JavaScript function to call from another thread. It must be provided if NULL is passed to call_js_cb.
  • [in] async_resource: An optional object associated with the async work that will be passed to possible async_hooks init hooks.
  • [in] async_resource_name: A JavaScript string to provide an identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API.
  • [in] max_queue_size: Maximum size of the queue. 0 for no limit.
  • [in] initial_thread_count: The initial number of threads, including the main thread, which will be making use of this function.
  • [in] thread_finalize_data: Optional data to be passed to thread_finalize_cb.
  • [in] thread_finalize_cb: Optional function to call when the napi_threadsafe_function is being destroyed.
  • [in] context: Optional data to attach to the resulting napi_threadsafe_function.
  • [in] call_js_cb: Optional callback which calls the JavaScript function in response to a call on a different thread. This callback will be called on the main thread. If not given, the JavaScript function will be called with no parameters and with undefined as its this value.
  • [out] result: The asynchronous thread-safe JavaScript function.

napi_get_threadsafe_function_context#

Added in: v10.6.0 N-API version: 4 
NAPI_EXTERN napi_status
napi_get_threadsafe_function_context(napi_threadsafe_function func,
                                     void** result);
  • [in] func: The thread-safe function for which to retrieve the context.
  • [out] result: The location where to store the context.

This API may be called from any thread which makes use of func.

napi_call_threadsafe_function#

History
VersionChanges
v14.1.0

Return napi_would_deadlock when called with napi_tsfn_blocking from the main thread or a worker thread and the queue is full.

v10.6.0

Added in: v10.6.0

N-API version: 4 
NAPI_EXTERN napi_status
napi_call_threadsafe_function(napi_threadsafe_function func,
                              void* data,
                              napi_threadsafe_function_call_mode is_blocking);
  • [in] func: The asynchronous thread-safe JavaScript function to invoke.
  • [in] data: Data to send into JavaScript via the callback call_js_cb provided during the creation of the thread-safe JavaScript function.
  • [in] is_blocking: Flag whose value can be either napi_tsfn_blocking to indicate that the call should block if the queue is full or napi_tsfn_nonblocking to indicate that the call should return immediately with a status of napi_queue_full whenever the queue is full.

This API will return napi_would_deadlock if called with napi_tsfn_blocking from the main thread and the queue is full.

This API will return napi_closing if napi_release_threadsafe_function() was called with abort set to napi_tsfn_abort from any thread.

The value is only added to the queue if the API returns napi_ok.

This API may be called from any thread which makes use of func.

napi_acquire_threadsafe_function#

Added in: v10.6.0 N-API version: 4 
NAPI_EXTERN napi_status
napi_acquire_threadsafe_function(napi_threadsafe_function func);
  • [in] func: The asynchronous thread-safe JavaScript function to start making use of.

A thread should call this API before passing func to any other thread-safe function APIs to indicate that it will be making use of func. This prevents func from being destroyed when all other threads have stopped making use of it.

This API may be called from any thread which will start making use of func.

napi_release_threadsafe_function#

Added in: v10.6.0 N-API version: 4 
NAPI_EXTERN napi_status
napi_release_threadsafe_function(napi_threadsafe_function func,
                                 napi_threadsafe_function_release_mode mode);
  • [in] func: The asynchronous thread-safe JavaScript function whose reference count to decrement.
  • [in] mode: Flag whose value can be either napi_tsfn_release to indicate that the current thread will make no further calls to the thread-safe function, or napi_tsfn_abort to indicate that in addition to the current thread, no other thread should make any further calls to the thread-safe function. If set to napi_tsfn_abort, further calls to napi_call_threadsafe_function() will return napi_closing, and no further values will be placed in the queue.

A thread should call this API when it stops making use of func. Passing func to any thread-safe APIs after having called this API has undefined results, as func may have been destroyed.

This API may be called from any thread which will stop making use of func.

napi_ref_threadsafe_function#

Added in: v10.6.0 N-API version: 4 
NAPI_EXTERN napi_status
napi_ref_threadsafe_function(napi_env env, napi_threadsafe_function func);
  • [in] env: The environment that the API is invoked under.
  • [in] func: The thread-safe function to reference.

This API is used to indicate that the event loop running on the main thread should not exit until func has been destroyed. Similar to uv_ref it is also idempotent.

This API may only be called from the main thread.

napi_unref_threadsafe_function#

Added in: v10.6.0 N-API version: 4 
NAPI_EXTERN napi_status
napi_unref_threadsafe_function(napi_env env, napi_threadsafe_function func);
  • [in] env: The environment that the API is invoked under.
  • [in] func: The thread-safe function to unreference.

This API is used to indicate that the event loop running on the main thread may exit before func is destroyed. Similar to uv_unref it is also idempotent.

This API may only be called from the main thread.

C++ Embedder API#

Node.js provides a number of C++ APIs that can be used to execute JavaScript in a Node.js environment from other C++ software.

The documentation for these APIs can be found in src/node.h in the Node.js source tree. In addition to the APIs exposed by Node.js, some required concepts are provided by the V8 embedder API.

Because using Node.js as an embedded library is different from writing code that is executed by Node.js, breaking changes do not follow typical Node.js deprecation policy and may occur on each semver-major release without prior warning.

Example embedding application#

The following sections will provide an overview over how to use these APIs to create an application from scratch that will perform the equivalent of node -e <code>, i.e. that will take a piece of JavaScript and run it in a Node.js-specific environment.

The full code can be found in the Node.js source tree.

Setting up per-process state#

Node.js requires some per-process state management in order to run:

  • Arguments parsing for Node.js CLI options,
  • V8 per-process requirements, such as a v8::Platform instance.

The following example shows how these can be set up. Some class names are from the node and v8 C++ namespaces, respectively.

int main(int argc, char** argv) {
  std::vector<std::string> args(argv, argv + argc);
  std::vector<std::string> exec_args;
  std::vector<std::string> errors;
  // Parse Node.js CLI options, and print any errors that have occurred while
  // trying to parse them.
  int exit_code = node::InitializeNodeWithArgs(&args, &exec_args, &errors);
  for (const std::string& error : errors)
    fprintf(stderr, "%s: %s\n", args[0].c_str(), error.c_str());
  if (exit_code != 0) {
    return exit_code;
  }

  // Create a v8::Platform instance. `MultiIsolatePlatform::Create()` is a way
  // to create a v8::Platform instance that Node.js can use when creating
  // Worker threads. When no `MultiIsolatePlatform` instance is present,
  // Worker threads are disabled.
  std::unique_ptr<MultiIsolatePlatform> platform =
      MultiIsolatePlatform::Create(4);
  V8::InitializePlatform(platform.get());
  V8::Initialize();

  // See below for the contents of this function.
  int ret = RunNodeInstance(platform.get(), args, exec_args);

  V8::Dispose();
  V8::ShutdownPlatform();
  return ret;
}

Per-instance state#

Node.js has a concept of a “Node.js instance”, that is commonly being referred to as node::Environment. Each node::Environment is associated with:

  • Exactly one v8::Isolate, i.e. one JS Engine instance,
  • Exactly one uv_loop_t, i.e. one event loop, and
  • A number of v8::Contexts, but exactly one main v8::Context.
  • One node::IsolateData instance that contains information that could be shared by multiple node::Environments that use the same v8::Isolate. Currently, no testing if performed for this scenario.

In order to set up a v8::Isolate, an v8::ArrayBuffer::Allocator needs to be provided. One possible choice is the default Node.js allocator, which can be created through node::ArrayBufferAllocator::Create(). Using the Node.js allocator allows minor performance optimizations when addons use the Node.js C++ Buffer API, and is required in order to track ArrayBuffer memory in process.memoryUsage().

Additionally, each v8::Isolate that is used for a Node.js instance needs to be registered and unregistered with the MultiIsolatePlatform instance, if one is being used, in order for the platform to know which event loop to use for tasks scheduled by the v8::Isolate.

The node::NewIsolate() helper function creates a v8::Isolate, sets it up with some Node.js-specific hooks (e.g. the Node.js error handler), and registers it with the platform automatically.

int RunNodeInstance(MultiIsolatePlatform* platform,
                    const std::vector<std::string>& args,
                    const std::vector<std::string>& exec_args) {
  int exit_code = 0;
  // Set up a libuv event loop.
  uv_loop_t loop;
  int ret = uv_loop_init(&loop);
  if (ret != 0) {
    fprintf(stderr, "%s: Failed to initialize loop: %s\n",
            args[0].c_str(),
            uv_err_name(ret));
    return 1;
  }

  std::shared_ptr<ArrayBufferAllocator> allocator =
      ArrayBufferAllocator::Create();

  Isolate* isolate = NewIsolate(allocator, &loop, platform);
  if (isolate == nullptr) {
    fprintf(stderr, "%s: Failed to initialize V8 Isolate\n", args[0].c_str());
    return 1;
  }

  {
    Locker locker(isolate);
    Isolate::Scope isolate_scope(isolate);

    // Create a node::IsolateData instance that will later be released using
    // node::FreeIsolateData().
    std::unique_ptr<IsolateData, decltype(&node::FreeIsolateData)> isolate_data(
        node::CreateIsolateData(isolate, &loop, platform, allocator.get()),
        node::FreeIsolateData);

    // Set up a new v8::Context.
    HandleScope handle_scope(isolate);
    Local<Context> context = node::NewContext(isolate);
    if (context.IsEmpty()) {
      fprintf(stderr, "%s: Failed to initialize V8 Context\n", args[0].c_str());
      return 1;
    }

    // The v8::Context needs to be entered when node::CreateEnvironment() and
    // node::LoadEnvironment() are being called.
    Context::Scope context_scope(context);

    // Create a node::Environment instance that will later be released using
    // node::FreeEnvironment().
    std::unique_ptr<Environment, decltype(&node::FreeEnvironment)> env(
        node::CreateEnvironment(isolate_data.get(), context, args, exec_args),
        node::FreeEnvironment);

    // Set up the Node.js instance for execution, and run code inside of it.
    // There is also a variant that takes a callback and provides it with
    // the `require` and `process` objects, so that it can manually compile
    // and run scripts as needed.
    // The `require` function inside this script does *not* access the file
    // system, and can only load built-in Node.js modules.
    // `module.createRequire()` is being used to create one that is able to
    // load files from the disk, and uses the standard CommonJS file loader
    // instead of the internal-only `require` function.
    MaybeLocal<Value> loadenv_ret = node::LoadEnvironment(
        env.get(),
        "const publicRequire ="
        "  require('module').createRequire(process.cwd() + '/');"
        "globalThis.require = publicRequire;"
        "require('vm').runInThisContext(process.argv[1]);");

    if (loadenv_ret.IsEmpty())  // There has been a JS exception.
      return 1;

    {
      // SealHandleScope protects against handle leaks from callbacks.
      SealHandleScope seal(isolate);
      bool more;
      do {
        uv_run(&loop, UV_RUN_DEFAULT);

        // V8 tasks on background threads may end up scheduling new tasks in the
        // foreground, which in turn can keep the event loop going. For example,
        // WebAssembly.compile() may do so.
        platform->DrainTasks(isolate);

        // If there are new tasks, continue.
        more = uv_loop_alive(&loop);
        if (more) continue;

        // node::EmitBeforeExit() is used to emit the 'beforeExit' event on
        // the `process` object.
        node::EmitBeforeExit(env.get());

        // 'beforeExit' can also schedule new work that keeps the event loop
        // running.
        more = uv_loop_alive(&loop);
      } while (more == true);
    }

    // node::EmitExit() returns the current exit code.
    exit_code = node::EmitExit(env.get());

    // node::Stop() can be used to explicitly stop the event loop and keep
    // further JavaScript from running. It can be called from any thread,
    // and will act like worker.terminate() if called from another thread.
    node::Stop(env.get());
  }

  // Unregister the Isolate with the platform and add a listener that is called
  // when the Platform is done cleaning up any state it had associated with
  // the Isolate.
  bool platform_finished = false;
  platform->AddIsolateFinishedCallback(isolate, [](void* data) {
    *static_cast<bool*>(data) = true;
  }, &platform_finished);
  platform->UnregisterIsolate(isolate);
  isolate->Dispose();

  // Wait until the platform has cleaned up all relevant resources.
  while (!platform_finished)
    uv_run(&loop, UV_RUN_ONCE);
  int err = uv_loop_close(&loop);
  assert(err == 0);

  return exit_code;
}

Child Process#

Stability: 2 - Stable

The child_process module provides the ability to spawn child processes in a manner that is similar, but not identical, to popen(3). This capability is primarily provided by the child_process.spawn() function:

const { spawn } = require('child_process');
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.stderr.on('data', (data) => {
  console.error(`stderr: ${data}`);
});

ls.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});

By default, pipes for stdin, stdout, and stderr are established between the parent Node.js process and the spawned child. These pipes have limited (and platform-specific) capacity. If the child process writes to stdout in excess of that limit without the output being captured, the child process will block waiting for the pipe buffer to accept more data. This is identical to the behavior of pipes in the shell. Use the { stdio: 'ignore' } option if the output will not be consumed.

The command lookup will be performed using options.env.PATH environment variable if passed in options object, otherwise process.env.PATH will be used. To account for the fact that Windows environment variables are case-insensitive Node.js will lexicographically sort all env keys and choose the first one case-insensitively matching PATH to perform command lookup. This may lead to issues on Windows when passing objects to env option that have multiple variants of PATH variable.

The child_process.spawn() method spawns the child process asynchronously, without blocking the Node.js event loop. The child_process.spawnSync() function provides equivalent functionality in a synchronous manner that blocks the event loop until the spawned process either exits or is terminated.

For convenience, the child_process module provides a handful of synchronous and asynchronous alternatives to child_process.spawn() and child_process.spawnSync(). Each of these alternatives are implemented on top of child_process.spawn() or child_process.spawnSync().

For certain use cases, such as automating shell scripts, the synchronous counterparts may be more convenient. In many cases, however, the synchronous methods can have significant impact on performance due to stalling the event loop while spawned processes complete.

Asynchronous Process Creation#

The child_process.spawn(), child_process.fork(), child_process.exec(), and child_process.execFile() methods all follow the idiomatic asynchronous programming pattern typical of other Node.js APIs.

Each of the methods returns a ChildProcess instance. These objects implement the Node.js EventEmitter API, allowing the parent process to register listener functions that are called when certain events occur during the life cycle of the child process.

The child_process.exec() and child_process.execFile() methods additionally allow for an optional callback function to be specified that is invoked when the child process terminates.

Spawning .bat and .cmd files on Windows#

The importance of the distinction between child_process.exec() and child_process.execFile() can vary based on platform. On Unix-type operating systems (Unix, Linux, macOS) child_process.execFile() can be more efficient because it does not spawn a shell by default. On Windows, however, .bat and .cmd files are not executable on their own without a terminal, and therefore cannot be launched using child_process.execFile(). When running on Windows, .bat and .cmd files can be invoked using child_process.spawn() with the shell option set, with child_process.exec(), or by spawning cmd.exe and passing the .bat or .cmd file as an argument (which is what the shell option and child_process.exec() do). In any case, if the script filename contains spaces it needs to be quoted.

// On Windows Only...
const { spawn } = require('child_process');
const bat = spawn('cmd.exe', ['/c', 'my.bat']);

bat.stdout.on('data', (data) => {
  console.log(data.toString());
});

bat.stderr.on('data', (data) => {
  console.error(data.toString());
});

bat.on('exit', (code) => {
  console.log(`Child exited with code ${code}`);
});

// OR...
const { exec, spawn } = require('child_process');
exec('my.bat', (err, stdout, stderr) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(stdout);
});

// Script with spaces in the filename:
const bat = spawn('"my script.cmd"', ['a', 'b'], { shell: true });
// or:
exec('"my script.cmd" a b', (err, stdout, stderr) => {
  // ...
});

child_process.exec(command[, options][, callback])#

History
VersionChanges
v8.8.0

The windowsHide option is supported now.

v0.1.90

Added in: v0.1.90

Spawns a shell then executes the command within that shell, buffering any generated output. The command string passed to the exec function is processed directly by the shell and special characters (vary based on shell) need to be dealt with accordingly:

exec('"/path/to/test file/test.sh" arg1 arg2');
// Double quotes are used so that the space in the path is not interpreted as
// a delimiter of multiple arguments.

exec('echo "The \\$HOME variable is $HOME"');
// The $HOME variable is escaped in the first instance, but not in the second.

Never pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

If a callback function is provided, it is called with the arguments (error, stdout, stderr). On success, error will be null. On error, error will be an instance of Error. The error.code property will be the exit code of the child process while error.signal will be set to the signal that terminated the process. Any exit code other than 0 is considered to be an error.

The stdout and stderr arguments passed to the callback will contain the stdout and stderr output of the child process. By default, Node.js will decode the output as UTF-8 and pass strings to the callback. The encoding option can be used to specify the character encoding used to decode the stdout and stderr output. If encoding is 'buffer', or an unrecognized character encoding, Buffer objects will be passed to the callback instead.

const { exec } = require('child_process');
exec('cat *.js missing_file | wc -l', (error, stdout, stderr) => {
  if (error) {
    console.error(`exec error: ${error}`);
    return;
  }
  console.log(`stdout: ${stdout}`);
  console.error(`stderr: ${stderr}`);
});

If timeout is greater than 0, the parent will send the signal identified by the killSignal property (the default is 'SIGTERM') if the child runs longer than timeout milliseconds.

Unlike the exec(3) POSIX system call, child_process.exec() does not replace the existing process and uses a shell to execute the command.

If this method is invoked as its util.promisify()ed version, it returns a Promise for an Object with stdout and stderr properties. The returned ChildProcess instance is attached to the Promise as a child property. In case of an error (including any error resulting in an exit code other than 0), a rejected promise is returned, with the same error object given in the callback, but with two additional properties stdout and stderr.

const util = require('util');
const exec = util.promisify(require('child_process').exec);

async function lsExample() {
  const { stdout, stderr } = await exec('ls');
  console.log('stdout:', stdout);
  console.error('stderr:', stderr);
}
lsExample();

child_process.execFile(file[, args][, options][, callback])#

History
VersionChanges
v8.8.0

The windowsHide option is supported now.

v0.1.91

Added in: v0.1.91

The child_process.execFile() function is similar to child_process.exec() except that it does not spawn a shell by default. Rather, the specified executable file is spawned directly as a new process making it slightly more efficient than child_process.exec().

The same options as child_process.exec() are supported. Since a shell is not spawned, behaviors such as I/O redirection and file globbing are not supported.

const { execFile } = require('child_process');
const child = execFile('node', ['--version'], (error, stdout, stderr) => {
  if (error) {
    throw error;
  }
  console.log(stdout);
});

The stdout and stderr arguments passed to the callback will contain the stdout and stderr output of the child process. By default, Node.js will decode the output as UTF-8 and pass strings to the callback. The encoding option can be used to specify the character encoding used to decode the stdout and stderr output. If encoding is 'buffer', or an unrecognized character encoding, Buffer objects will be passed to the callback instead.

If this method is invoked as its util.promisify()ed version, it returns a Promise for an Object with stdout and stderr properties. The returned ChildProcess instance is attached to the Promise as a child property. In case of an error (including any error resulting in an exit code other than 0), a rejected promise is returned, with the same error object given in the callback, but with two additional properties stdout and stderr.

const util = require('util');
const execFile = util.promisify(require('child_process').execFile);
async function getVersion() {
  const { stdout } = await execFile('node', ['--version']);
  console.log(stdout);
}
getVersion();

If the shell option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

child_process.fork(modulePath[, args][, options])#

History
VersionChanges
v13.2.0, v12.16.0

The serialization option is supported now.

v8.0.0

The stdio option can now be a string.

v6.4.0

The stdio option is supported now.

v0.5.0

Added in: v0.5.0

  • modulePath <string> The module to run in the child.
  • args <string[]> List of string arguments.

  • options <Object>

    • cwd <string> Current working directory of the child process.
    • detached <boolean> Prepare child to run independently of its parent process. Specific behavior depends on the platform, see options.detached).
    • env <Object> Environment key-value pairs. Default: process.env.
    • execPath <string> Executable used to create the child process.
    • execArgv <string[]> List of string arguments passed to the executable. Default: process.execArgv.
    • serialization <string> Specify the kind of serialization used for sending messages between processes. Possible values are 'json' and 'advanced'. See Advanced Serialization for more details. Default: 'json'.
    • silent <boolean> If true, stdin, stdout, and stderr of the child will be piped to the parent, otherwise they will be inherited from the parent, see the 'pipe' and 'inherit' options for child_process.spawn()'s stdio for more details. Default: false.
    • stdio <Array> | <string> See child_process.spawn()'s stdio. When this option is provided, it overrides silent. If the array variant is used, it must contain exactly one item with value 'ipc' or an error will be thrown. For instance [0, 1, 2, 'ipc'].
    • windowsVerbatimArguments <boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. Default: false.
    • uid <number> Sets the user identity of the process (see setuid(2)).
    • gid <number> Sets the group identity of the process (see setgid(2)).
  • Returns: <ChildProcess>

The child_process.fork() method is a special case of child_process.spawn() used specifically to spawn new Node.js processes. Like child_process.spawn(), a ChildProcess object is returned. The returned ChildProcess will have an additional communication channel built-in that allows messages to be passed back and forth between the parent and child. See subprocess.send() for details.

Keep in mind that spawned Node.js child processes are independent of the parent with exception of the IPC communication channel that is established between the two. Each process has its own memory, with their own V8 instances. Because of the additional resource allocations required, spawning a large number of child Node.js processes is not recommended.

By default, child_process.fork() will spawn new Node.js instances using the process.execPath of the parent process. The execPath property in the options object allows for an alternative execution path to be used.

Node.js processes launched with a custom execPath will communicate with the parent process using the file descriptor (fd) identified using the environment variable NODE_CHANNEL_FD on the child process.

Unlike the fork(2) POSIX system call, child_process.fork() does not clone the current process.

The shell option available in child_process.spawn() is not supported by child_process.fork() and will be ignored if set.

child_process.spawn(command[, args][, options])#

History
VersionChanges
v13.2.0, v12.16.0

The serialization option is supported now.

v8.8.0

The windowsHide option is supported now.

v6.4.0

The argv0 option is supported now.

v5.7.0

The shell option is supported now.

v0.1.90

Added in: v0.1.90

  • command <string> The command to run.
  • args <string[]> List of string arguments.

  • options <Object>

    • cwd <string> Current working directory of the child process.
    • env <Object> Environment key-value pairs. Default: process.env.
    • argv0 <string> Explicitly set the value of argv[0] sent to the child process. This will be set to command if not specified.
    • stdio <Array> | <string> Child's stdio configuration (see options.stdio).
    • detached <boolean> Prepare child to run independently of its parent process. Specific behavior depends on the platform, see options.detached).
    • uid <number> Sets the user identity of the process (see setuid(2)).
    • gid <number> Sets the group identity of the process (see setgid(2)).
    • serialization <string> Specify the kind of serialization used for sending messages between processes. Possible values are 'json' and 'advanced'. See Advanced Serialization for more details. Default: 'json'.
    • shell <boolean> | <string> If true, runs command inside of a shell. Uses '/bin/sh' on Unix, and process.env.ComSpec on Windows. A different shell can be specified as a string. See Shell Requirements and Default Windows Shell. Default: false (no shell).
    • windowsVerbatimArguments <boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. This is set to true automatically when shell is specified and is CMD. Default: false.
    • windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false.
  • Returns: <ChildProcess>

The child_process.spawn() method spawns a new process using the given command, with command line arguments in args. If omitted, args defaults to an empty array.

If the shell option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

A third argument may be used to specify additional options, with these defaults:

const defaults = {
  cwd: undefined,
  env: process.env
};

Use cwd to specify the working directory from which the process is spawned. If not given, the default is to inherit the current working directory.

Use env to specify environment variables that will be visible to the new process, the default is process.env.

undefined values in env will be ignored.

Example of running ls -lh /usr, capturing stdout, stderr, and the exit code:

const { spawn } = require('child_process');
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.stderr.on('data', (data) => {
  console.error(`stderr: ${data}`);
});

ls.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});

Example: A very elaborate way to run ps ax | grep ssh

const { spawn } = require('child_process');
const ps = spawn('ps', ['ax']);
const grep = spawn('grep', ['ssh']);

ps.stdout.on('data', (data) => {
  grep.stdin.write(data);
});

ps.stderr.on('data', (data) => {
  console.error(`ps stderr: ${data}`);
});

ps.on('close', (code) => {
  if (code !== 0) {
    console.log(`ps process exited with code ${code}`);
  }
  grep.stdin.end();
});

grep.stdout.on('data', (data) => {
  console.log(data.toString());
});

grep.stderr.on('data', (data) => {
  console.error(`grep stderr: ${data}`);
});

grep.on('close', (code) => {
  if (code !== 0) {
    console.log(`grep process exited with code ${code}`);
  }
});

Example of checking for failed spawn:

const { spawn } = require('child_process');
const subprocess = spawn('bad_command');

subprocess.on('error', (err) => {
  console.error('Failed to start subprocess.');
});

Certain platforms (macOS, Linux) will use the value of argv[0] for the process title while others (Windows, SunOS) will use command.

Node.js currently overwrites argv[0] with process.execPath on startup, so process.argv[0] in a Node.js child process will not match the argv0 parameter passed to spawn from the parent, retrieve it with the process.argv0 property instead.

options.detached#

Added in: v0.7.10

On Windows, setting options.detached to true makes it possible for the child process to continue running after the parent exits. The child will have its own console window. Once enabled for a child process, it cannot be disabled.

On non-Windows platforms, if options.detached is set to true, the child process will be made the leader of a new process group and session. Child processes may continue running after the parent exits regardless of whether they are detached or not. See setsid(2) for more information.

By default, the parent will wait for the detached child to exit. To prevent the parent from waiting for a given subprocess to exit, use the subprocess.unref() method. Doing so will cause the parent's event loop to not include the child in its reference count, allowing the parent to exit independently of the child, unless there is an established IPC channel between the child and the parent.

When using the detached option to start a long-running process, the process will not stay running in the background after the parent exits unless it is provided with a stdio configuration that is not connected to the parent. If the parent's stdio is inherited, the child will remain attached to the controlling terminal.

Example of a long-running process, by detaching and also ignoring its parent stdio file descriptors, in order to ignore the parent's termination:

const { spawn } = require('child_process');

const subprocess = spawn(process.argv[0], ['child_program.js'], {
  detached: true,
  stdio: 'ignore'
});

subprocess.unref();

Alternatively one can redirect the child process' output into files:

const fs = require('fs');
const { spawn } = require('child_process');
const out = fs.openSync('./out.log', 'a');
const err = fs.openSync('./out.log', 'a');

const subprocess = spawn('prg', [], {
  detached: true,
  stdio: [ 'ignore', out, err ]
});

subprocess.unref();

options.stdio#

History
VersionChanges
v3.3.1

The value 0 is now accepted as a file descriptor.

v0.7.10

Added in: v0.7.10

The options.stdio option is used to configure the pipes that are established between the parent and child process. By default, the child's stdin, stdout, and stderr are redirected to corresponding subprocess.stdin, subprocess.stdout, and subprocess.stderr streams on the ChildProcess object. This is equivalent to setting the options.stdio equal to ['pipe', 'pipe', 'pipe'].

For convenience, options.stdio may be one of the following strings:

  • 'pipe': equivalent to ['pipe', 'pipe', 'pipe'] (the default)
  • 'ignore': equivalent to ['ignore', 'ignore', 'ignore']
  • 'inherit': equivalent to ['inherit', 'inherit', 'inherit'] or [0, 1, 2]

Otherwise, the value of options.stdio is an array where each index corresponds to an fd in the child. The fds 0, 1, and 2 correspond to stdin, stdout, and stderr, respectively. Additional fds can be specified to create additional pipes between the parent and child. The value is one of the following:

  1. 'pipe': Create a pipe between the child process and the parent process. The parent end of the pipe is exposed to the parent as a property on the child_process object as subprocess.stdio[fd]. Pipes created for fds 0, 1, and 2 are also available as subprocess.stdin, subprocess.stdout and subprocess.stderr, respectively.

  2. 'ipc': Create an IPC channel for passing messages/file descriptors between parent and child. A ChildProcess may have at most one IPC stdio file descriptor. Setting this option enables the subprocess.send() method. If the child is a Node.js process, the presence of an IPC channel will enable process.send() and process.disconnect() methods, as well as 'disconnect' and 'message' events within the child.

    Accessing the IPC channel fd in any way other than process.send() or using the IPC channel with a child process that is not a Node.js instance is not supported.

  3. 'ignore': Instructs Node.js to ignore the fd in the child. While Node.js will always open fds 0, 1, and 2 for the processes it spawns, setting the fd to 'ignore' will cause Node.js to open /dev/null and attach it to the child's fd.

  4. 'inherit': Pass through the corresponding stdio stream to/from the parent process. In the first three positions, this is equivalent to process.stdin, process.stdout, and process.stderr, respectively. In any other position, equivalent to 'ignore'.

  5. <Stream> object: Share a readable or writable stream that refers to a tty, file, socket, or a pipe with the child process. The stream's underlying file descriptor is duplicated in the child process to the fd that corresponds to the index in the stdio array. The stream must have an underlying descriptor (file streams do not until the 'open' event has occurred).

  6. Positive integer: The integer value is interpreted as a file descriptor that is currently open in the parent process. It is shared with the child process, similar to how <Stream> objects can be shared. Passing sockets is not supported on Windows.

  7. null, undefined: Use default value. For stdio fds 0, 1, and 2 (in other words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the default is 'ignore'.

const { spawn } = require('child_process');

// Child will use parent's stdios.
spawn('prg', [], { stdio: 'inherit' });

// Spawn child sharing only stderr.
spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });

// Open an extra fd=4, to interact with programs presenting a
// startd-style interface.
spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });

It is worth noting that when an IPC channel is established between the parent and child processes, and the child is a Node.js process, the child is launched with the IPC channel unreferenced (using unref()) until the child registers an event handler for the 'disconnect' event or the 'message' event. This allows the child to exit normally without the process being held open by the open IPC channel.

On Unix-like operating systems, the child_process.spawn() method performs memory operations synchronously before decoupling the event loop from the child. Applications with a large memory footprint may find frequent child_process.spawn() calls to be a bottleneck. For more information, see V8 issue 7381.

See also: child_process.exec() and child_process.fork().

Synchronous Process Creation#

The child_process.spawnSync(), child_process.execSync(), and child_process.execFileSync() methods are synchronous and will block the Node.js event loop, pausing execution of any additional code until the spawned process exits.

Blocking calls like these are mostly useful for simplifying general-purpose scripting tasks and for simplifying the loading/processing of application configuration at startup.

child_process.execFileSync(file[, args][, options])#

History
VersionChanges
v10.10.0

The input option can now be any TypedArray or a DataView.

v8.8.0

The windowsHide option is supported now.

v8.0.0

The input option can now be a Uint8Array.

v6.2.1, v4.5.0

The encoding option can now explicitly be set to buffer.

v0.11.12

Added in: v0.11.12

  • file <string> The name or path of the executable file to run.
  • args <string[]> List of string arguments.

  • options <Object>

    • cwd <string> Current working directory of the child process.
    • input <string> | <Buffer> | <TypedArray> | <DataView> The value which will be passed as stdin to the spawned process. Supplying this value will override stdio[0].
    • stdio <string> | <Array> Child's stdio configuration. stderr by default will be output to the parent process' stderr unless stdio is specified. Default: 'pipe'.
    • env <Object> Environment key-value pairs. Default: process.env.
    • uid <number> Sets the user identity of the process (see setuid(2)).
    • gid <number> Sets the group identity of the process (see setgid(2)).
    • timeout <number> In milliseconds the maximum amount of time the process is allowed to run. Default: undefined.
    • killSignal <string> | <integer> The signal value to be used when the spawned process will be killed. Default: 'SIGTERM'.
    • maxBuffer <number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated. See caveat at maxBuffer and Unicode. Default: 1024 * 1024.
    • encoding <string> The encoding used for all stdio inputs and outputs. Default: 'buffer'.
    • windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false.
    • shell <boolean> | <string> If true, runs command inside of a shell. Uses '/bin/sh' on Unix, and process.env.ComSpec on Windows. A different shell can be specified as a string. See Shell Requirements and Default Windows Shell. Default: false (no shell).
  • Returns: <Buffer> | <string> The stdout from the command.

The child_process.execFileSync() method is generally identical to child_process.execFile() with the exception that the method will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited.

If the child process intercepts and handles the SIGTERM signal and does not exit, the parent process will still wait until the child process has exited.

If the process times out or has a non-zero exit code, this method will throw an Error that will include the full result of the underlying child_process.spawnSync().

If the shell option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

child_process.execSync(command[, options])#

History
VersionChanges
v10.10.0

The input option can now be any TypedArray or a DataView.

v8.8.0

The windowsHide option is supported now.

v8.0.0

The input option can now be a Uint8Array.

v0.11.12

Added in: v0.11.12

  • command <string> The command to run.

  • options <Object>

    • cwd <string> Current working directory of the child process.
    • input <string> | <Buffer> | <TypedArray> | <DataView> The value which will be passed as stdin to the spawned process. Supplying this value will override stdio[0].
    • stdio <string> | <Array> Child's stdio configuration. stderr by default will be output to the parent process' stderr unless stdio is specified. Default: 'pipe'.
    • env <Object> Environment key-value pairs. Default: process.env.
    • shell <string> Shell to execute the command with. See Shell Requirements and Default Windows Shell. Default: '/bin/sh' on Unix, process.env.ComSpec on Windows.
    • uid <number> Sets the user identity of the process. (See setuid(2)).
    • gid <number> Sets the group identity of the process. (See setgid(2)).
    • timeout <number> In milliseconds the maximum amount of time the process is allowed to run. Default: undefined.
    • killSignal <string> | <integer> The signal value to be used when the spawned process will be killed. Default: 'SIGTERM'.
    • maxBuffer <number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at maxBuffer and Unicode. Default: 1024 * 1024.
    • encoding <string> The encoding used for all stdio inputs and outputs. Default: 'buffer'.
    • windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false.
  • Returns: <Buffer> | <string> The stdout from the command.

The child_process.execSync() method is generally identical to child_process.exec() with the exception that the method will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. If the child process intercepts and handles the SIGTERM signal and doesn't exit, the parent process will wait until the child process has exited.

If the process times out or has a non-zero exit code, this method will throw. The Error object will contain the entire result from child_process.spawnSync().

Never pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

child_process.spawnSync(command[, args][, options])#

History
VersionChanges
v10.10.0

The input option can now be any TypedArray or a DataView.

v8.8.0

The windowsHide option is supported now.

v8.0.0

The input option can now be a Uint8Array.

v6.2.1, v4.5.0

The encoding option can now explicitly be set to buffer.

v5.7.0

The shell option is supported now.

v0.11.12

Added in: v0.11.12

  • command <string> The command to run.
  • args <string[]> List of string arguments.

  • options <Object>

    • cwd <string> Current working directory of the child process.
    • input <string> | <Buffer> | <TypedArray> | <DataView> The value which will be passed as stdin to the spawned process. Supplying this value will override stdio[0].
    • argv0 <string> Explicitly set the value of argv[0] sent to the child process. This will be set to command if not specified.
    • stdio <string> | <Array> Child's stdio configuration.
    • env <Object> Environment key-value pairs. Default: process.env.
    • uid <number> Sets the user identity of the process (see setuid(2)).
    • gid <number> Sets the group identity of the process (see setgid(2)).
    • timeout <number> In milliseconds the maximum amount of time the process is allowed to run. Default: undefined.
    • killSignal <string> | <integer> The signal value to be used when the spawned process will be killed. Default: 'SIGTERM'.
    • maxBuffer <number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at maxBuffer and Unicode. Default: 1024 * 1024.
    • encoding <string> The encoding used for all stdio inputs and outputs. Default: 'buffer'.
    • shell <boolean> | <string> If true, runs command inside of a shell. Uses '/bin/sh' on Unix, and process.env.ComSpec on Windows. A different shell can be specified as a string. See Shell Requirements and Default Windows Shell. Default: false (no shell).
    • windowsVerbatimArguments <boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. This is set to true automatically when shell is specified and is CMD. Default: false.
    • windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false.

  • Returns: <Object>

    • pid <number> Pid of the child process.
    • output <Array> Array of results from stdio output.
    • stdout <Buffer> | <string> The contents of output[1].
    • stderr <Buffer> | <string> The contents of output[2].
    • status <number> | <null> The exit code of the subprocess, or null if the subprocess terminated due to a signal.
    • signal <string> | <null> The signal used to kill the subprocess, or null if the subprocess did not terminate due to a signal.
    • error <Error> The error object if the child process failed or timed out.

The child_process.spawnSync() method is generally identical to child_process.spawn() with the exception that the function will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. If the process intercepts and handles the SIGTERM signal and doesn't exit, the parent process will wait until the child process has exited.

If the shell option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

Class: ChildProcess#

Added in: v2.2.0

Instances of the ChildProcess represent spawned child processes.

Instances of ChildProcess are not intended to be created directly. Rather, use the child_process.spawn(), child_process.exec(), child_process.execFile(), or child_process.fork() methods to create instances of ChildProcess.

Event: 'close'#

Added in: v0.7.7
  • code <number> The exit code if the child exited on its own.
  • signal <string> The signal by which the child process was terminated.

The 'close' event is emitted when the stdio streams of a child process have been closed. This is distinct from the 'exit' event, since multiple processes might share the same stdio streams.

const { spawn } = require('child_process');
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.on('close', (code) => {
  console.log(`child process close all stdio with code ${code}`);
});

ls.on('exit', (code) => {
  console.log(`child process exited with code ${code}`);
});

Event: 'disconnect'#

Added in: v0.7.2

The 'disconnect' event is emitted after calling the subprocess.disconnect() method in parent process or process.disconnect() in child process. After disconnecting it is no longer possible to send or receive messages, and the subprocess.connected property is false.

Event: 'error'#

The 'error' event is emitted whenever:

  1. The process could not be spawned, or
  2. The process could not be killed, or
  3. Sending a message to the child process failed.

The 'exit' event may or may not fire after an error has occurred. When listening to both the 'exit' and 'error' events, guard against accidentally invoking handler functions multiple times.

See also subprocess.kill() and subprocess.send().

Event: 'exit'#

Added in: v0.1.90
  • code <number> The exit code if the child exited on its own.
  • signal <string> The signal by which the child process was terminated.

The 'exit' event is emitted after the child process ends. If the process exited, code is the final exit code of the process, otherwise null. If the process terminated due to receipt of a signal, signal is the string name of the signal, otherwise null. One of the two will always be non-null.

When the 'exit' event is triggered, child process stdio streams might still be open.

Node.js establishes signal handlers for SIGINT and SIGTERM and Node.js processes will not terminate immediately due to receipt of those signals. Rather, Node.js will perform a sequence of cleanup actions and then will re-raise the handled signal.

See waitpid(2).

Event: 'message'#

Added in: v0.5.9

The 'message' event is triggered when a child process uses process.send() to send messages.

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

If the serialization option was set to 'advanced' used when spawning the child process, the message argument can contain data that JSON is not able to represent. See Advanced Serialization for more details.

subprocess.channel#

History
VersionChanges
v14.0.0

The object no longer accidentally exposes native C++ bindings.

v7.1.0

Added in: v7.1.0

  • <Object> A pipe representing the IPC channel to the child process.

The subprocess.channel property is a reference to the child's IPC channel. If no IPC channel currently exists, this property is undefined.

subprocess.channel.ref()#

Added in: v7.1.0

This method makes the IPC channel keep the event loop of the parent process running if .unref() has been called before.

subprocess.channel.unref()#

Added in: v7.1.0

This method makes the IPC channel not keep the event loop of the parent process running, and lets it finish even while the channel is open.

subprocess.connected#

Added in: v0.7.2
  • <boolean> Set to false after subprocess.disconnect() is called.

The subprocess.connected property indicates whether it is still possible to send and receive messages from a child process. When subprocess.connected is false, it is no longer possible to send or receive messages.

subprocess.disconnect()#

Added in: v0.7.2

Closes the IPC channel between parent and child, allowing the child to exit gracefully once there are no other connections keeping it alive. After calling this method the subprocess.connected and process.connected properties in both the parent and child (respectively) will be set to false, and it will be no longer possible to pass messages between the processes.

The 'disconnect' event will be emitted when there are no messages in the process of being received. This will most often be triggered immediately after calling subprocess.disconnect().

When the child process is a Node.js instance (e.g. spawned using child_process.fork()), the process.disconnect() method can be invoked within the child process to close the IPC channel as well.

subprocess.exitCode#

The subprocess.exitCode property indicates the exit code of the child process. If the child process is still running, the field will be null.

subprocess.kill([signal])#

Added in: v0.1.90

The subprocess.kill() method sends a signal to the child process. If no argument is given, the process will be sent the 'SIGTERM' signal. See signal(7) for a list of available signals. This function returns true if kill(2) succeeds, and false otherwise.

const { spawn } = require('child_process');
const grep = spawn('grep', ['ssh']);

grep.on('close', (code, signal) => {
  console.log(
    `child process terminated due to receipt of signal ${signal}`);
});

// Send SIGHUP to process.
grep.kill('SIGHUP');

The ChildProcess object may emit an 'error' event if the signal cannot be delivered. Sending a signal to a child process that has already exited is not an error but may have unforeseen consequences. Specifically, if the process identifier (PID) has been reassigned to another process, the signal will be delivered to that process instead which can have unexpected results.

While the function is called kill, the signal delivered to the child process may not actually terminate the process.

See kill(2) for reference.

On Linux, child processes of child processes will not be terminated when attempting to kill their parent. This is likely to happen when running a new process in a shell or with the use of the shell option of ChildProcess:

'use strict';
const { spawn } = require('child_process');

const subprocess = spawn(
  'sh',
  [
    '-c',
    `node -e "setInterval(() => {
      console.log(process.pid, 'is alive')
    }, 500);"`
  ], {
    stdio: ['inherit', 'inherit', 'inherit']
  }
);

setTimeout(() => {
  subprocess.kill(); // Does not terminate the Node.js process in the shell.
}, 2000);

subprocess.killed#

Added in: v0.5.10
  • <boolean> Set to true after subprocess.kill() is used to successfully send a signal to the child process.

The subprocess.killed property indicates whether the child process successfully received a signal from subprocess.kill(). The killed property does not indicate that the child process has been terminated.

subprocess.pid#

Added in: v0.1.90

Returns the process identifier (PID) of the child process.

const { spawn } = require('child_process');
const grep = spawn('grep', ['ssh']);

console.log(`Spawned child pid: ${grep.pid}`);
grep.stdin.end();

subprocess.ref()#

Added in: v0.7.10

Calling subprocess.ref() after making a call to subprocess.unref() will restore the removed reference count for the child process, forcing the parent to wait for the child to exit before exiting itself.

const { spawn } = require('child_process');

const subprocess = spawn(process.argv[0], ['child_program.js'], {
  detached: true,
  stdio: 'ignore'
});

subprocess.unref();
subprocess.ref();

subprocess.send(message[, sendHandle[, options]][, callback])#

History
VersionChanges
v5.8.0

The options parameter, and the keepOpen option in particular, is supported now.

v5.0.0

This method returns a boolean for flow control now.

v4.0.0

The callback parameter is supported now.

v0.5.9

Added in: v0.5.9

  • message <Object>
  • sendHandle <Handle>

  • options <Object> The options argument, if present, is an object used to parameterize the sending of certain types of handles. options supports the following properties:

    • keepOpen <boolean> A value that can be used when passing instances of net.Socket. When true, the socket is kept open in the sending process. Default: false.
  • callback <Function>
  • Returns: <boolean>

When an IPC channel has been established between the parent and child ( i.e. when using child_process.fork()), the subprocess.send() method can be used to send messages to the child process. When the child process is a Node.js instance, these messages can be received via the 'message' event.

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

For example, in the parent script:

const cp = require('child_process');
const n = cp.fork(`${__dirname}/sub.js`);

n.on('message', (m) => {
  console.log('PARENT got message:', m);
});

// Causes the child to print: CHILD got message: { hello: 'world' }
n.send({ hello: 'world' });

And then the child script, 'sub.js' might look like this:

process.on('message', (m) => {
  console.log('CHILD got message:', m);
});

// Causes the parent to print: PARENT got message: { foo: 'bar', baz: null }
process.send({ foo: 'bar', baz: NaN });

Child Node.js processes will have a process.send() method of their own that allows the child to send messages back to the parent.

There is a special case when sending a {cmd: 'NODE_foo'} message. Messages containing a NODE_ prefix in the cmd property are reserved for use within Node.js core and will not be emitted in the child's 'message' event. Rather, such messages are emitted using the 'internalMessage' event and are consumed internally by Node.js. Applications should avoid using such messages or listening for 'internalMessage' events as it is subject to change without notice.

The optional sendHandle argument that may be passed to subprocess.send() is for passing a TCP server or socket object to the child process. The child will receive the object as the second argument passed to the callback function registered on the 'message' event. Any data that is received and buffered in the socket will not be sent to the child.

The optional callback is a function that is invoked after the message is sent but before the child may have received it. The function is called with a single argument: null on success, or an Error object on failure.

If no callback function is provided and the message cannot be sent, an 'error' event will be emitted by the ChildProcess object. This can happen, for instance, when the child process has already exited.

subprocess.send() will return false if the channel has closed or when the backlog of unsent messages exceeds a threshold that makes it unwise to send more. Otherwise, the method returns true. The callback function can be used to implement flow control.

Example: sending a server object#

The sendHandle argument can be used, for instance, to pass the handle of a TCP server object to the child process as illustrated in the example below:

const subprocess = require('child_process').fork('subprocess.js');

// Open up the server object and send the handle.
const server = require('net').createServer();
server.on('connection', (socket) => {
  socket.end('handled by parent');
});
server.listen(1337, () => {
  subprocess.send('server', server);
});

The child would then receive the server object as:

process.on('message', (m, server) => {
  if (m === 'server') {
    server.on('connection', (socket) => {
      socket.end('handled by child');
    });
  }
});

Once the server is now shared between the parent and child, some connections can be handled by the parent and some by the child.

While the example above uses a server created using the net module, dgram module servers use exactly the same workflow with the exceptions of listening on a 'message' event instead of 'connection' and using server.bind() instead of server.listen(). This is, however, currently only supported on Unix platforms.

Example: sending a socket object#

Similarly, the sendHandler argument can be used to pass the handle of a socket to the child process. The example below spawns two children that each handle connections with "normal" or "special" priority:

const { fork } = require('child_process');
const normal = fork('subprocess.js', ['normal']);
const special = fork('subprocess.js', ['special']);

// Open up the server and send sockets to child. Use pauseOnConnect to prevent
// the sockets from being read before they are sent to the child process.
const server = require('net').createServer({ pauseOnConnect: true });
server.on('connection', (socket) => {

  // If this is special priority...
  if (socket.remoteAddress === '74.125.127.100') {
    special.send('socket', socket);
    return;
  }
  // This is normal priority.
  normal.send('socket', socket);
});
server.listen(1337);

The subprocess.js would receive the socket handle as the second argument passed to the event callback function:

process.on('message', (m, socket) => {
  if (m === 'socket') {
    if (socket) {
      // Check that the client socket exists.
      // It is possible for the socket to be closed between the time it is
      // sent and the time it is received in the child process.
      socket.end(`Request handled with ${process.argv[2]} priority`);
    }
  }
});

Once a socket has been passed to a child, the parent is no longer capable of tracking when the socket is destroyed. To indicate this, the .connections property becomes null. It is recommended not to use .maxConnections when this occurs.

It is also recommended that any 'message' handlers in the child process verify that socket exists, as the connection may have been closed during the time it takes to send the connection to the child.

subprocess.signalCode#

The subprocess.signalCode property indicates the signal number received by the child process if any, else null.

subprocess.spawnargs#

The subprocess.spawnargs property represents the full list of command line arguments the child process was launched with.

subprocess.spawnfile#

The subprocess.spawnfile property indicates the executable file name of the child process that is launched.

For child_process.fork(), its value will be equal to process.execPath. For child_process.spawn(), its value will be the name of the executable file. For child_process.exec(), its value will be the name of the shell in which the child process is launched.

subprocess.stderr#

Added in: v0.1.90

A Readable Stream that represents the child process's stderr.

If the child was spawned with stdio[2] set to anything other than 'pipe', then this will be null.

subprocess.stderr is an alias for subprocess.stdio[2]. Both properties will refer to the same value.

subprocess.stdin#

Added in: v0.1.90

A Writable Stream that represents the child process's stdin.

If a child process waits to read all of its input, the child will not continue until this stream has been closed via end().

If the child was spawned with stdio[0] set to anything other than 'pipe', then this will be null.

subprocess.stdin is an alias for subprocess.stdio[0]. Both properties will refer to the same value.

subprocess.stdio#

Added in: v0.7.10

A sparse array of pipes to the child process, corresponding with positions in the stdio option passed to child_process.spawn() that have been set to the value 'pipe'. subprocess.stdio[0], subprocess.stdio[1], and subprocess.stdio[2] are also available as subprocess.stdin, subprocess.stdout, and subprocess.stderr, respectively.

In the following example, only the child's fd 1 (stdout) is configured as a pipe, so only the parent's subprocess.stdio[1] is a stream, all other values in the array are null.

const assert = require('assert');
const fs = require('fs');
const child_process = require('child_process');

const subprocess = child_process.spawn('ls', {
  stdio: [
    0, // Use parent's stdin for child.
    'pipe', // Pipe child's stdout to parent.
    fs.openSync('err.out', 'w') // Direct child's stderr to a file.
  ]
});

assert.strictEqual(subprocess.stdio[0], null);
assert.strictEqual(subprocess.stdio[0], subprocess.stdin);

assert(subprocess.stdout);
assert.strictEqual(subprocess.stdio[1], subprocess.stdout);

assert.strictEqual(subprocess.stdio[2], null);
assert.strictEqual(subprocess.stdio[2], subprocess.stderr);

subprocess.stdout#

Added in: v0.1.90

A Readable Stream that represents the child process's stdout.

If the child was spawned with stdio[1] set to anything other than 'pipe', then this will be null.

subprocess.stdout is an alias for subprocess.stdio[1]. Both properties will refer to the same value.

const { spawn } = require('child_process');

const subprocess = spawn('ls');

subprocess.stdout.on('data', (data) => {
  console.log(`Received chunk ${data}`);
});

subprocess.unref()#

Added in: v0.7.10

By default, the parent will wait for the detached child to exit. To prevent the parent from waiting for a given subprocess to exit, use the subprocess.unref() method. Doing so will cause the parent's event loop to not include the child in its reference count, allowing the parent to exit independently of the child, unless there is an established IPC channel between the child and the parent.

const { spawn } = require('child_process');

const subprocess = spawn(process.argv[0], ['child_program.js'], {
  detached: true,
  stdio: 'ignore'
});

subprocess.unref();

maxBuffer and Unicode#

The maxBuffer option specifies the largest number of bytes allowed on stdout or stderr. If this value is exceeded, then the child process is terminated. This impacts output that includes multibyte character encodings such as UTF-8 or UTF-16. For instance, console.log('中文测试') will send 13 UTF-8 encoded bytes to stdout although there are only 4 characters.

Shell Requirements#

The shell should understand the -c switch. If the shell is 'cmd.exe', it should understand the /d /s /c switches and command line parsing should be compatible.

Default Windows Shell#

Although Microsoft specifies %COMSPEC% must contain the path to 'cmd.exe' in the root environment, child processes are not always subject to the same requirement. Thus, in child_process functions where a shell can be spawned, 'cmd.exe' is used as a fallback if process.env.ComSpec is unavailable.

Advanced Serialization#

Added in: v13.2.0, v12.16.0

Child processes support a serialization mechanism for IPC that is based on the serialization API of the v8 module, based on the HTML structured clone algorithm. This is generally more powerful and supports more built-in JavaScript object types, such as BigInt, Map and Set, ArrayBuffer and TypedArray, Buffer, Error, RegExp etc.

However, this format is not a full superset of JSON, and e.g. properties set on objects of such built-in types will not be passed on through the serialization step. Additionally, performance may not be equivalent to that of JSON, depending on the structure of the passed data. Therefore, this feature requires opting in by setting the serialization option to 'advanced' when calling child_process.spawn() or child_process.fork().

Cluster#

Stability: 2 - Stable

A single instance of Node.js runs in a single thread. To take advantage of multi-core systems, the user will sometimes want to launch a cluster of Node.js processes to handle the load.

The cluster module allows easy creation of child processes that all share server ports.

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  console.log(`Master ${process.pid} is running`);

  // Fork workers.
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`worker ${worker.process.pid} died`);
  });
} else {
  // Workers can share any TCP connection
  // In this case it is an HTTP server
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');
  }).listen(8000);

  console.log(`Worker ${process.pid} started`);
}

Running Node.js will now share port 8000 between the workers:

$ node server.js
Master 3596 is running
Worker 4324 started
Worker 4520 started
Worker 6056 started
Worker 5644 started

On Windows, it is not yet possible to set up a named pipe server in a worker.

How It Works#

The worker processes are spawned using the child_process.fork() method, so that they can communicate with the parent via IPC and pass server handles back and forth.

The cluster module supports two methods of distributing incoming connections.

The first one (and the default one on all platforms except Windows), is the round-robin approach, where the master process listens on a port, accepts new connections and distributes them across the workers in a round-robin fashion, with some built-in smarts to avoid overloading a worker process.

The second approach is where the master process creates the listen socket and sends it to interested workers. The workers then accept incoming connections directly.

The second approach should, in theory, give the best performance. In practice however, distribution tends to be very unbalanced due to operating system scheduler vagaries. Loads have been observed where over 70% of all connections ended up in just two processes, out of a total of eight.

Because server.listen() hands off most of the work to the master process, there are three cases where the behavior between a normal Node.js process and a cluster worker differs:

  1. server.listen({fd: 7}) Because the message is passed to the master, file descriptor 7 in the parent will be listened on, and the handle passed to the worker, rather than listening to the worker's idea of what the number 7 file descriptor references.
  2. server.listen(handle) Listening on handles explicitly will cause the worker to use the supplied handle, rather than talk to the master process.
  3. server.listen(0) Normally, this will cause servers to listen on a random port. However, in a cluster, each worker will receive the same "random" port each time they do listen(0). In essence, the port is random the first time, but predictable thereafter. To listen on a unique port, generate a port number based on the cluster worker ID.

Node.js does not provide routing logic. It is, therefore important to design an application such that it does not rely too heavily on in-memory data objects for things like sessions and login.

Because workers are all separate processes, they can be killed or re-spawned depending on a program's needs, without affecting other workers. As long as there are some workers still alive, the server will continue to accept connections. If no workers are alive, existing connections will be dropped and new connections will be refused. Node.js does not automatically manage the number of workers, however. It is the application's responsibility to manage the worker pool based on its own needs.

Although a primary use case for the cluster module is networking, it can also be used for other use cases requiring worker processes.

Class: Worker#

Added in: v0.7.0

A Worker object contains all public information and method about a worker. In the master it can be obtained using cluster.workers. In a worker it can be obtained using cluster.worker.

Event: 'disconnect'#

Added in: v0.7.7

Similar to the cluster.on('disconnect') event, but specific to this worker.

cluster.fork().on('disconnect', () => {
  // Worker has disconnected
});

Event: 'error'#

Added in: v0.7.3

This event is the same as the one provided by child_process.fork().

Within a worker, process.on('error') may also be used.

Event: 'exit'#

Added in: v0.11.2
  • code <number> The exit code, if it exited normally.
  • signal <string> The name of the signal (e.g. 'SIGHUP') that caused the process to be killed.

Similar to the cluster.on('exit') event, but specific to this worker.

const worker = cluster.fork();
worker.on('exit', (code, signal) => {
  if (signal) {
    console.log(`worker was killed by signal: ${signal}`);
  } else if (code !== 0) {
    console.log(`worker exited with error code: ${code}`);
  } else {
    console.log('worker success!');
  }
});

Event: 'listening'#

Added in: v0.7.0

Similar to the cluster.on('listening') event, but specific to this worker.

cluster.fork().on('listening', (address) => {
  // Worker is listening
});

It is not emitted in the worker.

Event: 'message'#

Added in: v0.7.0

Similar to the 'message' event of cluster, but specific to this worker.

Within a worker, process.on('message') may also be used.

See process event: 'message'.

Here is an example using the message system. It keeps a count in the master process of the number of HTTP requests received by the workers:

const cluster = require('cluster');
const http = require('http');

if (cluster.isMaster) {

  // Keep track of http requests
  let numReqs = 0;
  setInterval(() => {
    console.log(`numReqs = ${numReqs}`);
  }, 1000);

  // Count requests
  function messageHandler(msg) {
    if (msg.cmd && msg.cmd === 'notifyRequest') {
      numReqs += 1;
    }
  }

  // Start workers and listen for messages containing notifyRequest
  const numCPUs = require('os').cpus().length;
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  for (const id in cluster.workers) {
    cluster.workers[id].on('message', messageHandler);
  }

} else {

  // Worker processes have a http server.
  http.Server((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');

    // Notify master about the request
    process.send({ cmd: 'notifyRequest' });
  }).listen(8000);
}

Event: 'online'#

Added in: v0.7.0

Similar to the cluster.on('online') event, but specific to this worker.

cluster.fork().on('online', () => {
  // Worker is online
});

It is not emitted in the worker.

worker.disconnect()#

History
VersionChanges
v7.3.0

This method now returns a reference to worker.

v0.7.7

Added in: v0.7.7

In a worker, this function will close all servers, wait for the 'close' event on those servers, and then disconnect the IPC channel.

In the master, an internal message is sent to the worker causing it to call .disconnect() on itself.

Causes .exitedAfterDisconnect to be set.

After a server is closed, it will no longer accept new connections, but connections may be accepted by any other listening worker. Existing connections will be allowed to close as usual. When no more connections exist, see server.close(), the IPC channel to the worker will close allowing it to die gracefully.

The above applies only to server connections, client connections are not automatically closed by workers, and disconnect does not wait for them to close before exiting.

In a worker, process.disconnect exists, but it is not this function; it is disconnect().

Because long living server connections may block workers from disconnecting, it may be useful to send a message, so application specific actions may be taken to close them. It also may be useful to implement a timeout, killing a worker if the 'disconnect' event has not been emitted after some time.

if (cluster.isMaster) {
  const worker = cluster.fork();
  let timeout;

  worker.on('listening', (address) => {
    worker.send('shutdown');
    worker.disconnect();
    timeout = setTimeout(() => {
      worker.kill();
    }, 2000);
  });

  worker.on('disconnect', () => {
    clearTimeout(timeout);
  });

} else if (cluster.isWorker) {
  const net = require('net');
  const server = net.createServer((socket) => {
    // Connections never end
  });

  server.listen(8000);

  process.on('message', (msg) => {
    if (msg === 'shutdown') {
      // Initiate graceful close of any connections to server
    }
  });
}

worker.exitedAfterDisconnect#

Added in: v6.0.0

This property is true if the worker exited due to .kill() or .disconnect(). If the worker exited any other way, it is false. If the worker has not exited, it is undefined.

The boolean worker.exitedAfterDisconnect allows distinguishing between voluntary and accidental exit, the master may choose not to respawn a worker based on this value.

cluster.on('exit', (worker, code, signal) => {
  if (worker.exitedAfterDisconnect === true) {
    console.log('Oh, it was just voluntary – no need to worry');
  }
});

// kill worker
worker.kill();

worker.id#

Added in: v0.8.0

Each new worker is given its own unique id, this id is stored in the id.

While a worker is alive, this is the key that indexes it in cluster.workers.

worker.isConnected()#

Added in: v0.11.14

This function returns true if the worker is connected to its master via its IPC channel, false otherwise. A worker is connected to its master after it has been created. It is disconnected after the 'disconnect' event is emitted.

worker.isDead()#

Added in: v0.11.14

This function returns true if the worker's process has terminated (either because of exiting or being signaled). Otherwise, it returns false.

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  console.log(`Master ${process.pid} is running`);

  // Fork workers.
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('fork', (worker) => {
    console.log('worker is dead:', worker.isDead());
  });

  cluster.on('exit', (worker, code, signal) => {
    console.log('worker is dead:', worker.isDead());
  });
} else {
  // Workers can share any TCP connection. In this case, it is an HTTP server.
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end(`Current process\n ${process.pid}`);
    process.kill(process.pid);
  }).listen(8000);
}

worker.kill([signal='SIGTERM'])#

Added in: v0.9.12
  • signal <string> Name of the kill signal to send to the worker process.

This function will kill the worker. In the master, it does this by disconnecting the worker.process, and once disconnected, killing with signal. In the worker, it does it by disconnecting the channel, and then exiting with code 0.

Because kill() attempts to gracefully disconnect the worker process, it is susceptible to waiting indefinitely for the disconnect to complete. For example, if the worker enters an infinite loop, a graceful disconnect will never occur. If the graceful disconnect behavior is not needed, use worker.process.kill().

Causes .exitedAfterDisconnect to be set.

This method is aliased as worker.destroy() for backwards compatibility.

In a worker, process.kill() exists, but it is not this function; it is kill().

worker.process#

Added in: v0.7.0

All workers are created using child_process.fork(), the returned object from this function is stored as .process. In a worker, the global process is stored.

See: Child Process module.

Workers will call process.exit(0) if the 'disconnect' event occurs on process and .exitedAfterDisconnect is not true. This protects against accidental disconnection.

worker.send(message[, sendHandle[, options]][, callback])#

History
VersionChanges
v4.0.0

The callback parameter is supported now.

v0.7.0

Added in: v0.7.0

  • message <Object>
  • sendHandle <Handle>

  • options <Object> The options argument, if present, is an object used to parameterize the sending of certain types of handles. options supports the following properties:

    • keepOpen <boolean> A value that can be used when passing instances of net.Socket. When true, the socket is kept open in the sending process. Default: false.
  • callback <Function>
  • Returns: <boolean>

Send a message to a worker or master, optionally with a handle.

In the master this sends a message to a specific worker. It is identical to ChildProcess.send().

In a worker this sends a message to the master. It is identical to process.send().

This example will echo back all messages from the master:

if (cluster.isMaster) {
  const worker = cluster.fork();
  worker.send('hi there');

} else if (cluster.isWorker) {
  process.on('message', (msg) => {
    process.send(msg);
  });
}

Event: 'disconnect'#

Added in: v0.7.9

Emitted after the worker IPC channel has disconnected. This can occur when a worker exits gracefully, is killed, or is disconnected manually (such as with worker.disconnect()).

There may be a delay between the 'disconnect' and 'exit' events. These events can be used to detect if the process is stuck in a cleanup or if there are long-living connections.

cluster.on('disconnect', (worker) => {
  console.log(`The worker #${worker.id} has disconnected`);
});

Event: 'exit'#

Added in: v0.7.9
  • worker <cluster.Worker>
  • code <number> The exit code, if it exited normally.
  • signal <string> The name of the signal (e.g. 'SIGHUP') that caused the process to be killed.

When any of the workers die the cluster module will emit the 'exit' event.

This can be used to restart the worker by calling .fork() again.

cluster.on('exit', (worker, code, signal) => {
  console.log('worker %d died (%s). restarting...',
              worker.process.pid, signal || code);
  cluster.fork();
});

See child_process event: 'exit'.

Event: 'fork'#

Added in: v0.7.0

When a new worker is forked the cluster module will emit a 'fork' event. This can be used to log worker activity, and create a custom timeout.

const timeouts = [];
function errorMsg() {
  console.error('Something must be wrong with the connection ...');
}

cluster.on('fork', (worker) => {
  timeouts[worker.id] = setTimeout(errorMsg, 2000);
});
cluster.on('listening', (worker, address) => {
  clearTimeout(timeouts[worker.id]);
});
cluster.on('exit', (worker, code, signal) => {
  clearTimeout(timeouts[worker.id]);
  errorMsg();
});

Event: 'listening'#

Added in: v0.7.0

After calling listen() from a worker, when the 'listening' event is emitted on the server a 'listening' event will also be emitted on cluster in the master.

The event handler is executed with two arguments, the worker contains the worker object and the address object contains the following connection properties: address, port and addressType. This is very useful if the worker is listening on more than one address.

cluster.on('listening', (worker, address) => {
  console.log(
    `A worker is now connected to ${address.address}:${address.port}`);
});

The addressType is one of:

  • 4 (TCPv4)
  • 6 (TCPv6)
  • -1 (Unix domain socket)
  • 'udp4' or 'udp6' (UDP v4 or v6)

Event: 'message'#

History
VersionChanges
v6.0.0

The worker parameter is passed now; see below for details.

v2.5.0

Added in: v2.5.0

Emitted when the cluster master receives a message from any worker.

See child_process event: 'message'.

Event: 'online'#

Added in: v0.7.0

After forking a new worker, the worker should respond with an online message. When the master receives an online message it will emit this event. The difference between 'fork' and 'online' is that fork is emitted when the master forks a worker, and 'online' is emitted when the worker is running.

cluster.on('online', (worker) => {
  console.log('Yay, the worker responded after it was forked');
});

Event: 'setup'#

Added in: v0.7.1

Emitted every time .setupMaster() is called.

The settings object is the cluster.settings object at the time .setupMaster() was called and is advisory only, since multiple calls to .setupMaster() can be made in a single tick.

If accuracy is important, use cluster.settings.

cluster.disconnect([callback])#

Added in: v0.7.7
  • callback <Function> Called when all workers are disconnected and handles are closed.

Calls .disconnect() on each worker in cluster.workers.

When they are disconnected all internal handles will be closed, allowing the master process to die gracefully if no other event is waiting.

The method takes an optional callback argument which will be called when finished.

This can only be called from the master process.

cluster.fork([env])#

Added in: v0.6.0

Spawn a new worker process.

This can only be called from the master process.

cluster.isMaster#

Added in: v0.8.1

True if the process is a master. This is determined by the process.env.NODE_UNIQUE_ID. If process.env.NODE_UNIQUE_ID is undefined, then isMaster is true.

cluster.isWorker#

Added in: v0.6.0

True if the process is not a master (it is the negation of cluster.isMaster).

cluster.schedulingPolicy#

Added in: v0.11.2

The scheduling policy, either cluster.SCHED_RR for round-robin or cluster.SCHED_NONE to leave it to the operating system. This is a global setting and effectively frozen once either the first worker is spawned, or .setupMaster() is called, whichever comes first.

SCHED_RR is the default on all operating systems except Windows. Windows will change to SCHED_RR once libuv is able to effectively distribute IOCP handles without incurring a large performance hit.

cluster.schedulingPolicy can also be set through the NODE_CLUSTER_SCHED_POLICY environment variable. Valid values are 'rr' and 'none'.

cluster.settings#

History
VersionChanges
v13.2.0, v12.16.0

The serialization option is supported now.

v9.5.0

The cwd option is supported now.

v9.4.0

The windowsHide option is supported now.

v8.2.0

The inspectPort option is supported now.

v6.4.0

The stdio option is supported now.

v0.7.1

Added in: v0.7.1

  • <Object>

    • execArgv <string[]> List of string arguments passed to the Node.js executable. Default: process.execArgv.
    • exec <string> File path to worker file. Default: process.argv[1].
    • args <string[]> String arguments passed to worker. Default: process.argv.slice(2).
    • cwd <string> Current working directory of the worker process. Default: undefined (inherits from parent process).
    • serialization <string> Specify the kind of serialization used for sending messages between processes. Possible values are 'json' and 'advanced'. See Advanced Serialization for child_process for more details. Default: false.
    • silent <boolean> Whether or not to send output to parent's stdio. Default: false.
    • stdio <Array> Configures the stdio of forked processes. Because the cluster module relies on IPC to function, this configuration must contain an 'ipc' entry. When this option is provided, it overrides silent.
    • uid <number> Sets the user identity of the process. (See setuid(2).)
    • gid <number> Sets the group identity of the process. (See setgid(2).)
    • inspectPort <number> | <Function> Sets inspector port of worker. This can be a number, or a function that takes no arguments and returns a number. By default each worker gets its own port, incremented from the master's process.debugPort.
    • windowsHide <boolean> Hide the forked processes console window that would normally be created on Windows systems. Default: false.

After calling .setupMaster() (or .fork()) this settings object will contain the settings, including the default values.

This object is not intended to be changed or set manually.

cluster.setupMaster([settings])#

History
VersionChanges
v6.4.0

The stdio option is supported now.

v0.7.1

Added in: v0.7.1

setupMaster is used to change the default 'fork' behavior. Once called, the settings will be present in cluster.settings.

Any settings changes only affect future calls to .fork() and have no effect on workers that are already running.

The only attribute of a worker that cannot be set via .setupMaster() is the env passed to .fork().

The defaults above apply to the first call only; the defaults for later calls are the current values at the time of cluster.setupMaster() is called.

const cluster = require('cluster');
cluster.setupMaster({
  exec: 'worker.js',
  args: ['--use', 'https'],
  silent: true
});
cluster.fork(); // https worker
cluster.setupMaster({
  exec: 'worker.js',
  args: ['--use', 'http']
});
cluster.fork(); // http worker

This can only be called from the master process.

cluster.worker#

Added in: v0.7.0

A reference to the current worker object. Not available in the master process.

const cluster = require('cluster');

if (cluster.isMaster) {
  console.log('I am master');
  cluster.fork();
  cluster.fork();
} else if (cluster.isWorker) {
  console.log(`I am worker #${cluster.worker.id}`);
}

cluster.workers#

Added in: v0.7.0

A hash that stores the active worker objects, keyed by id field. Makes it easy to loop through all the workers. It is only available in the master process.

A worker is removed from cluster.workers after the worker has disconnected and exited. The order between these two events cannot be determined in advance. However, it is guaranteed that the removal from the cluster.workers list happens before last 'disconnect' or 'exit' event is emitted.

// Go through all workers
function eachWorker(callback) {
  for (const id in cluster.workers) {
    callback(cluster.workers[id]);
  }
}
eachWorker((worker) => {
  worker.send('big announcement to all workers');
});

Using the worker's unique id is the easiest way to locate the worker.

socket.on('data', (id) => {
  const worker = cluster.workers[id];
});

Command Line Options#

Node.js comes with a variety of CLI options. These options expose built-in debugging, multiple ways to execute scripts, and other helpful runtime options.

To view this documentation as a manual page in a terminal, run man node.

Synopsis#

node [options] [V8 options] [script.js | -e "script" | -] [--] [arguments]

node inspect [script.js | -e "script" | <host>:<port>] …

node --v8-options

Execute without arguments to start the REPL.

For more info about node inspect, please see the debugger documentation.

Options#

History
VersionChanges
v10.12.0

Underscores instead of dashes are now allowed for Node.js options as well, in addition to V8 options.

All options, including V8 options, allow words to be separated by both dashes (-) or underscores (_).

For example, --pending-deprecation is equivalent to --pending_deprecation.

-#

Added in: v8.0.0

Alias for stdin. Analogous to the use of - in other command line utilities, meaning that the script will be read from stdin, and the rest of the options are passed to that script.

--#

Added in: v6.11.0

Indicate the end of node options. Pass the rest of the arguments to the script. If no script filename or eval/print script is supplied prior to this, then the next argument will be used as a script filename.

--abort-on-uncaught-exception#

Added in: v0.10.8

Aborting instead of exiting causes a core file to be generated for post-mortem analysis using a debugger (such as lldb, gdb, and mdb).

If this flag is passed, the behavior can still be set to not abort through process.setUncaughtExceptionCaptureCallback() (and through usage of the domain module that uses it).

--completion-bash#

Added in: v10.12.0

Print source-able bash completion script for Node.js.

$ node --completion-bash > node_bash_completion
$ source node_bash_completion

--cpu-prof#

Added in: v12.0.0

Stability: 1 - Experimental

Starts the V8 CPU profiler on start up, and writes the CPU profile to disk before exit.

If --cpu-prof-dir is not specified, the generated profile will be placed in the current working directory.

If --cpu-prof-name is not specified, the generated profile will be named CPU.${yyyymmdd}.${hhmmss}.${pid}.${tid}.${seq}.cpuprofile.

$ node --cpu-prof index.js
$ ls *.cpuprofile
CPU.20190409.202950.15293.0.0.cpuprofile

--cpu-prof-dir#

Added in: v12.0.0

Stability: 1 - Experimental

Specify the directory where the CPU profiles generated by --cpu-prof will be placed.

--cpu-prof-interval#

Added in: v12.2.0

Stability: 1 - Experimental

Specify the sampling interval in microseconds for the CPU profiles generated by --cpu-prof. The default is 1000 microseconds.

--cpu-prof-name#

Added in: v12.0.0

Stability: 1 - Experimental

Specify the file name of the CPU profile generated by --cpu-prof.

--disable-proto=mode#

Disable the Object.prototype.__proto__ property. If mode is delete, the property will be removed entirely. If mode is throw, accesses to the property will throw an exception with the code ERR_PROTO_ACCESS.

--disallow-code-generation-from-strings#

Added in: v9.8.0

Make built-in language features like eval and new Function that generate code from strings throw an exception instead. This does not affect the Node.js vm module.

--enable-fips#

Added in: v6.0.0

Enable FIPS-compliant crypto at startup. (Requires Node.js to be built with ./configure --openssl-fips.)

--enable-source-maps#

Added in: v12.12.0

Stability: 1 - Experimental

Enable experimental Source Map V3 support for stack traces.

Currently, overriding Error.prepareStackTrace is ignored when the --enable-source-maps flag is set.

--experimental-import-meta-resolve#

Added in: v13.9.0, v12.16.2

Enable experimental import.meta.resolve() support.

--experimental-json-modules#

Added in: v12.9.0

Enable experimental JSON support for the ES Module loader.

--experimental-loader=module#

Added in: v9.0.0

Specify the module of a custom experimental ECMAScript Module loader. module may be either a path to a file, or an ECMAScript Module name.

--experimental-modules#

Added in: v8.5.0

Enable latest experimental modules features (deprecated).

--experimental-policy#

Added in: v11.8.0

Use the specified file as a security policy.

--experimental-repl-await#

Added in: v10.0.0

Enable experimental top-level await keyword support in REPL.

--experimental-specifier-resolution=mode#

Added in: v13.4.0, v12.16.0

Sets the resolution algorithm for resolving ES module specifiers. Valid options are explicit and node.

The default is explicit, which requires providing the full path to a module. The node mode will enable support for optional file extensions and the ability to import a directory that has an index file.

Please see customizing ESM specifier resolution for example usage.

--experimental-vm-modules#

Added in: v9.6.0

Enable experimental ES Module support in the vm module.

--experimental-wasi-unstable-preview1#

History
VersionChanges
v13.6.0

changed from --experimental-wasi-unstable-preview0 to --experimental-wasi-unstable-preview1

v13.3.0, v12.16.0

Added in: v13.3.0, v12.16.0

Enable experimental WebAssembly System Interface (WASI) support.

--experimental-wasm-modules#

Added in: v12.3.0

Enable experimental WebAssembly module support.

--force-fips#

Added in: v6.0.0

Force FIPS-compliant crypto on startup. (Cannot be disabled from script code.) (Same requirements as --enable-fips.)

--frozen-intrinsics#

Added in: v11.12.0

Stability: 1 - Experimental

Enable experimental frozen intrinsics like Array and Object.

Support is currently only provided for the root context and no guarantees are currently provided that global.Array is indeed the default intrinsic reference. Code may break under this flag.

--require runs prior to freezing intrinsics in order to allow polyfills to be added.

--heapsnapshot-signal=signal#

Added in: v12.0.0

Enables a signal handler that causes the Node.js process to write a heap dump when the specified signal is received. signal must be a valid signal name. Disabled by default.

$ node --heapsnapshot-signal=SIGUSR2 index.js &
$ ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
node         1  5.5  6.1 787252 247004 ?       Ssl  16:43   0:02 node --heapsnapshot-signal=SIGUSR2 index.js
$ kill -USR2 1
$ ls
Heap.20190718.133405.15554.0.001.heapsnapshot

--heap-prof#

Added in: v12.4.0

Stability: 1 - Experimental

Starts the V8 heap profiler on start up, and writes the heap profile to disk before exit.

If --heap-prof-dir is not specified, the generated profile will be placed in the current working directory.

If --heap-prof-name is not specified, the generated profile will be named Heap.${yyyymmdd}.${hhmmss}.${pid}.${tid}.${seq}.heapprofile.

$ node --heap-prof index.js
$ ls *.heapprofile
Heap.20190409.202950.15293.0.001.heapprofile

--heap-prof-dir#

Added in: v12.4.0

Stability: 1 - Experimental

Specify the directory where the heap profiles generated by --heap-prof will be placed.

--heap-prof-interval#

Added in: v12.4.0

Stability: 1 - Experimental

Specify the average sampling interval in bytes for the heap profiles generated by --heap-prof. The default is 512 * 1024 bytes.

--heap-prof-name#

Added in: v12.4.0

Stability: 1 - Experimental

Specify the file name of the heap profile generated by --heap-prof.

--icu-data-dir=file#

Added in: v0.11.15

Specify ICU data load path. (Overrides NODE_ICU_DATA.)

--input-type=type#

Added in: v12.0.0

This configures Node.js to interpret string input as CommonJS or as an ES module. String input is input via --eval, --print, or STDIN.

Valid values are "commonjs" and "module". The default is "commonjs".

--inspect-brk[=[host:]port]#

Added in: v7.6.0

Activate inspector on host:port and break at start of user script. Default host:port is 127.0.0.1:9229.

--inspect-port=[host:]port#

Added in: v7.6.0

Set the host:port to be used when the inspector is activated. Useful when activating the inspector by sending the SIGUSR1 signal.

Default host is 127.0.0.1.

See the security warning below regarding the host parameter usage.

--inspect[=[host:]port]#

Added in: v6.3.0

Activate inspector on host:port. Default is 127.0.0.1:9229.

V8 inspector integration allows tools such as Chrome DevTools and IDEs to debug and profile Node.js instances. The tools attach to Node.js instances via a tcp port and communicate using the Chrome DevTools Protocol.

Warning: binding inspector to a public IP:port combination is insecure#

Binding the inspector to a public IP (including 0.0.0.0) with an open port is insecure, as it allows external hosts to connect to the inspector and perform a remote code execution attack.

If specifying a host, make sure that either:

  • The host is not accessible from public networks.
  • A firewall disallows unwanted connections on the port.

More specifically, --inspect=0.0.0.0 is insecure if the port (9229 by default) is not firewall-protected.

See the debugging security implications section for more information.

--inspect-publish-uid=stderr,http#

Specify ways of the inspector web socket url exposure.

By default inspector websocket url is available in stderr and under /json/list endpoint on http://host:port/json/list.

--insecure-http-parser#

Added in: v13.4.0, v12.15.0, v10.19.0

Use an insecure HTTP parser that accepts invalid HTTP headers. This may allow interoperability with non-conformant HTTP implementations. It may also allow request smuggling and other HTTP attacks that rely on invalid headers being accepted. Avoid using this option.

--jitless#

Added in: v12.0.0

Disable runtime allocation of executable memory. This may be required on some platforms for security reasons. It can also reduce attack surface on other platforms, but the performance impact may be severe.

This flag is inherited from V8 and is subject to change upstream. It may disappear in a non-semver-major release.

--max-http-header-size=size#

History
VersionChanges
v14.0.0

Change maximum default size of HTTP headers from 8KB to 16KB.

v11.6.0, v10.15.0

Added in: v11.6.0, v10.15.0

Specify the maximum size, in bytes, of HTTP headers. Defaults to 16KB.

--napi-modules#

Added in: v7.10.0

This option is a no-op. It is kept for compatibility.

--no-deprecation#

Added in: v0.8.0

Silence deprecation warnings.

--no-force-async-hooks-checks#

Added in: v9.0.0

Disables runtime checks for async_hooks. These will still be enabled dynamically when async_hooks is enabled.

--no-warnings#

Added in: v6.0.0

Silence all process warnings (including deprecations).

--force-context-aware#

Added in: v12.12.0

Disable loading native addons that are not context-aware.

--openssl-config=file#

Added in: v6.9.0

Load an OpenSSL configuration file on startup. Among other uses, this can be used to enable FIPS-compliant crypto if Node.js is built with ./configure --openssl-fips.

--pending-deprecation#

Added in: v8.0.0

Emit pending deprecation warnings.

Pending deprecations are generally identical to a runtime deprecation with the notable exception that they are turned off by default and will not be emitted unless either the --pending-deprecation command line flag, or the NODE_PENDING_DEPRECATION=1 environment variable, is set. Pending deprecations are used to provide a kind of selective "early warning" mechanism that developers may leverage to detect deprecated API usage.

--policy-integrity=sri#

Added in: v12.7.0

Stability: 1 - Experimental

Instructs Node.js to error prior to running any code if the policy does not have the specified integrity. It expects a Subresource Integrity string as a parameter.

--preserve-symlinks#

Added in: v6.3.0

Instructs the module loader to preserve symbolic links when resolving and caching modules.

By default, when Node.js loads a module from a path that is symbolically linked to a different on-disk location, Node.js will dereference the link and use the actual on-disk "real path" of the module as both an identifier and as a root path to locate other dependency modules. In most cases, this default behavior is acceptable. However, when using symbolically linked peer dependencies, as illustrated in the example below, the default behavior causes an exception to be thrown if moduleA attempts to require moduleB as a peer dependency:

{appDir}
 ├── app
 │   ├── index.js
 │   └── node_modules
 │       ├── moduleA -> {appDir}/moduleA
 │       └── moduleB
 │           ├── index.js
 │           └── package.json
 └── moduleA
     ├── index.js
     └── package.json

The --preserve-symlinks command line flag instructs Node.js to use the symlink path for modules as opposed to the real path, allowing symbolically linked peer dependencies to be found.

Note, however, that using --preserve-symlinks can have other side effects. Specifically, symbolically linked native modules can fail to load if those are linked from more than one location in the dependency tree (Node.js would see those as two separate modules and would attempt to load the module multiple times, causing an exception to be thrown).

The --preserve-symlinks flag does not apply to the main module, which allows node --preserve-symlinks node_module/.bin/<foo> to work. To apply the same behavior for the main module, also use --preserve-symlinks-main.

--preserve-symlinks-main#

Added in: v10.2.0

Instructs the module loader to preserve symbolic links when resolving and caching the main module (require.main).

This flag exists so that the main module can be opted-in to the same behavior that --preserve-symlinks gives to all other imports; they are separate flags, however, for backward compatibility with older Node.js versions.

--preserve-symlinks-main does not imply --preserve-symlinks; it is expected that --preserve-symlinks-main will be used in addition to --preserve-symlinks when it is not desirable to follow symlinks before resolving relative paths.

See --preserve-symlinks for more information.

--prof#

Added in: v2.0.0

Generate V8 profiler output.

--prof-process#

Added in: v5.2.0

Process V8 profiler output generated using the V8 option --prof.

--redirect-warnings=file#

Added in: v8.0.0

Write process warnings to the given file instead of printing to stderr. The file will be created if it does not exist, and will be appended to if it does. If an error occurs while attempting to write the warning to the file, the warning will be written to stderr instead.

--report-compact#

Added in: v13.12.0

Write reports in a compact format, single-line JSON, more easily consumable by log processing systems than the default multi-line format designed for human consumption.

--report-directory=directory#

History
VersionChanges
v13.12.0

This option is no longer considered experimental.

v12.0.0

Changed from --diagnostic-report-directory to --report-directory

v11.8.0

Added in: v11.8.0

Location at which the report will be generated.

--report-filename=filename#

History
VersionChanges
v13.12.0

This option is no longer considered experimental.

v12.0.0

changed from --diagnostic-report-filename to --report-filename

v11.8.0

Added in: v11.8.0

Name of the file to which the report will be written.

--report-on-fatalerror#

History
VersionChanges
v14.0.0

This option is no longer considered experimental.

v12.0.0

changed from --diagnostic-report-on-fatalerror to --report-on-fatalerror

v11.8.0

Added in: v11.8.0

Enables the report to be triggered on fatal errors (internal errors within the Node.js runtime such as out of memory) that lead to termination of the application. Useful to inspect various diagnostic data elements such as heap, stack, event loop state, resource consumption etc. to reason about the fatal error.

--report-on-signal#

History
VersionChanges
v13.12.0

This option is no longer considered experimental.

v12.0.0

changed from --diagnostic-report-on-signal to --report-on-signal

v11.8.0

Added in: v11.8.0

Enables report to be generated upon receiving the specified (or predefined) signal to the running Node.js process. The signal to trigger the report is specified through --report-signal.

--report-signal=signal#

History
VersionChanges
v13.12.0

This option is no longer considered experimental.

v12.0.0

changed from --diagnostic-report-signal to --report-signal

v11.8.0

Added in: v11.8.0

Sets or resets the signal for report generation (not supported on Windows). Default signal is SIGUSR2.

--report-uncaught-exception#

History
VersionChanges
v13.12.0

This option is no longer considered experimental.

v12.0.0

changed from --diagnostic-report-uncaught-exception to --report-uncaught-exception

v11.8.0

Added in: v11.8.0

Enables report to be generated on uncaught exceptions. Useful when inspecting the JavaScript stack in conjunction with native stack and other runtime environment data.

--throw-deprecation#

Added in: v0.11.14

Throw errors for deprecations.

--title=title#

Added in: v10.7.0

Set process.title on startup.

--tls-cipher-list=list#

Added in: v4.0.0

Specify an alternative default TLS cipher list. Requires Node.js to be built with crypto support (default).

--tls-keylog=file#

Added in: v13.2.0, v12.16.0

Log TLS key material to a file. The key material is in NSS SSLKEYLOGFILE format and can be used by software (such as Wireshark) to decrypt the TLS traffic.

--tls-max-v1.2#

Added in: v12.0.0, v10.20.0

Set tls.DEFAULT_MAX_VERSION to 'TLSv1.2'. Use to disable support for TLSv1.3.

--tls-max-v1.3#

Added in: v12.0.0

Set default tls.DEFAULT_MAX_VERSION to 'TLSv1.3'. Use to enable support for TLSv1.3.

--tls-min-v1.0#

Added in: v12.0.0, v10.20.0

Set default tls.DEFAULT_MIN_VERSION to 'TLSv1'. Use for compatibility with old TLS clients or servers.

--tls-min-v1.1#

Added in: v12.0.0, v10.20.0

Set default tls.DEFAULT_MIN_VERSION to 'TLSv1.1'. Use for compatibility with old TLS clients or servers.

--tls-min-v1.2#

Added in: v12.2.0, v10.20.0

Set default tls.DEFAULT_MIN_VERSION to 'TLSv1.2'. This is the default for 12.x and later, but the option is supported for compatibility with older Node.js versions.

--tls-min-v1.3#

Added in: v12.0.0

Set default tls.DEFAULT_MIN_VERSION to 'TLSv1.3'. Use to disable support for TLSv1.2, which is not as secure as TLSv1.3.

--trace-atomics-wait#

Added in: v14.3.0

Print short summaries of calls to Atomics.wait() to stderr. The output could look like this:

(node:15701) [Thread 0] Atomics.wait(<address> + 0, 1, inf) started
(node:15701) [Thread 0] Atomics.wait(<address> + 0, 1, inf) did not wait because the values mismatched
(node:15701) [Thread 0] Atomics.wait(<address> + 0, 0, 10) started
(node:15701) [Thread 0] Atomics.wait(<address> + 0, 0, 10) timed out
(node:15701) [Thread 0] Atomics.wait(<address> + 4, 0, inf) started
(node:15701) [Thread 1] Atomics.wait(<address> + 4, -1, inf) started
(node:15701) [Thread 0] Atomics.wait(<address> + 4, 0, inf) was woken up by another thread
(node:15701) [Thread 1] Atomics.wait(<address> + 4, -1, inf) was woken up by another thread

The fields here correspond to:

  • The thread id as given by worker_threads.threadId
  • The base address of the SharedArrayBuffer in question, as well as the byte offset corresponding to the index passed to Atomics.wait()
  • The expected value that was passed to Atomics.wait()
  • The timeout passed to Atomics.wait

--trace-deprecation#

Added in: v0.8.0

Print stack traces for deprecations.

--trace-event-categories#

Added in: v7.7.0

A comma separated list of categories that should be traced when trace event tracing is enabled using --trace-events-enabled.

--trace-event-file-pattern#

Added in: v9.8.0

Template string specifying the filepath for the trace event data, it supports ${rotation} and ${pid}.

--trace-events-enabled#

Added in: v7.7.0

Enables the collection of trace event tracing information.

--trace-exit#

Added in: v13.5.0, v12.16.0

Prints a stack trace whenever an environment is exited proactively, i.e. invoking process.exit().

--trace-sigint#

Added in: v13.9.0

Prints a stack trace on SIGINT.

--trace-sync-io#

Added in: v2.1.0

Prints a stack trace whenever synchronous I/O is detected after the first turn of the event loop.

--trace-tls#

Added in: v12.2.0

Prints TLS packet trace information to stderr. This can be used to debug TLS connection problems.

--trace-uncaught#

Added in: v13.1.0

Print stack traces for uncaught exceptions; usually, the stack trace associated with the creation of an Error is printed, whereas this makes Node.js also print the stack trace associated with throwing the value (which does not need to be an Error instance).

Enabling this option may affect garbage collection behavior negatively.

--trace-warnings#

Added in: v6.0.0

Print stack traces for process warnings (including deprecations).

--track-heap-objects#

Added in: v2.4.0

Track heap object allocations for heap snapshots.

--unhandled-rejections=mode#

Added in: v12.0.0, v10.17.0

By default all unhandled rejections trigger a warning plus a deprecation warning for the very first unhandled rejection in case no unhandledRejection hook is used.

Using this flag allows to change what should happen when an unhandled rejection occurs. One of three modes can be chosen:

  • strict: Raise the unhandled rejection as an uncaught exception.
  • warn: Always trigger a warning, no matter if the unhandledRejection hook is set or not but do not print the deprecation warning.
  • none: Silence all warnings.

--use-bundled-ca, --use-openssl-ca#

Added in: v6.11.0

Use bundled Mozilla CA store as supplied by current Node.js version or use OpenSSL's default CA store. The default store is selectable at build-time.

The bundled CA store, as supplied by Node.js, is a snapshot of Mozilla CA store that is fixed at release time. It is identical on all supported platforms.

Using OpenSSL store allows for external modifications of the store. For most Linux and BSD distributions, this store is maintained by the distribution maintainers and system administrators. OpenSSL CA store location is dependent on configuration of the OpenSSL library but this can be altered at runtime using environment variables.

See SSL_CERT_DIR and SSL_CERT_FILE.

--use-largepages=mode#

Added in: v13.6.0

Re-map the Node.js static code to large memory pages at startup. If supported on the target system, this will cause the Node.js static code to be moved onto 2 MiB pages instead of 4 KiB pages.

The following values are valid for mode:

  • off: No mapping will be attempted. This is the default.
  • on: If supported by the OS, mapping will be attempted. Failure to map will be ignored and a message will be printed to standard error.
  • silent: If supported by the OS, mapping will be attempted. Failure to map will be ignored and will not be reported.

--v8-options#

Added in: v0.1.3

Print V8 command line options.

--v8-pool-size=num#

Added in: v5.10.0

Set V8's thread pool size which will be used to allocate background jobs.

If set to 0 then V8 will choose an appropriate size of the thread pool based on the number of online processors.

If the value provided is larger than V8's maximum, then the largest value will be chosen.

--zero-fill-buffers#

Added in: v6.0.0

Automatically zero-fills all newly allocated Buffer and SlowBuffer instances.

-c, --check#

History
VersionChanges
v10.0.0

The --require option is now supported when checking a file.

v5.0.0, v4.2.0

Added in: v5.0.0, v4.2.0

Syntax check the script without executing.

-e, --eval "script"#

History
VersionChanges
v5.11.0

Built-in libraries are now available as predefined variables.

v0.5.2

Added in: v0.5.2

Evaluate the following argument as JavaScript. The modules which are predefined in the REPL can also be used in script.

On Windows, using cmd.exe a single quote will not work correctly because it only recognizes double " for quoting. In Powershell or Git bash, both ' and " are usable.

-h, --help#

Added in: v0.1.3

Print node command line options. The output of this option is less detailed than this document.

-i, --interactive#

Added in: v0.7.7

Opens the REPL even if stdin does not appear to be a terminal.

-p, --print "script"#

History
VersionChanges
v5.11.0

Built-in libraries are now available as predefined variables.

v0.6.4

Added in: v0.6.4

Identical to -e but prints the result.

-r, --require module#

Added in: v1.6.0

Preload the specified module at startup.

Follows require()'s module resolution rules. module may be either a path to a file, or a node module name.

-v, --version#

Added in: v0.1.3

Print node's version.

Environment Variables#

NODE_DEBUG=module[,…]#

Added in: v0.1.32

','-separated list of core modules that should print debug information.

NODE_DEBUG_NATIVE=module[,…]#

','-separated list of core C++ modules that should print debug information.

NODE_DISABLE_COLORS=1#

Added in: v0.3.0

When set, colors will not be used in the REPL.

NODE_EXTRA_CA_CERTS=file#

Added in: v7.3.0

When set, the well known "root" CAs (like VeriSign) will be extended with the extra certificates in file. The file should consist of one or more trusted certificates in PEM format. A message will be emitted (once) with process.emitWarning() if the file is missing or malformed, but any errors are otherwise ignored.

Neither the well known nor extra certificates are used when the ca options property is explicitly specified for a TLS or HTTPS client or server.

This environment variable is ignored when node runs as setuid root or has Linux file capabilities set.

NODE_ICU_DATA=file#

Added in: v0.11.15

Data path for ICU (Intl object) data. Will extend linked-in data when compiled with small-icu support.

NODE_NO_WARNINGS=1#

Added in: v6.11.0

When set to 1, process warnings are silenced.

NODE_OPTIONS=options...#

Added in: v8.0.0

A space-separated list of command line options. options... are interpreted before command line options, so command line options will override or compound after anything in options.... Node.js will exit with an error if an option that is not allowed in the environment is used, such as -p or a script file.

In case an option value happens to contain a space (for example a path listed in --require), it must be escaped using double quotes. For example:

NODE_OPTIONS='--require "./my path/file.js"'

A singleton flag passed as a command line option will override the same flag passed into NODE_OPTIONS:

# The inspector will be available on port 5555
NODE_OPTIONS='--inspect=localhost:4444' node --inspect=localhost:5555

A flag that can be passed multiple times will be treated as if its NODE_OPTIONS instances were passed first, and then its command line instances afterwards:

NODE_OPTIONS='--require "./a.js"' node --require "./b.js"
# is equivalent to:
node --require "./a.js" --require "./b.js"

Node.js options that are allowed are:

  • --disable-proto
  • --enable-fips
  • --enable-source-maps
  • --experimental-import-meta-resolve
  • --experimental-json-modules
  • --experimental-loader
  • --experimental-modules
  • --experimental-policy
  • --experimental-repl-await
  • --experimental-specifier-resolution
  • --experimental-vm-modules
  • --experimental-wasi-unstable-preview1
  • --experimental-wasm-modules
  • --force-context-aware
  • --force-fips
  • --frozen-intrinsics
  • --heapsnapshot-signal
  • --http-parser
  • --icu-data-dir
  • --input-type
  • --insecure-http-parser
  • --inspect-brk
  • --inspect-port, --debug-port
  • --inspect-publish-uid
  • --inspect
  • --max-http-header-size
  • --napi-modules
  • --no-deprecation
  • --no-force-async-hooks-checks
  • --no-warnings
  • --openssl-config
  • --pending-deprecation
  • --policy-integrity
  • --preserve-symlinks-main
  • --preserve-symlinks
  • --prof-process
  • --redirect-warnings
  • --report-compact
  • --report-directory
  • --report-filename
  • --report-on-fatalerror
  • --report-on-signal
  • --report-signal
  • --report-uncaught-exception
  • --require, -r
  • --throw-deprecation
  • --title
  • --tls-cipher-list
  • --tls-keylog
  • --tls-max-v1.2
  • --tls-max-v1.3
  • --tls-min-v1.0
  • --tls-min-v1.1
  • --tls-min-v1.2
  • --tls-min-v1.3
  • --trace-atomics-wait
  • --trace-deprecation
  • --trace-event-categories
  • --trace-event-file-pattern
  • --trace-events-enabled
  • --trace-exit
  • --trace-sigint
  • --trace-sync-io
  • --trace-tls
  • --trace-uncaught
  • --trace-warnings
  • --track-heap-objects
  • --unhandled-rejections
  • --use-bundled-ca
  • --use-largepages
  • --use-openssl-ca
  • --v8-pool-size

  • --zero-fill-buffers

V8 options that are allowed are:

  • --abort-on-uncaught-exception
  • --disallow-code-generation-from-strings
  • --huge-max-old-generation-size
  • --interpreted-frames-native-stack
  • --jitless
  • --max-old-space-size
  • --perf-basic-prof-only-functions
  • --perf-basic-prof
  • --perf-prof-unwinding-info
  • --perf-prof

  • --stack-trace-limit

--perf-basic-prof-only-functions, --perf-basic-prof, --perf-prof-unwinding-info, and --perf-prof are only available on Linux.

NODE_PATH=path[:…]#

Added in: v0.1.32

':'-separated list of directories prefixed to the module search path.

On Windows, this is a ';'-separated list instead.

NODE_PENDING_DEPRECATION=1#

Added in: v8.0.0

When set to 1, emit pending deprecation warnings.

Pending deprecations are generally identical to a runtime deprecation with the notable exception that they are turned off by default and will not be emitted unless either the --pending-deprecation command line flag, or the NODE_PENDING_DEPRECATION=1 environment variable, is set. Pending deprecations are used to provide a kind of selective "early warning" mechanism that developers may leverage to detect deprecated API usage.

NODE_PENDING_PIPE_INSTANCES=instances#

Set the number of pending pipe instance handles when the pipe server is waiting for connections. This setting applies to Windows only.

NODE_PRESERVE_SYMLINKS=1#

Added in: v7.1.0

When set to 1, instructs the module loader to preserve symbolic links when resolving and caching modules.

NODE_REDIRECT_WARNINGS=file#

Added in: v8.0.0

When set, process warnings will be emitted to the given file instead of printing to stderr. The file will be created if it does not exist, and will be appended to if it does. If an error occurs while attempting to write the warning to the file, the warning will be written to stderr instead. This is equivalent to using the --redirect-warnings=file command-line flag.

NODE_REPL_HISTORY=file#

Added in: v3.0.0

Path to the file used to store the persistent REPL history. The default path is ~/.node_repl_history, which is overridden by this variable. Setting the value to an empty string ('' or ' ') disables persistent REPL history.

NODE_REPL_EXTERNAL_MODULE=file#

Added in: v13.0.0, v12.16.0

Path to a Node.js module which will be loaded in place of the built-in REPL. Overriding this value to an empty string ('') will use the built-in REPL.

NODE_TLS_REJECT_UNAUTHORIZED=value#

If value equals '0', certificate validation is disabled for TLS connections. This makes TLS, and HTTPS by extension, insecure. The use of this environment variable is strongly discouraged.

NODE_V8_COVERAGE=dir#

When set, Node.js will begin outputting V8 JavaScript code coverage and Source Map data to the directory provided as an argument (coverage information is written as JSON to files with a coverage prefix).

NODE_V8_COVERAGE will automatically propagate to subprocesses, making it easier to instrument applications that call the child_process.spawn() family of functions. NODE_V8_COVERAGE can be set to an empty string, to prevent propagation.

Coverage Output#

Coverage is output as an array of ScriptCoverage objects on the top-level key result:

{
  "result": [
    {
      "scriptId": "67",
      "url": "internal/tty.js",
      "functions": []
    }
  ]
}

Source Map Cache#

Stability: 1 - Experimental

If found, Source Map data is appended to the top-level key source-map-cache on the JSON coverage object.

source-map-cache is an object with keys representing the files source maps were extracted from, and values which include the raw source-map URL (in the key url), the parsed Source Map V3 information (in the key data), and the line lengths of the source file (in the key lineLengths).

{
  "result": [
    {
      "scriptId": "68",
      "url": "file:///absolute/path/to/source.js",
      "functions": []
    }
  ],
  "source-map-cache": {
    "file:///absolute/path/to/source.js": {
      "url": "./path-to-map.json",
      "data": {
        "version": 3,
        "sources": [
          "file:///absolute/path/to/original.js"
        ],
        "names": [
          "Foo",
          "console",
          "info"
        ],
        "mappings": "MAAMA,IACJC,YAAaC",
        "sourceRoot": "./"
      },
      "lineLengths": [
        13,
        62,
        38,
        27
      ]
    }
  }
}

OPENSSL_CONF=file#

Added in: v6.11.0

Load an OpenSSL configuration file on startup. Among other uses, this can be used to enable FIPS-compliant crypto if Node.js is built with ./configure --openssl-fips.

If the --openssl-config command line option is used, the environment variable is ignored.

SSL_CERT_DIR=dir#

Added in: v7.7.0

If --use-openssl-ca is enabled, this overrides and sets OpenSSL's directory containing trusted certificates.

Be aware that unless the child environment is explicitly set, this environment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node.

SSL_CERT_FILE=file#

Added in: v7.7.0

If --use-openssl-ca is enabled, this overrides and sets OpenSSL's file containing trusted certificates.

Be aware that unless the child environment is explicitly set, this environment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node.

UV_THREADPOOL_SIZE=size#

Set the number of threads used in libuv's threadpool to size threads.

Asynchronous system APIs are used by Node.js whenever possible, but where they do not exist, libuv's threadpool is used to create asynchronous node APIs based on synchronous system APIs. Node.js APIs that use the threadpool are:

  • all fs APIs, other than the file watcher APIs and those that are explicitly synchronous
  • asynchronous crypto APIs such as crypto.pbkdf2(), crypto.scrypt(), crypto.randomBytes(), crypto.randomFill(), crypto.generateKeyPair()
  • dns.lookup()
  • all zlib APIs, other than those that are explicitly synchronous

Because libuv's threadpool has a fixed size, it means that if for whatever reason any of these APIs takes a long time, other (seemingly unrelated) APIs that run in libuv's threadpool will experience degraded performance. In order to mitigate this issue, one potential solution is to increase the size of libuv's threadpool by setting the 'UV_THREADPOOL_SIZE' environment variable to a value greater than 4 (its current default value). For more information, see the libuv threadpool documentation.

Useful V8 options#

V8 has its own set of CLI options. Any V8 CLI option that is provided to node will be passed on to V8 to handle. V8's options have no stability guarantee. The V8 team themselves don't consider them to be part of their formal API, and reserve the right to change them at any time. Likewise, they are not covered by the Node.js stability guarantees. Many of the V8 options are of interest only to V8 developers. Despite this, there is a small set of V8 options that are widely applicable to Node.js, and they are documented here:

--max-old-space-size=SIZE (in Mbytes)#

Sets the max memory size of V8's old memory section. As memory consumption approaches the limit, V8 will spend more time on garbage collection in an effort to free unused memory.

On a machine with 2GB of memory, consider setting this to 1536 (1.5GB) to leave some memory for other uses and avoid swapping.

$ node --max-old-space-size=1536 index.js

Console#

Stability: 2 - Stable

The console module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers.

The module exports two specific components:

  • A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
  • A global console instance configured to write to process.stdout and process.stderr. The global console can be used without calling require('console').

Warning: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the note on process I/O for more information.

Example using the global console:

console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to stderr

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr

Example using the Console class:

const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err

Class: Console#

History
VersionChanges
v8.0.0

Errors that occur while writing to the underlying streams will now be ignored by default.

The Console class can be used to create a simple logger with configurable output streams and can be accessed using either require('console').Console or console.Console (or their destructured counterparts):

const { Console } = require('console');

const { Console } = console;

new Console(stdout[, stderr][, ignoreErrors])#

new Console(options)#

History
VersionChanges
v14.2.0

The groupIndentation option was introduced.

v11.7.0

The inspectOptions option is introduced.

v10.0.0

The Console constructor now supports an options argument, and the colorMode option was introduced.

v8.0.0

The ignoreErrors option was introduced.

  • options <Object>

    • stdout <stream.Writable>
    • stderr <stream.Writable>
    • ignoreErrors <boolean> Ignore errors when writing to the underlying streams. Default: true.
    • colorMode <boolean> | <string> Set color support for this Console instance. Setting to true enables coloring while inspecting values. Setting to false disables coloring while inspecting values. Setting to 'auto' makes color support depend on the value of the isTTY property and the value returned by getColorDepth() on the respective stream. This option can not be used, if inspectOptions.colors is set as well. Default: 'auto'.
    • inspectOptions <Object> Specifies options that are passed along to util.inspect().
    • groupIndentation <number> Set group indentation. Default: 2.

Creates a new Console with one or two writable stream instances. stdout is a writable stream to print log or info output. stderr is used for warning or error output. If stderr is not provided, stdout is used for stderr.

const output = fs.createWriteStream('./stdout.log');
const errorOutput = fs.createWriteStream('./stderr.log');
// Custom simple logger
const logger = new Console({ stdout: output, stderr: errorOutput });
// use it like console
const count = 5;
logger.log('count: %d', count);
// In stdout.log: count 5

The global console is a special Console whose output is sent to process.stdout and process.stderr. It is equivalent to calling:

new Console({ stdout: process.stdout, stderr: process.stderr });

console.assert(value[, ...message])#

History
VersionChanges
v10.0.0

The implementation is now spec compliant and does not throw anymore.

v0.1.101

Added in: v0.1.101

  • value <any> The value tested for being truthy.
  • ...message <any> All arguments besides value are used as error message.

A simple assertion test that verifies whether value is truthy. If it is not, Assertion failed is logged. If provided, the error message is formatted using util.format() by passing along all message arguments. The output is used as the error message.

console.assert(true, 'does nothing');
// OK
console.assert(false, 'Whoops %s work', 'didn\'t');
// Assertion failed: Whoops didn't work

Calling console.assert() with a falsy assertion will only cause the message to be printed to the console without interrupting execution of subsequent code.

console.clear()#

Added in: v8.3.0

When stdout is a TTY, calling console.clear() will attempt to clear the TTY. When stdout is not a TTY, this method does nothing.

The specific operation of console.clear() can vary across operating systems and terminal types. For most Linux operating systems, console.clear() operates similarly to the clear shell command. On Windows, console.clear() will clear only the output in the current terminal viewport for the Node.js binary.

console.count([label])#

Added in: v8.3.0
  • label <string> The display label for the counter. Default: 'default'.

Maintains an internal counter specific to label and outputs to stdout the number of times console.count() has been called with the given label.

> console.count()
default: 1
undefined
> console.count('default')
default: 2
undefined
> console.count('abc')
abc: 1
undefined
> console.count('xyz')
xyz: 1
undefined
> console.count('abc')
abc: 2
undefined
> console.count()
default: 3
undefined
>

console.countReset([label])#

Added in: v8.3.0
  • label <string> The display label for the counter. Default: 'default'.

Resets the internal counter specific to label.

> console.count('abc');
abc: 1
undefined
> console.countReset('abc');
undefined
> console.count('abc');
abc: 1
undefined
>

console.debug(data[, ...args])#

History
VersionChanges
v8.10.0

console.debug is now an alias for console.log.

v8.0.0

Added in: v8.0.0

The console.debug() function is an alias for console.log().

console.dir(obj[, options])#

Added in: v0.1.101
  • obj <any>

  • options <Object>

    • showHidden <boolean> If true then the object's non-enumerable and symbol properties will be shown too. Default: false.
    • depth <number> Tells util.inspect() how many times to recurse while formatting the object. This is useful for inspecting large complicated objects. To make it recurse indefinitely, pass null. Default: 2.
    • colors <boolean> If true, then the output will be styled with ANSI color codes. Colors are customizable; see customizing util.inspect() colors. Default: false.

Uses util.inspect() on obj and prints the resulting string to stdout. This function bypasses any custom inspect() function defined on obj.

console.dirxml(...data)#

History
VersionChanges
v9.3.0

console.dirxml now calls console.log for its arguments.

v8.0.0

Added in: v8.0.0

This method calls console.log() passing it the arguments received. This method does not produce any XML formatting.

console.error([data][, ...args])#

Added in: v0.1.100

Prints to stderr with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format()).

const code = 5;
console.error('error #%d', code);
// Prints: error #5, to stderr
console.error('error', code);
// Prints: error 5, to stderr

If formatting elements (e.g. %d) are not found in the first string then util.inspect() is called on each argument and the resulting string values are concatenated. See util.format() for more information.

console.group([...label])#

Added in: v8.5.0

Increases indentation of subsequent lines by spaces for groupIndentation length.

If one or more labels are provided, those are printed first without the additional indentation.

console.groupCollapsed()#

Added in: v8.5.0

An alias for console.group().

console.groupEnd()#

Added in: v8.5.0

Decreases indentation of subsequent lines by spaces for groupIndentation length.

console.info([data][, ...args])#

Added in: v0.1.100

The console.info() function is an alias for console.log().

console.log([data][, ...args])#

Added in: v0.1.100

Prints to stdout with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format()).

const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout

See util.format() for more information.

console.table(tabularData[, properties])#

Added in: v10.0.0
  • tabularData <any>
  • properties <string[]> Alternate properties for constructing the table.

Try to construct a table with the columns of the properties of tabularData (or use properties) and rows of tabularData and log it. Falls back to just logging the argument if it can’t be parsed as tabular.

// These can't be parsed as tabular data
console.table(Symbol());
// Symbol()

console.table(undefined);
// undefined

console.table([{ a: 1, b: 'Y' }, { a: 'Z', b: 2 }]);
// ┌─────────┬─────┬─────┐
// │ (index) │  a  │  b  │
// ├─────────┼─────┼─────┤
// │    0    │  1  │ 'Y' │
// │    1    │ 'Z' │  2  │
// └─────────┴─────┴─────┘

console.table([{ a: 1, b: 'Y' }, { a: 'Z', b: 2 }], ['a']);
// ┌─────────┬─────┐
// │ (index) │  a  │
// ├─────────┼─────┤
// │    0    │  1  │
// │    1    │ 'Z' │
// └─────────┴─────┘

console.time([label])#

Added in: v0.1.104

Starts a timer that can be used to compute the duration of an operation. Timers are identified by a unique label. Use the same label when calling console.timeEnd() to stop the timer and output the elapsed time in milliseconds to stdout. Timer durations are accurate to the sub-millisecond.

console.timeEnd([label])#

History
VersionChanges
v6.0.0

This method no longer supports multiple calls that don’t map to individual console.time() calls; see below for details.

v0.1.104

Added in: v0.1.104

Stops a timer that was previously started by calling console.time() and prints the result to stdout:

console.time('100-elements');
for (let i = 0; i < 100; i++) {}
console.timeEnd('100-elements');
// prints 100-elements: 225.438ms

console.timeLog([label][, ...data])#

Added in: v10.7.0

For a timer that was previously started by calling console.time(), prints the elapsed time and other data arguments to stdout:

console.time('process');
const value = expensiveProcess1(); // Returns 42
console.timeLog('process', value);
// Prints "process: 365.227ms 42".
doExpensiveProcess2(value);
console.timeEnd('process');

console.trace([message][, ...args])#

Added in: v0.1.104

Prints to stderr the string 'Trace: ', followed by the util.format() formatted message and stack trace to the current position in the code.

console.trace('Show me');
// Prints: (stack trace will vary based on where trace is called)
//  Trace: Show me
//    at repl:2:9
//    at REPLServer.defaultEval (repl.js:248:27)
//    at bound (domain.js:287:14)
//    at REPLServer.runBound [as eval] (domain.js:300:12)
//    at REPLServer.<anonymous> (repl.js:412:12)
//    at emitOne (events.js:82:20)
//    at REPLServer.emit (events.js:169:7)
//    at REPLServer.Interface._onLine (readline.js:210:10)
//    at REPLServer.Interface._line (readline.js:549:8)
//    at REPLServer.Interface._ttyWrite (readline.js:826:14)

console.warn([data][, ...args])#

Added in: v0.1.100

The console.warn() function is an alias for console.error().

Inspector only methods#

The following methods are exposed by the V8 engine in the general API but do not display anything unless used in conjunction with the inspector (--inspect flag).

console.profile([label])#

Added in: v8.0.0

This method does not display anything unless used in the inspector. The console.profile() method starts a JavaScript CPU profile with an optional label until console.profileEnd() is called. The profile is then added to the Profile panel of the inspector.

console.profile('MyLabel');
// Some code
console.profileEnd('MyLabel');
// Adds the profile 'MyLabel' to the Profiles panel of the inspector.

console.profileEnd([label])#

Added in: v8.0.0

This method does not display anything unless used in the inspector. Stops the current JavaScript CPU profiling session if one has been started and prints the report to the Profiles panel of the inspector. See console.profile() for an example.

If this method is called without a label, the most recently started profile is stopped.

console.timeStamp([label])#

Added in: v8.0.0

This method does not display anything unless used in the inspector. The console.timeStamp() method adds an event with the label 'label' to the Timeline panel of the inspector.

Crypto#

Stability: 2 - Stable

The crypto module provides cryptographic functionality that includes a set of wrappers for OpenSSL's hash, HMAC, cipher, decipher, sign, and verify functions.

Use require('crypto') to access this module.

const crypto = require('crypto');

const secret = 'abcdefg';
const hash = crypto.createHmac('sha256', secret)
                   .update('I love cupcakes')
                   .digest('hex');
console.log(hash);
// Prints:
//   c0fa1bc00531bd78ef38c628449c5102aeabd49b5dc3a2a516ea6ea959d6658e

Determining if crypto support is unavailable#

It is possible for Node.js to be built without including support for the crypto module. In such cases, calling require('crypto') will result in an error being thrown.

let crypto;
try {
  crypto = require('crypto');
} catch (err) {
  console.log('crypto support is disabled!');
}

Class: Certificate#

Added in: v0.11.8

SPKAC is a Certificate Signing Request mechanism originally implemented by Netscape and was specified formally as part of HTML5's keygen element.

<keygen> is deprecated since HTML 5.2 and new projects should not use this element anymore.

The crypto module provides the Certificate class for working with SPKAC data. The most common usage is handling output generated by the HTML5 <keygen> element. Node.js uses OpenSSL's SPKAC implementation internally.

Certificate.exportChallenge(spkac)#

Added in: v9.0.0 
const { Certificate } = require('crypto');
const spkac = getSpkacSomehow();
const challenge = Certificate.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 string

Certificate.exportPublicKey(spkac[, encoding])#

Added in: v9.0.0 
const { Certificate } = require('crypto');
const spkac = getSpkacSomehow();
const publicKey = Certificate.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>

Certificate.verifySpkac(spkac)#

Added in: v9.0.0 
const { Certificate } = require('crypto');
const spkac = getSpkacSomehow();
console.log(Certificate.verifySpkac(Buffer.from(spkac)));
// Prints: true or false

Legacy API#

As a still supported legacy interface, it is possible (but not recommended) to create new instances of the crypto.Certificate class as illustrated in the examples below.

new crypto.Certificate()#

Instances of the Certificate class can be created using the new keyword or by calling crypto.Certificate() as a function:

const crypto = require('crypto');

const cert1 = new crypto.Certificate();
const cert2 = crypto.Certificate();

certificate.exportChallenge(spkac)#

Added in: v0.11.8 
const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
const challenge = cert.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 string

certificate.exportPublicKey(spkac)#

Added in: v0.11.8 
const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
const publicKey = cert.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>

certificate.verifySpkac(spkac)#

Added in: v0.11.8 
const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
console.log(cert.verifySpkac(Buffer.from(spkac)));
// Prints: true or false

Class: Cipher#

Added in: v0.1.94

Instances of the Cipher class are used to encrypt data. The class can be used in one of two ways:

  • As a stream that is both readable and writable, where plain unencrypted data is written to produce encrypted data on the readable side, or
  • Using the cipher.update() and cipher.final() methods to produce the encrypted data.

The crypto.createCipher() or crypto.createCipheriv() methods are used to create Cipher instances. Cipher objects are not to be created directly using the new keyword.

Example: Using Cipher objects as streams:

const crypto = require('crypto');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Key length is dependent on the algorithm. In this case for aes192, it is
// 24 bytes (192 bits).
// Use async `crypto.scrypt()` instead.
const key = crypto.scryptSync(password, 'salt', 24);
// Use `crypto.randomBytes()` to generate a random iv instead of the static iv
// shown here.
const iv = Buffer.alloc(16, 0); // Initialization vector.

const cipher = crypto.createCipheriv(algorithm, key, iv);

let encrypted = '';
cipher.on('readable', () => {
  let chunk;
  while (null !== (chunk = cipher.read())) {
    encrypted += chunk.toString('hex');
  }
});
cipher.on('end', () => {
  console.log(encrypted);
  // Prints: e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa
});

cipher.write('some clear text data');
cipher.end();

Example: Using Cipher and piped streams:

const crypto = require('crypto');
const fs = require('fs');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Use the async `crypto.scrypt()` instead.
const key = crypto.scryptSync(password, 'salt', 24);
// Use `crypto.randomBytes()` to generate a random iv instead of the static iv
// shown here.
const iv = Buffer.alloc(16, 0); // Initialization vector.

const cipher = crypto.createCipheriv(algorithm, key, iv);

const input = fs.createReadStream('test.js');
const output = fs.createWriteStream('test.enc');

input.pipe(cipher).pipe(output);

Example: Using the cipher.update() and cipher.final() methods:

const crypto = require('crypto');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Use the async `crypto.scrypt()` instead.
const key = crypto.scryptSync(password, 'salt', 24);
// Use `crypto.randomBytes` to generate a random iv instead of the static iv
// shown here.
const iv = Buffer.alloc(16, 0); // Initialization vector.

const cipher = crypto.createCipheriv(algorithm, key, iv);

let encrypted = cipher.update('some clear text data', 'utf8', 'hex');
encrypted += cipher.final('hex');
console.log(encrypted);
// Prints: e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa

cipher.final([outputEncoding])#

Added in: v0.1.94
  • outputEncoding <string> The encoding of the return value.
  • Returns: <Buffer> | <string> Any remaining enciphered contents. If outputEncoding is specified, a string is returned. If an outputEncoding is not provided, a Buffer is returned.

Once the cipher.final() method has been called, the Cipher object can no longer be used to encrypt data. Attempts to call cipher.final() more than once will result in an error being thrown.

cipher.setAAD(buffer[, options])#

Added in: v1.0.0

When using an authenticated encryption mode (GCM, CCM and OCB are currently supported), the cipher.setAAD() method sets the value used for the additional authenticated data (AAD) input parameter.

The options argument is optional for GCM and OCB. When using CCM, the plaintextLength option must be specified and its value must match the length of the plaintext in bytes. See CCM mode.

The cipher.setAAD() method must be called before cipher.update().

cipher.getAuthTag()#

Added in: v1.0.0
  • Returns: <Buffer> When using an authenticated encryption mode (GCM, CCM and OCB are currently supported), the cipher.getAuthTag() method returns a Buffer containing the authentication tag that has been computed from the given data.

The cipher.getAuthTag() method should only be called after encryption has been completed using the cipher.final() method.

cipher.setAutoPadding([autoPadding])#

Added in: v0.7.1

When using block encryption algorithms, the Cipher class will automatically add padding to the input data to the appropriate block size. To disable the default padding call cipher.setAutoPadding(false).

When autoPadding is false, the length of the entire input data must be a multiple of the cipher's block size or cipher.final() will throw an error. Disabling automatic padding is useful for non-standard padding, for instance using 0x0 instead of PKCS padding.

The cipher.setAutoPadding() method must be called before cipher.final().

cipher.update(data[, inputEncoding][, outputEncoding])#

History
VersionChanges
v6.0.0

The default inputEncoding changed from binary to utf8.

v0.1.94

Added in: v0.1.94

Updates the cipher with data. If the inputEncoding argument is given, the data argument is a string using the specified encoding. If the inputEncoding argument is not given, data must be a Buffer, TypedArray, or DataView. If data is a Buffer, TypedArray, or DataView, then inputEncoding is ignored.

The outputEncoding specifies the output format of the enciphered data. If the outputEncoding is specified, a string using the specified encoding is returned. If no outputEncoding is provided, a Buffer is returned.

The cipher.update() method can be called multiple times with new data until cipher.final() is called. Calling cipher.update() after cipher.final() will result in an error being thrown.

Class: Decipher#

Added in: v0.1.94

Instances of the Decipher class are used to decrypt data. The class can be used in one of two ways:

  • As a stream that is both readable and writable, where plain encrypted data is written to produce unencrypted data on the readable side, or
  • Using the decipher.update() and decipher.final() methods to produce the unencrypted data.

The crypto.createDecipher() or crypto.createDecipheriv() methods are used to create Decipher instances. Decipher objects are not to be created directly using the new keyword.

Example: Using Decipher objects as streams:

const crypto = require('crypto');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Key length is dependent on the algorithm. In this case for aes192, it is
// 24 bytes (192 bits).
// Use the async `crypto.scrypt()` instead.
const key = crypto.scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.

const decipher = crypto.createDecipheriv(algorithm, key, iv);

let decrypted = '';
decipher.on('readable', () => {
  while (null !== (chunk = decipher.read())) {
    decrypted += chunk.toString('utf8');
  }
});
decipher.on('end', () => {
  console.log(decrypted);
  // Prints: some clear text data
});

// Encrypted with same algorithm, key and iv.
const encrypted =
  'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa';
decipher.write(encrypted, 'hex');
decipher.end();

Example: Using Decipher and piped streams:

const crypto = require('crypto');
const fs = require('fs');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Use the async `crypto.scrypt()` instead.
const key = crypto.scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.

const decipher = crypto.createDecipheriv(algorithm, key, iv);

const input = fs.createReadStream('test.enc');
const output = fs.createWriteStream('test.js');

input.pipe(decipher).pipe(output);

Example: Using the decipher.update() and decipher.final() methods:

const crypto = require('crypto');

const algorithm = 'aes-192-cbc';
const password = 'Password used to generate key';
// Use the async `crypto.scrypt()` instead.
const key = crypto.scryptSync(password, 'salt', 24);
// The IV is usually passed along with the ciphertext.
const iv = Buffer.alloc(16, 0); // Initialization vector.

const decipher = crypto.createDecipheriv(algorithm, key, iv);

// Encrypted using same algorithm, key and iv.
const encrypted =
  'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa';
let decrypted = decipher.update(encrypted, 'hex', 'utf8');
decrypted += decipher.final('utf8');
console.log(decrypted);
// Prints: some clear text data

decipher.final([outputEncoding])#

Added in: v0.1.94
  • outputEncoding <string> The encoding of the return value.
  • Returns: <Buffer> | <string> Any remaining deciphered contents. If outputEncoding is specified, a string is returned. If an outputEncoding is not provided, a Buffer is returned.

Once the decipher.final() method has been called, the Decipher object can no longer be used to decrypt data. Attempts to call decipher.final() more than once will result in an error being thrown.

decipher.setAAD(buffer[, options])#

History
VersionChanges
v7.2.0

This method now returns a reference to decipher.

v1.0.0

Added in: v1.0.0

When using an authenticated encryption mode (GCM, CCM and OCB are currently supported), the decipher.setAAD() method sets the value used for the additional authenticated data (AAD) input parameter.

The options argument is optional for GCM. When using CCM, the plaintextLength option must be specified and its value must match the length of the plaintext in bytes. See CCM mode.

The decipher.setAAD() method must be called before decipher.update().

decipher.setAuthTag(buffer)#

History
VersionChanges
v11.0.0

This method now throws if the GCM tag length is invalid.

v7.2.0

This method now returns a reference to decipher.

v1.0.0

Added in: v1.0.0

When using an authenticated encryption mode (GCM, CCM and OCB are currently supported), the decipher.setAuthTag() method is used to pass in the received authentication tag. If no tag is provided, or if the cipher text has been tampered with, decipher.final() will throw, indicating that the cipher text should be discarded due to failed authentication. If the tag length is invalid according to NIST SP 800-38D or does not match the value of the authTagLength option, decipher.setAuthTag() will throw an error.

The decipher.setAuthTag() method must be called before decipher.update() for CCM mode or before decipher.final() for GCM and OCB modes. decipher.setAuthTag() can only be called once.

decipher.setAutoPadding([autoPadding])#

Added in: v0.7.1

When data has been encrypted without standard block padding, calling decipher.setAutoPadding(false) will disable automatic padding to prevent decipher.final() from checking for and removing padding.

Turning auto padding off will only work if the input data's length is a multiple of the ciphers block size.

The decipher.setAutoPadding() method must be called before decipher.final().

decipher.update(data[, inputEncoding][, outputEncoding])#

History
VersionChanges
v6.0.0

The default inputEncoding changed from binary to utf8.

v0.1.94

Added in: v0.1.94

Updates the decipher with data. If the inputEncoding argument is given, the data argument is a string using the specified encoding. If the inputEncoding argument is not given, data must be a Buffer. If data is a Buffer then inputEncoding is ignored.

The outputEncoding specifies the output format of the enciphered data. If the outputEncoding is specified, a string using the specified encoding is returned. If no outputEncoding is provided, a Buffer is returned.

The decipher.update() method can be called multiple times with new data until decipher.final() is called. Calling decipher.update() after decipher.final() will result in an error being thrown.

Class: DiffieHellman#

Added in: v0.5.0

The DiffieHellman class is a utility for creating Diffie-Hellman key exchanges.

Instances of the DiffieHellman class can be created using the crypto.createDiffieHellman() function.

const crypto = require('crypto');
const assert = require('assert');

// Generate Alice's keys...
const alice = crypto.createDiffieHellman(2048);
const aliceKey = alice.generateKeys();

// Generate Bob's keys...
const bob = crypto.createDiffieHellman(alice.getPrime(), alice.getGenerator());
const bobKey = bob.generateKeys();

// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);

// OK
assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));

diffieHellman.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])#

Added in: v0.5.0

Computes the shared secret using otherPublicKey as the other party's public key and returns the computed shared secret. The supplied key is interpreted using the specified inputEncoding, and secret is encoded using specified outputEncoding. If the inputEncoding is not provided, otherPublicKey is expected to be a Buffer, TypedArray, or DataView.

If outputEncoding is given a string is returned; otherwise, a Buffer is returned.

diffieHellman.generateKeys([encoding])#

Added in: v0.5.0

Generates private and public Diffie-Hellman key values, and returns the public key in the specified encoding. This key should be transferred to the other party. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getGenerator([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman generator in the specified encoding. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getPrime([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman prime in the specified encoding. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getPrivateKey([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman private key in the specified encoding. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getPublicKey([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman public key in the specified encoding. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.setPrivateKey(privateKey[, encoding])#

Added in: v0.5.0

Sets the Diffie-Hellman private key. If the encoding argument is provided, privateKey is expected to be a string. If no encoding is provided, privateKey is expected to be a Buffer, TypedArray, or DataView.

diffieHellman.setPublicKey(publicKey[, encoding])#

Added in: v0.5.0

Sets the Diffie-Hellman public key. If the encoding argument is provided, publicKey is expected to be a string. If no encoding is provided, publicKey is expected to be a Buffer, TypedArray, or DataView.

diffieHellman.verifyError#

Added in: v0.11.12

A bit field containing any warnings and/or errors resulting from a check performed during initialization of the DiffieHellman object.

The following values are valid for this property (as defined in constants module):

  • DH_CHECK_P_NOT_SAFE_PRIME
  • DH_CHECK_P_NOT_PRIME
  • DH_UNABLE_TO_CHECK_GENERATOR
  • DH_NOT_SUITABLE_GENERATOR

Class: DiffieHellmanGroup#

Added in: v0.7.5

The DiffieHellmanGroup class takes a well-known modp group as its argument but otherwise works the same as DiffieHellman.

const name = 'modp1';
const dh = crypto.createDiffieHellmanGroup(name);

name is taken from RFC 2412 (modp1 and 2) and RFC 3526:

$ perl -ne 'print "$1\n" if /"(modp\d+)"/' src/node_crypto_groups.h
modp1  #  768 bits
modp2  # 1024 bits
modp5  # 1536 bits
modp14 # 2048 bits
modp15 # etc.
modp16
modp17
modp18

Class: ECDH#

Added in: v0.11.14

The ECDH class is a utility for creating Elliptic Curve Diffie-Hellman (ECDH) key exchanges.

Instances of the ECDH class can be created using the crypto.createECDH() function.

const crypto = require('crypto');
const assert = require('assert');

// Generate Alice's keys...
const alice = crypto.createECDH('secp521r1');
const aliceKey = alice.generateKeys();

// Generate Bob's keys...
const bob = crypto.createECDH('secp521r1');
const bobKey = bob.generateKeys();

// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);

assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
// OK

Class Method: ECDH.convertKey(key, curve[, inputEncoding[, outputEncoding[, format]]])#

Added in: v10.0.0

Converts the EC Diffie-Hellman public key specified by key and curve to the format specified by format. The format argument specifies point encoding and can be 'compressed', 'uncompressed' or 'hybrid'. The supplied key is interpreted using the specified inputEncoding, and the returned key is encoded using the specified outputEncoding.

Use crypto.getCurves() to obtain a list of available curve names. On recent OpenSSL releases, openssl ecparam -list_curves will also display the name and description of each available elliptic curve.

If format is not specified the point will be returned in 'uncompressed' format.

If the inputEncoding is not provided, key is expected to be a Buffer, TypedArray, or DataView.

Example (uncompressing a key):

const { createECDH, ECDH } = require('crypto');

const ecdh = createECDH('secp256k1');
ecdh.generateKeys();

const compressedKey = ecdh.getPublicKey('hex', 'compressed');

const uncompressedKey = ECDH.convertKey(compressedKey,
                                        'secp256k1',
                                        'hex',
                                        'hex',
                                        'uncompressed');

// The converted key and the uncompressed public key should be the same
console.log(uncompressedKey === ecdh.getPublicKey('hex'));

ecdh.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])#

History
VersionChanges
v10.0.0

Changed error format to better support invalid public key error

v6.0.0

The default inputEncoding changed from binary to utf8

v0.11.14

Added in: v0.11.14

Computes the shared secret using otherPublicKey as the other party's public key and returns the computed shared secret. The supplied key is interpreted using specified inputEncoding, and the returned secret is encoded using the specified outputEncoding. If the inputEncoding is not provided, otherPublicKey is expected to be a Buffer, TypedArray, or DataView.

If outputEncoding is given a string will be returned; otherwise a Buffer is returned.

ecdh.computeSecret will throw an ERR_CRYPTO_ECDH_INVALID_PUBLIC_KEY error when otherPublicKey lies outside of the elliptic curve. Since otherPublicKey is usually supplied from a remote user over an insecure network, its recommended for developers to handle this exception accordingly.

ecdh.generateKeys([encoding[, format]])#

Added in: v0.11.14

Generates private and public EC Diffie-Hellman key values, and returns the public key in the specified format and encoding. This key should be transferred to the other party.

The format argument specifies point encoding and can be 'compressed' or 'uncompressed'. If format is not specified, the point will be returned in 'uncompressed' format.

If encoding is provided a string is returned; otherwise a Buffer is returned.

ecdh.getPrivateKey([encoding])#

Added in: v0.11.14

If encoding is specified, a string is returned; otherwise a Buffer is returned.

ecdh.getPublicKey([encoding][, format])#

Added in: v0.11.14

The format argument specifies point encoding and can be 'compressed' or 'uncompressed'. If format is not specified the point will be returned in 'uncompressed' format.

If encoding is specified, a string is returned; otherwise a Buffer is returned.

ecdh.setPrivateKey(privateKey[, encoding])#

Added in: v0.11.14

Sets the EC Diffie-Hellman private key. If encoding is provided, privateKey is expected to be a string; otherwise privateKey is expected to be a Buffer, TypedArray, or DataView.

If privateKey is not valid for the curve specified when the ECDH object was created, an error is thrown. Upon setting the private key, the associated public point (key) is also generated and set in the ECDH object.

ecdh.setPublicKey(publicKey[, encoding])#

Added in: v0.11.14Deprecated since: v5.2.0

Stability: 0 - Deprecated

Sets the EC Diffie-Hellman public key. If encoding is provided publicKey is expected to be a string; otherwise a Buffer, TypedArray, or DataView is expected.

There is not normally a reason to call this method because ECDH only requires a private key and the other party's public key to compute the shared secret. Typically either ecdh.generateKeys() or ecdh.setPrivateKey() will be called. The ecdh.setPrivateKey() method attempts to generate the public point/key associated with the private key being set.

Example (obtaining a shared secret):

const crypto = require('crypto');
const alice = crypto.createECDH('secp256k1');
const bob = crypto.createECDH('secp256k1');

// This is a shortcut way of specifying one of Alice's previous private
// keys. It would be unwise to use such a predictable private key in a real
// application.
alice.setPrivateKey(
  crypto.createHash('sha256').update('alice', 'utf8').digest()
);

// Bob uses a newly generated cryptographically strong
// pseudorandom key pair
bob.generateKeys();

const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

// aliceSecret and bobSecret should be the same shared secret value
console.log(aliceSecret === bobSecret);

Class: Hash#

Added in: v0.1.92

The Hash class is a utility for creating hash digests of data. It can be used in one of two ways:

  • As a stream that is both readable and writable, where data is written to produce a computed hash digest on the readable side, or
  • Using the hash.update() and hash.digest() methods to produce the computed hash.

The crypto.createHash() method is used to create Hash instances. Hash objects are not to be created directly using the new keyword.

Example: Using Hash objects as streams:

const crypto = require('crypto');
const hash = crypto.createHash('sha256');

hash.on('readable', () => {
  // Only one element is going to be produced by the
  // hash stream.
  const data = hash.read();
  if (data) {
    console.log(data.toString('hex'));
    // Prints:
    //   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
  }
});

hash.write('some data to hash');
hash.end();

Example: Using Hash and piped streams:

const crypto = require('crypto');
const fs = require('fs');
const hash = crypto.createHash('sha256');

const input = fs.createReadStream('test.js');
input.pipe(hash).pipe(process.stdout);

Example: Using the hash.update() and hash.digest() methods:

const crypto = require('crypto');
const hash = crypto.createHash('sha256');

hash.update('some data to hash');
console.log(hash.digest('hex'));
// Prints:
//   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50

hash.copy([options])#

Added in: v13.1.0

Creates a new Hash object that contains a deep copy of the internal state of the current Hash object.

The optional options argument controls stream behavior. For XOF hash functions such as 'shake256', the outputLength option can be used to specify the desired output length in bytes.

An error is thrown when an attempt is made to copy the Hash object after its hash.digest() method has been called.

// Calculate a rolling hash.
const crypto = require('crypto');
const hash = crypto.createHash('sha256');

hash.update('one');
console.log(hash.copy().digest('hex'));

hash.update('two');
console.log(hash.copy().digest('hex'));

hash.update('three');
console.log(hash.copy().digest('hex'));

// Etc.

hash.digest([encoding])#

Added in: v0.1.92

Calculates the digest of all of the data passed to be hashed (using the hash.update() method). If encoding is provided a string will be returned; otherwise a Buffer is returned.

The Hash object can not be used again after hash.digest() method has been called. Multiple calls will cause an error to be thrown.

hash.update(data[, inputEncoding])#

History
VersionChanges
v6.0.0

The default inputEncoding changed from binary to utf8.

v0.1.92

Added in: v0.1.92

Updates the hash content with the given data, the encoding of which is given in inputEncoding. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer, TypedArray, or DataView, then inputEncoding is ignored.

This can be called many times with new data as it is streamed.

Class: Hmac#

Added in: v0.1.94

The Hmac class is a utility for creating cryptographic HMAC digests. It can be used in one of two ways:

  • As a stream that is both readable and writable, where data is written to produce a computed HMAC digest on the readable side, or
  • Using the hmac.update() and hmac.digest() methods to produce the computed HMAC digest.

The crypto.createHmac() method is used to create Hmac instances. Hmac objects are not to be created directly using the new keyword.

Example: Using Hmac objects as streams:

const crypto = require('crypto');
const hmac = crypto.createHmac('sha256', 'a secret');

hmac.on('readable', () => {
  // Only one element is going to be produced by the
  // hash stream.
  const data = hmac.read();
  if (data) {
    console.log(data.toString('hex'));
    // Prints:
    //   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
  }
});

hmac.write('some data to hash');
hmac.end();

Example: Using Hmac and piped streams:

const crypto = require('crypto');
const fs = require('fs');
const hmac = crypto.createHmac('sha256', 'a secret');

const input = fs.createReadStream('test.js');
input.pipe(hmac).pipe(process.stdout);

Example: Using the hmac.update() and hmac.digest() methods:

const crypto = require('crypto');
const hmac = crypto.createHmac('sha256', 'a secret');

hmac.update('some data to hash');
console.log(hmac.digest('hex'));
// Prints:
//   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e

hmac.digest([encoding])#

Added in: v0.1.94

Calculates the HMAC digest of all of the data passed using hmac.update(). If encoding is provided a string is returned; otherwise a Buffer is returned;

The Hmac object can not be used again after hmac.digest() has been called. Multiple calls to hmac.digest() will result in an error being thrown.

hmac.update(data[, inputEncoding])#

History
VersionChanges
v6.0.0

The default inputEncoding changed from binary to utf8.

v0.1.94

Added in: v0.1.94

Updates the Hmac content with the given data, the encoding of which is given in inputEncoding. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer, TypedArray, or DataView, then inputEncoding is ignored.

This can be called many times with new data as it is streamed.

Class: KeyObject#

History
VersionChanges
v11.13.0

This class is now exported.

v11.6.0

Added in: v11.6.0

Node.js uses a KeyObject class to represent a symmetric or asymmetric key, and each kind of key exposes different functions. The crypto.createSecretKey(), crypto.createPublicKey() and crypto.createPrivateKey() methods are used to create KeyObject instances. KeyObject objects are not to be created directly using the new keyword.

Most applications should consider using the new KeyObject API instead of passing keys as strings or Buffers due to improved security features.

keyObject.asymmetricKeyType#

History
VersionChanges
v13.9.0

Added support for 'dh'.

v12.0.0

Added support for 'rsa-pss'

v12.0.0

This property now returns undefined for KeyObject instances of unrecognized type instead of aborting.

v12.0.0

Added support for 'x25519' and 'x448'

v12.0.0

Added support for 'ed25519' and 'ed448'.

v11.6.0

Added in: v11.6.0

For asymmetric keys, this property represents the type of the key. Supported key types are:

  • 'rsa' (OID 1.2.840.113549.1.1.1)
  • 'rsa-pss' (OID 1.2.840.113549.1.1.10)
  • 'dsa' (OID 1.2.840.10040.4.1)
  • 'ec' (OID 1.2.840.10045.2.1)
  • 'x25519' (OID 1.3.101.110)
  • 'x448' (OID 1.3.101.111)
  • 'ed25519' (OID 1.3.101.112)
  • 'ed448' (OID 1.3.101.113)
  • 'dh' (OID 1.2.840.113549.1.3.1)

This property is undefined for unrecognized KeyObject types and symmetric keys.

keyObject.export([options])#

Added in: v11.6.0

For symmetric keys, this function allocates a Buffer containing the key material and ignores any options.

For asymmetric keys, the options parameter is used to determine the export format.

For public keys, the following encoding options can be used:

  • type: <string> Must be one of 'pkcs1' (RSA only) or 'spki'.
  • format: <string> Must be 'pem' or 'der'.

For private keys, the following encoding options can be used:

  • type: <string> Must be one of 'pkcs1' (RSA only), 'pkcs8' or 'sec1' (EC only).
  • format: <string> Must be 'pem' or 'der'.
  • cipher: <string> If specified, the private key will be encrypted with the given cipher and passphrase using PKCS#5 v2.0 password based encryption.
  • passphrase: <string> | <Buffer> The passphrase to use for encryption, see cipher.

When PEM encoding was selected, the result will be a string, otherwise it will be a buffer containing the data encoded as DER.

PKCS#1, SEC1, and PKCS#8 type keys can be encrypted by using a combination of the cipher and format options. The PKCS#8 type can be used with any format to encrypt any key algorithm (RSA, EC, or DH) by specifying a cipher. PKCS#1 and SEC1 can only be encrypted by specifying a cipher when the PEM format is used. For maximum compatibility, use PKCS#8 for encrypted private keys. Since PKCS#8 defines its own encryption mechanism, PEM-level encryption is not supported when encrypting a PKCS#8 key. See RFC 5208 for PKCS#8 encryption and RFC 1421 for PKCS#1 and SEC1 encryption.

keyObject.symmetricKeySize#

Added in: v11.6.0

For secret keys, this property represents the size of the key in bytes. This property is undefined for asymmetric keys.

keyObject.type#

Added in: v11.6.0

Depending on the type of this KeyObject, this property is either 'secret' for secret (symmetric) keys, 'public' for public (asymmetric) keys or 'private' for private (asymmetric) keys.

Class: Sign#

Added in: v0.1.92

The Sign class is a utility for generating signatures. It can be used in one of two ways:

  • As a writable stream, where data to be signed is written and the sign.sign() method is used to generate and return the signature, or
  • Using the sign.update() and sign.sign() methods to produce the signature.

The crypto.createSign() method is used to create Sign instances. The argument is the string name of the hash function to use. Sign objects are not to be created directly using the new keyword.

Example: Using Sign and Verify objects as streams:

const crypto = require('crypto');

const { privateKey, publicKey } = crypto.generateKeyPairSync('ec', {
  namedCurve: 'sect239k1'
});

const sign = crypto.createSign('SHA256');
sign.write('some data to sign');
sign.end();
const signature = sign.sign(privateKey, 'hex');

const verify = crypto.createVerify('SHA256');
verify.write('some data to sign');
verify.end();
console.log(verify.verify(publicKey, signature, 'hex'));
// Prints: true

Example: Using the sign.update() and verify.update() methods:

const crypto = require('crypto');

const { privateKey, publicKey } = crypto.generateKeyPairSync('rsa', {
  modulusLength: 2048,
});

const sign = crypto.createSign('SHA256');
sign.update('some data to sign');
sign.end();
const signature = sign.sign(privateKey);

const verify = crypto.createVerify('SHA256');
verify.update('some data to sign');
verify.end();
console.log(verify.verify(publicKey, signature));
// Prints: true

sign.sign(privateKey[, outputEncoding])#

History
VersionChanges
v12.0.0

This function now supports RSA-PSS keys.

v11.6.0

This function now supports key objects.

v8.0.0

Support for RSASSA-PSS and additional options was added.

v0.1.92

Added in: v0.1.92

Calculates the signature on all the data passed through using either sign.update() or sign.write().

If privateKey is not a KeyObject, this function behaves as if privateKey had been passed to crypto.createPrivateKey(). If it is an object, the following additional properties can be passed:

  • dsaEncoding <string> For DSA and ECDSA, this option specifies the format of the generated signature. It can be one of the following:

    • 'der' (default): DER-encoded ASN.1 signature structure encoding (r, s).
    • 'ieee-p1363': Signature format r || s as proposed in IEEE-P1363.

  • padding <integer> Optional padding value for RSA, one of the following:

    • crypto.constants.RSA_PKCS1_PADDING (default)
    • crypto.constants.RSA_PKCS1_PSS_PADDING

    RSA_PKCS1_PSS_PADDING will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055, unless an MGF1 hash function has been specified as part of the key in compliance with section 3.3 of RFC 4055.

  • saltLength <integer> Salt length for when padding is RSA_PKCS1_PSS_PADDING. The special value crypto.constants.RSA_PSS_SALTLEN_DIGEST sets the salt length to the digest size, crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN (default) sets it to the maximum permissible value.

If outputEncoding is provided a string is returned; otherwise a Buffer is returned.

The Sign object can not be again used after sign.sign() method has been called. Multiple calls to sign.sign() will result in an error being thrown.

sign.update(data[, inputEncoding])#

History
VersionChanges
v6.0.0

The default inputEncoding changed from binary to utf8.

v0.1.92

Added in: v0.1.92

Updates the Sign content with the given data, the encoding of which is given in inputEncoding. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer, TypedArray, or DataView, then inputEncoding is ignored.

This can be called many times with new data as it is streamed.

Class: Verify#

Added in: v0.1.92

The Verify class is a utility for verifying signatures. It can be used in one of two ways:

The crypto.createVerify() method is used to create Verify instances. Verify objects are not to be created directly using the new keyword.

See Sign for examples.

verify.update(data[, inputEncoding])#

History
VersionChanges
v6.0.0

The default inputEncoding changed from binary to utf8.

v0.1.92

Added in: v0.1.92

Updates the Verify content with the given data, the encoding of which is given in inputEncoding. If inputEncoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer, TypedArray, or DataView, then inputEncoding is ignored.

This can be called many times with new data as it is streamed.

verify.verify(object, signature[, signatureEncoding])#

History
VersionChanges
v12.0.0

This function now supports RSA-PSS keys.

v11.7.0

The key can now be a private key.

v8.0.0

Support for RSASSA-PSS and additional options was added.

v0.1.92

Added in: v0.1.92

Verifies the provided data using the given object and signature.

If object is not a KeyObject, this function behaves as if object had been passed to crypto.createPublicKey(). If it is an object, the following additional properties can be passed:

  • dsaEncoding <string> For DSA and ECDSA, this option specifies the format of the generated signature. It can be one of the following:

    • 'der' (default): DER-encoded ASN.1 signature structure encoding (r, s).
    • 'ieee-p1363': Signature format r || s as proposed in IEEE-P1363.

  • padding <integer> Optional padding value for RSA, one of the following:

    • crypto.constants.RSA_PKCS1_PADDING (default)
    • crypto.constants.RSA_PKCS1_PSS_PADDING

    RSA_PKCS1_PSS_PADDING will use MGF1 with the same hash function used to verify the message as specified in section 3.1 of RFC 4055, unless an MGF1 hash function has been specified as part of the key in compliance with section 3.3 of RFC 4055.

  • saltLength <integer> Salt length for when padding is RSA_PKCS1_PSS_PADDING. The special value crypto.constants.RSA_PSS_SALTLEN_DIGEST sets the salt length to the digest size, crypto.constants.RSA_PSS_SALTLEN_AUTO (default) causes it to be determined automatically.

The signature argument is the previously calculated signature for the data, in the signatureEncoding. If a signatureEncoding is specified, the signature is expected to be a string; otherwise signature is expected to be a Buffer, TypedArray, or DataView.

The verify object can not be used again after verify.verify() has been called. Multiple calls to verify.verify() will result in an error being thrown.

Because public keys can be derived from private keys, a private key may be passed instead of a public key.

crypto module methods and properties#

crypto.constants#

Added in: v6.3.0
  • Returns: <Object> An object containing commonly used constants for crypto and security related operations. The specific constants currently defined are described in Crypto Constants.

crypto.DEFAULT_ENCODING#

Added in: v0.9.3Deprecated since: v10.0.0

Stability: 0 - Deprecated

The default encoding to use for functions that can take either strings or buffers. The default value is 'buffer', which makes methods default to Buffer objects.

The crypto.DEFAULT_ENCODING mechanism is provided for backwards compatibility with legacy programs that expect 'latin1' to be the default encoding.

New applications should expect the default to be 'buffer'.

This property is deprecated.

crypto.fips#

Added in: v6.0.0Deprecated since: v10.0.0

Stability: 0 - Deprecated

Property for checking and controlling whether a FIPS compliant crypto provider is currently in use. Setting to true requires a FIPS build of Node.js.

This property is deprecated. Please use crypto.setFips() and crypto.getFips() instead.

crypto.createCipher(algorithm, password[, options])#

History
VersionChanges
v10.10.0

Ciphers in OCB mode are now supported.

v10.2.0

The authTagLength option can now be used to produce shorter authentication tags in GCM mode and defaults to 16 bytes.

v10.0.0

Deprecated since: v10.0.0

v0.1.94

Added in: v0.1.94

Stability: 0 - Deprecated: Use crypto.createCipheriv() instead.

Creates and returns a Cipher object that uses the given algorithm and password.

The options argument controls stream behavior and is optional except when a cipher in CCM or OCB mode is used (e.g. 'aes-128-ccm'). In that case, the authTagLength option is required and specifies the length of the authentication tag in bytes, see CCM mode. In GCM mode, the authTagLength option is not required but can be used to set the length of the authentication tag that will be returned by getAuthTag() and defaults to 16 bytes.

The algorithm is dependent on OpenSSL, examples are 'aes192', etc. On recent OpenSSL releases, openssl list -cipher-algorithms (openssl list-cipher-algorithms for older versions of OpenSSL) will display the available cipher algorithms.

The password is used to derive the cipher key and initialization vector (IV). The value must be either a 'latin1' encoded string, a Buffer, a TypedArray, or a DataView.

The implementation of crypto.createCipher() derives keys using the OpenSSL function EVP_BytesToKey with the digest algorithm set to MD5, one iteration, and no salt. The lack of salt allows dictionary attacks as the same password always creates the same key. The low iteration count and non-cryptographically secure hash algorithm allow passwords to be tested very rapidly.

In line with OpenSSL's recommendation to use a more modern algorithm instead of EVP_BytesToKey it is recommended that developers derive a key and IV on their own using crypto.scrypt() and to use crypto.createCipheriv() to create the Cipher object. Users should not use ciphers with counter mode (e.g. CTR, GCM, or CCM) in crypto.createCipher(). A warning is emitted when they are used in order to avoid the risk of IV reuse that causes vulnerabilities. For the case when IV is reused in GCM, see Nonce-Disrespecting Adversaries for details.

crypto.createCipheriv(algorithm, key, iv[, options])#

History
VersionChanges
v11.6.0

The key argument can now be a KeyObject.

v11.2.0, v10.17.0

The cipher chacha20-poly1305 is now supported.

v10.10.0

Ciphers in OCB mode are now supported.

v10.2.0

The authTagLength option can now be used to produce shorter authentication tags in GCM mode and defaults to 16 bytes.

v9.9.0

The iv parameter may now be null for ciphers which do not need an initialization vector.

v0.1.94

Added in: v0.1.94

Creates and returns a Cipher object, with the given algorithm, key and initialization vector (iv).

The options argument controls stream behavior and is optional except when a cipher in CCM or OCB mode is used (e.g. 'aes-128-ccm'). In that case, the authTagLength option is required and specifies the length of the authentication tag in bytes, see CCM mode. In GCM mode, the authTagLength option is not required but can be used to set the length of the authentication tag that will be returned by getAuthTag() and defaults to 16 bytes.

The algorithm is dependent on OpenSSL, examples are 'aes192', etc. On recent OpenSSL releases, openssl list -cipher-algorithms (openssl list-cipher-algorithms for older versions of OpenSSL) will display the available cipher algorithms.

The key is the raw key used by the algorithm and iv is an initialization vector. Both arguments must be 'utf8' encoded strings, Buffers, TypedArray, or DataViews. The key may optionally be a KeyObject of type secret. If the cipher does not need an initialization vector, iv may be null.

Initialization vectors should be unpredictable and unique; ideally, they will be cryptographically random. They do not have to be secret: IVs are typically just added to ciphertext messages unencrypted. It may sound contradictory that something has to be unpredictable and unique, but does not have to be secret; remember that an attacker must not be able to predict ahead of time what a given IV will be.

crypto.createDecipher(algorithm, password[, options])#

History
VersionChanges
v10.10.0

Ciphers in OCB mode are now supported.

v10.0.0

Deprecated since: v10.0.0

v0.1.94

Added in: v0.1.94

Stability: 0 - Deprecated: Use crypto.createDecipheriv() instead.

Creates and returns a Decipher object that uses the given algorithm and password (key).

The options argument controls stream behavior and is optional except when a cipher in CCM or OCB mode is used (e.g. 'aes-128-ccm'). In that case, the authTagLength option is required and specifies the length of the authentication tag in bytes, see CCM mode.

The implementation of crypto.createDecipher() derives keys using the OpenSSL function EVP_BytesToKey with the digest algorithm set to MD5, one iteration, and no salt. The lack of salt allows dictionary attacks as the same password always creates the same key. The low iteration count and non-cryptographically secure hash algorithm allow passwords to be tested very rapidly.

In line with OpenSSL's recommendation to use a more modern algorithm instead of EVP_BytesToKey it is recommended that developers derive a key and IV on their own using crypto.scrypt() and to use crypto.createDecipheriv() to create the Decipher object.

crypto.createDecipheriv(algorithm, key, iv[, options])#

History
VersionChanges
v11.6.0

The key argument can now be a KeyObject.

v11.2.0, v10.17.0

The cipher chacha20-poly1305 is now supported.

v10.10.0

Ciphers in OCB mode are now supported.

v10.2.0

The authTagLength option can now be used to restrict accepted GCM authentication tag lengths.

v9.9.0

The iv parameter may now be null for ciphers which do not need an initialization vector.

v0.1.94

Added in: v0.1.94

Creates and returns a Decipher object that uses the given algorithm, key and initialization vector (iv).

The options argument controls stream behavior and is optional except when a cipher in CCM or OCB mode is used (e.g. 'aes-128-ccm'). In that case, the authTagLength option is required and specifies the length of the authentication tag in bytes, see CCM mode. In GCM mode, the authTagLength option is not required but can be used to restrict accepted authentication tags to those with the specified length.

The algorithm is dependent on OpenSSL, examples are 'aes192', etc. On recent OpenSSL releases, openssl list -cipher-algorithms (openssl list-cipher-algorithms for older versions of OpenSSL) will display the available cipher algorithms.

The key is the raw key used by the algorithm and iv is an initialization vector. Both arguments must be 'utf8' encoded strings, Buffers, TypedArray, or DataViews. The key may optionally be a KeyObject of type secret. If the cipher does not need an initialization vector, iv may be null.

Initialization vectors should be unpredictable and unique; ideally, they will be cryptographically random. They do not have to be secret: IVs are typically just added to ciphertext messages unencrypted. It may sound contradictory that something has to be unpredictable and unique, but does not have to be secret; remember that an attacker must not be able to predict ahead of time what a given IV will be.

crypto.createDiffieHellman(prime[, primeEncoding][, generator][, generatorEncoding])#

History
VersionChanges
v8.0.0

The prime argument can be any TypedArray or DataView now.

v8.0.0

The prime argument can be a Uint8Array now.

v6.0.0

The default for the encoding parameters changed from binary to utf8.

v0.11.12

Added in: v0.11.12

Creates a DiffieHellman key exchange object using the supplied prime and an optional specific generator.

The generator argument can be a number, string, or Buffer. If generator is not specified, the value 2 is used.

If primeEncoding is specified, prime is expected to be a string; otherwise a Buffer, TypedArray, or DataView is expected.

If generatorEncoding is specified, generator is expected to be a string; otherwise a number, Buffer, TypedArray, or DataView is expected.

crypto.createDiffieHellman(primeLength[, generator])#

Added in: v0.5.0

Creates a DiffieHellman key exchange object and generates a prime of primeLength bits using an optional specific numeric generator. If generator is not specified, the value 2 is used.

crypto.createDiffieHellmanGroup(name)#

Added in: v0.9.3

An alias for crypto.getDiffieHellman()

crypto.createECDH(curveName)#

Added in: v0.11.14

Creates an Elliptic Curve Diffie-Hellman (ECDH) key exchange object using a predefined curve specified by the curveName string. Use crypto.getCurves() to obtain a list of available curve names. On recent OpenSSL releases, openssl ecparam -list_curves will also display the name and description of each available elliptic curve.

crypto.createHash(algorithm[, options])#

History
VersionChanges
v12.8.0

The outputLength option was added for XOF hash functions.

v0.1.92

Added in: v0.1.92

Creates and returns a Hash object that can be used to generate hash digests using the given algorithm. Optional options argument controls stream behavior. For XOF hash functions such as 'shake256', the outputLength option can be used to specify the desired output length in bytes.

The algorithm is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are 'sha256', 'sha512', etc. On recent releases of OpenSSL, openssl list -digest-algorithms (openssl list-message-digest-algorithms for older versions of OpenSSL) will display the available digest algorithms.

Example: generating the sha256 sum of a file

const filename = process.argv[2];
const crypto = require('crypto');
const fs = require('fs');

const hash = crypto.createHash('sha256');

const input = fs.createReadStream(filename);
input.on('readable', () => {
  // Only one element is going to be produced by the
  // hash stream.
  const data = input.read();
  if (data)
    hash.update(data);
  else {
    console.log(`${hash.digest('hex')} ${filename}`);
  }
});

crypto.createHmac(algorithm, key[, options])#

History
VersionChanges
v11.6.0

The key argument can now be a KeyObject.

v0.1.94

Added in: v0.1.94

Creates and returns an Hmac object that uses the given algorithm and key. Optional options argument controls stream behavior.

The algorithm is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are 'sha256', 'sha512', etc. On recent releases of OpenSSL, openssl list -digest-algorithms (openssl list-message-digest-algorithms for older versions of OpenSSL) will display the available digest algorithms.

The key is the HMAC key used to generate the cryptographic HMAC hash. If it is a KeyObject, its type must be secret.

Example: generating the sha256 HMAC of a file

const filename = process.argv[2];
const crypto = require('crypto');
const fs = require('fs');

const hmac = crypto.createHmac('sha256', 'a secret');

const input = fs.createReadStream(filename);
input.on('readable', () => {
  // Only one element is going to be produced by the
  // hash stream.
  const data = input.read();
  if (data)
    hmac.update(data);
  else {
    console.log(`${hmac.digest('hex')} ${filename}`);
  }
});

crypto.createPrivateKey(key)#

Added in: v11.6.0

Creates and returns a new key object containing a private key. If key is a string or Buffer, format is assumed to be 'pem'; otherwise, key must be an object with the properties described above.

If the private key is encrypted, a passphrase must be specified. The length of the passphrase is limited to 1024 bytes.

crypto.createPublicKey(key)#

History
VersionChanges
v11.13.0

The key argument can now be a KeyObject with type private.

v11.7.0

The key argument can now be a private key.

v11.6.0

Added in: v11.6.0

Creates and returns a new key object containing a public key. If key is a string or Buffer, format is assumed to be 'pem'; if key is a KeyObject with type 'private', the public key is derived from the given private key; otherwise, key must be an object with the properties described above.

If the format is 'pem', the 'key' may also be an X.509 certificate.

Because public keys can be derived from private keys, a private key may be passed instead of a public key. In that case, this function behaves as if crypto.createPrivateKey() had been called, except that the type of the returned KeyObject will be 'public' and that the private key cannot be extracted from the returned KeyObject. Similarly, if a KeyObject with type 'private' is given, a new KeyObject with type 'public' will be returned and it will be impossible to extract the private key from the returned object.

crypto.createSecretKey(key)#

Added in: v11.6.0

Creates and returns a new key object containing a secret key for symmetric encryption or Hmac.

crypto.createSign(algorithm[, options])#

Added in: v0.1.92

Creates and returns a Sign object that uses the given algorithm. Use crypto.getHashes() to obtain the names of the available digest algorithms. Optional options argument controls the stream.Writable behavior.

In some cases, a Sign instance can be created using the name of a signature algorithm, such as 'RSA-SHA256', instead of a digest algorithm. This will use the corresponding digest algorithm. This does not work for all signature algorithms, such as 'ecdsa-with-SHA256', so it is best to always use digest algorithm names.

crypto.createVerify(algorithm[, options])#

Added in: v0.1.92

Creates and returns a Verify object that uses the given algorithm. Use crypto.getHashes() to obtain an array of names of the available signing algorithms. Optional options argument controls the stream.Writable behavior.

In some cases, a Verify instance can be created using the name of a signature algorithm, such as 'RSA-SHA256', instead of a digest algorithm. This will use the corresponding digest algorithm. This does not work for all signature algorithms, such as 'ecdsa-with-SHA256', so it is best to always use digest algorithm names.

crypto.diffieHellman(options)#

Added in: v13.9.0

Computes the Diffie-Hellman secret based on a privateKey and a publicKey. Both keys must have the same asymmetricKeyType, which must be one of 'dh' (for Diffie-Hellman), 'ec' (for ECDH), 'x448', or 'x25519' (for ECDH-ES).

crypto.generateKeyPair(type, options, callback)#

History
VersionChanges
v13.9.0

Add support for Diffie-Hellman.

v12.0.0

Add ability to generate X25519 and X448 key pairs.

v12.0.0

Add ability to generate Ed25519 and Ed448 key pairs.

v11.6.0

The generateKeyPair and generateKeyPairSync functions now produce key objects if no encoding was specified.

v10.12.0

Added in: v10.12.0

Generates a new asymmetric key pair of the given type. RSA, DSA, EC, Ed25519, Ed448, X25519, X448, and DH are currently supported.

If a publicKeyEncoding or privateKeyEncoding was specified, this function behaves as if keyObject.export() had been called on its result. Otherwise, the respective part of the key is returned as a KeyObject.

It is recommended to encode public keys as 'spki' and private keys as 'pkcs8' with encryption for long-term storage:

const { generateKeyPair } = require('crypto');
generateKeyPair('rsa', {
  modulusLength: 4096,
  publicKeyEncoding: {
    type: 'spki',
    format: 'pem'
  },
  privateKeyEncoding: {
    type: 'pkcs8',
    format: 'pem',
    cipher: 'aes-256-cbc',
    passphrase: 'top secret'
  }
}, (err, publicKey, privateKey) => {
  // Handle errors and use the generated key pair.
});

On completion, callback will be called with err set to undefined and publicKey / privateKey representing the generated key pair.

If this method is invoked as its util.promisify()ed version, it returns a Promise for an Object with publicKey and privateKey properties.

crypto.generateKeyPairSync(type, options)#

History
VersionChanges
v13.9.0

Add support for Diffie-Hellman.

v12.0.0

Add ability to generate Ed25519 and Ed448 key pairs.

v11.6.0

The generateKeyPair and generateKeyPairSync functions now produce key objects if no encoding was specified.

v10.12.0

Added in: v10.12.0

Generates a new asymmetric key pair of the given type. RSA, DSA, EC, Ed25519, Ed448, X25519, X448, and DH are currently supported.

If a publicKeyEncoding or privateKeyEncoding was specified, this function behaves as if keyObject.export() had been called on its result. Otherwise, the respective part of the key is returned as a KeyObject.

When encoding public keys, it is recommended to use 'spki'. When encoding private keys, it is recommended to use 'pkcs8' with a strong passphrase, and to keep the passphrase confidential.

const { generateKeyPairSync } = require('crypto');
const { publicKey, privateKey } = generateKeyPairSync('rsa', {
  modulusLength: 4096,
  publicKeyEncoding: {
    type: 'spki',
    format: 'pem'
  },
  privateKeyEncoding: {
    type: 'pkcs8',
    format: 'pem',
    cipher: 'aes-256-cbc',
    passphrase: 'top secret'
  }
});

The return value { publicKey, privateKey } represents the generated key pair. When PEM encoding was selected, the respective key will be a string, otherwise it will be a buffer containing the data encoded as DER.

crypto.getCiphers()#

Added in: v0.9.3
  • Returns: <string[]> An array with the names of the supported cipher algorithms.
const ciphers = crypto.getCiphers();
console.log(ciphers); // ['aes-128-cbc', 'aes-128-ccm', ...]

crypto.getCurves()#

Added in: v2.3.0
  • Returns: <string[]> An array with the names of the supported elliptic curves.
const curves = crypto.getCurves();
console.log(curves); // ['Oakley-EC2N-3', 'Oakley-EC2N-4', ...]

crypto.getDiffieHellman(groupName)#

Added in: v0.7.5

Creates a predefined DiffieHellmanGroup key exchange object. The supported groups are: 'modp1', 'modp2', 'modp5' (defined in RFC 2412, but see Caveats) and 'modp14', 'modp15', 'modp16', 'modp17', 'modp18' (defined in RFC 3526). The returned object mimics the interface of objects created by crypto.createDiffieHellman(), but will not allow changing the keys (with diffieHellman.setPublicKey(), for example). The advantage of using this method is that the parties do not have to generate nor exchange a group modulus beforehand, saving both processor and communication time.

Example (obtaining a shared secret):

const crypto = require('crypto');
const alice = crypto.getDiffieHellman('modp14');
const bob = crypto.getDiffieHellman('modp14');

alice.generateKeys();
bob.generateKeys();

const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

/* aliceSecret and bobSecret should be the same */
console.log(aliceSecret === bobSecret);

crypto.getFips()#

Added in: v10.0.0
  • Returns: <number> 1 if and only if a FIPS compliant crypto provider is currently in use, 0 otherwise. A future semver-major release may change the return type of this API to a <boolean>.

crypto.getHashes()#

Added in: v0.9.3
  • Returns: <string[]> An array of the names of the supported hash algorithms, such as 'RSA-SHA256'. Hash algorithms are also called "digest" algorithms.
const hashes = crypto.getHashes();
console.log(hashes); // ['DSA', 'DSA-SHA', 'DSA-SHA1', ...]

crypto.pbkdf2(password, salt, iterations, keylen, digest, callback)#

History
VersionChanges
v14.0.0

The iterations parameter is now restricted to positive values. Earlier releases treated other values as one.

v8.0.0

The digest parameter is always required now.

v6.0.0

Calling this function without passing the digest parameter is deprecated now and will emit a warning.

v6.0.0

The default encoding for password if it is a string changed from binary to utf8.

v0.5.5

Added in: v0.5.5

Provides an asynchronous Password-Based Key Derivation Function 2 (PBKDF2) implementation. A selected HMAC digest algorithm specified by digest is applied to derive a key of the requested byte length (keylen) from the password, salt and iterations.

The supplied callback function is called with two arguments: err and derivedKey. If an error occurs while deriving the key, err will be set; otherwise err will be null. By default, the successfully generated derivedKey will be passed to the callback as a Buffer. An error will be thrown if any of the input arguments specify invalid values or types.

If digest is null, 'sha1' will be used. This behavior is deprecated, please specify a digest explicitly.

The iterations argument must be a number set as high as possible. The higher the number of iterations, the more secure the derived key will be, but will take a longer amount of time to complete.

The salt should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details.

const crypto = require('crypto');
crypto.pbkdf2('secret', 'salt', 100000, 64, 'sha512', (err, derivedKey) => {
  if (err) throw err;
  console.log(derivedKey.toString('hex'));  // '3745e48...08d59ae'
});

The crypto.DEFAULT_ENCODING property can be used to change the way the derivedKey is passed to the callback. This property, however, has been deprecated and use should be avoided.

const crypto = require('crypto');
crypto.DEFAULT_ENCODING = 'hex';
crypto.pbkdf2('secret', 'salt', 100000, 512, 'sha512', (err, derivedKey) => {
  if (err) throw err;
  console.log(derivedKey);  // '3745e48...aa39b34'
});

An array of supported digest functions can be retrieved using crypto.getHashes().

This API uses libuv's threadpool, which can have surprising and negative performance implications for some applications; see the UV_THREADPOOL_SIZE documentation for more information.

crypto.pbkdf2Sync(password, salt, iterations, keylen, digest)#

History
VersionChanges
v14.0.0

The iterations parameter is now restricted to positive values. Earlier releases treated other values as one.

v6.0.0

Calling this function without passing the digest parameter is deprecated now and will emit a warning.

v6.0.0

The default encoding for password if it is a string changed from binary to utf8.

v0.9.3

Added in: v0.9.3

Provides a synchronous Password-Based Key Derivation Function 2 (PBKDF2) implementation. A selected HMAC digest algorithm specified by digest is applied to derive a key of the requested byte length (keylen) from the password, salt and iterations.

If an error occurs an Error will be thrown, otherwise the derived key will be returned as a Buffer.

If digest is null, 'sha1' will be used. This behavior is deprecated, please specify a digest explicitly.

The iterations argument must be a number set as high as possible. The higher the number of iterations, the more secure the derived key will be, but will take a longer amount of time to complete.

The salt should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details.

const crypto = require('crypto');
const key = crypto.pbkdf2Sync('secret', 'salt', 100000, 64, 'sha512');
console.log(key.toString('hex'));  // '3745e48...08d59ae'

The crypto.DEFAULT_ENCODING property may be used to change the way the derivedKey is returned. This property, however, is deprecated and use should be avoided.

const crypto = require('crypto');
crypto.DEFAULT_ENCODING = 'hex';
const key = crypto.pbkdf2Sync('secret', 'salt', 100000, 512, 'sha512');
console.log(key);  // '3745e48...aa39b34'

An array of supported digest functions can be retrieved using crypto.getHashes().

crypto.privateDecrypt(privateKey, buffer)#

History
VersionChanges
v12.11.0

The oaepLabel option was added.

v12.9.0

The oaepHash option was added.

v11.6.0

This function now supports key objects.

v0.11.14

Added in: v0.11.14

Decrypts buffer with privateKey. buffer was previously encrypted using the corresponding public key, for example using crypto.publicEncrypt().

If privateKey is not a KeyObject, this function behaves as if privateKey had been passed to crypto.createPrivateKey(). If it is an object, the padding property can be passed. Otherwise, this function uses RSA_PKCS1_OAEP_PADDING.

crypto.privateEncrypt(privateKey, buffer)#

History
VersionChanges
v11.6.0

This function now supports key objects.

v1.1.0

Added in: v1.1.0

Encrypts buffer with privateKey. The returned data can be decrypted using the corresponding public key, for example using crypto.publicDecrypt().

If privateKey is not a KeyObject, this function behaves as if privateKey had been passed to crypto.createPrivateKey(). If it is an object, the padding property can be passed. Otherwise, this function uses RSA_PKCS1_PADDING.

crypto.publicDecrypt(key, buffer)#

History
VersionChanges
v11.6.0

This function now supports key objects.

v1.1.0

Added in: v1.1.0

Decrypts buffer with key.buffer was previously encrypted using the corresponding private key, for example using crypto.privateEncrypt().

If key is not a KeyObject, this function behaves as if key had been passed to crypto.createPublicKey(). If it is an object, the padding property can be passed. Otherwise, this function uses RSA_PKCS1_PADDING.

Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key.

crypto.publicEncrypt(key, buffer)#

History
VersionChanges
v12.11.0

The oaepLabel option was added.

v12.9.0

The oaepHash option was added.

v11.6.0

This function now supports key objects.

v0.11.14

Added in: v0.11.14

Encrypts the content of buffer with key and returns a new Buffer with encrypted content. The returned data can be decrypted using the corresponding private key, for example using crypto.privateDecrypt().

If key is not a KeyObject, this function behaves as if key had been passed to crypto.createPublicKey(). If it is an object, the padding property can be passed. Otherwise, this function uses RSA_PKCS1_OAEP_PADDING.

Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key.

crypto.randomBytes(size[, callback])#

History
VersionChanges
v9.0.0

Passing null as the callback argument now throws ERR_INVALID_CALLBACK.

v0.5.8

Added in: v0.5.8

Generates cryptographically strong pseudo-random data. The size argument is a number indicating the number of bytes to generate.

If a callback function is provided, the bytes are generated asynchronously and the callback function is invoked with two arguments: err and buf. If an error occurs, err will be an Error object; otherwise it is null. The buf argument is a Buffer containing the generated bytes.

// Asynchronous
const crypto = require('crypto');
crypto.randomBytes(256, (err, buf) => {
  if (err) throw err;
  console.log(`${buf.length} bytes of random data: ${buf.toString('hex')}`);
});

If the callback function is not provided, the random bytes are generated synchronously and returned as a Buffer. An error will be thrown if there is a problem generating the bytes.

// Synchronous
const buf = crypto.randomBytes(256);
console.log(
  `${buf.length} bytes of random data: ${buf.toString('hex')}`);

The crypto.randomBytes() method will not complete until there is sufficient entropy available. This should normally never take longer than a few milliseconds. The only time when generating the random bytes may conceivably block for a longer period of time is right after boot, when the whole system is still low on entropy.

This API uses libuv's threadpool, which can have surprising and negative performance implications for some applications; see the UV_THREADPOOL_SIZE documentation for more information.

The asynchronous version of crypto.randomBytes() is carried out in a single threadpool request. To minimize threadpool task length variation, partition large randomBytes requests when doing so as part of fulfilling a client request.

crypto.randomFillSync(buffer[, offset][, size])#

History
VersionChanges
v9.0.0

The buffer argument may be any TypedArray or DataView.

v7.10.0, v6.13.0

Added in: v7.10.0, v6.13.0

Synchronous version of crypto.randomFill().

const buf = Buffer.alloc(10);
console.log(crypto.randomFillSync(buf).toString('hex'));

crypto.randomFillSync(buf, 5);
console.log(buf.toString('hex'));

// The above is equivalent to the following:
crypto.randomFillSync(buf, 5, 5);
console.log(buf.toString('hex'));

Any TypedArray or DataView instance may be passed as buffer.

const a = new Uint32Array(10);
console.log(Buffer.from(crypto.randomFillSync(a).buffer,
                        a.byteOffset, a.byteLength).toString('hex'));

const b = new Float64Array(10);
console.log(Buffer.from(crypto.randomFillSync(b).buffer,
                        b.byteOffset, b.byteLength).toString('hex'));

const c = new DataView(new ArrayBuffer(10));
console.log(Buffer.from(crypto.randomFillSync(c).buffer,
                        c.byteOffset, c.byteLength).toString('hex'));

crypto.randomFill(buffer[, offset][, size], callback)#

History
VersionChanges
v9.0.0

The buffer argument may be any TypedArray or DataView.

v7.10.0, v6.13.0

Added in: v7.10.0, v6.13.0

This function is similar to crypto.randomBytes() but requires the first argument to be a Buffer that will be filled. It also requires that a callback is passed in.

If the callback function is not provided, an error will be thrown.

const buf = Buffer.alloc(10);
crypto.randomFill(buf, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

crypto.randomFill(buf, 5, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

// The above is equivalent to the following:
crypto.randomFill(buf, 5, 5, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

Any TypedArray or DataView instance may be passed as buffer.

const a = new Uint32Array(10);
crypto.randomFill(a, (err, buf) => {
  if (err) throw err;
  console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
    .toString('hex'));
});

const b = new Float64Array(10);
crypto.randomFill(b, (err, buf) => {
  if (err) throw err;
  console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
    .toString('hex'));
});

const c = new DataView(new ArrayBuffer(10));
crypto.randomFill(c, (err, buf) => {
  if (err) throw err;
  console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength)
    .toString('hex'));
});

This API uses libuv's threadpool, which can have surprising and negative performance implications for some applications; see the UV_THREADPOOL_SIZE documentation for more information.

The asynchronous version of crypto.randomFill() is carried out in a single threadpool request. To minimize threadpool task length variation, partition large randomFill requests when doing so as part of fulfilling a client request.

crypto.scrypt(password, salt, keylen[, options], callback)#

History
VersionChanges
v12.8.0, v10.17.0

The maxmem value can now be any safe integer.

v10.9.0

The cost, blockSize and parallelization option names have been added.

v10.5.0

Added in: v10.5.0

Provides an asynchronous scrypt implementation. Scrypt is a password-based key derivation function that is designed to be expensive computationally and memory-wise in order to make brute-force attacks unrewarding.

The salt should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details.

The callback function is called with two arguments: err and derivedKey. err is an exception object when key derivation fails, otherwise err is null. derivedKey is passed to the callback as a Buffer.

An exception is thrown when any of the input arguments specify invalid values or types.

const crypto = require('crypto');
// Using the factory defaults.
crypto.scrypt('secret', 'salt', 64, (err, derivedKey) => {
  if (err) throw err;
  console.log(derivedKey.toString('hex'));  // '3745e48...08d59ae'
});
// Using a custom N parameter. Must be a power of two.
crypto.scrypt('secret', 'salt', 64, { N: 1024 }, (err, derivedKey) => {
  if (err) throw err;
  console.log(derivedKey.toString('hex'));  // '3745e48...aa39b34'
});

crypto.scryptSync(password, salt, keylen[, options])#

History
VersionChanges
v12.8.0, v10.17.0

The maxmem value can now be any safe integer.

v10.9.0

The cost, blockSize and parallelization option names have been added.

v10.5.0

Added in: v10.5.0

Provides a synchronous scrypt implementation. Scrypt is a password-based key derivation function that is designed to be expensive computationally and memory-wise in order to make brute-force attacks unrewarding.

The salt should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details.

An exception is thrown when key derivation fails, otherwise the derived key is returned as a Buffer.

An exception is thrown when any of the input arguments specify invalid values or types.

const crypto = require('crypto');
// Using the factory defaults.
const key1 = crypto.scryptSync('secret', 'salt', 64);
console.log(key1.toString('hex'));  // '3745e48...08d59ae'
// Using a custom N parameter. Must be a power of two.
const key2 = crypto.scryptSync('secret', 'salt', 64, { N: 1024 });
console.log(key2.toString('hex'));  // '3745e48...aa39b34'

crypto.setEngine(engine[, flags])#

Added in: v0.11.11

Load and set the engine for some or all OpenSSL functions (selected by flags).

engine could be either an id or a path to the engine's shared library.

The optional flags argument uses ENGINE_METHOD_ALL by default. The flags is a bit field taking one of or a mix of the following flags (defined in crypto.constants):

  • crypto.constants.ENGINE_METHOD_RSA
  • crypto.constants.ENGINE_METHOD_DSA
  • crypto.constants.ENGINE_METHOD_DH
  • crypto.constants.ENGINE_METHOD_RAND
  • crypto.constants.ENGINE_METHOD_EC
  • crypto.constants.ENGINE_METHOD_CIPHERS
  • crypto.constants.ENGINE_METHOD_DIGESTS
  • crypto.constants.ENGINE_METHOD_PKEY_METHS
  • crypto.constants.ENGINE_METHOD_PKEY_ASN1_METHS
  • crypto.constants.ENGINE_METHOD_ALL
  • crypto.constants.ENGINE_METHOD_NONE

The flags below are deprecated in OpenSSL-1.1.0.

  • crypto.constants.ENGINE_METHOD_ECDH
  • crypto.constants.ENGINE_METHOD_ECDSA
  • crypto.constants.ENGINE_METHOD_STORE

crypto.setFips(bool)#

Added in: v10.0.0

Enables the FIPS compliant crypto provider in a FIPS-enabled Node.js build. Throws an error if FIPS mode is not available.

crypto.sign(algorithm, data, key)#

Added in: v12.0.0

Calculates and returns the signature for data using the given private key and algorithm. If algorithm is null or undefined, then the algorithm is dependent upon the key type (especially Ed25519 and Ed448).

If key is not a KeyObject, this function behaves as if key had been passed to crypto.createPrivateKey(). If it is an object, the following additional properties can be passed:

  • dsaEncoding <string> For DSA and ECDSA, this option specifies the format of the generated signature. It can be one of the following:

    • 'der' (default): DER-encoded ASN.1 signature structure encoding (r, s).
    • 'ieee-p1363': Signature format r || s as proposed in IEEE-P1363.

  • padding <integer> Optional padding value for RSA, one of the following:

    • crypto.constants.RSA_PKCS1_PADDING (default)
    • crypto.constants.RSA_PKCS1_PSS_PADDING

    RSA_PKCS1_PSS_PADDING will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055.

  • saltLength <integer> Salt length for when padding is RSA_PKCS1_PSS_PADDING. The special value crypto.constants.RSA_PSS_SALTLEN_DIGEST sets the salt length to the digest size, crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN (default) sets it to the maximum permissible value.

crypto.timingSafeEqual(a, b)#

Added in: v6.6.0

This function is based on a constant-time algorithm. Returns true if a is equal to b, without leaking timing information that would allow an attacker to guess one of the values. This is suitable for comparing HMAC digests or secret values like authentication cookies or capability urls.

a and b must both be Buffers, TypedArrays, or DataViews, and they must have the same length.

Use of crypto.timingSafeEqual does not guarantee that the surrounding code is timing-safe. Care should be taken to ensure that the surrounding code does not introduce timing vulnerabilities.

crypto.verify(algorithm, data, key, signature)#

Added in: v12.0.0

Verifies the given signature for data using the given key and algorithm. If algorithm is null or undefined, then the algorithm is dependent upon the key type (especially Ed25519 and Ed448).

If key is not a KeyObject, this function behaves as if key had been passed to crypto.createPublicKey(). If it is an object, the following additional properties can be passed:

  • dsaEncoding <string> For DSA and ECDSA, this option specifies the format of the generated signature. It can be one of the following:

    • 'der' (default): DER-encoded ASN.1 signature structure encoding (r, s).
    • 'ieee-p1363': Signature format r || s as proposed in IEEE-P1363.

  • padding <integer> Optional padding value for RSA, one of the following:

    • crypto.constants.RSA_PKCS1_PADDING (default)
    • crypto.constants.RSA_PKCS1_PSS_PADDING

    RSA_PKCS1_PSS_PADDING will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055.

  • saltLength <integer> Salt length for when padding is RSA_PKCS1_PSS_PADDING. The special value crypto.constants.RSA_PSS_SALTLEN_DIGEST sets the salt length to the digest size, crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN (default) sets it to the maximum permissible value.

The signature argument is the previously calculated signature for the data.

Because public keys can be derived from private keys, a private key or a public key may be passed for key.

Notes#

Legacy Streams API (pre Node.js v0.10)#

The Crypto module was added to Node.js before there was the concept of a unified Stream API, and before there were Buffer objects for handling binary data. As such, the many of the crypto defined classes have methods not typically found on other Node.js classes that implement the streams API (e.g. update(), final(), or digest()). Also, many methods accepted and returned 'latin1' encoded strings by default rather than Buffers. This default was changed after Node.js v0.8 to use Buffer objects by default instead.

Recent ECDH Changes#

Usage of ECDH with non-dynamically generated key pairs has been simplified. Now, ecdh.setPrivateKey() can be called with a preselected private key and the associated public point (key) will be computed and stored in the object. This allows code to only store and provide the private part of the EC key pair. ecdh.setPrivateKey() now also validates that the private key is valid for the selected curve.

The ecdh.setPublicKey() method is now deprecated as its inclusion in the API is not useful. Either a previously stored private key should be set, which automatically generates the associated public key, or ecdh.generateKeys() should be called. The main drawback of using ecdh.setPublicKey() is that it can be used to put the ECDH key pair into an inconsistent state.

Support for weak or compromised algorithms#

The crypto module still supports some algorithms which are already compromised and are not currently recommended for use. The API also allows the use of ciphers and hashes with a small key size that are too weak for safe use.

Users should take full responsibility for selecting the crypto algorithm and key size according to their security requirements.

Based on the recommendations of NIST SP 800-131A:

  • MD5 and SHA-1 are no longer acceptable where collision resistance is required such as digital signatures.
  • The key used with RSA, DSA, and DH algorithms is recommended to have at least 2048 bits and that of the curve of ECDSA and ECDH at least 224 bits, to be safe to use for several years.
  • The DH groups of modp1, modp2 and modp5 have a key size smaller than 2048 bits and are not recommended.

See the reference for other recommendations and details.

CCM mode#

CCM is one of the supported AEAD algorithms. Applications which use this mode must adhere to certain restrictions when using the cipher API:

  • The authentication tag length must be specified during cipher creation by setting the authTagLength option and must be one of 4, 6, 8, 10, 12, 14 or 16 bytes.
  • The length of the initialization vector (nonce) N must be between 7 and 13 bytes (7 ≤ N ≤ 13).
  • The length of the plaintext is limited to 2 ** (8 * (15 - N)) bytes.
  • When decrypting, the authentication tag must be set via setAuthTag() before calling update(). Otherwise, decryption will fail and final() will throw an error in compliance with section 2.6 of RFC 3610.
  • Using stream methods such as write(data), end(data) or pipe() in CCM mode might fail as CCM cannot handle more than one chunk of data per instance.
  • When passing additional authenticated data (AAD), the length of the actual message in bytes must be passed to setAAD() via the plaintextLength option. This is not necessary if no AAD is used.
  • As CCM processes the whole message at once, update() can only be called once.
  • Even though calling update() is sufficient to encrypt/decrypt the message, applications must call final() to compute or verify the authentication tag.
const crypto = require('crypto');

const key = 'keykeykeykeykeykeykeykey';
const nonce = crypto.randomBytes(12);

const aad = Buffer.from('0123456789', 'hex');

const cipher = crypto.createCipheriv('aes-192-ccm', key, nonce, {
  authTagLength: 16
});
const plaintext = 'Hello world';
cipher.setAAD(aad, {
  plaintextLength: Buffer.byteLength(plaintext)
});
const ciphertext = cipher.update(plaintext, 'utf8');
cipher.final();
const tag = cipher.getAuthTag();

// Now transmit { ciphertext, nonce, tag }.

const decipher = crypto.createDecipheriv('aes-192-ccm', key, nonce, {
  authTagLength: 16
});
decipher.setAuthTag(tag);
decipher.setAAD(aad, {
  plaintextLength: ciphertext.length
});
const receivedPlaintext = decipher.update(ciphertext, null, 'utf8');

try {
  decipher.final();
} catch (err) {
  console.error('Authentication failed!');
  return;
}

console.log(receivedPlaintext);

Crypto Constants#

The following constants exported by crypto.constants apply to various uses of the crypto, tls, and https modules and are generally specific to OpenSSL.

OpenSSL Options#

Constant Description
SSL_OP_ALL Applies multiple bug workarounds within OpenSSL. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html for detail.
SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION Allows legacy insecure renegotiation between OpenSSL and unpatched clients or servers. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html.
SSL_OP_CIPHER_SERVER_PREFERENCE Attempts to use the server's preferences instead of the client's when selecting a cipher. Behavior depends on protocol version. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html.
SSL_OP_CISCO_ANYCONNECT Instructs OpenSSL to use Cisco's "speshul" version of DTLS_BAD_VER.
SSL_OP_COOKIE_EXCHANGE Instructs OpenSSL to turn on cookie exchange.
SSL_OP_CRYPTOPRO_TLSEXT_BUG Instructs OpenSSL to add server-hello extension from an early version of the cryptopro draft.
SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS Instructs OpenSSL to disable a SSL 3.0/TLS 1.0 vulnerability workaround added in OpenSSL 0.9.6d.
SSL_OP_EPHEMERAL_RSA Instructs OpenSSL to always use the tmp_rsa key when performing RSA operations.
SSL_OP_LEGACY_SERVER_CONNECT Allows initial connection to servers that do not support RI.
SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
SSL_OP_MICROSOFT_SESS_ID_BUG
SSL_OP_MSIE_SSLV2_RSA_PADDING Instructs OpenSSL to disable the workaround for a man-in-the-middle protocol-version vulnerability in the SSL 2.0 server implementation.
SSL_OP_NETSCAPE_CA_DN_BUG
SSL_OP_NETSCAPE_CHALLENGE_BUG
SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
SSL_OP_NO_COMPRESSION Instructs OpenSSL to disable support for SSL/TLS compression.
SSL_OP_NO_QUERY_MTU
SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION Instructs OpenSSL to always start a new session when performing renegotiation.
SSL_OP_NO_SSLv2 Instructs OpenSSL to turn off SSL v2
SSL_OP_NO_SSLv3 Instructs OpenSSL to turn off SSL v3
SSL_OP_NO_TICKET Instructs OpenSSL to disable use of RFC4507bis tickets.
SSL_OP_NO_TLSv1 Instructs OpenSSL to turn off TLS v1
SSL_OP_NO_TLSv1_1 Instructs OpenSSL to turn off TLS v1.1
SSL_OP_NO_TLSv1_2 Instructs OpenSSL to turn off TLS v1.2
SSL_OP_PKCS1_CHECK_1
SSL_OP_PKCS1_CHECK_2
SSL_OP_SINGLE_DH_USE Instructs OpenSSL to always create a new key when using temporary/ephemeral DH parameters.
SSL_OP_SINGLE_ECDH_USE Instructs OpenSSL to always create a new key when using temporary/ephemeral ECDH parameters.
SSL_OP_SSLEAY_080_CLIENT_DH_BUG
SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
SSL_OP_TLS_BLOCK_PADDING_BUG
SSL_OP_TLS_D5_BUG
SSL_OP_TLS_ROLLBACK_BUG Instructs OpenSSL to disable version rollback attack detection.

OpenSSL Engine Constants#

Constant Description
ENGINE_METHOD_RSA Limit engine usage to RSA
ENGINE_METHOD_DSA Limit engine usage to DSA
ENGINE_METHOD_DH Limit engine usage to DH
ENGINE_METHOD_RAND Limit engine usage to RAND
ENGINE_METHOD_EC Limit engine usage to EC
ENGINE_METHOD_CIPHERS Limit engine usage to CIPHERS
ENGINE_METHOD_DIGESTS Limit engine usage to DIGESTS
ENGINE_METHOD_PKEY_METHS Limit engine usage to PKEY_METHDS
ENGINE_METHOD_PKEY_ASN1_METHS Limit engine usage to PKEY_ASN1_METHS
ENGINE_METHOD_ALL
ENGINE_METHOD_NONE

Other OpenSSL Constants#

Constant Description
DH_CHECK_P_NOT_SAFE_PRIME
DH_CHECK_P_NOT_PRIME
DH_UNABLE_TO_CHECK_GENERATOR
DH_NOT_SUITABLE_GENERATOR
ALPN_ENABLED
RSA_PKCS1_PADDING
RSA_SSLV23_PADDING
RSA_NO_PADDING
RSA_PKCS1_OAEP_PADDING
RSA_X931_PADDING
RSA_PKCS1_PSS_PADDING
RSA_PSS_SALTLEN_DIGEST Sets the salt length for RSA_PKCS1_PSS_PADDING to the digest size when signing or verifying.
RSA_PSS_SALTLEN_MAX_SIGN Sets the salt length for RSA_PKCS1_PSS_PADDING to the maximum permissible value when signing data.
RSA_PSS_SALTLEN_AUTO Causes the salt length for RSA_PKCS1_PSS_PADDING to be determined automatically when verifying a signature.
POINT_CONVERSION_COMPRESSED
POINT_CONVERSION_UNCOMPRESSED
POINT_CONVERSION_HYBRID

Node.js Crypto Constants#

Constant Description
defaultCoreCipherList Specifies the built-in default cipher list used by Node.js.
defaultCipherList Specifies the active default cipher list used by the current Node.js process.

Debugger#

Stability: 2 - Stable

Node.js includes an out-of-process debugging utility accessible via a V8 Inspector and built-in debugging client. To use it, start Node.js with the inspect argument followed by the path to the script to debug; a prompt will be displayed indicating successful launch of the debugger:

$ node inspect myscript.js
< Debugger listening on ws://127.0.0.1:9229/80e7a814-7cd3-49fb-921a-2e02228cd5ba
< For help, see: https://nodejs.org/en/docs/inspector
< Debugger attached.
Break on start in myscript.js:1
> 1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
  2 setTimeout(() => {
  3   console.log('world');
debug>

The Node.js debugger client is not a full-featured debugger, but simple step and inspection are possible.

Inserting the statement debugger; into the source code of a script will enable a breakpoint at that position in the code:

// myscript.js
global.x = 5;
setTimeout(() => {
  debugger;
  console.log('world');
}, 1000);
console.log('hello');

Once the debugger is run, a breakpoint will occur at line 3:

$ node inspect myscript.js
< Debugger listening on ws://127.0.0.1:9229/80e7a814-7cd3-49fb-921a-2e02228cd5ba
< For help, see: https://nodejs.org/en/docs/inspector
< Debugger attached.
Break on start in myscript.js:1
> 1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
  2 setTimeout(() => {
  3   debugger;
debug> cont
< hello
break in myscript.js:3
  1 (function (exports, require, module, __filename, __dirname) { global.x = 5;
  2 setTimeout(() => {
> 3   debugger;
  4   console.log('world');
  5 }, 1000);
debug> next
break in myscript.js:4
  2 setTimeout(() => {
  3   debugger;
> 4   console.log('world');
  5 }, 1000);
  6 console.log('hello');
debug> repl
Press Ctrl + C to leave debug repl
> x
5
> 2 + 2
4
debug> next
< world
break in myscript.js:5
  3   debugger;
  4   console.log('world');
> 5 }, 1000);
  6 console.log('hello');
  7
debug> .exit

The repl command allows code to be evaluated remotely. The next command steps to the next line. Type help to see what other commands are available.

Pressing enter without typing a command will repeat the previous debugger command.

Watchers#

It is possible to watch expression and variable values while debugging. On every breakpoint, each expression from the watchers list will be evaluated in the current context and displayed immediately before the breakpoint's source code listing.

To begin watching an expression, type watch('my_expression'). The command watchers will print the active watchers. To remove a watcher, type unwatch('my_expression').

Command reference#

Stepping#

  • cont, c: Continue execution
  • next, n: Step next
  • step, s: Step in
  • out, o: Step out
  • pause: Pause running code (like pause button in Developer Tools)

Breakpoints#

  • setBreakpoint(), sb(): Set breakpoint on current line
  • setBreakpoint(line), sb(line): Set breakpoint on specific line
  • setBreakpoint('fn()'), sb(...): Set breakpoint on a first statement in functions body
  • setBreakpoint('script.js', 1), sb(...): Set breakpoint on first line of script.js
  • clearBreakpoint('script.js', 1), cb(...): Clear breakpoint in script.js on line 1

It is also possible to set a breakpoint in a file (module) that is not loaded yet:

$ node inspect main.js
< Debugger listening on ws://127.0.0.1:9229/4e3db158-9791-4274-8909-914f7facf3bd
< For help, see: https://nodejs.org/en/docs/inspector
< Debugger attached.
Break on start in main.js:1
> 1 (function (exports, require, module, __filename, __dirname) { const mod = require('./mod.js');
  2 mod.hello();
  3 mod.hello();
debug> setBreakpoint('mod.js', 22)
Warning: script 'mod.js' was not loaded yet.
debug> c
break in mod.js:22
 20 // USE OR OTHER DEALINGS IN THE SOFTWARE.
 21
>22 exports.hello = function() {
 23   return 'hello from module';
 24 };
debug>

Information#

  • backtrace, bt: Print backtrace of current execution frame
  • list(5): List scripts source code with 5 line context (5 lines before and after)
  • watch(expr): Add expression to watch list
  • unwatch(expr): Remove expression from watch list
  • watchers: List all watchers and their values (automatically listed on each breakpoint)
  • repl: Open debugger's repl for evaluation in debugging script's context
  • exec expr: Execute an expression in debugging script's context

Execution control#

  • run: Run script (automatically runs on debugger's start)
  • restart: Restart script
  • kill: Kill script

Various#

  • scripts: List all loaded scripts
  • version: Display V8's version

Advanced Usage#

V8 Inspector Integration for Node.js#

V8 Inspector integration allows attaching Chrome DevTools to Node.js instances for debugging and profiling. It uses the Chrome DevTools Protocol.

V8 Inspector can be enabled by passing the --inspect flag when starting a Node.js application. It is also possible to supply a custom port with that flag, e.g. --inspect=9222 will accept DevTools connections on port 9222.

To break on the first line of the application code, pass the --inspect-brk flag instead of --inspect.

$ node --inspect index.js
Debugger listening on 127.0.0.1:9229.
To start debugging, open the following URL in Chrome:
    chrome-devtools://devtools/bundled/js_app.html?experiments=true&v8only=true&ws=127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29

(In the example above, the UUID dc9010dd-f8b8-4ac5-a510-c1a114ec7d29 at the end of the URL is generated on the fly, it varies in different debugging sessions.)

If the Chrome browser is older than 66.0.3345.0, use inspector.html instead of js_app.html in the above URL.

Chrome DevTools doesn't support debugging Worker Threads yet. ndb can be used to debug them.

Deprecated APIs#

Node.js may deprecate APIs for any of the following reasons:

  • Use of the API is unsafe.
  • An improved alternative API is available.
  • Breaking changes to the API are expected in a future major release.

Node.js utilizes three kinds of Deprecations:

  • Documentation-only
  • Runtime
  • End-of-Life

A Documentation-only deprecation is one that is expressed only within the Node.js API docs. These generate no side-effects while running Node.js. Some Documentation-only deprecations trigger a runtime warning when launched with --pending-deprecation flag (or its alternative, NODE_PENDING_DEPRECATION=1 environment variable), similarly to Runtime deprecations below. Documentation-only deprecations that support that flag are explicitly labeled as such in the list of Deprecated APIs.

A Runtime deprecation will, by default, generate a process warning that will be printed to stderr the first time the deprecated API is used. When the --throw-deprecation command-line flag is used, a Runtime deprecation will cause an error to be thrown.

An End-of-Life deprecation is used when functionality is or will soon be removed from Node.js.

Revoking deprecations#

Occasionally, the deprecation of an API may be reversed. In such situations, this document will be updated with information relevant to the decision. However, the deprecation identifier will not be modified.

List of Deprecated APIs#

DEP0001: http.OutgoingMessage.prototype.flush#

History
VersionChanges
v14.0.0

End-of-Life.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v1.6.0

Runtime deprecation.

Type: End-of-Life

OutgoingMessage.prototype.flush() has been removed. Use OutgoingMessage.prototype.flushHeaders() instead.

DEP0002: require('_linklist')#

History
VersionChanges
v8.0.0

End-of-Life.

v6.12.0

A deprecation code has been assigned.

v5.0.0

Runtime deprecation.

Type: End-of-Life

The _linklist module is deprecated. Please use a userland alternative.

DEP0003: _writableState.buffer#

History
VersionChanges
v14.0.0

End-of-Life

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.11.15

Runtime deprecation.

Type: End-of-Life

The _writableState.buffer has been removed. Use _writableState.getBuffer() instead.

DEP0004: CryptoStream.prototype.readyState#

History
VersionChanges
v10.0.0

End-of-Life.

v4.8.6, v6.12.0

A deprecation code has been assigned.

0.4.0

Documentation-only deprecation.

Type: End-of-Life

The CryptoStream.prototype.readyState property was removed.

DEP0005: Buffer() constructor#

History
VersionChanges
v10.0.0

Runtime deprecation.

v6.12.0

A deprecation code has been assigned.

v6.0.0

Documentation-only deprecation.

Type: Runtime (supports --pending-deprecation)

The Buffer() function and new Buffer() constructor are deprecated due to API usability issues that can lead to accidental security issues.

As an alternative, use one of the following methods of constructing Buffer objects:

Without --pending-deprecation, runtime warnings occur only for code not in node_modules. This means there will not be deprecation warnings for Buffer() usage in dependencies. With --pending-deprecation, a runtime warning results no matter where the Buffer() usage occurs.

DEP0006: child_process options.customFds#

History
VersionChanges
v12.0.0

End-of-Life.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.11.14

Runtime deprecation.

v0.5.11

Documentation-only deprecation.

Type: End-of-Life

Within the child_process module's spawn(), fork(), and exec() methods, the options.customFds option is deprecated. The options.stdio option should be used instead.

DEP0007: Replace cluster worker.suicide with worker.exitedAfterDisconnect#

History
VersionChanges
v9.0.0

End-of-Life.

v7.0.0

Runtime deprecation.

v6.12.0

A deprecation code has been assigned.

v6.0.0

Documentation-only deprecation.

Type: End-of-Life

In an earlier version of the Node.js cluster, a boolean property with the name suicide was added to the Worker object. The intent of this property was to provide an indication of how and why the Worker instance exited. In Node.js 6.0.0, the old property was deprecated and replaced with a new worker.exitedAfterDisconnect property. The old property name did not precisely describe the actual semantics and was unnecessarily emotion-laden.

DEP0008: require('constants')#

History
VersionChanges
v6.12.0

A deprecation code has been assigned.

v6.3.0

Documentation-only deprecation.

Type: Documentation-only

The constants module is deprecated. When requiring access to constants relevant to specific Node.js builtin modules, developers should instead refer to the constants property exposed by the relevant module. For instance, require('fs').constants and require('os').constants.

DEP0009: crypto.pbkdf2 without digest#

History
VersionChanges
v14.0.0

End-of-Life (for digest === null)

v11.0.0

Runtime deprecation (for digest === null).

v8.0.0

End-of-Life (for digest === undefined).

v6.12.0

A deprecation code has been assigned.

v6.0.0

Runtime deprecation (for digest === undefined).

Type: End-of-Life

Use of the crypto.pbkdf2() API without specifying a digest was deprecated in Node.js 6.0 because the method defaulted to using the non-recommended 'SHA1' digest. Previously, a deprecation warning was printed. Starting in Node.js 8.0.0, calling crypto.pbkdf2() or crypto.pbkdf2Sync() with digest set to undefined will throw a TypeError.

Beginning in Node.js v11.0.0, calling these functions with digest set to null would print a deprecation warning to align with the behavior when digest is undefined.

Now, however, passing either undefined or null will throw a TypeError.

DEP0010: crypto.createCredentials#

History
VersionChanges
v11.0.0

End-of-Life.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.11.13

Runtime deprecation.

Type: End-of-Life

The crypto.createCredentials() API was removed. Please use tls.createSecureContext() instead.

DEP0011: crypto.Credentials#

History
VersionChanges
v11.0.0

End-of-Life.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.11.13

Runtime deprecation.

Type: End-of-Life

The crypto.Credentials class was removed. Please use tls.SecureContext instead.

DEP0012: Domain.dispose#

History
VersionChanges
v9.0.0

End-of-Life.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.11.7

Runtime deprecation.

Type: End-of-Life

Domain.dispose() has been removed. Recover from failed I/O actions explicitly via error event handlers set on the domain instead.

DEP0013: fs asynchronous function without callback#

History
VersionChanges
v10.0.0

End-of-Life.

v7.0.0

Runtime deprecation.

Type: End-of-Life

Calling an asynchronous function without a callback throws a TypeError in Node.js 10.0.0 onwards. See https://github.com/nodejs/node/pull/12562.

DEP0014: fs.read legacy String interface#

History
VersionChanges
v8.0.0

End-of-Life.

v6.0.0

Runtime deprecation.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.1.96

Documentation-only deprecation.

Type: End-of-Life

The fs.read() legacy String interface is deprecated. Use the Buffer API as mentioned in the documentation instead.

DEP0015: fs.readSync legacy String interface#

History
VersionChanges
v8.0.0

End-of-Life.

v6.0.0

Runtime deprecation.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.1.96

Documentation-only deprecation.

Type: End-of-Life

The fs.readSync() legacy String interface is deprecated. Use the Buffer API as mentioned in the documentation instead.

DEP0016: GLOBAL/root#

History
VersionChanges
v14.0.0

End-of-Life

v6.12.0

A deprecation code has been assigned.

v6.0.0

Runtime deprecation.

Type: End-of-Life

The GLOBAL and root aliases for the global property were deprecated in Node.js 6.0.0 and have since been removed.

DEP0017: Intl.v8BreakIterator#

History
VersionChanges
v9.0.0

End-of-Life.

v7.0.0

Runtime deprecation.

Type: End-of-Life

Intl.v8BreakIterator was a non-standard extension and has been removed. See Intl.Segmenter.

DEP0018: Unhandled promise rejections#

History
VersionChanges
v7.0.0

Runtime deprecation.

Type: Runtime

Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

DEP0019: require('.') resolved outside directory#

History
VersionChanges
v12.0.0

Removed functionality.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v1.8.1

Runtime deprecation.

Type: End-of-Life

In certain cases, require('.') could resolve outside the package directory. This behavior has been removed.

DEP0020: Server.connections#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.9.7

Runtime deprecation.

Type: Runtime

The Server.connections property is deprecated. Please use the Server.getConnections() method instead.

DEP0021: Server.listenFD#

History
VersionChanges
v12.0.0

End-of-Life.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.7.12

Runtime deprecation.

Type: End-of-Life

The Server.listenFD() method was deprecated and removed. Please use Server.listen({fd: <number>}) instead.

DEP0022: os.tmpDir()#

History
VersionChanges
v14.0.0

End-of-Life.

v7.0.0

Runtime deprecation.

Type: End-of-Life

The os.tmpDir() API was deprecated in Node.js 7.0.0 and has since been removed. Please use os.tmpdir() instead.

DEP0023: os.getNetworkInterfaces()#

History
VersionChanges
v12.0.0

End-of-Life.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.6.0

Runtime deprecation.

Type: End-of-Life

The os.getNetworkInterfaces() method is deprecated. Please use the os.networkInterfaces() method instead.

DEP0024: REPLServer.prototype.convertToContext()#

History
VersionChanges
v9.0.0

End-of-Life.

v7.0.0

Runtime deprecation.

Type: End-of-Life

The REPLServer.prototype.convertToContext() API has been removed.

DEP0025: require('sys')#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v1.0.0

Runtime deprecation.

Type: Runtime

The sys module is deprecated. Please use the util module instead.

DEP0026: util.print()#

History
VersionChanges
v12.0.0

End-of-Life.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.11.3

Runtime deprecation.

Type: End-of-Life

util.print() has been removed. Please use console.log() instead.

DEP0027: util.puts()#

History
VersionChanges
v12.0.0

End-of-Life.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.11.3

Runtime deprecation.

Type: End-of-Life

util.puts() has been removed. Please use console.log() instead.

DEP0028: util.debug()#

History
VersionChanges
v12.0.0

End-of-Life.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.11.3

Runtime deprecation.

Type: End-of-Life

util.debug() has been removed. Please use console.error() instead.

DEP0029: util.error()#

History
VersionChanges
v12.0.0

End-of-Life.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.11.3

Runtime deprecation.

Type: End-of-Life

util.error() has been removed. Please use console.error() instead.

DEP0030: SlowBuffer#

History
VersionChanges
v6.12.0

A deprecation code has been assigned.

v6.0.0

Documentation-only deprecation.

Type: Documentation-only

The SlowBuffer class is deprecated. Please use Buffer.allocUnsafeSlow(size) instead.

DEP0031: ecdh.setPublicKey()#

History
VersionChanges
v6.12.0

A deprecation code has been assigned.

v5.2.0

Documentation-only deprecation.

Type: Documentation-only

The ecdh.setPublicKey() method is now deprecated as its inclusion in the API is not useful.

DEP0032: domain module#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v1.4.2

Documentation-only deprecation.

Type: Documentation-only

The domain module is deprecated and should not be used.

DEP0033: EventEmitter.listenerCount()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.2.0

Documentation-only deprecation.

Type: Documentation-only

The EventEmitter.listenerCount(emitter, eventName) API is deprecated. Please use emitter.listenerCount(eventName) instead.

DEP0034: fs.exists(path, callback)#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v1.0.0

Documentation-only deprecation.

Type: Documentation-only

The fs.exists(path, callback) API is deprecated. Please use fs.stat() or fs.access() instead.

DEP0035: fs.lchmod(path, mode, callback)#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.4.7

Documentation-only deprecation.

Type: Documentation-only

The fs.lchmod(path, mode, callback) API is deprecated.

DEP0036: fs.lchmodSync(path, mode)#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.4.7

Documentation-only deprecation.

Type: Documentation-only

The fs.lchmodSync(path, mode) API is deprecated.

DEP0037: fs.lchown(path, uid, gid, callback)#

History
VersionChanges
v10.6.0

Deprecation revoked.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.4.7

Documentation-only deprecation.

Type: Deprecation revoked

The fs.lchown(path, uid, gid, callback) API was deprecated. The deprecation was revoked because the requisite supporting APIs were added in libuv.

DEP0038: fs.lchownSync(path, uid, gid)#

History
VersionChanges
v10.6.0

Deprecation revoked.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.4.7

Documentation-only deprecation.

Type: Deprecation revoked

The fs.lchownSync(path, uid, gid) API was deprecated. The deprecation was revoked because the requisite supporting APIs were added in libuv.

DEP0039: require.extensions#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.10.6

Documentation-only deprecation.

Type: Documentation-only

The require.extensions property is deprecated.

DEP0040: punycode module#

History
VersionChanges
v7.0.0

Documentation-only deprecation.

Type: Documentation-only

The punycode module is deprecated. Please use a userland alternative instead.

DEP0041: NODE_REPL_HISTORY_FILE environment variable#

History
VersionChanges
v10.0.0

End-of-Life.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.0.0

Documentation-only deprecation.

Type: End-of-Life

The NODE_REPL_HISTORY_FILE environment variable was removed. Please use NODE_REPL_HISTORY instead.

DEP0042: tls.CryptoStream#

History
VersionChanges
v10.0.0

End-of-Life.

v4.8.6, v6.12.0

A deprecation code has been assigned.

v0.11.3

Documentation-only deprecation.

Type: End-of-Life

The tls.CryptoStream class was removed. Please use tls.TLSSocket instead.

DEP0043: tls.SecurePair#

History
VersionChanges
v8.0.0

Runtime deprecation.

v6.12.0

A deprecation code has been assigned.

v6.0.0

Documentation-only deprecation.

v0.11.15

Deprecation revoked.

v0.11.3

Runtime deprecation.

Type: Documentation-only

The tls.SecurePair class is deprecated. Please use tls.TLSSocket instead.

DEP0044: util.isArray()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isArray() API is deprecated. Please use Array.isArray() instead.

DEP0045: util.isBoolean()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isBoolean() API is deprecated.

DEP0046: util.isBuffer()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isBuffer() API is deprecated. Please use Buffer.isBuffer() instead.

DEP0047: util.isDate()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isDate() API is deprecated.

DEP0048: util.isError()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isError() API is deprecated.

DEP0049: util.isFunction()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isFunction() API is deprecated.

DEP0050: util.isNull()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isNull() API is deprecated.

DEP0051: util.isNullOrUndefined()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isNullOrUndefined() API is deprecated.

DEP0052: util.isNumber()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isNumber() API is deprecated.

DEP0053 util.isObject()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isObject() API is deprecated.

DEP0054: util.isPrimitive()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isPrimitive() API is deprecated.

DEP0055: util.isRegExp()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isRegExp() API is deprecated.

DEP0056: util.isString()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isString() API is deprecated.

DEP0057: util.isSymbol()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isSymbol() API is deprecated.

DEP0058: util.isUndefined()#

History
VersionChanges
v4.8.6, v6.12.0

A deprecation code has been assigned.

v3.3.1, v4.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.isUndefined() API is deprecated.

DEP0059: util.log()#

History
VersionChanges
v6.12.0

A deprecation code has been assigned.

v6.0.0

Documentation-only deprecation.

Type: Documentation-only

The util.log() API is deprecated.

DEP0060: util._extend()#

History
VersionChanges
v6.12.0

A deprecation code has been assigned.

v6.0.0

Documentation-only deprecation.

Type: Documentation-only

The util._extend() API is deprecated.

DEP0061: fs.SyncWriteStream#

History
VersionChanges
v11.0.0

End-of-Life.

v8.0.0

Runtime deprecation.

v7.0.0

Documentation-only deprecation.

Type: End-of-Life

The fs.SyncWriteStream class was never intended to be a publicly accessible API and has been removed. No alternative API is available. Please use a userland alternative.

DEP0062: node --debug#

History
VersionChanges
v12.0.0

End-of-Life.

v8.0.0

Runtime deprecation.

Type: End-of-Life

--debug activates the legacy V8 debugger interface, which was removed as of V8 5.8. It is replaced by Inspector which is activated with --inspect instead.

DEP0063: ServerResponse.prototype.writeHeader()#

History
VersionChanges
v8.0.0

Documentation-only deprecation.

Type: Documentation-only

The http module ServerResponse.prototype.writeHeader() API is deprecated. Please use ServerResponse.prototype.writeHead() instead.

The ServerResponse.prototype.writeHeader() method was never documented as an officially supported API.

DEP0064: tls.createSecurePair()#

History
VersionChanges
v8.0.0

Runtime deprecation.

v6.12.0

A deprecation code has been assigned.

v6.0.0

Documentation-only deprecation.

v0.11.15

Deprecation revoked.

v0.11.3

Runtime deprecation.

Type: Runtime

The tls.createSecurePair() API was deprecated in documentation in Node.js 0.11.3. Users should use tls.Socket instead.

DEP0065: repl.REPL_MODE_MAGIC and NODE_REPL_MODE=magic#

History
VersionChanges
v10.0.0

End-of-Life.

v8.0.0

Documentation-only deprecation.

Type: End-of-Life

The repl module's REPL_MODE_MAGIC constant, used for replMode option, has been removed. Its behavior has been functionally identical to that of REPL_MODE_SLOPPY since Node.js 6.0.0, when V8 5.0 was imported. Please use REPL_MODE_SLOPPY instead.

The NODE_REPL_MODE environment variable is used to set the underlying replMode of an interactive node session. Its value, magic, is also removed. Please use sloppy instead.

DEP0066: OutgoingMessage.prototype._headers, OutgoingMessage.prototype._headerNames#

History
VersionChanges
v12.0.0

Runtime deprecation.

v8.0.0

Documentation-only deprecation.

Type: Runtime

The http module OutgoingMessage.prototype._headers and OutgoingMessage.prototype._headerNames properties are deprecated. Use one of the public methods (e.g. OutgoingMessage.prototype.getHeader(), OutgoingMessage.prototype.getHeaders(), OutgoingMessage.prototype.getHeaderNames(), OutgoingMessage.prototype.hasHeader(), OutgoingMessage.prototype.removeHeader(), OutgoingMessage.prototype.setHeader()) for working with outgoing headers.

The OutgoingMessage.prototype._headers and OutgoingMessage.prototype._headerNames properties were never documented as officially supported properties.

DEP0067: OutgoingMessage.prototype._renderHeaders#

History
VersionChanges
v8.0.0

Documentation-only deprecation.

Type: Documentation-only

The http module OutgoingMessage.prototype._renderHeaders() API is deprecated.

The OutgoingMessage.prototype._renderHeaders property was never documented as an officially supported API.

DEP0068: node debug#

History
VersionChanges
v8.0.0

Runtime deprecation.

Type: Runtime

node debug corresponds to the legacy CLI debugger which has been replaced with a V8-inspector based CLI debugger available through node inspect.

DEP0069: vm.runInDebugContext(string)#

History
VersionChanges
v10.0.0

End-of-Life.

v9.0.0

Runtime deprecation.

v8.0.0

Documentation-only deprecation.

Type: End-of-Life

DebugContext has been removed in V8 and is not available in Node.js 10+.

DebugContext was an experimental API.

DEP0070: async_hooks.currentId()#

History
VersionChanges
v9.0.0

End-of-Life.

v8.2.0

Runtime deprecation.

Type: End-of-Life

async_hooks.currentId() was renamed to async_hooks.executionAsyncId() for clarity.

This change was made while async_hooks was an experimental API.

DEP0071: async_hooks.triggerId()#

History
VersionChanges
v9.0.0

End-of-Life.

v8.2.0

Runtime deprecation.

Type: End-of-Life

async_hooks.triggerId() was renamed to async_hooks.triggerAsyncId() for clarity.

This change was made while async_hooks was an experimental API.

DEP0072: async_hooks.AsyncResource.triggerId()#

History
VersionChanges
v9.0.0

End-of-Life.

v8.2.0

Runtime deprecation.

Type: End-of-Life

async_hooks.AsyncResource.triggerId() was renamed to async_hooks.AsyncResource.triggerAsyncId() for clarity.

This change was made while async_hooks was an experimental API.

DEP0073: Several internal properties of net.Server#

History
VersionChanges
v10.0.0

End-of-Life.

v9.0.0

Runtime deprecation.

Type: End-of-Life

Accessing several internal, undocumented properties of net.Server instances with inappropriate names is deprecated.

As the original API was undocumented and not generally useful for non-internal code, no replacement API is provided.

DEP0074: REPLServer.bufferedCommand#

History
VersionChanges
v9.0.0

Runtime deprecation.

Type: Runtime

The REPLServer.bufferedCommand property was deprecated in favor of REPLServer.clearBufferedCommand().

DEP0075: REPLServer.parseREPLKeyword()#

History
VersionChanges
v9.0.0

Runtime deprecation.

Type: Runtime

REPLServer.parseREPLKeyword() was removed from userland visibility.

DEP0076: tls.parseCertString()#

History
VersionChanges
v9.0.0

Runtime deprecation.

v8.6.0

Documentation-only deprecation.

Type: Runtime

tls.parseCertString() is a trivial parsing helper that was made public by mistake. This function can usually be replaced with:

const querystring = require('querystring');
querystring.parse(str, '\n', '=');

This function is not completely equivalent to querystring.parse(). One difference is that querystring.parse() does url decoding:

> querystring.parse('%E5%A5%BD=1', '\n', '=');
{ '好': '1' }
> tls.parseCertString('%E5%A5%BD=1');
{ '%E5%A5%BD': '1' }

DEP0077: Module._debug()#

History
VersionChanges
v9.0.0

Runtime deprecation.

Type: Runtime

Module._debug() is deprecated.

The Module._debug() function was never documented as an officially supported API.

DEP0078: REPLServer.turnOffEditorMode()#

History
VersionChanges
v9.0.0

Runtime deprecation.

Type: Runtime

REPLServer.turnOffEditorMode() was removed from userland visibility.

DEP0079: Custom inspection function on Objects via .inspect()#

History
VersionChanges
v11.0.0

End-of-Life.

v10.0.0

Runtime deprecation.

v8.7.0

Documentation-only deprecation.

Type: End-of-Life

Using a property named inspect on an object to specify a custom inspection function for util.inspect() is deprecated. Use util.inspect.custom instead. For backward compatibility with Node.js prior to version 6.4.0, both may be specified.

DEP0080: path._makeLong()#

History
VersionChanges
v9.0.0

Documentation-only deprecation.

Type: Documentation-only

The internal path._makeLong() was not intended for public use. However, userland modules have found it useful. The internal API is deprecated and replaced with an identical, public path.toNamespacedPath() method.

DEP0081: fs.truncate() using a file descriptor#

History
VersionChanges
v9.0.0

Runtime deprecation.

Type: Runtime

fs.truncate() fs.truncateSync() usage with a file descriptor is deprecated. Please use fs.ftruncate() or fs.ftruncateSync() to work with file descriptors.

DEP0082: REPLServer.prototype.memory()#

History
VersionChanges
v9.0.0

Runtime deprecation.

Type: Runtime

REPLServer.prototype.memory() is only necessary for the internal mechanics of the REPLServer itself. Do not use this function.

DEP0083: Disabling ECDH by setting ecdhCurve to false#

History
VersionChanges
v10.0.0

End-of-Life.

v9.2.0

Runtime deprecation.

Type: End-of-Life.

The ecdhCurve option to tls.createSecureContext() and tls.TLSSocket could be set to false to disable ECDH entirely on the server only. This mode was deprecated in preparation for migrating to OpenSSL 1.1.0 and consistency with the client and is now unsupported. Use the ciphers parameter instead.

DEP0084: requiring bundled internal dependencies#

History
VersionChanges
v12.0.0

This functionality has been removed.

v10.0.0

Runtime deprecation.

Type: End-of-Life

Since Node.js versions 4.4.0 and 5.2.0, several modules only intended for internal usage were mistakenly exposed to user code through require(). These modules were:

  • v8/tools/codemap
  • v8/tools/consarray
  • v8/tools/csvparser
  • v8/tools/logreader
  • v8/tools/profile_view
  • v8/tools/profile
  • v8/tools/SourceMap
  • v8/tools/splaytree
  • v8/tools/tickprocessor-driver
  • v8/tools/tickprocessor
  • node-inspect/lib/_inspect (from 7.6.0)
  • node-inspect/lib/internal/inspect_client (from 7.6.0)
  • node-inspect/lib/internal/inspect_repl (from 7.6.0)

The v8/* modules do not have any exports, and if not imported in a specific order would in fact throw errors. As such there are virtually no legitimate use cases for importing them through require().

On the other hand, node-inspect may be installed locally through a package manager, as it is published on the npm registry under the same name. No source code modification is necessary if that is done.

DEP0085: AsyncHooks Sensitive API#

History
VersionChanges
10.0.0

End-of-Life.

v8.10.0, v9.4.0

Runtime deprecation.

Type: End-of-Life

The AsyncHooks Sensitive API was never documented and had various minor issues. Use the AsyncResource API instead. See https://github.com/nodejs/node/issues/15572.

DEP0086: Remove runInAsyncIdScope#

History
VersionChanges
10.0.0

End-of-Life.

v8.10.0, v9.4.0

Runtime deprecation.

Type: End-of-Life

runInAsyncIdScope doesn't emit the 'before' or 'after' event and can thus cause a lot of issues. See https://github.com/nodejs/node/issues/14328.

DEP0089: require('assert')#

History
VersionChanges
v12.8.0

Deprecation revoked.

v9.9.0, v10.0.0

Documentation-only deprecation.

Type: Deprecation revoked

Importing assert directly was not recommended as the exposed functions use loose equality checks. The deprecation was revoked because use of the assert module is not discouraged, and the deprecation caused end user confusion.

DEP0090: Invalid GCM authentication tag lengths#

History
VersionChanges
v11.0.0

End-of-Life.

v10.0.0

Runtime deprecation.

Type: End-of-Life

Node.js used to support all GCM authentication tag lengths which are accepted by OpenSSL when calling decipher.setAuthTag(). Beginning with Node.js v11.0.0, only authentication tag lengths of 128, 120, 112, 104, 96, 64, and 32 bits are allowed. Authentication tags of other lengths are invalid per NIST SP 800-38D.

DEP0091: crypto.DEFAULT_ENCODING#

History
VersionChanges
v10.0.0

Runtime deprecation.

Type: Runtime

The crypto.DEFAULT_ENCODING property is deprecated.

DEP0092: Top-level this bound to module.exports#

History
VersionChanges
v10.0.0

Documentation-only deprecation.

Type: Documentation-only

Assigning properties to the top-level this as an alternative to module.exports is deprecated. Developers should use exports or module.exports instead.

DEP0093: crypto.fips is deprecated and replaced.#

History
VersionChanges
v10.0.0

Documentation-only deprecation.

Type: Documentation-only

The crypto.fips property is deprecated. Please use crypto.setFips() and crypto.getFips() instead.

DEP0094: Using assert.fail() with more than one argument.#

History
VersionChanges
v10.0.0

Runtime deprecation.

Type: Runtime

Using assert.fail() with more than one argument is deprecated. Use assert.fail() with only one argument or use a different assert module method.

DEP0095: timers.enroll()#

History
VersionChanges
v10.0.0

Runtime deprecation.

Type: Runtime

timers.enroll() is deprecated. Please use the publicly documented setTimeout() or setInterval() instead.

DEP0096: timers.unenroll()#

History
VersionChanges
v10.0.0

Runtime deprecation.

Type: Runtime

timers.unenroll() is deprecated. Please use the publicly documented clearTimeout() or clearInterval() instead.

DEP0097: MakeCallback with domain property#

History
VersionChanges
v10.0.0

Runtime deprecation.

Type: Runtime

Users of MakeCallback that add the domain property to carry context, should start using the async_context variant of MakeCallback or CallbackScope, or the high-level AsyncResource class.

DEP0098: AsyncHooks Embedder AsyncResource.emitBefore and AsyncResource.emitAfter APIs#

History
VersionChanges
v12.0.0

End-of-Life

v8.12.0, v9.6.0, v10.0.0

Runtime deprecation.

Type: End-of-Life

The embedded API provided by AsyncHooks exposes .emitBefore() and .emitAfter() methods which are very easy to use incorrectly which can lead to unrecoverable errors.

Use asyncResource.runInAsyncScope() API instead which provides a much safer, and more convenient, alternative. See https://github.com/nodejs/node/pull/18513.

DEP0099: async context-unaware node::MakeCallback C++ APIs#

History
VersionChanges
v10.0.0

Compile-time deprecation.

Type: Compile-time

Certain versions of node::MakeCallback APIs available to native modules are deprecated. Please use the versions of the API that accept an async_context parameter.

DEP0100: process.assert()#

History
VersionChanges
v10.0.0

Runtime deprecation.

v0.3.7

Documentation-only deprecation.

Type: Runtime

process.assert() is deprecated. Please use the assert module instead.

This was never a documented feature.

DEP0101: --with-lttng#

History
VersionChanges
v10.0.0

End-of-Life.

Type: End-of-Life

The --with-lttng compile-time option has been removed.

DEP0102: Using noAssert in Buffer#(read|write) operations.#

History
VersionChanges
v10.0.0

End-of-Life.

Type: End-of-Life

Using the noAssert argument has no functionality anymore. All input is going to be verified, no matter if it is set to true or not. Skipping the verification could lead to hard to find errors and crashes.

DEP0103: process.binding('util').is[...] typechecks#

History
VersionChanges
v10.9.0

Superseded by DEP0111.

v10.0.0

Documentation-only deprecation.

Type: Documentation-only (supports --pending-deprecation)

Using process.binding() in general should be avoided. The type checking methods in particular can be replaced by using util.types.

This deprecation has been superseded by the deprecation of the process.binding() API (DEP0111).

DEP0104: process.env string coercion#

History
VersionChanges
v10.0.0

Documentation-only deprecation.

Type: Documentation-only (supports --pending-deprecation)

When assigning a non-string property to process.env, the assigned value is implicitly converted to a string. This behavior is deprecated if the assigned value is not a string, boolean, or number. In the future, such assignment may result in a thrown error. Please convert the property to a string before assigning it to process.env.

DEP0105: decipher.finaltol#

History
VersionChanges
v11.0.0

End-of-Life.

v10.0.0

Runtime deprecation.

Type: End-of-Life

decipher.finaltol() has never been documented and was an alias for decipher.final(). This API has been removed, and it is recommended to use decipher.final() instead.

DEP0106: crypto.createCipher and crypto.createDecipher#

History
VersionChanges
v11.0.0

Runtime deprecation.

v10.0.0

Documentation-only deprecation.

Type: Runtime

Using crypto.createCipher() and crypto.createDecipher() should be avoided as they use a weak key derivation function (MD5 with no salt) and static initialization vectors. It is recommended to derive a key using crypto.pbkdf2() or crypto.scrypt() and to use crypto.createCipheriv() and crypto.createDecipheriv() to obtain the Cipher and Decipher objects respectively.

DEP0107: tls.convertNPNProtocols()#

History
VersionChanges
v11.0.0

End-of-Life.

v10.0.0

Runtime deprecation.

Type: End-of-Life

This was an undocumented helper function not intended for use outside Node.js core and obsoleted by the removal of NPN (Next Protocol Negotiation) support.

DEP0108: zlib.bytesRead#

History
VersionChanges
v11.0.0

Runtime deprecation.

v10.0.0

Documentation-only deprecation.

Type: Runtime

Deprecated alias for zlib.bytesWritten. This original name was chosen because it also made sense to interpret the value as the number of bytes read by the engine, but is inconsistent with other streams in Node.js that expose values under these names.

DEP0109: http, https, and tls support for invalid URLs#

History
VersionChanges
v11.0.0

Runtime deprecation.

Type: Runtime

Some previously supported (but strictly invalid) URLs were accepted through the http.request(), http.get(), https.request(), https.get(), and tls.checkServerIdentity() APIs because those were accepted by the legacy url.parse() API. The mentioned APIs now use the WHATWG URL parser that requires strictly valid URLs. Passing an invalid URL is deprecated and support will be removed in the future.

DEP0110: vm.Script cached data#

History
VersionChanges
v10.6.0

Documentation-only deprecation.

Type: Documentation-only

The produceCachedData option is deprecated. Use script.createCachedData() instead.

DEP0111: process.binding()#

History
VersionChanges
v11.12.0

Added support for --pending-deprecation.

v10.9.0

Documentation-only deprecation.

Type: Documentation-only (supports --pending-deprecation)

process.binding() is for use by Node.js internal code only.

DEP0112: dgram private APIs#

History
VersionChanges
v11.0.0

Runtime deprecation.

Type: Runtime

The dgram module previously contained several APIs that were never meant to accessed outside of Node.js core: Socket.prototype._handle, Socket.prototype._receiving, Socket.prototype._bindState, Socket.prototype._queue, Socket.prototype._reuseAddr, Socket.prototype._healthCheck(), Socket.prototype._stopReceiving(), and dgram._createSocketHandle().

DEP0113: Cipher.setAuthTag(), Decipher.getAuthTag()#

History
VersionChanges
v12.0.0

End-of-Life.

v11.0.0

Runtime deprecation.

Type: End-of-Life

Cipher.setAuthTag() and Decipher.getAuthTag() are no longer available. They were never documented and would throw when called.

DEP0114: crypto._toBuf()#

History
VersionChanges
v12.0.0

End-of-Life.

v11.0.0

Runtime deprecation.

Type: End-of-Life

The crypto._toBuf() function was not designed to be used by modules outside of Node.js core and was removed.

DEP0115: crypto.prng(), crypto.pseudoRandomBytes(), crypto.rng()#

History
VersionChanges
v11.0.0

Added documentation-only deprecation with --pending-deprecation support.

Type: Documentation-only (supports --pending-deprecation)

In recent versions of Node.js, there is no difference between crypto.randomBytes() and crypto.pseudoRandomBytes(). The latter is deprecated along with the undocumented aliases crypto.prng() and crypto.rng() in favor of crypto.randomBytes() and may be removed in a future release.

DEP0116: Legacy URL API#

History
VersionChanges
v11.0.0

Documentation-only deprecation.

Type: Documentation-only

The Legacy URL API is deprecated. This includes url.format(), url.parse(), url.resolve(), and the legacy urlObject. Please use the WHATWG URL API instead.

DEP0117: Native crypto handles#

History
VersionChanges
v12.0.0

End-of-Life.

v11.0.0

Runtime deprecation.

Type: End-of-Life

Previous versions of Node.js exposed handles to internal native objects through the _handle property of the Cipher, Decipher, DiffieHellman, DiffieHellmanGroup, ECDH, Hash, Hmac, Sign, and Verify classes. The _handle property has been removed because improper use of the native object can lead to crashing the application.

DEP0118: dns.lookup() support for a falsy host name#

History
VersionChanges
v11.0.0

Runtime deprecation.

Type: Runtime

Previous versions of Node.js supported dns.lookup() with a falsy host name like dns.lookup(false) due to backward compatibility. This behavior is undocumented and is thought to be unused in real world apps. It will become an error in future versions of Node.js.

DEP0119: process.binding('uv').errname() private API#

History
VersionChanges
v11.0.0

Documentation-only deprecation.

Type: Documentation-only (supports --pending-deprecation)

process.binding('uv').errname() is deprecated. Please use util.getSystemErrorName() instead.

DEP0120: Windows Performance Counter Support#

History
VersionChanges
v12.0.0

End-of-Life.

v11.0.0

Runtime deprecation.

Type: End-of-Life

Windows Performance Counter support has been removed from Node.js. The undocumented COUNTER_NET_SERVER_CONNECTION(), COUNTER_NET_SERVER_CONNECTION_CLOSE(), COUNTER_HTTP_SERVER_REQUEST(), COUNTER_HTTP_SERVER_RESPONSE(), COUNTER_HTTP_CLIENT_REQUEST(), and COUNTER_HTTP_CLIENT_RESPONSE() functions have been deprecated.

DEP0121: net._setSimultaneousAccepts()#

History
VersionChanges
v12.0.0

Runtime deprecation.

Type: Runtime

The undocumented net._setSimultaneousAccepts() function was originally intended for debugging and performance tuning when using the child_process and cluster modules on Windows. The function is not generally useful and is being removed. See discussion here: https://github.com/nodejs/node/issues/18391

DEP0122: tls Server.prototype.setOptions()#

History
VersionChanges
v12.0.0

Runtime deprecation.

Type: Runtime

Please use Server.prototype.setSecureContext() instead.

DEP0123: setting the TLS ServerName to an IP address#

History
VersionChanges
v12.0.0

Runtime deprecation.

Type: Runtime

Setting the TLS ServerName to an IP address is not permitted by RFC 6066. This will be ignored in a future version.

DEP0124: using REPLServer.rli#

History
VersionChanges
v12.0.0

Runtime deprecation.

Type: Runtime

This property is a reference to the instance itself.

DEP0125: require('_stream_wrap')#

History
VersionChanges
v12.0.0

Runtime deprecation.

Type: Runtime

The _stream_wrap module is deprecated.

DEP0126: timers.active()#

History
VersionChanges
v11.14.0

Runtime deprecation.

Type: Runtime

The previously undocumented timers.active() is deprecated. Please use the publicly documented timeout.refresh() instead. If re-referencing the timeout is necessary, timeout.ref() can be used with no performance impact since Node.js 10.

DEP0127: timers._unrefActive()#

History
VersionChanges
v11.14.0

Runtime deprecation.

Type: Runtime

The previously undocumented and "private" timers._unrefActive() is deprecated. Please use the publicly documented timeout.refresh() instead. If unreferencing the timeout is necessary, timeout.unref() can be used with no performance impact since Node.js 10.

DEP0128: modules with an invalid main entry and an index.js file#

History
VersionChanges
v12.0.0

Documentation-only.

Type: Documentation-only (supports --pending-deprecation)

Modules that have an invalid main entry (e.g., ./does-not-exist.js) and also have an index.js file in the top level directory will resolve the index.js file. That is deprecated and is going to throw an error in future Node.js versions.

DEP0129: ChildProcess._channel#

History
VersionChanges
v13.0.0

Runtime deprecation.

v11.14.0

Documentation-only.

Type: Runtime

The _channel property of child process objects returned by spawn() and similar functions is not intended for public use. Use ChildProcess.channel instead.

DEP0130: Module.createRequireFromPath()#

History
VersionChanges
v13.0.0

Runtime deprecation.

v12.2.0

Documentation-only.

Type: Runtime

Module.createRequireFromPath() is deprecated. Please use module.createRequire() instead.

DEP0131: Legacy HTTP parser#

History
VersionChanges
v13.0.0

This feature has been removed.

v12.3.0

Documentation-only.

Type: End-of-Life

The legacy HTTP parser, used by default in versions of Node.js prior to 12.0.0, is deprecated and has been removed in v13.0.0. Prior to v13.0.0, the --http-parser=legacy command-line flag could be used to revert to using the legacy parser.

DEP0132: worker.terminate() with callback#

History
VersionChanges
v12.5.0

Runtime deprecation.

Type: Runtime

Passing a callback to worker.terminate() is deprecated. Use the returned Promise instead, or a listener to the worker’s 'exit' event.

DEP0133: http connection#

History
VersionChanges
v12.12.0

Documentation-only deprecation.

Type: Documentation-only

Prefer response.socket over response.connection and request.socket over request.connection.

DEP0134: process._tickCallback#

History
VersionChanges
v12.12.0

Documentation-only deprecation.

Type: Documentation-only (supports --pending-deprecation)

The process._tickCallback property was never documented as an officially supported API.

DEP0135: WriteStream.open() and ReadStream.open() are internal#

History
VersionChanges
v13.0.0

Type: Runtime

WriteStream.open() and ReadStream.open() are undocumented internal APIs that do not make sense to use in userland. File streams should always be opened through their corresponding factory methods fs.createWriteStream() and fs.createReadStream()) or by passing a file descriptor in options.

DEP0136: http finished#

History
VersionChanges
v13.4.0, v12.16.0

Documentation-only deprecation.

Type: Documentation-only

response.finished indicates whether response.end() has been called, not whether 'finish' has been emitted and the underlying data is flushed.

Use response.writableFinished or response.writableEnded accordingly instead to avoid the ambigiuty.

To maintain existing behaviour response.finished should be replaced with response.writableEnded.

DEP0137: Closing fs.FileHandle on garbage collection#

History
VersionChanges
v14.0.0

Runtime deprecation.

Type: Runtime

Allowing a fs.FileHandle object to be closed on garbage collection is deprecated. In the future, doing so may result in a thrown error that will terminate the process.

Please ensure that all fs.FileHandle objects are explicitly closed using FileHandle.prototype.close() when the fs.FileHandle is no longer needed:

const fsPromises = require('fs').promises;
async function openAndClose() {
  let filehandle;
  try {
    filehandle = await fsPromises.open('thefile.txt', 'r');
  } finally {
    if (filehandle !== undefined)
      await filehandle.close();
  }
}

DEP0138: process.mainModule#

History
VersionChanges
v14.0.0

Documentation-only deprecation.

Type: Documentation-only

process.mainModule is a CommonJS-only feature while process global object is shared with non-CommonJS environment. Its use within ECMAScript modules is unsupported.

It is deprecated in favor of require.main, because it serves the same purpose and is only available on CommonJS environment.

DEP0139: process.umask() with no arguments#

History
VersionChanges
v14.0.0

Documentation-only deprecation.

Type: Documentation-only

Calling process.umask() with no argument causes the process-wide umask to be written twice. This introduces a race condition between threads, and is a potential security vulnerability. There is no safe, cross-platform alternative API.

DEP0140: Use request.destroy() instead of request.abort()#

History
VersionChanges
v14.1.0

Documentation-only deprecation.

Type: Documentation-only

Use request.destroy() instead of request.abort().

DEP0141: repl.inputStream and repl.outputStream#

History
VersionChanges
v14.3.0

Documentation-only (supports [--pending-deprecation][]).

Type: Documentation-only (supports --pending-deprecation)

The repl module exported the input and output stream twice. Use .input instead of .inputStream and .output instead of .outputStream.

DEP0142: repl._builtinLibs#

History
VersionChanges
v14.3.0

Documentation-only (supports [--pending-deprecation][]).

Type: Documentation-only

The repl module exports a _builtinLibs property that contains an array with native modules. It was incomplete so far and instead it's better to rely upon require('module').builtinModules.

DNS#

Stability: 2 - Stable

The dns module enables name resolution. For example, use it to look up IP addresses of host names.

Although named for the Domain Name System (DNS), it does not always use the DNS protocol for lookups. dns.lookup() uses the operating system facilities to perform name resolution. It may not need to perform any network communication. Developers looking to perform name resolution in the same way that other applications on the same operating system behave should use dns.lookup().

const dns = require('dns');

dns.lookup('example.org', (err, address, family) => {
  console.log('address: %j family: IPv%s', address, family);
});
// address: "93.184.216.34" family: IPv4

All other functions in the dns module connect to an actual DNS server to perform name resolution. They will always use the network to perform DNS queries. These functions do not use the same set of configuration files used by dns.lookup() (e.g. /etc/hosts). These functions should be used by developers who do not want to use the underlying operating system's facilities for name resolution, and instead want to always perform DNS queries.

const dns = require('dns');

dns.resolve4('archive.org', (err, addresses) => {
  if (err) throw err;

  console.log(`addresses: ${JSON.stringify(addresses)}`);

  addresses.forEach((a) => {
    dns.reverse(a, (err, hostnames) => {
      if (err) {
        throw err;
      }
      console.log(`reverse for ${a}: ${JSON.stringify(hostnames)}`);
    });
  });
});

See the Implementation considerations section for more information.

Class: dns.Resolver#

Added in: v8.3.0

An independent resolver for DNS requests.

Creating a new resolver uses the default server settings. Setting the servers used for a resolver using resolver.setServers() does not affect other resolvers:

const { Resolver } = require('dns');
const resolver = new Resolver();
resolver.setServers(['4.4.4.4']);

// This request will use the server at 4.4.4.4, independent of global settings.
resolver.resolve4('example.org', (err, addresses) => {
  // ...
});

The following methods from the dns module are available:

resolver.cancel()#

Added in: v8.3.0

Cancel all outstanding DNS queries made by this resolver. The corresponding callbacks will be called with an error with code ECANCELLED.

dns.getServers()#

Added in: v0.11.3

Returns an array of IP address strings, formatted according to RFC 5952, that are currently configured for DNS resolution. A string will include a port section if a custom port is used.

[
  '4.4.4.4',
  '2001:4860:4860::8888',
  '4.4.4.4:1053',
  '[2001:4860:4860::8888]:1053'
]

dns.lookup(hostname[, options], callback)#

History
VersionChanges
v8.5.0

The verbatim option is supported now.

v1.2.0

The all option is supported now.

v0.1.90

Added in: v0.1.90

  • hostname <string>

  • options <integer> | <Object>

    • family <integer> The record family. Must be 4, 6, or 0. The value 0 indicates that IPv4 and IPv6 addresses are both returned. Default: 0.
    • hints <number> One or more supported getaddrinfo flags. Multiple flags may be passed by bitwise ORing their values.
    • all <boolean> When true, the callback returns all resolved addresses in an array. Otherwise, returns a single address. Default: false.
    • verbatim <boolean> When true, the callback receives IPv4 and IPv6 addresses in the order the DNS resolver returned them. When false, IPv4 addresses are placed before IPv6 addresses. Default: currently false (addresses are reordered) but this is expected to change in the not too distant future. New code should use { verbatim: true }.

  • callback <Function>

    • err <Error>
    • address <string> A string representation of an IPv4 or IPv6 address.
    • family <integer> 4 or 6, denoting the family of address, or 0 if the address is not an IPv4 or IPv6 address. 0 is a likely indicator of a bug in the name resolution service used by the operating system.

Resolves a host name (e.g. 'nodejs.org') into the first found A (IPv4) or AAAA (IPv6) record. All option properties are optional. If options is an integer, then it must be 4 or 6 – if options is not provided, then IPv4 and IPv6 addresses are both returned if found.

With the all option set to true, the arguments for callback change to (err, addresses), with addresses being an array of objects with the properties address and family.

On error, err is an Error object, where err.code is the error code. Keep in mind that err.code will be set to 'ENOTFOUND' not only when the host name does not exist but also when the lookup fails in other ways such as no available file descriptors.

dns.lookup() does not necessarily have anything to do with the DNS protocol. The implementation uses an operating system facility that can associate names with addresses, and vice versa. This implementation can have subtle but important consequences on the behavior of any Node.js program. Please take some time to consult the Implementation considerations section before using dns.lookup().

Example usage:

const dns = require('dns');
const options = {
  family: 6,
  hints: dns.ADDRCONFIG | dns.V4MAPPED,
};
dns.lookup('example.com', options, (err, address, family) =>
  console.log('address: %j family: IPv%s', address, family));
// address: "2606:2800:220:1:248:1893:25c8:1946" family: IPv6

// When options.all is true, the result will be an Array.
options.all = true;
dns.lookup('example.com', options, (err, addresses) =>
  console.log('addresses: %j', addresses));
// addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}]

If this method is invoked as its util.promisify()ed version, and all is not set to true, it returns a Promise for an Object with address and family properties.

Supported getaddrinfo flags#

History
VersionChanges
v14.0.0

Added support for the dns.ALL flag.

The following flags can be passed as hints to dns.lookup().

  • dns.ADDRCONFIG: Returned address types are determined by the types of addresses supported by the current system. For example, IPv4 addresses are only returned if the current system has at least one IPv4 address configured. Loopback addresses are not considered.
  • dns.V4MAPPED: If the IPv6 family was specified, but no IPv6 addresses were found, then return IPv4 mapped IPv6 addresses. It is not supported on some operating systems (e.g FreeBSD 10.1).
  • dns.ALL: If dns.V4MAPPED is specified, return resolved IPv6 addresses as well as IPv4 mapped IPv6 addresses.

dns.lookupService(address, port, callback)#

Added in: v0.11.14

Resolves the given address and port into a host name and service using the operating system's underlying getnameinfo implementation.

If address is not a valid IP address, a TypeError will be thrown. The port will be coerced to a number. If it is not a legal port, a TypeError will be thrown.

On an error, err is an Error object, where err.code is the error code.

const dns = require('dns');
dns.lookupService('127.0.0.1', 22, (err, hostname, service) => {
  console.log(hostname, service);
  // Prints: localhost ssh
});

If this method is invoked as its util.promisify()ed version, it returns a Promise for an Object with hostname and service properties.

dns.resolve(hostname[, rrtype], callback)#

Added in: v0.1.27

Uses the DNS protocol to resolve a host name (e.g. 'nodejs.org') into an array of the resource records. The callback function has arguments (err, records). When successful, records will be an array of resource records. The type and structure of individual results varies based on rrtype:

rrtyperecords containsResult typeShorthand method
'A'IPv4 addresses (default)<string>dns.resolve4()
'AAAA'IPv6 addresses<string>dns.resolve6()
'ANY'any records<Object>dns.resolveAny()
'CNAME'canonical name records<string>dns.resolveCname()
'MX'mail exchange records<Object>dns.resolveMx()
'NAPTR'name authority pointer records<Object>dns.resolveNaptr()
'NS'name server records<string>dns.resolveNs()
'PTR'pointer records<string>dns.resolvePtr()
'SOA'start of authority records<Object>dns.resolveSoa()
'SRV'service records<Object>dns.resolveSrv()
'TXT'text records<string[]>dns.resolveTxt()

On error, err is an Error object, where err.code is one of the DNS error codes.

dns.resolve4(hostname[, options], callback)#

History
VersionChanges
v7.2.0

This method now supports passing options, specifically options.ttl.

v0.1.16

Added in: v0.1.16

  • hostname <string> Host name to resolve.

  • options <Object>

    • ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true, the callback receives an array of { address: '1.2.3.4', ttl: 60 } objects rather than an array of strings, with the TTL expressed in seconds.

  • callback <Function>

Uses the DNS protocol to resolve a IPv4 addresses (A records) for the hostname. The addresses argument passed to the callback function will contain an array of IPv4 addresses (e.g. ['74.125.79.104', '74.125.79.105', '74.125.79.106']).

dns.resolve6(hostname[, options], callback)#

History
VersionChanges
v7.2.0

This method now supports passing options, specifically options.ttl.

v0.1.16

Added in: v0.1.16

  • hostname <string> Host name to resolve.

  • options <Object>

    • ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true, the callback receives an array of { address: '0:1:2:3:4:5:6:7', ttl: 60 } objects rather than an array of strings, with the TTL expressed in seconds.

  • callback <Function>

Uses the DNS protocol to resolve a IPv6 addresses (AAAA records) for the hostname. The addresses argument passed to the callback function will contain an array of IPv6 addresses.

dns.resolveAny(hostname, callback)#

Uses the DNS protocol to resolve all records (also known as ANY or * query). The ret argument passed to the callback function will be an array containing various types of records. Each object has a property type that indicates the type of the current record. And depending on the type, additional properties will be present on the object:

TypeProperties
'A'address/ttl
'AAAA'address/ttl
'CNAME'value
'MX'Refer to dns.resolveMx()
'NAPTR'Refer to dns.resolveNaptr()
'NS'value
'PTR'value
'SOA'Refer to dns.resolveSoa()
'SRV'Refer to dns.resolveSrv()
'TXT'This type of record contains an array property called entries which refers to dns.resolveTxt(), e.g. { entries: ['...'], type: 'TXT' }

Here is an example of the ret object passed to the callback:

[ { type: 'A', address: '127.0.0.1', ttl: 299 },
  { type: 'CNAME', value: 'example.com' },
  { type: 'MX', exchange: 'alt4.aspmx.l.example.com', priority: 50 },
  { type: 'NS', value: 'ns1.example.com' },
  { type: 'TXT', entries: [ 'v=spf1 include:_spf.example.com ~all' ] },
  { type: 'SOA',
    nsname: 'ns1.example.com',
    hostmaster: 'admin.example.com',
    serial: 156696742,
    refresh: 900,
    retry: 900,
    expire: 1800,
    minttl: 60 } ]

DNS server operators may choose not to respond to ANY queries. It may be better to call individual methods like dns.resolve4(), dns.resolveMx(), and so on. For more details, see RFC 8482.

dns.resolveCname(hostname, callback)#

Added in: v0.3.2

Uses the DNS protocol to resolve CNAME records for the hostname. The addresses argument passed to the callback function will contain an array of canonical name records available for the hostname (e.g. ['bar.example.com']).

dns.resolveMx(hostname, callback)#

Added in: v0.1.27

Uses the DNS protocol to resolve mail exchange records (MX records) for the hostname. The addresses argument passed to the callback function will contain an array of objects containing both a priority and exchange property (e.g. [{priority: 10, exchange: 'mx.example.com'}, ...]).

dns.resolveNaptr(hostname, callback)#

Added in: v0.9.12

Uses the DNS protocol to resolve regular expression based records (NAPTR records) for the hostname. The addresses argument passed to the callback function will contain an array of objects with the following properties:

  • flags
  • service
  • regexp
  • replacement
  • order
  • preference
{
  flags: 's',
  service: 'SIP+D2U',
  regexp: '',
  replacement: '_sip._udp.example.com',
  order: 30,
  preference: 100
}

dns.resolveNs(hostname, callback)#

Added in: v0.1.90

Uses the DNS protocol to resolve name server records (NS records) for the hostname. The addresses argument passed to the callback function will contain an array of name server records available for hostname (e.g. ['ns1.example.com', 'ns2.example.com']).

dns.resolvePtr(hostname, callback)#

Added in: v6.0.0

Uses the DNS protocol to resolve pointer records (PTR records) for the hostname. The addresses argument passed to the callback function will be an array of strings containing the reply records.

dns.resolveSoa(hostname, callback)#

Added in: v0.11.10

Uses the DNS protocol to resolve a start of authority record (SOA record) for the hostname. The address argument passed to the callback function will be an object with the following properties:

  • nsname
  • hostmaster
  • serial
  • refresh
  • retry
  • expire
  • minttl
{
  nsname: 'ns.example.com',
  hostmaster: 'root.example.com',
  serial: 2013101809,
  refresh: 10000,
  retry: 2400,
  expire: 604800,
  minttl: 3600
}

dns.resolveSrv(hostname, callback)#

Added in: v0.1.27

Uses the DNS protocol to resolve service records (SRV records) for the hostname. The addresses argument passed to the callback function will be an array of objects with the following properties:

  • priority
  • weight
  • port
  • name
{
  priority: 10,
  weight: 5,
  port: 21223,
  name: 'service.example.com'
}

dns.resolveTxt(hostname, callback)#

Added in: v0.1.27

Uses the DNS protocol to resolve text queries (TXT records) for the hostname. The records argument passed to the callback function is a two-dimensional array of the text records available for hostname (e.g. [ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ]). Each sub-array contains TXT chunks of one record. Depending on the use case, these could be either joined together or treated separately.

dns.reverse(ip, callback)#

Added in: v0.1.16

Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of host names.

On error, err is an Error object, where err.code is one of the DNS error codes.

dns.setServers(servers)#

Added in: v0.11.3

Sets the IP address and port of servers to be used when performing DNS resolution. The servers argument is an array of RFC 5952 formatted addresses. If the port is the IANA default DNS port (53) it can be omitted.

dns.setServers([
  '4.4.4.4',
  '[2001:4860:4860::8888]',
  '4.4.4.4:1053',
  '[2001:4860:4860::8888]:1053'
]);

An error will be thrown if an invalid address is provided.

The dns.setServers() method must not be called while a DNS query is in progress.

The dns.setServers() method affects only dns.resolve(), dns.resolve*() and dns.reverse() (and specifically not dns.lookup()).

This method works much like resolve.conf. That is, if attempting to resolve with the first server provided results in a NOTFOUND error, the resolve() method will not attempt to resolve with subsequent servers provided. Fallback DNS servers will only be used if the earlier ones time out or result in some other error.

DNS Promises API#

The dns.promises API provides an alternative set of asynchronous DNS methods that return Promise objects rather than using callbacks. The API is accessible via require('dns').promises.

Class: dnsPromises.Resolver#

Added in: v10.6.0

An independent resolver for DNS requests.

Creating a new resolver uses the default server settings. Setting the servers used for a resolver using resolver.setServers() does not affect other resolvers:

const { Resolver } = require('dns').promises;
const resolver = new Resolver();
resolver.setServers(['4.4.4.4']);

// This request will use the server at 4.4.4.4, independent of global settings.
resolver.resolve4('example.org').then((addresses) => {
  // ...
});

// Alternatively, the same code can be written using async-await style.
(async function() {
  const addresses = await resolver.resolve4('example.org');
})();

The following methods from the dnsPromises API are available:

dnsPromises.getServers()#

Added in: v10.6.0

Returns an array of IP address strings, formatted according to RFC 5952, that are currently configured for DNS resolution. A string will include a port section if a custom port is used.

[
  '4.4.4.4',
  '2001:4860:4860::8888',
  '4.4.4.4:1053',
  '[2001:4860:4860::8888]:1053'
]

dnsPromises.lookup(hostname[, options])#

Added in: v10.6.0
  • hostname <string>

  • options <integer> | <Object>

    • family <integer> The record family. Must be 4, 6, or 0. The value 0 indicates that IPv4 and IPv6 addresses are both returned. Default: 0.
    • hints <number> One or more supported getaddrinfo flags. Multiple flags may be passed by bitwise ORing their values.
    • all <boolean> When true, the Promise is resolved with all addresses in an array. Otherwise, returns a single address. Default: false.
    • verbatim <boolean> When true, the Promise is resolved with IPv4 and IPv6 addresses in the order the DNS resolver returned them. When false, IPv4 addresses are placed before IPv6 addresses. Default: currently false (addresses are reordered) but this is expected to change in the not too distant future. New code should use { verbatim: true }.

Resolves a host name (e.g. 'nodejs.org') into the first found A (IPv4) or AAAA (IPv6) record. All option properties are optional. If options is an integer, then it must be 4 or 6 – if options is not provided, then IPv4 and IPv6 addresses are both returned if found.

With the all option set to true, the Promise is resolved with addresses being an array of objects with the properties address and family.

On error, the Promise is rejected with an Error object, where err.code is the error code. Keep in mind that err.code will be set to 'ENOTFOUND' not only when the host name does not exist but also when the lookup fails in other ways such as no available file descriptors.

dnsPromises.lookup() does not necessarily have anything to do with the DNS protocol. The implementation uses an operating system facility that can associate names with addresses, and vice versa. This implementation can have subtle but important consequences on the behavior of any Node.js program. Please take some time to consult the Implementation considerations section before using dnsPromises.lookup().

Example usage:

const dns = require('dns');
const dnsPromises = dns.promises;
const options = {
  family: 6,
  hints: dns.ADDRCONFIG | dns.V4MAPPED,
};

dnsPromises.lookup('example.com', options).then((result) => {
  console.log('address: %j family: IPv%s', result.address, result.family);
  // address: "2606:2800:220:1:248:1893:25c8:1946" family: IPv6
});

// When options.all is true, the result will be an Array.
options.all = true;
dnsPromises.lookup('example.com', options).then((result) => {
  console.log('addresses: %j', result);
  // addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}]
});

dnsPromises.lookupService(address, port)#

Added in: v10.6.0

Resolves the given address and port into a host name and service using the operating system's underlying getnameinfo implementation.

If address is not a valid IP address, a TypeError will be thrown. The port will be coerced to a number. If it is not a legal port, a TypeError will be thrown.

On error, the Promise is rejected with an Error object, where err.code is the error code.

const dnsPromises = require('dns').promises;
dnsPromises.lookupService('127.0.0.1', 22).then((result) => {
  console.log(result.hostname, result.service);
  // Prints: localhost ssh
});

dnsPromises.resolve(hostname[, rrtype])#

Added in: v10.6.0
  • hostname <string> Host name to resolve.
  • rrtype <string> Resource record type. Default: 'A'.

Uses the DNS protocol to resolve a host name (e.g. 'nodejs.org') into an array of the resource records. When successful, the Promise is resolved with an array of resource records. The type and structure of individual results vary based on rrtype:

rrtyperecords containsResult typeShorthand method
'A'IPv4 addresses (default)<string>dnsPromises.resolve4()
'AAAA'IPv6 addresses<string>dnsPromises.resolve6()
'ANY'any records<Object>dnsPromises.resolveAny()
'CNAME'canonical name records<string>dnsPromises.resolveCname()
'MX'mail exchange records<Object>dnsPromises.resolveMx()
'NAPTR'name authority pointer records<Object>dnsPromises.resolveNaptr()
'NS'name server records<string>dnsPromises.resolveNs()
'PTR'pointer records<string>dnsPromises.resolvePtr()
'SOA'start of authority records<Object>dnsPromises.resolveSoa()
'SRV'service records<Object>dnsPromises.resolveSrv()
'TXT'text records<string[]>dnsPromises.resolveTxt()

On error, the Promise is rejected with an Error object, where err.code is one of the DNS error codes.

dnsPromises.resolve4(hostname[, options])#

Added in: v10.6.0
  • hostname <string> Host name to resolve.

  • options <Object>

    • ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true, the Promise is resolved with an array of { address: '1.2.3.4', ttl: 60 } objects rather than an array of strings, with the TTL expressed in seconds.

Uses the DNS protocol to resolve IPv4 addresses (A records) for the hostname. On success, the Promise is resolved with an array of IPv4 addresses (e.g. ['74.125.79.104', '74.125.79.105', '74.125.79.106']).

dnsPromises.resolve6(hostname[, options])#

Added in: v10.6.0
  • hostname <string> Host name to resolve.

  • options <Object>

    • ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true, the Promise is resolved with an array of { address: '0:1:2:3:4:5:6:7', ttl: 60 } objects rather than an array of strings, with the TTL expressed in seconds.

Uses the DNS protocol to resolve IPv6 addresses (AAAA records) for the hostname. On success, the Promise is resolved with an array of IPv6 addresses.

dnsPromises.resolveAny(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve all records (also known as ANY or * query). On success, the Promise is resolved with an array containing various types of records. Each object has a property type that indicates the type of the current record. And depending on the type, additional properties will be present on the object:

TypeProperties
'A'address/ttl
'AAAA'address/ttl
'CNAME'value
'MX'Refer to dnsPromises.resolveMx()
'NAPTR'Refer to dnsPromises.resolveNaptr()
'NS'value
'PTR'value
'SOA'Refer to dnsPromises.resolveSoa()
'SRV'Refer to dnsPromises.resolveSrv()
'TXT'This type of record contains an array property called entries which refers to dnsPromises.resolveTxt(), e.g. { entries: ['...'], type: 'TXT' }

Here is an example of the result object:

[ { type: 'A', address: '127.0.0.1', ttl: 299 },
  { type: 'CNAME', value: 'example.com' },
  { type: 'MX', exchange: 'alt4.aspmx.l.example.com', priority: 50 },
  { type: 'NS', value: 'ns1.example.com' },
  { type: 'TXT', entries: [ 'v=spf1 include:_spf.example.com ~all' ] },
  { type: 'SOA',
    nsname: 'ns1.example.com',
    hostmaster: 'admin.example.com',
    serial: 156696742,
    refresh: 900,
    retry: 900,
    expire: 1800,
    minttl: 60 } ]

dnsPromises.resolveCname(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve CNAME records for the hostname. On success, the Promise is resolved with an array of canonical name records available for the hostname (e.g. ['bar.example.com']).

dnsPromises.resolveMx(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve mail exchange records (MX records) for the hostname. On success, the Promise is resolved with an array of objects containing both a priority and exchange property (e.g. [{priority: 10, exchange: 'mx.example.com'}, ...]).

dnsPromises.resolveNaptr(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve regular expression based records (NAPTR records) for the hostname. On success, the Promise is resolved with an array of objects with the following properties:

  • flags
  • service
  • regexp
  • replacement
  • order
  • preference
{
  flags: 's',
  service: 'SIP+D2U',
  regexp: '',
  replacement: '_sip._udp.example.com',
  order: 30,
  preference: 100
}

dnsPromises.resolveNs(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve name server records (NS records) for the hostname. On success, the Promise is resolved with an array of name server records available for hostname (e.g. ['ns1.example.com', 'ns2.example.com']).

dnsPromises.resolvePtr(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve pointer records (PTR records) for the hostname. On success, the Promise is resolved with an array of strings containing the reply records.

dnsPromises.resolveSoa(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve a start of authority record (SOA record) for the hostname. On success, the Promise is resolved with an object with the following properties:

  • nsname
  • hostmaster
  • serial
  • refresh
  • retry
  • expire
  • minttl
{
  nsname: 'ns.example.com',
  hostmaster: 'root.example.com',
  serial: 2013101809,
  refresh: 10000,
  retry: 2400,
  expire: 604800,
  minttl: 3600
}

dnsPromises.resolveSrv(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve service records (SRV records) for the hostname. On success, the Promise is resolved with an array of objects with the following properties:

  • priority
  • weight
  • port
  • name
{
  priority: 10,
  weight: 5,
  port: 21223,
  name: 'service.example.com'
}

dnsPromises.resolveTxt(hostname)#

Added in: v10.6.0

Uses the DNS protocol to resolve text queries (TXT records) for the hostname. On success, the Promise is resolved with a two-dimensional array of the text records available for hostname (e.g. [ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ]). Each sub-array contains TXT chunks of one record. Depending on the use case, these could be either joined together or treated separately.

dnsPromises.reverse(ip)#

Added in: v10.6.0

Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of host names.

On error, the Promise is rejected with an Error object, where err.code is one of the DNS error codes.

dnsPromises.setServers(servers)#

Added in: v10.6.0

Sets the IP address and port of servers to be used when performing DNS resolution. The servers argument is an array of RFC 5952 formatted addresses. If the port is the IANA default DNS port (53) it can be omitted.

dnsPromises.setServers([
  '4.4.4.4',
  '[2001:4860:4860::8888]',
  '4.4.4.4:1053',
  '[2001:4860:4860::8888]:1053'
]);

An error will be thrown if an invalid address is provided.

The dnsPromises.setServers() method must not be called while a DNS query is in progress.

This method works much like resolve.conf. That is, if attempting to resolve with the first server provided results in a NOTFOUND error, the resolve() method will not attempt to resolve with subsequent servers provided. Fallback DNS servers will only be used if the earlier ones time out or result in some other error.

Error codes#

Each DNS query can return one of the following error codes:

  • dns.NODATA: DNS server returned answer with no data.
  • dns.FORMERR: DNS server claims query was misformatted.
  • dns.SERVFAIL: DNS server returned general failure.
  • dns.NOTFOUND: Domain name not found.
  • dns.NOTIMP: DNS server does not implement requested operation.
  • dns.REFUSED: DNS server refused query.
  • dns.BADQUERY: Misformatted DNS query.
  • dns.BADNAME: Misformatted host name.
  • dns.BADFAMILY: Unsupported address family.
  • dns.BADRESP: Misformatted DNS reply.
  • dns.CONNREFUSED: Could not contact DNS servers.
  • dns.TIMEOUT: Timeout while contacting DNS servers.
  • dns.EOF: End of file.
  • dns.FILE: Error reading file.
  • dns.NOMEM: Out of memory.
  • dns.DESTRUCTION: Channel is being destroyed.
  • dns.BADSTR: Misformatted string.
  • dns.BADFLAGS: Illegal flags specified.
  • dns.NONAME: Given host name is not numeric.
  • dns.BADHINTS: Illegal hints flags specified.
  • dns.NOTINITIALIZED: c-ares library initialization not yet performed.
  • dns.LOADIPHLPAPI: Error loading iphlpapi.dll.
  • dns.ADDRGETNETWORKPARAMS: Could not find GetNetworkParams function.
  • dns.CANCELLED: DNS query cancelled.

Implementation considerations#

Although dns.lookup() and the various dns.resolve*()/dns.reverse() functions have the same goal of associating a network name with a network address (or vice versa), their behavior is quite different. These differences can have subtle but significant consequences on the behavior of Node.js programs.

dns.lookup()#

Under the hood, dns.lookup() uses the same operating system facilities as most other programs. For instance, dns.lookup() will almost always resolve a given name the same way as the ping command. On most POSIX-like operating systems, the behavior of the dns.lookup() function can be modified by changing settings in nsswitch.conf(5) and/or resolv.conf(5), but changing these files will change the behavior of all other programs running on the same operating system.

Though the call to dns.lookup() will be asynchronous from JavaScript's perspective, it is implemented as a synchronous call to getaddrinfo(3) that runs on libuv's threadpool. This can have surprising negative performance implications for some applications, see the UV_THREADPOOL_SIZE documentation for more information.

Various networking APIs will call dns.lookup() internally to resolve host names. If that is an issue, consider resolving the host name to an address using dns.resolve() and using the address instead of a host name. Also, some networking APIs (such as socket.connect() and dgram.createSocket()) allow the default resolver, dns.lookup(), to be replaced.

dns.resolve(), dns.resolve*() and dns.reverse()#

These functions are implemented quite differently than dns.lookup(). They do not use getaddrinfo(3) and they always perform a DNS query on the network. This network communication is always done asynchronously, and does not use libuv's threadpool.

As a result, these functions cannot have the same negative impact on other processing that happens on libuv's threadpool that dns.lookup() can have.

They do not use the same set of configuration files than what dns.lookup() uses. For instance, they do not use the configuration from /etc/hosts.

Domain#

History
VersionChanges
v8.8.0

Any Promises created in VM contexts no longer have a .domain property. Their handlers are still executed in the proper domain, however, and Promises created in the main context still possess a .domain property.

v8.0.0

Handlers for Promises are now invoked in the domain in which the first promise of a chain was created.

Stability: 0 - Deprecated

This module is pending deprecation. Once a replacement API has been finalized, this module will be fully deprecated. Most end users should not have cause to use this module. Users who absolutely must have the functionality that domains provide may rely on it for the time being but should expect to have to migrate to a different solution in the future.

Domains provide a way to handle multiple different IO operations as a single group. If any of the event emitters or callbacks registered to a domain emit an 'error' event, or throw an error, then the domain object will be notified, rather than losing the context of the error in the process.on('uncaughtException') handler, or causing the program to exit immediately with an error code.

Warning: Don't Ignore Errors!#

Domain error handlers are not a substitute for closing down a process when an error occurs.

By the very nature of how throw works in JavaScript, there is almost never any way to safely "pick up where it left off", without leaking references, or creating some other sort of undefined brittle state.

The safest way to respond to a thrown error is to shut down the process. Of course, in a normal web server, there may be many open connections, and it is not reasonable to abruptly shut those down because an error was triggered by someone else.

The better approach is to send an error response to the request that triggered the error, while letting the others finish in their normal time, and stop listening for new requests in that worker.

In this way, domain usage goes hand-in-hand with the cluster module, since the master process can fork a new worker when a worker encounters an error. For Node.js programs that scale to multiple machines, the terminating proxy or service registry can take note of the failure, and react accordingly.

For example, this is not a good idea:

// XXX WARNING! BAD IDEA!

const d = require('domain').create();
d.on('error', (er) => {
  // The error won't crash the process, but what it does is worse!
  // Though we've prevented abrupt process restarting, we are leaking
  // resources like crazy if this ever happens.
  // This is no better than process.on('uncaughtException')!
  console.log(`error, but oh well ${er.message}`);
});
d.run(() => {
  require('http').createServer((req, res) => {
    handleRequest(req, res);
  }).listen(PORT);
});

By using the context of a domain, and the resilience of separating our program into multiple worker processes, we can react more appropriately, and handle errors with much greater safety.

// Much better!

const cluster = require('cluster');
const PORT = +process.env.PORT || 1337;

if (cluster.isMaster) {
  // A more realistic scenario would have more than 2 workers,
  // and perhaps not put the master and worker in the same file.
  //
  // It is also possible to get a bit fancier about logging, and
  // implement whatever custom logic is needed to prevent DoS
  // attacks and other bad behavior.
  //
  // See the options in the cluster documentation.
  //
  // The important thing is that the master does very little,
  // increasing our resilience to unexpected errors.

  cluster.fork();
  cluster.fork();

  cluster.on('disconnect', (worker) => {
    console.error('disconnect!');
    cluster.fork();
  });

} else {
  // the worker
  //
  // This is where we put our bugs!

  const domain = require('domain');

  // See the cluster documentation for more details about using
  // worker processes to serve requests. How it works, caveats, etc.

  const server = require('http').createServer((req, res) => {
    const d = domain.create();
    d.on('error', (er) => {
      console.error(`error ${er.stack}`);

      // We're in dangerous territory!
      // By definition, something unexpected occurred,
      // which we probably didn't want.
      // Anything can happen now! Be very careful!

      try {
        // Make sure we close down within 30 seconds
        const killtimer = setTimeout(() => {
          process.exit(1);
        }, 30000);
        // But don't keep the process open just for that!
        killtimer.unref();

        // Stop taking new requests.
        server.close();

        // Let the master know we're dead. This will trigger a
        // 'disconnect' in the cluster master, and then it will fork
        // a new worker.
        cluster.worker.disconnect();

        // Try to send an error to the request that triggered the problem
        res.statusCode = 500;
        res.setHeader('content-type', 'text/plain');
        res.end('Oops, there was a problem!\n');
      } catch (er2) {
        // Oh well, not much we can do at this point.
        console.error(`Error sending 500! ${er2.stack}`);
      }
    });

    // Because req and res were created before this domain existed,
    // we need to explicitly add them.
    // See the explanation of implicit vs explicit binding below.
    d.add(req);
    d.add(res);

    // Now run the handler function in the domain.
    d.run(() => {
      handleRequest(req, res);
    });
  });
  server.listen(PORT);
}

// This part is not important. Just an example routing thing.
// Put fancy application logic here.
function handleRequest(req, res) {
  switch (req.url) {
    case '/error':
      // We do some async stuff, and then...
      setTimeout(() => {
        // Whoops!
        flerb.bark();
      }, timeout);
      break;
    default:
      res.end('ok');
  }
}

Additions to Error objects#

Any time an Error object is routed through a domain, a few extra fields are added to it.

  • error.domain The domain that first handled the error.
  • error.domainEmitter The event emitter that emitted an 'error' event with the error object.
  • error.domainBound The callback function which was bound to the domain, and passed an error as its first argument.
  • error.domainThrown A boolean indicating whether the error was thrown, emitted, or passed to a bound callback function.

Implicit Binding#

If domains are in use, then all new EventEmitter objects (including Stream objects, requests, responses, etc.) will be implicitly bound to the active domain at the time of their creation.

Additionally, callbacks passed to lowlevel event loop requests (such as to fs.open(), or other callback-taking methods) will automatically be bound to the active domain. If they throw, then the domain will catch the error.

In order to prevent excessive memory usage, Domain objects themselves are not implicitly added as children of the active domain. If they were, then it would be too easy to prevent request and response objects from being properly garbage collected.

To nest Domain objects as children of a parent Domain they must be explicitly added.

Implicit binding routes thrown errors and 'error' events to the Domain's 'error' event, but does not register the EventEmitter on the Domain. Implicit binding only takes care of thrown errors and 'error' events.

Explicit Binding#

Sometimes, the domain in use is not the one that ought to be used for a specific event emitter. Or, the event emitter could have been created in the context of one domain, but ought to instead be bound to some other domain.

For example, there could be one domain in use for an HTTP server, but perhaps we would like to have a separate domain to use for each request.

That is possible via explicit binding.

// Create a top-level domain for the server
const domain = require('domain');
const http = require('http');
const serverDomain = domain.create();

serverDomain.run(() => {
  // Server is created in the scope of serverDomain
  http.createServer((req, res) => {
    // Req and res are also created in the scope of serverDomain
    // however, we'd prefer to have a separate domain for each request.
    // create it first thing, and add req and res to it.
    const reqd = domain.create();
    reqd.add(req);
    reqd.add(res);
    reqd.on('error', (er) => {
      console.error('Error', er, req.url);
      try {
        res.writeHead(500);
        res.end('Error occurred, sorry.');
      } catch (er2) {
        console.error('Error sending 500', er2, req.url);
      }
    });
  }).listen(1337);
});

domain.create()#

Class: Domain#

The Domain class encapsulates the functionality of routing errors and uncaught exceptions to the active Domain object.

To handle the errors that it catches, listen to its 'error' event.

domain.members#

An array of timers and event emitters that have been explicitly added to the domain.

domain.add(emitter)#

Explicitly adds an emitter to the domain. If any event handlers called by the emitter throw an error, or if the emitter emits an 'error' event, it will be routed to the domain's 'error' event, just like with implicit binding.

This also works with timers that are returned from setInterval() and setTimeout(). If their callback function throws, it will be caught by the domain 'error' handler.

If the Timer or EventEmitter was already bound to a domain, it is removed from that one, and bound to this one instead.

domain.bind(callback)#

The returned function will be a wrapper around the supplied callback function. When the returned function is called, any errors that are thrown will be routed to the domain's 'error' event.

const d = domain.create();

function readSomeFile(filename, cb) {
  fs.readFile(filename, 'utf8', d.bind((er, data) => {
    // If this throws, it will also be passed to the domain.
    return cb(er, data ? JSON.parse(data) : null);
  }));
}

d.on('error', (er) => {
  // An error occurred somewhere. If we throw it now, it will crash the program
  // with the normal line number and stack message.
});

domain.enter()#

The enter() method is plumbing used by the run(), bind(), and intercept() methods to set the active domain. It sets domain.active and process.domain to the domain, and implicitly pushes the domain onto the domain stack managed by the domain module (see domain.exit() for details on the domain stack). The call to enter() delimits the beginning of a chain of asynchronous calls and I/O operations bound to a domain.

Calling enter() changes only the active domain, and does not alter the domain itself. enter() and exit() can be called an arbitrary number of times on a single domain.

domain.exit()#

The exit() method exits the current domain, popping it off the domain stack. Any time execution is going to switch to the context of a different chain of asynchronous calls, it's important to ensure that the current domain is exited. The call to exit() delimits either the end of or an interruption to the chain of asynchronous calls and I/O operations bound to a domain.

If there are multiple, nested domains bound to the current execution context, exit() will exit any domains nested within this domain.

Calling exit() changes only the active domain, and does not alter the domain itself. enter() and exit() can be called an arbitrary number of times on a single domain.

domain.intercept(callback)#

This method is almost identical to domain.bind(callback). However, in addition to catching thrown errors, it will also intercept Error objects sent as the first argument to the function.

In this way, the common if (err) return callback(err); pattern can be replaced with a single error handler in a single place.

const d = domain.create();

function readSomeFile(filename, cb) {
  fs.readFile(filename, 'utf8', d.intercept((data) => {
    // Note, the first argument is never passed to the
    // callback since it is assumed to be the 'Error' argument
    // and thus intercepted by the domain.

    // If this throws, it will also be passed to the domain
    // so the error-handling logic can be moved to the 'error'
    // event on the domain instead of being repeated throughout
    // the program.
    return cb(null, JSON.parse(data));
  }));
}

d.on('error', (er) => {
  // An error occurred somewhere. If we throw it now, it will crash the program
  // with the normal line number and stack message.
});

domain.remove(emitter)#

The opposite of domain.add(emitter). Removes domain handling from the specified emitter.

domain.run(fn[, ...args])#

Run the supplied function in the context of the domain, implicitly binding all event emitters, timers, and lowlevel requests that are created in that context. Optionally, arguments can be passed to the function.

This is the most basic way to use a domain.

const domain = require('domain');
const fs = require('fs');
const d = domain.create();
d.on('error', (er) => {
  console.error('Caught error!', er);
});
d.run(() => {
  process.nextTick(() => {
    setTimeout(() => { // Simulating some various async stuff
      fs.open('non-existent file', 'r', (er, fd) => {
        if (er) throw er;
        // proceed...
      });
    }, 100);
  });
});

In this example, the d.on('error') handler will be triggered, rather than crashing the program.

Domains and Promises#

As of Node.js 8.0.0, the handlers of Promises are run inside the domain in which the call to .then() or .catch() itself was made:

const d1 = domain.create();
const d2 = domain.create();

let p;
d1.run(() => {
  p = Promise.resolve(42);
});

d2.run(() => {
  p.then((v) => {
    // running in d2
  });
});

A callback may be bound to a specific domain using domain.bind(callback):

const d1 = domain.create();
const d2 = domain.create();

let p;
d1.run(() => {
  p = Promise.resolve(42);
});

d2.run(() => {
  p.then(p.domain.bind((v) => {
    // running in d1
  }));
});

Domains will not interfere with the error handling mechanisms for Promises. In other words, no 'error' event will be emitted for unhandled Promise rejections.

ECMAScript Modules#

Stability: 1 - Experimental

Introduction#

ECMAScript modules are the official standard format to package JavaScript code for reuse. Modules are defined using a variety of import and export statements.

The following example of an ES module exports a function:

// addTwo.mjs
function addTwo(num) {
  return num + 2;
}

export { addTwo };

The following example of an ES module imports the function from addTwo.mjs:

// app.mjs
import { addTwo } from './addTwo.mjs';

// Prints: 6
console.log(addTwo(4));

Node.js fully supports ECMAScript modules as they are currently specified and provides limited interoperability between them and the existing module format, CommonJS.

Node.js contains support for ES Modules based upon the Node.js EP for ES Modules and the ECMAScript-modules implementation.

Expect major changes in the implementation including interoperability support, specifier resolution, and default behavior.

Enabling#

Experimental support for ECMAScript modules is enabled by default. Node.js will treat the following as ES modules when passed to node as the initial input, or when referenced by import statements within ES module code:

  • Files ending in .mjs.

  • Files ending in .js when the nearest parent package.json file contains a top-level field "type" with a value of "module".

  • Strings passed in as an argument to --eval, or piped to node via STDIN, with the flag --input-type=module.

Node.js will treat as CommonJS all other forms of input, such as .js files where the nearest parent package.json file contains no top-level "type" field, or string input without the flag --input-type. This behavior is to preserve backward compatibility. However, now that Node.js supports both CommonJS and ES modules, it is best to be explicit whenever possible. Node.js will treat the following as CommonJS when passed to node as the initial input, or when referenced by import statements within ES module code:

  • Files ending in .cjs.

  • Files ending in .js when the nearest parent package.json file contains a top-level field "type" with a value of "commonjs".

  • Strings passed in as an argument to --eval or --print, or piped to node via STDIN, with the flag --input-type=commonjs.

package.json "type" field#

Files ending with .js will be loaded as ES modules when the nearest parent package.json file contains a top-level field "type" with a value of "module".

The nearest parent package.json is defined as the first package.json found when searching in the current folder, that folder’s parent, and so on up until the root of the volume is reached.

// package.json
{
  "type": "module"
}

# In same folder as above package.json
node my-app.js # Runs as ES module

If the nearest parent package.json lacks a "type" field, or contains "type": "commonjs", .js files are treated as CommonJS. If the volume root is reached and no package.json is found, Node.js defers to the default, a package.json with no "type" field.

import statements of .js files are treated as ES modules if the nearest parent package.json contains "type": "module".

// my-app.js, part of the same example as above
import './startup.js'; // Loaded as ES module because of package.json

Package authors should include the "type" field, even in packages where all sources are CommonJS. Being explicit about the type of the package will future-proof the package in case the default type of Node.js ever changes, and it will also make things easier for build tools and loaders to determine how the files in the package should be interpreted.

Regardless of the value of the "type" field, .mjs files are always treated as ES modules and .cjs files are always treated as CommonJS.

Package Scope and File Extensions#

A folder containing a package.json file, and all subfolders below that folder down until the next folder containing another package.json, is considered a package scope. The "type" field defines how .js files should be treated within a particular package.json file’s package scope. Every package in a project’s node_modules folder contains its own package.json file, so each project’s dependencies have their own package scopes. A package.json lacking a "type" field is treated as if it contained "type": "commonjs".

The package scope applies not only to initial entry points (node my-app.js) but also to files referenced by import statements and import() expressions.

// my-app.js, in an ES module package scope because there is a package.json
// file in the same folder with "type": "module".

import './startup/init.js';
// Loaded as ES module since ./startup contains no package.json file,
// and therefore inherits the ES module package scope from one level up.

import 'commonjs-package';
// Loaded as CommonJS since ./node_modules/commonjs-package/package.json
// lacks a "type" field or contains "type": "commonjs".

import './node_modules/commonjs-package/index.js';
// Loaded as CommonJS since ./node_modules/commonjs-package/package.json
// lacks a "type" field or contains "type": "commonjs".

Files ending with .mjs are always loaded as ES modules regardless of package scope.

Files ending with .cjs are always loaded as CommonJS regardless of package scope.

import './legacy-file.cjs';
// Loaded as CommonJS since .cjs is always loaded as CommonJS.

import 'commonjs-package/src/index.mjs';
// Loaded as ES module since .mjs is always loaded as ES module.

The .mjs and .cjs extensions may be used to mix types within the same package scope:

  • Within a "type": "module" package scope, Node.js can be instructed to interpret a particular file as CommonJS by naming it with a .cjs extension (since both .js and .mjs files are treated as ES modules within a "module" package scope).

  • Within a "type": "commonjs" package scope, Node.js can be instructed to interpret a particular file as an ES module by naming it with an .mjs extension (since both .js and .cjs files are treated as CommonJS within a "commonjs" package scope).

--input-type flag#

Strings passed in as an argument to --eval (or -e), or piped to node via STDIN, will be treated as ES modules when the --input-type=module flag is set.

node --input-type=module --eval "import { sep } from 'path'; console.log(sep);"

echo "import { sep } from 'path'; console.log(sep);" | node --input-type=module

For completeness there is also --input-type=commonjs, for explicitly running string input as CommonJS. This is the default behavior if --input-type is unspecified.

Packages#

Package Entry Points#

In a package’s package.json file, two fields can define entry points for a package: "main" and "exports". The "main" field is supported in all versions of Node.js, but its capabilities are limited: it only defines the main entry point of the package.

The "exports" field provides an alternative to "main" where the package main entry point can be defined while also encapsulating the package, preventing any other entry points besides those defined in "exports". This encapsulation allows module authors to define a public interface for their package.

If both "exports" and "main" are defined, the "exports" field takes precedence over "main". "exports" are not specific to ES modules or CommonJS; "main" will be overridden by "exports" if it exists. As such "main" cannot be used as a fallback for CommonJS but it can be used as a fallback for legacy versions of Node.js that do not support the "exports" field.

Conditional Exports can be used within "exports" to define different package entry points per environment, including whether the package is referenced via require or via import. For more information about supporting both CommonJS and ES Modules in a single package please consult the dual CommonJS/ES module packages section.

Warning: Introducing the "exports" field prevents consumers of a package from using any entry points that are not defined, including the package.json (e.g. require('your-package/package.json'). This will likely be a breaking change.

To make the introduction of "exports" non-breaking, ensure that every previously supported entry point is exported. It is best to explicitly specify entry points so that the package’s public API is well-defined. For example, a project that previous exported main, lib, feature, and the package.json could use the following package.exports:

{
  "name": "my-mod",
  "exports": {
    ".": "./lib/index.js",
    "./lib": "./lib/index.js",
    "./lib/index": "./lib/index.js",
    "./lib/index.js": "./lib/index.js",
    "./feature": "./feature/index.js",
    "./feature/index.js": "./feature/index.js",
    "./package.json": "./package.json"
  }
}

Alternatively a project could choose to export entire folders:

{
  "name": "my-mod",
  "exports": {
    ".": "./lib/index.js",
    "./lib": "./lib/index.js",
    "./lib/": "./lib/",
    "./feature": "./feature/index.js",
    "./feature/": "./feature/",
    "./package.json": "./package.json"
  }
}

As a last resort, package encapsulation can be disabled entirely by creating an export for the root of the package "./": "./". This will expose every file in the package at the cost of disabling the encapsulation and potential tooling benefits this provides. As the ES Module loader in Node.js enforces the use of the full specifier path, exporting the root rather than being explicit about entry is less expressive than either of the prior examples. Not only will encapsulation be lost but module consumers will be unable to import feature from 'my-mod/feature' as they will need to provide the full path import feature from 'my-mod/feature/index.js.

Main Entry Point Export#

To set the main entry point for a package, it is advisable to define both "exports" and "main" in the package’s package.json file:

{
  "main": "./main.js",
  "exports": "./main.js"
}

The benefit of doing this is that when using the "exports" field all subpaths of the package will no longer be available to importers under require('pkg/subpath.js'), and instead they will get a new error, ERR_PACKAGE_PATH_NOT_EXPORTED.

This encapsulation of exports provides more reliable guarantees about package interfaces for tools and when handling semver upgrades for a package. It is not a strong encapsulation since a direct require of any absolute subpath of the package such as require('/path/to/node_modules/pkg/subpath.js') will still load subpath.js.

Subpath Exports#

When using the "exports" field, custom subpaths can be defined along with the main entry point by treating the main entry point as the "." subpath:

{
  "main": "./main.js",
  "exports": {
    ".": "./main.js",
    "./submodule": "./src/submodule.js"
  }
}

Now only the defined subpath in "exports" can be imported by a consumer:

import submodule from 'es-module-package/submodule';
// Loads ./node_modules/es-module-package/src/submodule.js

While other subpaths will error:

import submodule from 'es-module-package/private-module.js';
// Throws ERR_PACKAGE_PATH_NOT_EXPORTED

Entire folders can also be mapped with package exports:

// ./node_modules/es-module-package/package.json
{
  "exports": {
    "./features/": "./src/features/"
  }
}

With the above, all modules within the ./src/features/ folder are exposed deeply to import and require:

import feature from 'es-module-package/features/x.js';
// Loads ./node_modules/es-module-package/src/features/x.js

When using folder mappings, ensure that you do want to expose every module inside the subfolder. Any modules which are not public should be moved to another folder to retain the encapsulation benefits of exports.

Package Exports Fallbacks#

For possible new specifier support in future, array fallbacks are supported for all invalid specifiers:

{
  "exports": {
    "./submodule": ["not:valid", "./submodule.js"]
  }
}

Since "not:valid" is not a valid specifier, "./submodule.js" is used instead as the fallback, as if it were the only target.

Exports Sugar#

If the "." export is the only export, the "exports" field provides sugar for this case being the direct "exports" field value.

If the "." export has a fallback array or string value, then the "exports" field can be set to this value directly.

{
  "exports": {
    ".": "./main.js"
  }
}

can be written:

{
  "exports": "./main.js"
}

Conditional Exports#

Conditional exports provide a way to map to different paths depending on certain conditions. They are supported for both CommonJS and ES module imports.

For example, a package that wants to provide different ES module exports for require() and import can be written:

// package.json
{
  "main": "./main-require.cjs",
  "exports": {
    "import": "./main-module.js",
    "require": "./main-require.cjs"
  },
  "type": "module"
}

Node.js supports the following conditions:

  • "import" - matched when the package is loaded via import or import(). Can reference either an ES module or CommonJS file, as both import and import() can load either ES module or CommonJS sources.
  • "require" - matched when the package is loaded via require(). As require() only supports CommonJS, the referenced file must be CommonJS.
  • "node" - matched for any Node.js environment. Can be a CommonJS or ES module file. This condition should always come after "import" or "require".
  • "default" - the generic fallback that will always match. Can be a CommonJS or ES module file. This condition should always come last.

Condition matching is applied in object order from first to last within the "exports" object. The general rule is that conditions should be used from most specific to least specific in object order.

Other conditions such as "browser", "electron", "deno", "react-native", etc. are ignored by Node.js but may be used by other runtimes or tools. Further restrictions, definitions or guidance on condition names may be provided in the future.

Using the "import" and "require" conditions can lead to some hazards, which are explained further in the dual CommonJS/ES module packages section.

Conditional exports can also be extended to exports subpaths, for example:

{
  "main": "./main.js",
  "exports": {
    ".": "./main.js",
    "./feature": {
      "browser": "./feature-browser.js",
      "default": "./feature.js"
    }
  }
}

Defines a package where require('pkg/feature') and import 'pkg/feature' could provide different implementations between the browser and Node.js, given third-party tool support for a "browser" condition.

Nested conditions#

In addition to direct mappings, Node.js also supports nested condition objects.

For example, to define a package that only has dual mode entry points for use in Node.js but not the browser:

{
  "main": "./main.js",
  "exports": {
    "browser": "./feature-browser.mjs",
    "node": {
      "import": "./feature-node.mjs",
      "require": "./feature-node.cjs"
    }
  }
}

Conditions continue to be matched in order as with flat conditions. If a nested conditional does not have any mapping it will continue checking the remaining conditions of the parent condition. In this way nested conditions behave analogously to nested JavaScript if statements.

Self-referencing a package using its name#

Within a package, the values defined in the package’s package.json "exports" field can be referenced via the package’s name. For example, assuming the package.json is:

// package.json
{
  "name": "a-package",
  "exports": {
    ".": "./main.mjs",
    "./foo": "./foo.js"
  }
}

Then any module in that package can reference an export in the package itself:

// ./a-module.mjs
import { something } from 'a-package'; // Imports "something" from ./main.mjs.

Self-referencing is available only if package.json has exports, and will allow importing only what that exports (in the package.json) allows. So the code below, given the package above, will generate a runtime error:

// ./another-module.mjs

// Imports "another" from ./m.mjs. Fails because
// the "package.json" "exports" field
// does not provide an export named "./m.mjs".
import { another } from 'a-package/m.mjs';

Self-referencing is also available when using require, both in an ES module, and in a CommonJS one. For example, this code will also work:

// ./a-module.js
const { something } = require('a-package/foo'); // Loads from ./foo.js.

Dual CommonJS/ES Module Packages#

Prior to the introduction of support for ES modules in Node.js, it was a common pattern for package authors to include both CommonJS and ES module JavaScript sources in their package, with package.json "main" specifying the CommonJS entry point and package.json "module" specifying the ES module entry point. This enabled Node.js to run the CommonJS entry point while build tools such as bundlers used the ES module entry point, since Node.js ignored (and still ignores) the top-level "module" field.

Node.js can now run ES module entry points, and a package can contain both CommonJS and ES module entry points (either via separate specifiers such as 'pkg' and 'pkg/es-module', or both at the same specifier via Conditional Exports). Unlike in the scenario where "module" is only used by bundlers, or ES module files are transpiled into CommonJS on the fly before evaluation by Node.js, the files referenced by the ES module entry point are evaluated as ES modules.

Dual Package Hazard#

When an application is using a package that provides both CommonJS and ES module sources, there is a risk of certain bugs if both versions of the package get loaded. This potential comes from the fact that the pkgInstance created by const pkgInstance = require('pkg') is not the same as the pkgInstance created by import pkgInstance from 'pkg' (or an alternative main path like 'pkg/module'). This is the “dual package hazard,” where two versions of the same package can be loaded within the same runtime environment. While it is unlikely that an application or package would intentionally load both versions directly, it is common for an application to load one version while a dependency of the application loads the other version. This hazard can happen because Node.js supports intermixing CommonJS and ES modules, and can lead to unexpected behavior.

If the package main export is a constructor, an instanceof comparison of instances created by the two versions returns false, and if the export is an object, properties added to one (like pkgInstance.foo = 3) are not present on the other. This differs from how import and require statements work in all-CommonJS or all-ES module environments, respectively, and therefore is surprising to users. It also differs from the behavior users are familiar with when using transpilation via tools like Babel or esm.

Writing Dual Packages While Avoiding or Minimizing Hazards#

First, the hazard described in the previous section occurs when a package contains both CommonJS and ES module sources and both sources are provided for use in Node.js, either via separate main entry points or exported paths. A package could instead be written where any version of Node.js receives only CommonJS sources, and any separate ES module sources the package may contain could be intended only for other environments such as browsers. Such a package would be usable by any version of Node.js, since import can refer to CommonJS files; but it would not provide any of the advantages of using ES module syntax.

A package could also switch from CommonJS to ES module syntax in a breaking change version bump. This has the disadvantage that the newest version of the package would only be usable in ES module-supporting versions of Node.js.

Every pattern has tradeoffs, but there are two broad approaches that satisfy the following conditions:

  1. The package is usable via both require and import.
  2. The package is usable in both current Node.js and older versions of Node.js that lack support for ES modules.
  3. The package main entry point, e.g. 'pkg' can be used by both require to resolve to a CommonJS file and by import to resolve to an ES module file. (And likewise for exported paths, e.g. 'pkg/feature'.)
  4. The package provides named exports, e.g. import { name } from 'pkg' rather than import pkg from 'pkg'; pkg.name.
  5. The package is potentially usable in other ES module environments such as browsers.
  6. The hazards described in the previous section are avoided or minimized.
Approach #1: Use an ES Module Wrapper#

Write the package in CommonJS or transpile ES module sources into CommonJS, and create an ES module wrapper file that defines the named exports. Using Conditional Exports, the ES module wrapper is used for import and the CommonJS entry point for require.

// ./node_modules/pkg/package.json
{
  "type": "module",
  "main": "./index.cjs",
  "exports": {
    "import": "./wrapper.mjs",
    "require": "./index.cjs"
  }
}

// ./node_modules/pkg/index.cjs
exports.name = 'value';

// ./node_modules/pkg/wrapper.mjs
import cjsModule from './index.cjs';
export const name = cjsModule.name;

In this example, the name from import { name } from 'pkg' is the same singleton as the name from const { name } = require('pkg'). Therefore === returns true when comparing the two names and the divergent specifier hazard is avoided.

If the module is not simply a list of named exports, but rather contains a unique function or object export like module.exports = function () { ... }, or if support in the wrapper for the import pkg from 'pkg' pattern is desired, then the wrapper would instead be written to export the default optionally along with any named exports as well:

import cjsModule from './index.cjs';
export const name = cjsModule.name;
export default cjsModule;

This approach is appropriate for any of the following use cases:

  • The package is currently written in CommonJS and the author would prefer not to refactor it into ES module syntax, but wishes to provide named exports for ES module consumers.
  • The package has other packages that depend on it, and the end user might install both this package and those other packages. For example a utilities package is used directly in an application, and a utilities-plus package adds a few more functions to utilities. Because the wrapper exports underlying CommonJS files, it doesn’t matter if utilities-plus is written in CommonJS or ES module syntax; it will work either way.
  • The package stores internal state, and the package author would prefer not to refactor the package to isolate its state management. See the next section.

A variant of this approach not requiring conditional exports for consumers could be to add an export, e.g. "./module", to point to an all-ES module-syntax version of the package. This could be used via import 'pkg/module' by users who are certain that the CommonJS version will not be loaded anywhere in the application, such as by dependencies; or if the CommonJS version can be loaded but doesn’t affect the ES module version (for example, because the package is stateless):

// ./node_modules/pkg/package.json
{
  "type": "module",
  "main": "./index.cjs",
  "exports": {
    ".": "./index.cjs",
    "./module": "./wrapper.mjs"
  }
}
Approach #2: Isolate State#

A package.json file can define the separate CommonJS and ES module entry points directly:

// ./node_modules/pkg/package.json
{
  "type": "module",
  "main": "./index.cjs",
  "exports": {
    "import": "./index.mjs",
    "require": "./index.cjs"
  }
}

This can be done if both the CommonJS and ES module versions of the package are equivalent, for example because one is the transpiled output of the other; and the package’s management of state is carefully isolated (or the package is stateless).

The reason that state is an issue is because both the CommonJS and ES module versions of the package may get used within an application; for example, the user’s application code could import the ES module version while a dependency requires the CommonJS version. If that were to occur, two copies of the package would be loaded in memory and therefore two separate states would be present. This would likely cause hard-to-troubleshoot bugs.

Aside from writing a stateless package (if JavaScript’s Math were a package, for example, it would be stateless as all of its methods are static), there are some ways to isolate state so that it’s shared between the potentially loaded CommonJS and ES module instances of the package:

  1. If possible, contain all state within an instantiated object. JavaScript’s Date, for example, needs to be instantiated to contain state; if it were a package, it would be used like this:

    import Date from 'date';
const someDate = new Date();
// someDate contains state; Date does not

    The new keyword isn’t required; a package’s function can return a new object, or modify a passed-in object, to keep the state external to the package.

  2. Isolate the state in one or more CommonJS files that are shared between the CommonJS and ES module versions of the package. For example, if the CommonJS and ES module entry points are index.cjs and index.mjs, respectively:

    // ./node_modules/pkg/index.cjs
const state = require('./state.cjs');
module.exports.state = state;

    // ./node_modules/pkg/index.mjs
import state from './state.cjs';
export {
  state
};

    Even if pkg is used via both require and import in an application (for example, via import in application code and via require by a dependency) each reference of pkg will contain the same state; and modifying that state from either module system will apply to both.

Any plugins that attach to the package’s singleton would need to separately attach to both the CommonJS and ES module singletons.

This approach is appropriate for any of the following use cases:

  • The package is currently written in ES module syntax and the package author wants that version to be used wherever such syntax is supported.
  • The package is stateless or its state can be isolated without too much difficulty.
  • The package is unlikely to have other public packages that depend on it, or if it does, the package is stateless or has state that need not be shared between dependencies or with the overall application.

Even with isolated state, there is still the cost of possible extra code execution between the CommonJS and ES module versions of a package.

As with the previous approach, a variant of this approach not requiring conditional exports for consumers could be to add an export, e.g. "./module", to point to an all-ES module-syntax version of the package:

// ./node_modules/pkg/package.json
{
  "type": "module",
  "main": "./index.cjs",
  "exports": {
    ".": "./index.cjs",
    "./module": "./index.mjs"
  }
}

import Specifiers#

Terminology#

The specifier of an import statement is the string after the from keyword, e.g. 'path' in import { sep } from 'path'. Specifiers are also used in export from statements, and as the argument to an import() expression.

There are four types of specifiers:

  • Bare specifiers like 'some-package'. They refer to an entry point of a package by the package name.

  • Deep import specifiers like 'some-package/lib/shuffle.mjs'. They refer to a path within a package prefixed by the package name.

  • Relative specifiers like './startup.js' or '../config.mjs'. They refer to a path relative to the location of the importing file.

  • Absolute specifiers like 'file:///opt/nodejs/config.js'. They refer directly and explicitly to a full path.

Bare specifiers, and the bare specifier portion of deep import specifiers, are strings; but everything else in a specifier is a URL.

Only file: and data: URLs are supported. A specifier like 'https://example.com/app.js' may be supported by browsers but it is not supported in Node.js.

Specifiers may not begin with / or //. These are reserved for potential future use. The root of the current volume may be referenced via file:///.

data: Imports#

Added in: v12.10.0

data: URLs are supported for importing with the following MIME types:

  • text/javascript for ES Modules
  • application/json for JSON
  • application/wasm for WASM.

data: URLs only resolve Bare specifiers for builtin modules and Absolute specifiers. Resolving Relative specifiers will not work because data: is not a special scheme. For example, attempting to load ./foo from data:text/javascript,import "./foo"; will fail to resolve since there is no concept of relative resolution for data: URLs. An example of a data: URLs being used is:

import 'data:text/javascript,console.log("hello!");';
import _ from 'data:application/json,"world!"';

import.meta#

The import.meta metaproperty is an Object that contains the following property:

  • url <string> The absolute file: URL of the module.

Differences Between ES Modules and CommonJS#

Mandatory file extensions#

A file extension must be provided when using the import keyword. Directory indexes (e.g. './startup/index.js') must also be fully specified.

This behavior matches how import behaves in browser environments, assuming a typically configured server.

No NODE_PATH#

NODE_PATH is not part of resolving import specifiers. Please use symlinks if this behavior is desired.

No require, exports, module.exports, __filename, __dirname#

These CommonJS variables are not available in ES modules.

require can be imported into an ES module using module.createRequire().

Equivalents of __filename and __dirname can be created inside of each file via import.meta.url.

import { fileURLToPath } from 'url';
import { dirname } from 'path';

const __filename = fileURLToPath(import.meta.url);
const __dirname = dirname(__filename);

No require.resolve#

Former use cases relying on require.resolve to determine the resolved path of a module can be supported via import.meta.resolve, which is experimental and supported via the --experimental-import-meta-resolve flag:

(async () => {
  const dependencyAsset = await import.meta.resolve('component-lib/asset.css');
})();

import.meta.resolve also accepts a second argument which is the parent module from which to resolve from:

(async () => {
  // Equivalent to import.meta.resolve('./dep')
  await import.meta.resolve('./dep', import.meta.url);
})();

This function is asynchronous since the ES module resolver in Node.js is asynchronous. With the introduction of Top-Level Await, these use cases will be easier as they won't require an async function wrapper.

No require.extensions#

require.extensions is not used by import. The expectation is that loader hooks can provide this workflow in the future.

No require.cache#

require.cache is not used by import. It has a separate cache.

URL-based paths#

ES modules are resolved and cached based upon URL semantics. This means that files containing special characters such as # and ? need to be escaped.

Modules will be loaded multiple times if the import specifier used to resolve them have a different query or fragment.

import './foo.mjs?query=1'; // loads ./foo.mjs with query of "?query=1"
import './foo.mjs?query=2'; // loads ./foo.mjs with query of "?query=2"

For now, only modules using the file: protocol can be loaded.

Interoperability with CommonJS#

require#

require always treats the files it references as CommonJS. This applies whether require is used the traditional way within a CommonJS environment, or in an ES module environment using module.createRequire().

To include an ES module into CommonJS, use import().

import statements#

An import statement can reference an ES module or a CommonJS module. Other file types such as JSON or Native modules are not supported. For those, use module.createRequire().

import statements are permitted only in ES modules. For similar functionality in CommonJS, see import().

The specifier of an import statement (the string after the from keyword) can either be an URL-style relative path like './file.mjs' or a package name like 'fs'.

Like in CommonJS, files within packages can be accessed by appending a path to the package name; unless the package’s package.json contains an "exports" field, in which case files within packages need to be accessed via the path defined in "exports".

import { sin, cos } from 'geometry/trigonometry-functions.mjs';

Only the “default export” is supported for CommonJS files or packages:

import packageMain from 'commonjs-package'; // Works

import { method } from 'commonjs-package'; // Errors

It is also possible to import an ES or CommonJS module for its side effects only.

import() expressions#

Dynamic import() is supported in both CommonJS and ES modules. It can be used to include ES module files from CommonJS code.

CommonJS, JSON, and Native Modules#

CommonJS, JSON, and Native modules can be used with module.createRequire().

// cjs.cjs
module.exports = 'cjs';

// esm.mjs
import { createRequire } from 'module';

const require = createRequire(import.meta.url);

const cjs = require('./cjs.cjs');
cjs === 'cjs'; // true

Builtin modules#

Builtin modules will provide named exports of their public API. A default export is also provided which is the value of the CommonJS exports. The default export can be used for, among other things, modifying the named exports. Named exports of builtin modules are updated only by calling module.syncBuiltinESMExports().

import EventEmitter from 'events';
const e = new EventEmitter();

import { readFile } from 'fs';
readFile('./foo.txt', (err, source) => {
  if (err) {
    console.error(err);
  } else {
    console.log(source);
  }
});

import fs, { readFileSync } from 'fs';
import { syncBuiltinESMExports } from 'module';

fs.readFileSync = () => Buffer.from('Hello, ESM');
syncBuiltinESMExports();

fs.readFileSync === readFileSync;

Experimental JSON Modules#

Currently importing JSON modules are only supported in the commonjs mode and are loaded using the CJS loader. WHATWG JSON modules specification are still being standardized, and are experimentally supported by including the additional flag --experimental-json-modules when running Node.js.

When the --experimental-json-modules flag is included both the commonjs and module mode will use the new experimental JSON loader. The imported JSON only exposes a default, there is no support for named exports. A cache entry is created in the CommonJS cache, to avoid duplication. The same object will be returned in CommonJS if the JSON module has already been imported from the same path.

Assuming an index.mjs with

import packageConfig from './package.json';

The --experimental-json-modules flag is needed for the module to work.

node index.mjs # fails
node --experimental-json-modules index.mjs # works

Experimental Wasm Modules#

Importing Web Assembly modules is supported under the --experimental-wasm-modules flag, allowing any .wasm files to be imported as normal modules while also supporting their module imports.

This integration is in line with the ES Module Integration Proposal for Web Assembly.

For example, an index.mjs containing:

import * as M from './module.wasm';
console.log(M);

executed under:

node --experimental-wasm-modules index.mjs

would provide the exports interface for the instantiation of module.wasm.

Experimental Top-Level await#

When the --experimental-top-level-await flag is provided, await may be used in the top level (outside of async functions) within modules. This implements the ECMAScript Top-Level await proposal.

Assuming an a.mjs with

export const five = await Promise.resolve(5);

And a b.mjs with

import { five } from './a.mjs';

console.log(five); // Logs `5`

node b.mjs # fails
node --experimental-top-level-await b.mjs # works

Experimental Loaders#

Note: This API is currently being redesigned and will still change.

To customize the default module resolution, loader hooks can optionally be provided via a --experimental-loader ./loader-name.mjs argument to Node.js.

When hooks are used they only apply to ES module loading and not to any CommonJS modules loaded.

Hooks#

resolve hook#

Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.

The resolve hook returns the resolved file URL for a given module specifier and parent URL. The module specifier is the string in an import statement or import() expression, and the parent URL is the URL of the module that imported this one, or undefined if this is the main entry point for the application.

The conditions property on the context is an array of conditions for Conditional Exports that apply to this resolution request. They can be used for looking up conditional mappings elsewhere or to modify the list when calling the default resolution logic.

The current set of Node.js default conditions will always be in the context.conditions list passed to the hook. If the hook wants to ensure Node.js-compatible resolution logic, all items from this default condition list must be passed through to the defaultResolve function.

/**
 * @param {string} specifier
 * @param {object} context
 * @param {string} context.parentURL
 * @param {string[]} context.conditions
 * @param {function} defaultResolve
 * @returns {object} response
 * @returns {string} response.url
 */
export async function resolve(specifier, context, defaultResolve) {
  const { parentURL = null } = context;
  if (someCondition) {
    // For some or all specifiers, do some custom logic for resolving.
    // Always return an object of the form {url: <string>}
    return {
      url: (parentURL) ?
        new URL(specifier, parentURL).href : new URL(specifier).href
    };
  }
  if (anotherCondition) {
    // When calling the defaultResolve, the arguments can be modified. In this
    // case it's adding another value for matching conditional exports.
    return defaultResolve(specifier, {
      ...context,
      conditions: [...context.conditions, 'another-condition'],
    });
  }
  // Defer to Node.js for all other specifiers.
  return defaultResolve(specifier, context, defaultResolve);
}

getFormat hook#

Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.

The getFormat hook provides a way to define a custom method of determining how a URL should be interpreted. This can be one of the following:

formatDescription
'builtin'Load a Node.js builtin module
'commonjs'Load a Node.js CommonJS module
'dynamic'Use a dynamic instantiate hook
'json'Load a JSON file
'module'Load a standard JavaScript module (ES module)
'wasm'Load a WebAssembly module
/**
 * @param {string} url
 * @param {object} context (currently empty)
 * @param {function} defaultGetFormat
 * @returns {object} response
 * @returns {string} response.format
 */
export async function getFormat(url, context, defaultGetFormat) {
  if (someCondition) {
    // For some or all URLs, do some custom logic for determining format.
    // Always return an object of the form {format: <string>}, where the
    // format is one of the strings in the table above.
    return {
      format: 'module'
    };
  }
  // Defer to Node.js for all other URLs.
  return defaultGetFormat(url, context, defaultGetFormat);
}

getSource hook#

Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.

The getSource hook provides a way to define a custom method for retrieving the source code of an ES module specifier. This would allow a loader to potentially avoid reading files from disk.

/**
 * @param {string} url
 * @param {object} context
 * @param {string} context.format
 * @param {function} defaultGetSource
 * @returns {object} response
 * @returns {string|buffer} response.source
 */
export async function getSource(url, context, defaultGetSource) {
  const { format } = context;
  if (someCondition) {
    // For some or all URLs, do some custom logic for retrieving the source.
    // Always return an object of the form {source: <string|buffer>}.
    return {
      source: '...'
    };
  }
  // Defer to Node.js for all other URLs.
  return defaultGetSource(url, context, defaultGetSource);
}

transformSource hook#

Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.

The transformSource hook provides a way to modify the source code of a loaded ES module file after the source string has been loaded but before Node.js has done anything with it.

If this hook is used to convert unknown-to-Node.js file types into executable JavaScript, a resolve hook is also necessary in order to register any unknown-to-Node.js file extensions. See the transpiler loader example below.

/**
 * @param {string|buffer} source
 * @param {object} context
 * @param {string} context.url
 * @param {string} context.format
 * @param {function} defaultTransformSource
 * @returns {object} response
 * @returns {string|buffer} response.source
 */
export async function transformSource(source,
                                      context,
                                      defaultTransformSource) {
  const { url, format } = context;
  if (someCondition) {
    // For some or all URLs, do some custom logic for modifying the source.
    // Always return an object of the form {source: <string|buffer>}.
    return {
      source: '...'
    };
  }
  // Defer to Node.js for all other sources.
  return defaultTransformSource(
    source, context, defaultTransformSource);
}

getGlobalPreloadCode hook#

Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.

Sometimes it can be necessary to run some code inside of the same global scope that the application will run in. This hook allows to return a string that will be ran as sloppy-mode script on startup.

Similar to how CommonJS wrappers work, the code runs in an implicit function scope. The only argument is a require-like function that can be used to load builtins like "fs": getBuiltin(request: string).

If the code needs more advanced require features, it will have to construct its own require using module.createRequire().

/**
 * @returns {string} Code to run before application startup
 */
export function getGlobalPreloadCode() {
  return `\
globalThis.someInjectedProperty = 42;
console.log('I just set some globals!');

const { createRequire } = getBuiltin('module');

const require = createRequire(process.cwd() + '/<preload>');
// [...]
`;
}

dynamicInstantiate hook#

Note: The loaders API is being redesigned. This hook may disappear or its signature may change. Do not rely on the API described below.

To create a custom dynamic module that doesn't correspond to one of the existing format interpretations, the dynamicInstantiate hook can be used. This hook is called only for modules that return format: 'dynamic' from the getFormat hook.

/**
 * @param {string} url
 * @returns {object} response
 * @returns {array} response.exports
 * @returns {function} response.execute
 */
export async function dynamicInstantiate(url) {
  return {
    exports: ['customExportName'],
    execute: (exports) => {
      // Get and set functions provided for pre-allocated export names
      exports.customExportName.set('value');
    }
  };
}

With the list of module exports provided upfront, the execute function will then be called at the exact point of module evaluation order for that module in the import tree.

Examples#

The various loader hooks can be used together to accomplish wide-ranging customizations of Node.js’ code loading and evaluation behaviors.

HTTPS loader#

In current Node.js, specifiers starting with https:// are unsupported. The loader below registers hooks to enable rudimentary support for such specifiers. While this may seem like a significant improvement to Node.js core functionality, there are substantial downsides to actually using this loader: performance is much slower than loading files from disk, there is no caching, and there is no security.

// https-loader.mjs
import { get } from 'https';

export function resolve(specifier, context, defaultResolve) {
  const { parentURL = null } = context;

  // Normally Node.js would error on specifiers starting with 'https://', so
  // this hook intercepts them and converts them into absolute URLs to be
  // passed along to the later hooks below.
  if (specifier.startsWith('https://')) {
    return {
      url: specifier
    };
  } else if (parentURL && parentURL.startsWith('https://')) {
    return {
      url: new URL(specifier, parentURL).href
    };
  }

  // Let Node.js handle all other specifiers.
  return defaultResolve(specifier, context, defaultResolve);
}

export function getFormat(url, context, defaultGetFormat) {
  // This loader assumes all network-provided JavaScript is ES module code.
  if (url.startsWith('https://')) {
    return {
      format: 'module'
    };
  }

  // Let Node.js handle all other URLs.
  return defaultGetFormat(url, context, defaultGetFormat);
}

export function getSource(url, context, defaultGetSource) {
  // For JavaScript to be loaded over the network, we need to fetch and
  // return it.
  if (url.startsWith('https://')) {
    return new Promise((resolve, reject) => {
      get(url, (res) => {
        let data = '';
        res.on('data', (chunk) => data += chunk);
        res.on('end', () => resolve({ source: data }));
      }).on('error', (err) => reject(err));
    });
  }

  // Let Node.js handle all other URLs.
  return defaultGetSource(url, context, defaultGetSource);
}

// main.mjs
import { VERSION } from 'https://coffeescript.org/browser-compiler-modern/coffeescript.js';

console.log(VERSION);

With this loader, running:

node --experimental-loader ./https-loader.mjs ./main.js

Will print the current version of CoffeeScript per the module at the URL in main.mjs.

Transpiler loader#

Sources that are in formats Node.js doesn’t understand can be converted into JavaScript using the transformSource hook. Before that hook gets called, however, other hooks need to tell Node.js not to throw an error on unknown file types; and to tell Node.js how to load this new file type.

This is less performant than transpiling source files before running Node.js; a transpiler loader should only be used for development and testing purposes.

// coffeescript-loader.mjs
import { URL, pathToFileURL } from 'url';
import CoffeeScript from 'coffeescript';

const baseURL = pathToFileURL(`${process.cwd()}/`).href;

// CoffeeScript files end in .coffee, .litcoffee or .coffee.md.
const extensionsRegex = /\.coffee$|\.litcoffee$|\.coffee\.md$/;

export function resolve(specifier, context, defaultResolve) {
  const { parentURL = baseURL } = context;

  // Node.js normally errors on unknown file extensions, so return a URL for
  // specifiers ending in the CoffeeScript file extensions.
  if (extensionsRegex.test(specifier)) {
    return {
      url: new URL(specifier, parentURL).href
    };
  }

  // Let Node.js handle all other specifiers.
  return defaultResolve(specifier, context, defaultResolve);
}

export function getFormat(url, context, defaultGetFormat) {
  // Now that we patched resolve to let CoffeeScript URLs through, we need to
  // tell Node.js what format such URLs should be interpreted as. For the
  // purposes of this loader, all CoffeeScript URLs are ES modules.
  if (extensionsRegex.test(url)) {
    return {
      format: 'module'
    };
  }

  // Let Node.js handle all other URLs.
  return defaultGetFormat(url, context, defaultGetFormat);
}

export function transformSource(source, context, defaultTransformSource) {
  const { url, format } = context;

  if (extensionsRegex.test(url)) {
    return {
      source: CoffeeScript.compile(source, { bare: true })
    };
  }

  // Let Node.js handle all other sources.
  return defaultTransformSource(source, context, defaultTransformSource);
}

# main.coffee
import { scream } from './scream.coffee'
console.log scream 'hello, world'

import { version } from 'process'
console.log "Brought to you by Node.js version #{version}"

# scream.coffee
export scream = (str) -> str.toUpperCase()

With this loader, running:

node --experimental-loader ./coffeescript-loader.mjs main.coffee

Will cause main.coffee to be turned into JavaScript after its source code is loaded from disk but before Node.js executes it; and so on for any .coffee, .litcoffee or .coffee.md files referenced via import statements of any loaded file.

Resolution Algorithm#

Features#

The resolver has the following properties:

  • FileURL-based resolution as is used by ES modules
  • Support for builtin module loading
  • Relative and absolute URL resolution
  • No default extensions
  • No folder mains
  • Bare specifier package resolution lookup through node_modules

Resolver Algorithm#

The algorithm to load an ES module specifier is given through the ESM_RESOLVE method below. It returns the resolved URL for a module specifier relative to a parentURL.

The algorithm to determine the module format of a resolved URL is provided by ESM_FORMAT, which returns the unique module format for any file. The "module" format is returned for an ECMAScript Module, while the "commonjs" format is used to indicate loading through the legacy CommonJS loader. Additional formats such as "addon" can be extended in future updates.

In the following algorithms, all subroutine errors are propagated as errors of these top-level routines unless stated otherwise.

defaultEnv is the conditional environment name priority array, ["node", "import"].

The resolver can throw the following errors:

  • Invalid Module Specifier: Module specifier is an invalid URL, package name or package subpath specifier.
  • Invalid Package Configuration: package.json configuration is invalid or contains an invalid configuration.
  • Invalid Package Target: Package exports define a target module within the package that is an invalid type or string target.
  • Package Path Not Exported: Package exports do not define or permit a target subpath in the package for the given module.
  • Module Not Found: The package or module requested does not exist.
Resolver algorithm specification

ESM_RESOLVE(specifier, parentURL)

  1. Let resolvedURL be undefined.

  2. If specifier is a valid URL, then

    1. Set resolvedURL to the result of parsing and reserializing specifier as a URL.

  3. Otherwise, if specifier starts with "/", then

    1. Throw an Invalid Module Specifier error.

  4. Otherwise, if specifier starts with "./" or "../", then

    1. Set resolvedURL to the URL resolution of specifier relative to parentURL.

  5. Otherwise,

    1. Note: specifier is now a bare specifier.
    2. Set resolvedURL the result of PACKAGE_RESOLVE(specifier, parentURL).

  6. If resolvedURL contains any percent encodings of "/" or "\" ("%2f" and "%5C" respectively), then

    1. Throw an Invalid Module Specifier error.

  7. If the file at resolvedURL is a directory, then

    1. Throw an Unsupported Directory Import error.

  8. If the file at resolvedURL does not exist, then

    1. Throw a Module Not Found error.
  9. Set resolvedURL to the real path of resolvedURL.
  10. Let format be the result of ESM_FORMAT(resolvedURL).
  11. Load resolvedURL as module format, format.
  12. Return resolvedURL.

PACKAGE_RESOLVE(packageSpecifier, parentURL)

  1. Let packageName be undefined.
  2. Let packageSubpath be undefined.

  3. If packageSpecifier is an empty string, then

    1. Throw an Invalid Module Specifier error.

  4. Otherwise,

    1. If packageSpecifier does not contain a "/" separator, then

      1. Throw an Invalid Module Specifier error.
    2. Set packageName to the substring of packageSpecifier until the second "/" separator or the end of the string.

  5. If packageName starts with "." or contains "\" or "%", then

    1. Throw an Invalid Module Specifier error.
  6. Let packageSubpath be undefined.

  7. If the length of packageSpecifier is greater than the length of packageName, then

    1. Set packageSubpath to "." concatenated with the substring of packageSpecifier from the position at the length of packageName.

  8. If packageSubpath contains any "." or ".." segments or percent encoded strings for "/" or "\", then

    1. Throw an Invalid Module Specifier error.
  9. Set selfUrl to the result of SELF_REFERENCE_RESOLVE(packageName, packageSubpath, parentURL).
  10. If selfUrl isn't empty, return selfUrl.

  11. If packageSubpath is undefined and packageName is a Node.js builtin module, then

    1. Return the string "nodejs:" concatenated with packageSpecifier.

  12. While parentURL is not the file system root,

    1. Let packageURL be the URL resolution of "node_modules/" concatenated with packageSpecifier, relative to parentURL.
    2. Set parentURL to the parent folder URL of parentURL.

    3. If the folder at packageURL does not exist, then

      1. Set parentURL to the parent URL path of parentURL.
      2. Continue the next loop iteration.
    4. Let pjson be the result of READ_PACKAGE_JSON(packageURL).

    5. If packageSubpath is equal to "./", then

      1. Return packageURL + "/".

    6. If packageSubpath is _undefined__, then

      1. Return the result of PACKAGE_MAIN_RESOLVE(packageURL, pjson).

    7. Otherwise,

      1. If pjson is not null and pjson has an "exports" key, then

        1. Let exports be pjson.exports.

        2. If exports is not null or undefined, then

          1. Return PACKAGE_EXPORTS_RESOLVE(packageURL, packageSubpath, pjson.exports).
      2. Return the URL resolution of packageSubpath in packageURL.
  13. Throw a Module Not Found error.

SELF_REFERENCE_RESOLVE(packageName, packageSubpath, parentURL)

  1. Let packageURL be the result of READ_PACKAGE_SCOPE(parentURL).

  2. If packageURL is null, then

    1. Return undefined.
  3. Let pjson be the result of READ_PACKAGE_JSON(packageURL).

  4. If pjson does not include an "exports" property, then

    1. Return undefined.

  5. If pjson.name is equal to packageName, then

    1. If packageSubpath is equal to "./", then

      1. Return packageURL + "/".

    2. If packageSubpath is undefined, then

      1. Return the result of PACKAGE_MAIN_RESOLVE(packageURL, pjson).

    3. Otherwise,

      1. If pjson is not null and pjson has an "exports" key, then

        1. Let exports be pjson.exports.

        2. If exports is not null or undefined, then

          1. Return PACKAGE_EXPORTS_RESOLVE(packageURL, subpath, pjson.exports).
      2. Return the URL resolution of subpath in packageURL.
  6. Otherwise, return undefined.

PACKAGE_MAIN_RESOLVE(packageURL, pjson)

  1. If pjson is null, then

    1. Throw a Module Not Found error.

  2. If pjson.exports is not null or undefined, then

    1. If exports is an Object with both a key starting with "." and a key not starting with ".", throw an Invalid Package Configuration error.

    2. If pjson.exports is a String or Array, or an Object containing no keys starting with ".", then

      1. Return PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, pjson.exports, "").

    3. If pjson.exports is an Object containing a "." property, then

      1. Let mainExport be the "." property in pjson.exports.
      2. Return PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, mainExport, "").
    4. Throw a Package Path Not Exported error.
  3. Let legacyMainURL be the result applying the legacy LOAD_AS_DIRECTORY CommonJS resolver to packageURL, throwing a Module Not Found error for no resolution.
  4. Return legacyMainURL.

PACKAGE_EXPORTS_RESOLVE(packageURL, packagePath, exports)

  1. If exports is an Object with both a key starting with "." and a key not starting with ".", throw an Invalid Package Configuration error.

  2. If exports is an Object and all keys of exports start with ".", then

    1. Set packagePath to "./" concatenated with packagePath.

    2. If packagePath is a key of exports, then

      1. Let target be the value of exports[packagePath].
      2. Return PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, target, "", defaultEnv).
    3. Let directoryKeys be the list of keys of exports ending in "/", sorted by length descending.

    4. For each key directory in directoryKeys, do

      1. If packagePath starts with directory, then

        1. Let target be the value of exports[directory].
        2. Let subpath be the substring of target starting at the index of the length of directory.
        3. Return PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, target, subpath, defaultEnv).
  3. Throw a Package Path Not Exported error.

PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, target, subpath, env)

  1. If target is a String, then

    1. If target does not start with "./" or contains any "node_modules" segments including "node_modules" percent-encoding, throw an Invalid Package Target error.
    2. Let resolvedTarget be the URL resolution of the concatenation of packageURL and target.
    3. If resolvedTarget is not contained in packageURL, throw an Invalid Package Target error.
    4. If subpath has non-zero length and target does not end with "/", throw an Invalid Module Specifier error.
    5. Let resolved be the URL resolution of the concatenation of subpath and resolvedTarget.
    6. If resolved is not contained in resolvedTarget, throw an Invalid Module Specifier error.
    7. Return resolved.

  2. Otherwise, if target is a non-null Object, then

    1. If exports contains any index property keys, as defined in ECMA-262 6.1.7 Array Index, throw an Invalid Package Configuration error.

    2. For each property p of target, in object insertion order as,

      1. If p equals "default" or env contains an entry for p, then

        1. Let targetValue be the value of the p property in target.
        2. Return the result of PACKAGE_EXPORTS_TARGET_RESOLVE( packageURL, targetValue, subpath, env), continuing the loop on any Package Path Not Exported error.
    3. Throw a Package Path Not Exported error.

  3. Otherwise, if target is an Array, then

    1. If target.length is zero, throw a _Package Path Not Exported error.

    2. For each item targetValue in target, do

      1. If targetValue is an Array, continue the loop.
      2. Return the result of PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, targetValue, subpath, env), continuing the loop on any Package Path Not Exported or Invalid Package Target error.
    3. Throw the last fallback resolution error.
  4. Otherwise, if target is null, throw a Package Path Not Exported error.
  5. Otherwise throw an Invalid Package Target error.

ESM_FORMAT(url)

  1. Assert: url corresponds to an existing file.
  2. Let pjson be the result of READ_PACKAGE_SCOPE(url).

  3. If url ends in ".mjs", then

    1. Return "module".

  4. If url ends in ".cjs", then

    1. Return "commonjs".

  5. If pjson?.type exists and is "module", then

    1. If url ends in ".js", then

      1. Return "module".
    2. Throw an Unsupported File Extension error.

  6. Otherwise,

    1. Throw an Unsupported File Extension error.

READ_PACKAGE_SCOPE(url)

  1. Let scopeURL be url.

  2. While scopeURL is not the file system root,

    1. If scopeURL ends in a "node_modules" path segment, return null.
    2. Let pjson be the result of READ_PACKAGE_JSON(scopeURL).

    3. If pjson is not null, then

      1. Return pjson.
    4. Set scopeURL to the parent URL of scopeURL.
  3. Return null.

READ_PACKAGE_JSON(packageURL)

  1. Let pjsonURL be the resolution of "package.json" within packageURL.

  2. If the file at pjsonURL does not exist, then

    1. Return null.

  3. If the file at packageURL does not parse as valid JSON, then

    1. Throw an Invalid Package Configuration error.
  4. Return the parsed JSON source of the file at pjsonURL.

Customizing ESM specifier resolution algorithm#

The current specifier resolution does not support all default behavior of the CommonJS loader. One of the behavior differences is automatic resolution of file extensions and the ability to import directories that have an index file.

The --experimental-specifier-resolution=[mode] flag can be used to customize the extension resolution algorithm. The default mode is explicit, which requires the full path to a module be provided to the loader. To enable the automatic extension resolution and importing from directories that include an index file use the node mode.

$ node index.mjs
success!
$ node index # Failure!
Error: Cannot find module
$ node --experimental-specifier-resolution=node index
success!

Errors#

Applications running in Node.js will generally experience four categories of errors:

  • Standard JavaScript errors such as <EvalError>, <SyntaxError>, <RangeError>, <ReferenceError>, <TypeError>, and <URIError>.
  • System errors triggered by underlying operating system constraints such as attempting to open a file that does not exist or attempting to send data over a closed socket.
  • User-specified errors triggered by application code.
  • AssertionErrors are a special class of error that can be triggered when Node.js detects an exceptional logic violation that should never occur. These are raised typically by the assert module.

All JavaScript and System errors raised by Node.js inherit from, or are instances of, the standard JavaScript <Error> class and are guaranteed to provide at least the properties available on that class.

Error Propagation and Interception#

Node.js supports several mechanisms for propagating and handling errors that occur while an application is running. How these errors are reported and handled depends entirely on the type of Error and the style of the API that is called.

All JavaScript errors are handled as exceptions that immediately generate and throw an error using the standard JavaScript throw mechanism. These are handled using the try…catch construct provided by the JavaScript language.

// Throws with a ReferenceError because z is not defined.
try {
  const m = 1;
  const n = m + z;
} catch (err) {
  // Handle the error here.
}

Any use of the JavaScript throw mechanism will raise an exception that must be handled using try…catch or the Node.js process will exit immediately.

With few exceptions, Synchronous APIs (any blocking method that does not accept a callback function, such as fs.readFileSync), will use throw to report errors.

Errors that occur within Asynchronous APIs may be reported in multiple ways:

  • Most asynchronous methods that accept a callback function will accept an Error object passed as the first argument to that function. If that first argument is not null and is an instance of Error, then an error occurred that should be handled.
const fs = require('fs');
fs.readFile('a file that does not exist', (err, data) => {
  if (err) {
    console.error('There was an error reading the file!', err);
    return;
  }
  // Otherwise handle the data
});

  • When an asynchronous method is called on an object that is an EventEmitter, errors can be routed to that object's 'error' event.

    const net = require('net');
const connection = net.connect('localhost');

// Adding an 'error' event handler to a stream:
connection.on('error', (err) => {
  // If the connection is reset by the server, or if it can't
  // connect at all, or on any sort of error encountered by
  // the connection, the error will be sent here.
  console.error(err);
});

connection.pipe(process.stdout);

  • A handful of typically asynchronous methods in the Node.js API may still use the throw mechanism to raise exceptions that must be handled using try…catch. There is no comprehensive list of such methods; please refer to the documentation of each method to determine the appropriate error handling mechanism required.

The use of the 'error' event mechanism is most common for stream-based and event emitter-based APIs, which themselves represent a series of asynchronous operations over time (as opposed to a single operation that may pass or fail).

For all EventEmitter objects, if an 'error' event handler is not provided, the error will be thrown, causing the Node.js process to report an uncaught exception and crash unless either: The domain module is used appropriately or a handler has been registered for the 'uncaughtException' event.

const EventEmitter = require('events');
const ee = new EventEmitter();

setImmediate(() => {
  // This will crash the process because no 'error' event
  // handler has been added.
  ee.emit('error', new Error('This will crash'));
});

Errors generated in this way cannot be intercepted using try…catch as they are thrown after the calling code has already exited.

Developers must refer to the documentation for each method to determine exactly how errors raised by those methods are propagated.

Error-first callbacks#

Most asynchronous methods exposed by the Node.js core API follow an idiomatic pattern referred to as an error-first callback. With this pattern, a callback function is passed to the method as an argument. When the operation either completes or an error is raised, the callback function is called with the Error object (if any) passed as the first argument. If no error was raised, the first argument will be passed as null.

const fs = require('fs');

function errorFirstCallback(err, data) {
  if (err) {
    console.error('There was an error', err);
    return;
  }
  console.log(data);
}

fs.readFile('/some/file/that/does-not-exist', errorFirstCallback);
fs.readFile('/some/file/that/does-exist', errorFirstCallback);

The JavaScript try…catch mechanism cannot be used to intercept errors generated by asynchronous APIs. A common mistake for beginners is to try to use throw inside an error-first callback:

// THIS WILL NOT WORK:
const fs = require('fs');

try {
  fs.readFile('/some/file/that/does-not-exist', (err, data) => {
    // Mistaken assumption: throwing here...
    if (err) {
      throw err;
    }
  });
} catch (err) {
  // This will not catch the throw!
  console.error(err);
}

This will not work because the callback function passed to fs.readFile() is called asynchronously. By the time the callback has been called, the surrounding code, including the try…catch block, will have already exited. Throwing an error inside the callback can crash the Node.js process in most cases. If domains are enabled, or a handler has been registered with process.on('uncaughtException'), such errors can be intercepted.

Class: Error#

A generic JavaScript <Error> object that does not denote any specific circumstance of why the error occurred. Error objects capture a "stack trace" detailing the point in the code at which the Error was instantiated, and may provide a text description of the error.

All errors generated by Node.js, including all System and JavaScript errors, will either be instances of, or inherit from, the Error class.

new Error(message)#

Creates a new Error object and sets the error.message property to the provided text message. If an object is passed as message, the text message is generated by calling message.toString(). The error.stack property will represent the point in the code at which new Error() was called. Stack traces are dependent on V8's stack trace API. Stack traces extend only to either (a) the beginning of synchronous code execution, or (b) the number of frames given by the property Error.stackTraceLimit, whichever is smaller.

Error.captureStackTrace(targetObject[, constructorOpt])#

Creates a .stack property on targetObject, which when accessed returns a string representing the location in the code at which Error.captureStackTrace() was called.

const myObject = {};
Error.captureStackTrace(myObject);
myObject.stack;  // Similar to `new Error().stack`

The first line of the trace will be prefixed with ${myObject.name}: ${myObject.message}.

The optional constructorOpt argument accepts a function. If given, all frames above constructorOpt, including constructorOpt, will be omitted from the generated stack trace.

The constructorOpt argument is useful for hiding implementation details of error generation from an end user. For instance:

function MyError() {
  Error.captureStackTrace(this, MyError);
}

// Without passing MyError to captureStackTrace, the MyError
// frame would show up in the .stack property. By passing
// the constructor, we omit that frame, and retain all frames below it.
new MyError().stack;

Error.stackTraceLimit#

The Error.stackTraceLimit property specifies the number of stack frames collected by a stack trace (whether generated by new Error().stack or Error.captureStackTrace(obj)).

The default value is 10 but may be set to any valid JavaScript number. Changes will affect any stack trace captured after the value has been changed.

If set to a non-number value, or set to a negative number, stack traces will not capture any frames.

error.code#

The error.code property is a string label that identifies the kind of error. error.code is the most stable way to identify an error. It will only change between major versions of Node.js. In contrast, error.message strings may change between any versions of Node.js. See Node.js Error Codes for details about specific codes.

error.message#

The error.message property is the string description of the error as set by calling new Error(message). The message passed to the constructor will also appear in the first line of the stack trace of the Error, however changing this property after the Error object is created may not change the first line of the stack trace (for example, when error.stack is read before this property is changed).

const err = new Error('The message');
console.error(err.message);
// Prints: The message

error.stack#

The error.stack property is a string describing the point in the code at which the Error was instantiated.

Error: Things keep happening!
   at /home/gbusey/file.js:525:2
   at Frobnicator.refrobulate (/home/gbusey/business-logic.js:424:21)
   at Actor.<anonymous> (/home/gbusey/actors.js:400:8)
   at increaseSynergy (/home/gbusey/actors.js:701:6)

The first line is formatted as <error class name>: <error message>, and is followed by a series of stack frames (each line beginning with "at "). Each frame describes a call site within the code that lead to the error being generated. V8 attempts to display a name for each function (by variable name, function name, or object method name), but occasionally it will not be able to find a suitable name. If V8 cannot determine a name for the function, only location information will be displayed for that frame. Otherwise, the determined function name will be displayed with location information appended in parentheses.

Frames are only generated for JavaScript functions. If, for example, execution synchronously passes through a C++ addon function called cheetahify which itself calls a JavaScript function, the frame representing the cheetahify call will not be present in the stack traces:

const cheetahify = require('./native-binding.node');

function makeFaster() {
  // `cheetahify()` *synchronously* calls speedy.
  cheetahify(function speedy() {
    throw new Error('oh no!');
  });
}

makeFaster();
// will throw:
//   /home/gbusey/file.js:6
//       throw new Error('oh no!');
//           ^
//   Error: oh no!
//       at speedy (/home/gbusey/file.js:6:11)
//       at makeFaster (/home/gbusey/file.js:5:3)
//       at Object.<anonymous> (/home/gbusey/file.js:10:1)
//       at Module._compile (module.js:456:26)
//       at Object.Module._extensions..js (module.js:474:10)
//       at Module.load (module.js:356:32)
//       at Function.Module._load (module.js:312:12)
//       at Function.Module.runMain (module.js:497:10)
//       at startup (node.js:119:16)
//       at node.js:906:3

The location information will be one of:

  • native, if the frame represents a call internal to V8 (as in [].forEach).
  • plain-filename.js:line:column, if the frame represents a call internal to Node.js.
  • /absolute/path/to/file.js:line:column, if the frame represents a call in a user program, or its dependencies.

The string representing the stack trace is lazily generated when the error.stack property is accessed.

The number of frames captured by the stack trace is bounded by the smaller of Error.stackTraceLimit or the number of available frames on the current event loop tick.

Class: AssertionError#

Indicates the failure of an assertion. For details, see Class: assert.AssertionError.

Class: RangeError#

Indicates that a provided argument was not within the set or range of acceptable values for a function; whether that is a numeric range, or outside the set of options for a given function parameter.

require('net').connect(-1);
// Throws "RangeError: "port" option should be >= 0 and < 65536: -1"

Node.js will generate and throw RangeError instances immediately as a form of argument validation.

Class: ReferenceError#

Indicates that an attempt is being made to access a variable that is not defined. Such errors commonly indicate typos in code, or an otherwise broken program.

While client code may generate and propagate these errors, in practice, only V8 will do so.

doesNotExist;
// Throws ReferenceError, doesNotExist is not a variable in this program.

Unless an application is dynamically generating and running code, ReferenceError instances should always be considered a bug in the code or its dependencies.

Class: SyntaxError#

Indicates that a program is not valid JavaScript. These errors may only be generated and propagated as a result of code evaluation. Code evaluation may happen as a result of eval, Function, require, or vm. These errors are almost always indicative of a broken program.

try {
  require('vm').runInThisContext('binary ! isNotOk');
} catch (err) {
  // 'err' will be a SyntaxError.
}

SyntaxError instances are unrecoverable in the context that created them – they may only be caught by other contexts.

Class: SystemError#

Node.js generates system errors when exceptions occur within its runtime environment. These usually occur when an application violates an operating system constraint. For example, a system error will occur if an application attempts to read a file that does not exist.

  • address <string> If present, the address to which a network connection failed
  • code <string> The string error code
  • dest <string> If present, the file path destination when reporting a file system error
  • errno <number> The system-provided error number
  • info <Object> If present, extra details about the error condition
  • message <string> A system-provided human-readable description of the error
  • path <string> If present, the file path when reporting a file system error
  • port <number> If present, the network connection port that is not available
  • syscall <string> The name of the system call that triggered the error

error.address#

If present, error.address is a string describing the address to which a network connection failed.

error.code#

The error.code property is a string representing the error code.

error.dest#

If present, error.dest is the file path destination when reporting a file system error.

error.errno#

The error.errno property is a negative number which corresponds to the error code defined in libuv Error handling.

On Windows the error number provided by the system will be normalized by libuv.

To get the string representation of the error code, use util.getSystemErrorName(error.errno).

error.info#

If present, error.info is an object with details about the error condition.

error.message#

error.message is a system-provided human-readable description of the error.

error.path#

If present, error.path is a string containing a relevant invalid pathname.

error.port#

If present, error.port is the network connection port that is not available.

error.syscall#

The error.syscall property is a string describing the syscall that failed.

Common System Errors#

This is a list of system errors commonly-encountered when writing a Node.js program. For a comprehensive list, see the errno(3) man page.

  • EACCES (Permission denied): An attempt was made to access a file in a way forbidden by its file access permissions.

  • EADDRINUSE (Address already in use): An attempt to bind a server (net, http, or https) to a local address failed due to another server on the local system already occupying that address.

  • ECONNREFUSED (Connection refused): No connection could be made because the target machine actively refused it. This usually results from trying to connect to a service that is inactive on the foreign host.

  • ECONNRESET (Connection reset by peer): A connection was forcibly closed by a peer. This normally results from a loss of the connection on the remote socket due to a timeout or reboot. Commonly encountered via the http and net modules.

  • EEXIST (File exists): An existing file was the target of an operation that required that the target not exist.

  • EISDIR (Is a directory): An operation expected a file, but the given pathname was a directory.

  • EMFILE (Too many open files in system): Maximum number of file descriptors allowable on the system has been reached, and requests for another descriptor cannot be fulfilled until at least one has been closed. This is encountered when opening many files at once in parallel, especially on systems (in particular, macOS) where there is a low file descriptor limit for processes. To remedy a low limit, run ulimit -n 2048 in the same shell that will run the Node.js process.

  • ENOENT (No such file or directory): Commonly raised by fs operations to indicate that a component of the specified pathname does not exist. No entity (file or directory) could be found by the given path.

  • ENOTDIR (Not a directory): A component of the given pathname existed, but was not a directory as expected. Commonly raised by fs.readdir.

  • ENOTEMPTY (Directory not empty): A directory with entries was the target of an operation that requires an empty directory, usually fs.unlink.

  • ENOTFOUND (DNS lookup failed): Indicates a DNS failure of either EAI_NODATA or EAI_NONAME. This is not a standard POSIX error.

  • EPERM (Operation not permitted): An attempt was made to perform an operation that requires elevated privileges.

  • EPIPE (Broken pipe): A write on a pipe, socket, or FIFO for which there is no process to read the data. Commonly encountered at the net and http layers, indicative that the remote side of the stream being written to has been closed.

  • ETIMEDOUT (Operation timed out): A connect or send request failed because the connected party did not properly respond after a period of time. Usually encountered by http or net. Often a sign that a socket.end() was not properly called.

Class: TypeError#

Indicates that a provided argument is not an allowable type. For example, passing a function to a parameter which expects a string would be considered a TypeError.

require('url').parse(() => { });
// Throws TypeError, since it expected a string.

Node.js will generate and throw TypeError instances immediately as a form of argument validation.

Exceptions vs. Errors#

A JavaScript exception is a value that is thrown as a result of an invalid operation or as the target of a throw statement. While it is not required that these values are instances of Error or classes which inherit from Error, all exceptions thrown by Node.js or the JavaScript runtime will be instances of Error.

Some exceptions are unrecoverable at the JavaScript layer. Such exceptions will always cause the Node.js process to crash. Examples include assert() checks or abort() calls in the C++ layer.

OpenSSL Errors#

Errors originating in crypto or tls are of class Error, and in addition to the standard .code and .message properties, may have some additional OpenSSL-specific properties.

error.opensslErrorStack#

An array of errors that can give context to where in the OpenSSL library an error originates from.

error.function#

The OpenSSL function the error originates in.

error.library#

The OpenSSL library the error originates in.

error.reason#

A human-readable string describing the reason for the error.

Node.js Error Codes#

ERR_AMBIGUOUS_ARGUMENT#

A function argument is being used in a way that suggests that the function signature may be misunderstood. This is thrown by the assert module when the message parameter in assert.throws(block, message) matches the error message thrown by block because that usage suggests that the user believes message is the expected message rather than the message the AssertionError will display if block does not throw.

ERR_ARG_NOT_ITERABLE#

An iterable argument (i.e. a value that works with for...of loops) was required, but not provided to a Node.js API.

ERR_ASSERTION#

A special type of error that can be triggered whenever Node.js detects an exceptional logic violation that should never occur. These are raised typically by the assert module.

ERR_ASYNC_CALLBACK#

An attempt was made to register something that is not a function as an AsyncHooks callback.

ERR_ASYNC_TYPE#

The type of an asynchronous resource was invalid. Users are also able to define their own types if using the public embedder API.

ERR_BROTLI_COMPRESSION_FAILED#

Data passed to a Brotli stream was not successfully compressed.

ERR_BROTLI_INVALID_PARAM#

An invalid parameter key was passed during construction of a Brotli stream.

ERR_BUFFER_CONTEXT_NOT_AVAILABLE#

An attempt was made to create a Node.js Buffer instance from addon or embedder code, while in a JS engine Context that is not associated with a Node.js instance. The data passed to the Buffer method will have been released by the time the method returns.

When encountering this error, a possible alternative to creating a Buffer instance is to create a normal Uint8Array, which only differs in the prototype of the resulting object. Uint8Arrays are generally accepted in all Node.js core APIs where Buffers are; they are available in all Contexts.

ERR_BUFFER_OUT_OF_BOUNDS#

An operation outside the bounds of a Buffer was attempted.

ERR_BUFFER_TOO_LARGE#

An attempt has been made to create a Buffer larger than the maximum allowed size.

ERR_CANNOT_WATCH_SIGINT#

Node.js was unable to watch for the SIGINT signal.

ERR_CHILD_CLOSED_BEFORE_REPLY#

A child process was closed before the parent received a reply.

ERR_CHILD_PROCESS_IPC_REQUIRED#

Used when a child process is being forked without specifying an IPC channel.

ERR_CHILD_PROCESS_STDIO_MAXBUFFER#

Used when the main process is trying to read data from the child process's STDERR/STDOUT, and the data's length is longer than the maxBuffer option.

ERR_CONSOLE_WRITABLE_STREAM#

Console was instantiated without stdout stream, or Console has a non-writable stdout or stderr stream.

ERR_CONTEXT_NOT_INITIALIZED#

The vm context passed into the API is not yet initialized. This could happen when an error occurs (and is caught) during the creation of the context, for example, when the allocation fails or the maximum call stack size is reached when the context is created.

ERR_CONSTRUCT_CALL_REQUIRED#

A constructor for a class was called without new.

ERR_CONSTRUCT_CALL_INVALID#

A class constructor was called that is not callable.

ERR_CPU_USAGE#

The native call from process.cpuUsage could not be processed.

ERR_CRYPTO_CUSTOM_ENGINE_NOT_SUPPORTED#

A client certificate engine was requested that is not supported by the version of OpenSSL being used.

ERR_CRYPTO_ECDH_INVALID_FORMAT#

An invalid value for the format argument was passed to the crypto.ECDH() class getPublicKey() method.

ERR_CRYPTO_ECDH_INVALID_PUBLIC_KEY#

An invalid value for the key argument has been passed to the crypto.ECDH() class computeSecret() method. It means that the public key lies outside of the elliptic curve.

ERR_CRYPTO_ENGINE_UNKNOWN#

An invalid crypto engine identifier was passed to require('crypto').setEngine().

ERR_CRYPTO_FIPS_FORCED#

The --force-fips command-line argument was used but there was an attempt to enable or disable FIPS mode in the crypto module.

ERR_CRYPTO_FIPS_UNAVAILABLE#

An attempt was made to enable or disable FIPS mode, but FIPS mode was not available.

ERR_CRYPTO_HASH_FINALIZED#

hash.digest() was called multiple times. The hash.digest() method must be called no more than one time per instance of a Hash object.

ERR_CRYPTO_HASH_UPDATE_FAILED#

hash.update() failed for any reason. This should rarely, if ever, happen.

ERR_CRYPTO_INCOMPATIBLE_KEY#

The given crypto keys are incompatible with the attempted operation.

ERR_CRYPTO_INCOMPATIBLE_KEY_OPTIONS#

The selected public or private key encoding is incompatible with other options.

ERR_CRYPTO_INVALID_DIGEST#

An invalid crypto digest algorithm was specified.

ERR_CRYPTO_INVALID_KEY_OBJECT_TYPE#

The given crypto key object's type is invalid for the attempted operation.

ERR_CRYPTO_INVALID_STATE#

A crypto method was used on an object that was in an invalid state. For instance, calling cipher.getAuthTag() before calling cipher.final().

ERR_CRYPTO_PBKDF2_ERROR#

The PBKDF2 algorithm failed for unspecified reasons. OpenSSL does not provide more details and therefore neither does Node.js.

ERR_CRYPTO_SCRYPT_INVALID_PARAMETER#

One or more crypto.scrypt() or crypto.scryptSync() parameters are outside their legal range.

ERR_CRYPTO_SCRYPT_NOT_SUPPORTED#

Node.js was compiled without scrypt support. Not possible with the official release binaries but can happen with custom builds, including distro builds.

ERR_CRYPTO_SIGN_KEY_REQUIRED#

A signing key was not provided to the sign.sign() method.

ERR_CRYPTO_TIMING_SAFE_EQUAL_LENGTH#

crypto.timingSafeEqual() was called with Buffer, TypedArray, or DataView arguments of different lengths.

ERR_CRYPTO_UNKNOWN_CIPHER#

An unknown cipher was specified.

ERR_CRYPTO_UNKNOWN_DH_GROUP#

An unknown Diffie-Hellman group name was given. See crypto.getDiffieHellman() for a list of valid group names.

ERR_DIR_CLOSED#

The fs.Dir was previously closed.

ERR_DIR_CONCURRENT_OPERATION#

Added in: v14.3.0

A synchronous read or close call was attempted on an fs.Dir which has ongoing asynchronous operations.

ERR_DNS_SET_SERVERS_FAILED#

c-ares failed to set the DNS server.

ERR_DOMAIN_CALLBACK_NOT_AVAILABLE#

The domain module was not usable since it could not establish the required error handling hooks, because process.setUncaughtExceptionCaptureCallback() had been called at an earlier point in time.

ERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE#

process.setUncaughtExceptionCaptureCallback() could not be called because the domain module has been loaded at an earlier point in time.

The stack trace is extended to include the point in time at which the domain module had been loaded.

ERR_ENCODING_INVALID_ENCODED_DATA#

Data provided to TextDecoder() API was invalid according to the encoding provided.

ERR_ENCODING_NOT_SUPPORTED#

Encoding provided to TextDecoder() API was not one of the WHATWG Supported Encodings.

ERR_EVAL_ESM_CANNOT_PRINT#

--print cannot be used with ESM input.

ERR_EXECUTION_ENVIRONMENT_NOT_AVAILABLE#

The JS execution context is not associated with a Node.js environment. This may occur when Node.js is used as an embedded library and some hooks for the JS engine are not set up properly.

ERR_FALSY_VALUE_REJECTION#

A Promise that was callbackified via util.callbackify() was rejected with a falsy value.

ERR_FS_FILE_TOO_LARGE#

An attempt has been made to read a file whose size is larger than the maximum allowed size for a Buffer.

ERR_FS_INVALID_SYMLINK_TYPE#

An invalid symlink type was passed to the fs.symlink() or fs.symlinkSync() methods.

ERR_HTTP_HEADERS_SENT#

An attempt was made to add more headers after the headers had already been sent.

ERR_HTTP_INVALID_HEADER_VALUE#

An invalid HTTP header value was specified.

ERR_HTTP_INVALID_STATUS_CODE#

Status code was outside the regular status code range (100-999).

ERR_HTTP_TRAILER_INVALID#

The Trailer header was set even though the transfer encoding does not support that.

ERR_HTTP2_ALTSVC_INVALID_ORIGIN#

HTTP/2 ALTSVC frames require a valid origin.

ERR_HTTP2_ALTSVC_LENGTH#

HTTP/2 ALTSVC frames are limited to a maximum of 16,382 payload bytes.

ERR_HTTP2_CONNECT_AUTHORITY#

For HTTP/2 requests using the CONNECT method, the :authority pseudo-header is required.

ERR_HTTP2_CONNECT_PATH#

For HTTP/2 requests using the CONNECT method, the :path pseudo-header is forbidden.

ERR_HTTP2_CONNECT_SCHEME#

For HTTP/2 requests using the CONNECT method, the :scheme pseudo-header is forbidden.

ERR_HTTP2_ERROR#

A non-specific HTTP/2 error has occurred.

ERR_HTTP2_GOAWAY_SESSION#

New HTTP/2 Streams may not be opened after the Http2Session has received a GOAWAY frame from the connected peer.

ERR_HTTP2_HEADERS_AFTER_RESPOND#

An additional headers was specified after an HTTP/2 response was initiated.

ERR_HTTP2_HEADERS_SENT#

An attempt was made to send multiple response headers.

ERR_HTTP2_HEADER_SINGLE_VALUE#

Multiple values were provided for an HTTP/2 header field that was required to have only a single value.

ERR_HTTP2_INFO_STATUS_NOT_ALLOWED#

Informational HTTP status codes (1xx) may not be set as the response status code on HTTP/2 responses.

ERR_HTTP2_INVALID_CONNECTION_HEADERS#

HTTP/1 connection specific headers are forbidden to be used in HTTP/2 requests and responses.

ERR_HTTP2_INVALID_HEADER_VALUE#

An invalid HTTP/2 header value was specified.

ERR_HTTP2_INVALID_INFO_STATUS#

An invalid HTTP informational status code has been specified. Informational status codes must be an integer between 100 and 199 (inclusive).

ERR_HTTP2_INVALID_ORIGIN#

HTTP/2 ORIGIN frames require a valid origin.

ERR_HTTP2_INVALID_PACKED_SETTINGS_LENGTH#

Input Buffer and Uint8Array instances passed to the http2.getUnpackedSettings() API must have a length that is a multiple of six.

ERR_HTTP2_INVALID_PSEUDOHEADER#

Only valid HTTP/2 pseudoheaders (:status, :path, :authority, :scheme, and :method) may be used.

ERR_HTTP2_INVALID_SESSION#

An action was performed on an Http2Session object that had already been destroyed.

ERR_HTTP2_INVALID_SETTING_VALUE#

An invalid value has been specified for an HTTP/2 setting.

ERR_HTTP2_INVALID_STREAM#

An operation was performed on a stream that had already been destroyed.

ERR_HTTP2_MAX_PENDING_SETTINGS_ACK#

Whenever an HTTP/2 SETTINGS frame is sent to a connected peer, the peer is required to send an acknowledgment that it has received and applied the new SETTINGS. By default, a maximum number of unacknowledged SETTINGS frames may be sent at any given time. This error code is used when that limit has been reached.

ERR_HTTP2_NESTED_PUSH#

An attempt was made to initiate a new push stream from within a push stream. Nested push streams are not permitted.

ERR_HTTP2_NO_SOCKET_MANIPULATION#

An attempt was made to directly manipulate (read, write, pause, resume, etc.) a socket attached to an Http2Session.

ERR_HTTP2_ORIGIN_LENGTH#

HTTP/2 ORIGIN frames are limited to a length of 16382 bytes.

ERR_HTTP2_OUT_OF_STREAMS#

The number of streams created on a single HTTP/2 session reached the maximum limit.

ERR_HTTP2_PAYLOAD_FORBIDDEN#

A message payload was specified for an HTTP response code for which a payload is forbidden.

ERR_HTTP2_PING_CANCEL#

An HTTP/2 ping was canceled.

ERR_HTTP2_PING_LENGTH#

HTTP/2 ping payloads must be exactly 8 bytes in length.

ERR_HTTP2_PSEUDOHEADER_NOT_ALLOWED#

An HTTP/2 pseudo-header has been used inappropriately. Pseudo-headers are header key names that begin with the : prefix.

ERR_HTTP2_PUSH_DISABLED#

An attempt was made to create a push stream, which had been disabled by the client.

ERR_HTTP2_SEND_FILE#

An attempt was made to use the Http2Stream.prototype.responseWithFile() API to send a directory.

ERR_HTTP2_SEND_FILE_NOSEEK#

An attempt was made to use the Http2Stream.prototype.responseWithFile() API to send something other than a regular file, but offset or length options were provided.

ERR_HTTP2_SESSION_ERROR#

The Http2Session closed with a non-zero error code.

ERR_HTTP2_SETTINGS_CANCEL#

The Http2Session settings canceled.

ERR_HTTP2_SOCKET_BOUND#

An attempt was made to connect a Http2Session object to a net.Socket or tls.TLSSocket that had already been bound to another Http2Session object.

ERR_HTTP2_SOCKET_UNBOUND#

An attempt was made to use the socket property of an Http2Session that has already been closed.

ERR_HTTP2_STATUS_101#

Use of the 101 Informational status code is forbidden in HTTP/2.

ERR_HTTP2_STATUS_INVALID#

An invalid HTTP status code has been specified. Status codes must be an integer between 100 and 599 (inclusive).

ERR_HTTP2_STREAM_CANCEL#

An Http2Stream was destroyed before any data was transmitted to the connected peer.

ERR_HTTP2_STREAM_ERROR#

A non-zero error code was been specified in an RST_STREAM frame.

ERR_HTTP2_STREAM_SELF_DEPENDENCY#

When setting the priority for an HTTP/2 stream, the stream may be marked as a dependency for a parent stream. This error code is used when an attempt is made to mark a stream and dependent of itself.

ERR_HTTP2_TRAILERS_ALREADY_SENT#

Trailing headers have already been sent on the Http2Stream.

ERR_HTTP2_TRAILERS_NOT_READY#

The http2stream.sendTrailers() method cannot be called until after the 'wantTrailers' event is emitted on an Http2Stream object. The 'wantTrailers' event will only be emitted if the waitForTrailers option is set for the Http2Stream.

ERR_HTTP2_UNSUPPORTED_PROTOCOL#

http2.connect() was passed a URL that uses any protocol other than http: or https:.

ERR_INTERNAL_ASSERTION#

There was a bug in Node.js or incorrect usage of Node.js internals. To fix the error, open an issue at https://github.com/nodejs/node/issues.

ERR_INCOMPATIBLE_OPTION_PAIR#

An option pair is incompatible with each other and cannot be used at the same time.

ERR_INPUT_TYPE_NOT_ALLOWED#

Stability: 1 - Experimental

The --input-type flag was used to attempt to execute a file. This flag can only be used with input via --eval, --print or STDIN.

ERR_INSPECTOR_ALREADY_CONNECTED#

While using the inspector module, an attempt was made to connect when the inspector was already connected.

ERR_INSPECTOR_CLOSED#

While using the inspector module, an attempt was made to use the inspector after the session had already closed.

ERR_INSPECTOR_COMMAND#

An error occurred while issuing a command via the inspector module.

ERR_INSPECTOR_NOT_ACTIVE#

The inspector is not active when inspector.waitForDebugger() is called.

ERR_INSPECTOR_NOT_AVAILABLE#

The inspector module is not available for use.

ERR_INSPECTOR_NOT_CONNECTED#

While using the inspector module, an attempt was made to use the inspector before it was connected.

ERR_INSPECTOR_NOT_WORKER#

An API was called on the main thread that can only be used from the worker thread.

ERR_INVALID_ADDRESS_FAMILY#

The provided address family is not understood by the Node.js API.

ERR_INVALID_ARG_TYPE#

An argument of the wrong type was passed to a Node.js API.

ERR_INVALID_ARG_VALUE#

An invalid or unsupported value was passed for a given argument.

ERR_INVALID_ASYNC_ID#

An invalid asyncId or triggerAsyncId was passed using AsyncHooks. An id less than -1 should never happen.

ERR_INVALID_BUFFER_SIZE#

A swap was performed on a Buffer but its size was not compatible with the operation.

ERR_INVALID_CALLBACK#

A callback function was required but was not been provided to a Node.js API.

ERR_INVALID_CHAR#

Invalid characters were detected in headers.

ERR_INVALID_CURSOR_POS#

A cursor on a given stream cannot be moved to a specified row without a specified column.

ERR_INVALID_FD#

A file descriptor ('fd') was not valid (e.g. it was a negative value).

ERR_INVALID_FD_TYPE#

A file descriptor ('fd') type was not valid.

ERR_INVALID_FILE_URL_HOST#

A Node.js API that consumes file: URLs (such as certain functions in the fs module) encountered a file URL with an incompatible host. This situation can only occur on Unix-like systems where only localhost or an empty host is supported.

ERR_INVALID_FILE_URL_PATH#

A Node.js API that consumes file: URLs (such as certain functions in the fs module) encountered a file URL with an incompatible path. The exact semantics for determining whether a path can be used is platform-dependent.

ERR_INVALID_HANDLE_TYPE#

An attempt was made to send an unsupported "handle" over an IPC communication channel to a child process. See subprocess.send() and process.send() for more information.

ERR_INVALID_HTTP_TOKEN#

An invalid HTTP token was supplied.

ERR_INVALID_IP_ADDRESS#

An IP address is not valid.

ERR_INVALID_MODULE_SPECIFIER#

The imported module string is an invalid URL, package name, or package subpath specifier.

ERR_INVALID_OPT_VALUE#

An invalid or unexpected value was passed in an options object.

ERR_INVALID_OPT_VALUE_ENCODING#

An invalid or unknown file encoding was passed.

ERR_INVALID_PACKAGE_CONFIG#

An invalid package.json file was found which failed parsing.

ERR_INVALID_PACKAGE_TARGET#

The package.json exports field contains an invalid target mapping value for the attempted module resolution.

ERR_INVALID_PERFORMANCE_MARK#

While using the Performance Timing API (perf_hooks), a performance mark is invalid.

ERR_INVALID_PROTOCOL#

An invalid options.protocol was passed to http.request().

ERR_INVALID_REPL_EVAL_CONFIG#

Both breakEvalOnSigint and eval options were set in the REPL config, which is not supported.

ERR_INVALID_REPL_INPUT#

The input may not be used in the REPL. All prohibited inputs are documented in the REPL's documentation.

ERR_INVALID_RETURN_PROPERTY#

Thrown in case a function option does not provide a valid value for one of its returned object properties on execution.

ERR_INVALID_RETURN_PROPERTY_VALUE#

Thrown in case a function option does not provide an expected value type for one of its returned object properties on execution.

ERR_INVALID_RETURN_VALUE#

Thrown in case a function option does not return an expected value type on execution, such as when a function is expected to return a promise.

ERR_INVALID_SYNC_FORK_INPUT#

A Buffer, TypedArray, DataView or string was provided as stdio input to an asynchronous fork. See the documentation for the child_process module for more information.

ERR_INVALID_THIS#

A Node.js API function was called with an incompatible this value.

const urlSearchParams = new URLSearchParams('foo=bar&baz=new');

const buf = Buffer.alloc(1);
urlSearchParams.has.call(buf, 'foo');
// Throws a TypeError with code 'ERR_INVALID_THIS'

ERR_INVALID_TRANSFER_OBJECT#

An invalid transfer object was passed to postMessage().

ERR_INVALID_TUPLE#

An element in the iterable provided to the WHATWG URLSearchParams constructor did not represent a [name, value] tuple – that is, if an element is not iterable, or does not consist of exactly two elements.

ERR_INVALID_URI#

An invalid URI was passed.

ERR_INVALID_URL#

An invalid URL was passed to the WHATWG URL constructor to be parsed. The thrown error object typically has an additional property 'input' that contains the URL that failed to parse.

ERR_INVALID_URL_SCHEME#

An attempt was made to use a URL of an incompatible scheme (protocol) for a specific purpose. It is only used in the WHATWG URL API support in the fs module (which only accepts URLs with 'file' scheme), but may be used in other Node.js APIs as well in the future.

ERR_IPC_CHANNEL_CLOSED#

An attempt was made to use an IPC communication channel that was already closed.

ERR_IPC_DISCONNECTED#

An attempt was made to disconnect an IPC communication channel that was already disconnected. See the documentation for the child_process module for more information.

ERR_IPC_ONE_PIPE#

An attempt was made to create a child Node.js process using more than one IPC communication channel. See the documentation for the child_process module for more information.

ERR_IPC_SYNC_FORK#

An attempt was made to open an IPC communication channel with a synchronously forked Node.js process. See the documentation for the child_process module for more information.

ERR_MANIFEST_ASSERT_INTEGRITY#

An attempt was made to load a resource, but the resource did not match the integrity defined by the policy manifest. See the documentation for policy manifests for more information.

ERR_MANIFEST_DEPENDENCY_MISSING#

An attempt was made to load a resource, but the resource was not listed as a dependency from the location that attempted to load it. See the documentation for policy manifests for more information.

ERR_MANIFEST_INTEGRITY_MISMATCH#

An attempt was made to load a policy manifest, but the manifest had multiple entries for a resource which did not match each other. Update the manifest entries to match in order to resolve this error. See the documentation for policy manifests for more information.

ERR_MANIFEST_INVALID_RESOURCE_FIELD#

A policy manifest resource had an invalid value for one of its fields. Update the manifest entry to match in order to resolve this error. See the documentation for policy manifests for more information.

ERR_MANIFEST_PARSE_POLICY#

An attempt was made to load a policy manifest, but the manifest was unable to be parsed. See the documentation for policy manifests for more information.

ERR_MANIFEST_TDZ#

An attempt was made to read from a policy manifest, but the manifest initialization has not yet taken place. This is likely a bug in Node.js.

ERR_MANIFEST_UNKNOWN_ONERROR#

A policy manifest was loaded, but had an unknown value for its "onerror" behavior. See the documentation for policy manifests for more information.

ERR_MEMORY_ALLOCATION_FAILED#

An attempt was made to allocate memory (usually in the C++ layer) but it failed.

ERR_METHOD_NOT_IMPLEMENTED#

A method is required but not implemented.

ERR_MISSING_ARGS#

A required argument of a Node.js API was not passed. This is only used for strict compliance with the API specification (which in some cases may accept func(undefined) but not func()). In most native Node.js APIs, func(undefined) and func() are treated identically, and the ERR_INVALID_ARG_TYPE error code may be used instead.

ERR_MISSING_DYNAMIC_INSTANTIATE_HOOK#

Stability: 1 - Experimental

An ES Module loader hook specified format: 'dynamic' but did not provide a dynamicInstantiate hook.

ERR_MISSING_OPTION#

For APIs that accept options objects, some options might be mandatory. This code is thrown if a required option is missing.

ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST#

A MessagePort was found in the object passed to a postMessage() call, but not provided in the transferList for that call.

ERR_MISSING_PASSPHRASE#

An attempt was made to read an encrypted key without specifying a passphrase.

ERR_MISSING_PLATFORM_FOR_WORKER#

The V8 platform used by this instance of Node.js does not support creating Workers. This is caused by lack of embedder support for Workers. In particular, this error will not occur with standard builds of Node.js.

ERR_MODULE_NOT_FOUND#

Stability: 1 - Experimental

An ES Module could not be resolved.

ERR_MULTIPLE_CALLBACK#

A callback was called more than once.

A callback is almost always meant to only be called once as the query can either be fulfilled or rejected but not both at the same time. The latter would be possible by calling a callback more than once.

ERR_NAPI_CONS_FUNCTION#

While using N-API, a constructor passed was not a function.

ERR_NAPI_INVALID_DATAVIEW_ARGS#

While calling napi_create_dataview(), a given offset was outside the bounds of the dataview or offset + length was larger than a length of given buffer.

ERR_NAPI_INVALID_TYPEDARRAY_ALIGNMENT#

While calling napi_create_typedarray(), the provided offset was not a multiple of the element size.

ERR_NAPI_INVALID_TYPEDARRAY_LENGTH#

While calling napi_create_typedarray(), (length * size_of_element) + byte_offset was larger than the length of given buffer.

ERR_NAPI_TSFN_CALL_JS#

An error occurred while invoking the JavaScript portion of the thread-safe function.

ERR_NAPI_TSFN_GET_UNDEFINED#

An error occurred while attempting to retrieve the JavaScript undefined value.

ERR_NAPI_TSFN_START_IDLE_LOOP#

On the main thread, values are removed from the queue associated with the thread-safe function in an idle loop. This error indicates that an error has occurred when attempting to start the loop.

ERR_NAPI_TSFN_STOP_IDLE_LOOP#

Once no more items are left in the queue, the idle loop must be suspended. This error indicates that the idle loop has failed to stop.

ERR_NO_CRYPTO#

An attempt was made to use crypto features while Node.js was not compiled with OpenSSL crypto support.

ERR_NO_ICU#

An attempt was made to use features that require ICU, but Node.js was not compiled with ICU support.

ERR_NON_CONTEXT_AWARE_DISABLED#

A non-context-aware native addon was loaded in a process that disallows them.

ERR_OUT_OF_RANGE#

A given value is out of the accepted range.

ERR_PACKAGE_PATH_NOT_EXPORTED#

The package.json exports field does not export the requested subpath. Because exports are encapsulated, private internal modules that are not exported cannot be imported through the package resolution, unless using an absolute URL.

ERR_PROTO_ACCESS#

Accessing Object.prototype.__proto__ has been forbidden using --disable-proto=throw. Object.getPrototypeOf and Object.setPrototypeOf should be used to get and set the prototype of an object.

ERR_REQUIRE_ESM#

Stability: 1 - Experimental

An attempt was made to require() an ES Module.

ERR_SCRIPT_EXECUTION_INTERRUPTED#

Script execution was interrupted by SIGINT (For example, when Ctrl+C was pressed).

ERR_SCRIPT_EXECUTION_TIMEOUT#

Script execution timed out, possibly due to bugs in the script being executed.

ERR_SERVER_ALREADY_LISTEN#

The server.listen() method was called while a net.Server was already listening. This applies to all instances of net.Server, including HTTP, HTTPS, and HTTP/2 Server instances.

ERR_SERVER_NOT_RUNNING#

The server.close() method was called when a net.Server was not running. This applies to all instances of net.Server, including HTTP, HTTPS, and HTTP/2 Server instances.

ERR_SOCKET_ALREADY_BOUND#

An attempt was made to bind a socket that has already been bound.

ERR_SOCKET_BAD_BUFFER_SIZE#

An invalid (negative) size was passed for either the recvBufferSize or sendBufferSize options in dgram.createSocket().

ERR_SOCKET_BAD_PORT#

An API function expecting a port >= 0 and < 65536 received an invalid value.

ERR_SOCKET_BAD_TYPE#

An API function expecting a socket type (udp4 or udp6) received an invalid value.

ERR_SOCKET_BUFFER_SIZE#

While using dgram.createSocket(), the size of the receive or send Buffer could not be determined.

ERR_SOCKET_CLOSED#

An attempt was made to operate on an already closed socket.

ERR_SOCKET_DGRAM_IS_CONNECTED#

A dgram.connect() call was made on an already connected socket.

ERR_SOCKET_DGRAM_NOT_CONNECTED#

A dgram.disconnect() or dgram.remoteAddress() call was made on a disconnected socket.

ERR_SOCKET_DGRAM_NOT_RUNNING#

A call was made and the UDP subsystem was not running.

ERR_SRI_PARSE#

A string was provided for a Subresource Integrity check, but was unable to be parsed. Check the format of integrity attributes by looking at the Subresource Integrity specification.

ERR_STREAM_CANNOT_PIPE#

An attempt was made to call stream.pipe() on a Writable stream.

ERR_STREAM_DESTROYED#

A stream method was called that cannot complete because the stream was destroyed using stream.destroy().

ERR_STREAM_ALREADY_FINISHED#

A stream method was called that cannot complete because the stream was finished.

ERR_STREAM_NULL_VALUES#

An attempt was made to call stream.write() with a null chunk.

ERR_STREAM_PREMATURE_CLOSE#

An error returned by stream.finished() and stream.pipeline(), when a stream or a pipeline ends non gracefully with no explicit error.

ERR_STREAM_PUSH_AFTER_EOF#

An attempt was made to call stream.push() after a null(EOF) had been pushed to the stream.

ERR_STREAM_UNSHIFT_AFTER_END_EVENT#

An attempt was made to call stream.unshift() after the 'end' event was emitted.

ERR_STREAM_WRAP#

Prevents an abort if a string decoder was set on the Socket or if the decoder is in objectMode.

const Socket = require('net').Socket;
const instance = new Socket();

instance.setEncoding('utf8');

ERR_STREAM_WRITE_AFTER_END#

An attempt was made to call stream.write() after stream.end() has been called.

ERR_STRING_TOO_LONG#

An attempt has been made to create a string longer than the maximum allowed length.

ERR_SYNTHETIC#

An artificial error object used to capture the call stack for diagnostic reports.

ERR_SYSTEM_ERROR#

An unspecified or non-specific system error has occurred within the Node.js process. The error object will have an err.info object property with additional details.

ERR_TLS_CERT_ALTNAME_INVALID#

While using TLS, the host name/IP of the peer did not match any of the subjectAltNames in its certificate.

ERR_TLS_DH_PARAM_SIZE#

While using TLS, the parameter offered for the Diffie-Hellman (DH) key-agreement protocol is too small. By default, the key length must be greater than or equal to 1024 bits to avoid vulnerabilities, even though it is strongly recommended to use 2048 bits or larger for stronger security.

ERR_TLS_HANDSHAKE_TIMEOUT#

A TLS/SSL handshake timed out. In this case, the server must also abort the connection.

ERR_TLS_INVALID_CONTEXT#

Added in: v13.3.0

The context must be a SecureContext.

ERR_TLS_INVALID_STATE#

Added in: v13.10.0

The TLS socket must be connected and securily established. Ensure the 'secure' event is emitted before continuing.

ERR_TLS_INVALID_PROTOCOL_METHOD#

The specified secureProtocol method is invalid. It is either unknown, or disabled because it is insecure.

ERR_TLS_INVALID_PROTOCOL_VERSION#

Valid TLS protocol versions are 'TLSv1', 'TLSv1.1', or 'TLSv1.2'.

ERR_TLS_PROTOCOL_VERSION_CONFLICT#

Attempting to set a TLS protocol minVersion or maxVersion conflicts with an attempt to set the secureProtocol explicitly. Use one mechanism or the other.

ERR_TLS_RENEGOTIATION_DISABLED#

An attempt was made to renegotiate TLS on a socket instance with TLS disabled.

ERR_TLS_REQUIRED_SERVER_NAME#

While using TLS, the server.addContext() method was called without providing a host name in the first parameter.

ERR_TLS_SESSION_ATTACK#

An excessive amount of TLS renegotiations is detected, which is a potential vector for denial-of-service attacks.

ERR_TLS_SNI_FROM_SERVER#

An attempt was made to issue Server Name Indication from a TLS server-side socket, which is only valid from a client.

ERR_TLS_PSK_SET_IDENTIY_HINT_FAILED##

Failed to set PSK identity hint. Hint may be too long.

ERR_TRACE_EVENTS_CATEGORY_REQUIRED#

The trace_events.createTracing() method requires at least one trace event category.

ERR_TRACE_EVENTS_UNAVAILABLE#

The trace_events module could not be loaded because Node.js was compiled with the --without-v8-platform flag.

ERR_TRANSFERRING_EXTERNALIZED_SHAREDARRAYBUFFER#

A SharedArrayBuffer whose memory is not managed by the JavaScript engine or by Node.js was encountered during serialization. Such a SharedArrayBuffer cannot be serialized.

This can only happen when native addons create SharedArrayBuffers in "externalized" mode, or put existing SharedArrayBuffer into externalized mode.

ERR_TRANSFORM_ALREADY_TRANSFORMING#

A Transform stream finished while it was still transforming.

ERR_TRANSFORM_WITH_LENGTH_0#

A Transform stream finished with data still in the write buffer.

ERR_TTY_INIT_FAILED#

The initialization of a TTY failed due to a system error.

ERR_UNAVAILABLE_DURING_EXIT#

Function was called within a process.on('exit') handler that shouldn't be called within process.on('exit') handler.

ERR_UNCAUGHT_EXCEPTION_CAPTURE_ALREADY_SET#

process.setUncaughtExceptionCaptureCallback() was called twice, without first resetting the callback to null.

This error is designed to prevent accidentally overwriting a callback registered from another module.

ERR_UNESCAPED_CHARACTERS#

A string that contained unescaped characters was received.

ERR_UNHANDLED_ERROR#

An unhandled error occurred (for instance, when an 'error' event is emitted by an EventEmitter but an 'error' handler is not registered).

ERR_UNKNOWN_BUILTIN_MODULE#

Used to identify a specific kind of internal Node.js error that should not typically be triggered by user code. Instances of this error point to an internal bug within the Node.js binary itself.

ERR_UNKNOWN_CREDENTIAL#

A Unix group or user identifier that does not exist was passed.

ERR_UNKNOWN_ENCODING#

An invalid or unknown encoding option was passed to an API.

ERR_UNKNOWN_FILE_EXTENSION#

Stability: 1 - Experimental

An attempt was made to load a module with an unknown or unsupported file extension.

ERR_UNKNOWN_MODULE_FORMAT#

Stability: 1 - Experimental

An attempt was made to load a module with an unknown or unsupported format.

ERR_UNKNOWN_SIGNAL#

An invalid or unknown process signal was passed to an API expecting a valid signal (such as subprocess.kill()).

ERR_UNSUPPORTED_DIR_IMPORT#

import a directory URL is unsupported. Instead, you can self-reference a package using its name and define a custom subpath in the "exports" field of the package.json file.

import './'; // unsupported
import './index.js'; // supported
import 'package-name'; // supported

ERR_UNSUPPORTED_ESM_URL_SCHEME#

import with URL schemes other than file and data is unsupported.

ERR_V8BREAKITERATOR#

The V8 BreakIterator API was used but the full ICU data set is not installed.

ERR_VALID_PERFORMANCE_ENTRY_TYPE#

While using the Performance Timing API (perf_hooks), no valid performance entry types were found.

ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING#

A dynamic import callback was not specified.

ERR_VM_MODULE_ALREADY_LINKED#

The module attempted to be linked is not eligible for linking, because of one of the following reasons:

  • It has already been linked (linkingStatus is 'linked')
  • It is being linked (linkingStatus is 'linking')
  • Linking has failed for this module (linkingStatus is 'errored')

ERR_VM_MODULE_CACHED_DATA_REJECTED#

The cachedData option passed to a module constructor is invalid.

ERR_VM_MODULE_CANNOT_CREATE_CACHED_DATA#

Cached data cannot be created for modules which have already been evaluated.

ERR_VM_MODULE_DIFFERENT_CONTEXT#

The module being returned from the linker function is from a different context than the parent module. Linked modules must share the same context.

ERR_VM_MODULE_LINKING_ERRORED#

The linker function returned a module for which linking has failed.

ERR_VM_MODULE_NOT_MODULE#

The fulfilled value of a linking promise is not a vm.Module object.

ERR_VM_MODULE_STATUS#

The current module's status does not allow for this operation. The specific meaning of the error depends on the specific function.

ERR_WASI_ALREADY_STARTED#

The WASI instance has already started.

ERR_WASI_NOT_STARTED#

The WASI instance has not been started.

ERR_WORKER_INIT_FAILED#

The Worker initialization failed.

ERR_WORKER_INVALID_EXEC_ARGV#

The execArgv option passed to the Worker constructor contains invalid flags.

ERR_WORKER_NOT_RUNNING#

An operation failed because the Worker instance is not currently running.

ERR_WORKER_OUT_OF_MEMORY#

The Worker instance terminated because it reached its memory limit.

ERR_WORKER_PATH#

The path for the main script of a worker is neither an absolute path nor a relative path starting with ./ or ../.

ERR_WORKER_UNSERIALIZABLE_ERROR#

All attempts at serializing an uncaught exception from a worker thread failed.

ERR_WORKER_UNSUPPORTED_EXTENSION#

The pathname used for the main script of a worker has an unknown file extension.

ERR_WORKER_UNSUPPORTED_OPERATION#

The requested functionality is not supported in worker threads.

ERR_ZLIB_INITIALIZATION_FAILED#

Creation of a zlib object failed due to incorrect configuration.

HPE_HEADER_OVERFLOW#

History
VersionChanges
v11.4.0, v10.15.0

Max header size in http_parser was set to 8KB.

Too much HTTP header data was received. In order to protect against malicious or malconfigured clients, if more than 8KB of HTTP header data is received then HTTP parsing will abort without a request or response object being created, and an Error with this code will be emitted.

MODULE_NOT_FOUND#

History
VersionChanges
v12.0.0

Added requireStack property.

A module file could not be resolved while attempting a require() or import operation.

Legacy Node.js Error Codes#

Stability: 0 - Deprecated. These error codes are either inconsistent, or have been removed.

ERR_CANNOT_TRANSFER_OBJECT#

The value passed to postMessage() contained an object that is not supported for transferring.

ERR_CLOSED_MESSAGE_PORT#

Added in: v10.5.0Removed in: v11.12.0

There was an attempt to use a MessagePort instance in a closed state, usually after .close() has been called.

ERR_CRYPTO_HASH_DIGEST_NO_UTF16#

Added in: v9.0.0Removed in: v12.12.0

The UTF-16 encoding was used with hash.digest(). While the hash.digest() method does allow an encoding argument to be passed in, causing the method to return a string rather than a Buffer, the UTF-16 encoding (e.g. ucs or utf16le) is not supported.

ERR_HTTP2_FRAME_ERROR#

Added in: v9.0.0Removed in: v10.0.0

Used when a failure occurs sending an individual frame on the HTTP/2 session.

ERR_HTTP2_HEADERS_OBJECT#

Added in: v9.0.0Removed in: v10.0.0

Used when an HTTP/2 Headers Object is expected.

ERR_HTTP2_HEADER_REQUIRED#

Added in: v9.0.0Removed in: v10.0.0

Used when a required header is missing in an HTTP/2 message.

ERR_HTTP2_INFO_HEADERS_AFTER_RESPOND#

Added in: v9.0.0Removed in: v10.0.0

HTTP/2 informational headers must only be sent prior to calling the Http2Stream.prototype.respond() method.

ERR_HTTP2_STREAM_CLOSED#

Added in: v9.0.0Removed in: v10.0.0

Used when an action has been performed on an HTTP/2 Stream that has already been closed.

ERR_HTTP_INVALID_CHAR#

Added in: v9.0.0Removed in: v10.0.0

Used when an invalid character is found in an HTTP response status message (reason phrase).

ERR_INDEX_OUT_OF_RANGE#

Added in: v10.0.0Removed in: v11.0.0

A given index was out of the accepted range (e.g. negative offsets).

ERR_NAPI_CONS_PROTOTYPE_OBJECT#

Added in: v9.0.0Removed in: v10.0.0

Used by the N-API when Constructor.prototype is not an object.

ERR_NO_LONGER_SUPPORTED#

A Node.js API was called in an unsupported manner, such as Buffer.write(string, encoding, offset[, length]).

ERR_OUTOFMEMORY#

Added in: v9.0.0Removed in: v10.0.0

Used generically to identify that an operation caused an out of memory condition.

ERR_PARSE_HISTORY_DATA#

Added in: v9.0.0Removed in: v10.0.0

The repl module was unable to parse data from the REPL history file.

ERR_SOCKET_CANNOT_SEND#

Added in: v9.0.0Removed in: v14.0.0

Data could not be sent on a socket.

ERR_STDERR_CLOSE#

History
VersionChanges
v10.12.0

Rather than emitting an error, process.stderr.end() now only closes the stream side but not the underlying resource, making this error obsolete.

v10.12.0

Removed in: v10.12.0

An attempt was made to close the process.stderr stream. By design, Node.js does not allow stdout or stderr streams to be closed by user code.

ERR_STDOUT_CLOSE#

History
VersionChanges
v10.12.0

Rather than emitting an error, process.stderr.end() now only closes the stream side but not the underlying resource, making this error obsolete.

v10.12.0

Removed in: v10.12.0

An attempt was made to close the process.stdout stream. By design, Node.js does not allow stdout or stderr streams to be closed by user code.

ERR_STREAM_READ_NOT_IMPLEMENTED#

Added in: v9.0.0Removed in: v10.0.0

Used when an attempt is made to use a readable stream that has not implemented readable._read().

ERR_TLS_RENEGOTIATION_FAILED#

Added in: v9.0.0Removed in: v10.0.0

Used when a TLS renegotiation request has failed in a non-specific way.

ERR_UNKNOWN_BUILTIN_MODULE#

Added in: v8.0.0Removed in: v9.0.0

The 'ERR_UNKNOWN_BUILTIN_MODULE' error code is used to identify a specific kind of internal Node.js error that should not typically be triggered by user code. Instances of this error point to an internal bug within the Node.js binary itself.

ERR_UNKNOWN_STDIN_TYPE#

Added in: v8.0.0Removed in: v11.7.0

An attempt was made to launch a Node.js process with an unknown stdin file type. This error is usually an indication of a bug within Node.js itself, although it is possible for user code to trigger it.

ERR_UNKNOWN_STREAM_TYPE#

Added in: v8.0.0Removed in: v11.7.0

An attempt was made to launch a Node.js process with an unknown stdout or stderr file type. This error is usually an indication of a bug within Node.js itself, although it is possible for user code to trigger it.

ERR_VALUE_OUT_OF_RANGE#

Added in: v9.0.0Removed in: v10.0.0

Used when a given value is out of the accepted range.

ERR_VM_MODULE_NOT_LINKED#

The module must be successfully linked before instantiation.

ERR_ZLIB_BINDING_CLOSED#

Added in: v9.0.0Removed in: v10.0.0

Used when an attempt is made to use a zlib object after it has already been closed.

Other error codes#

These errors have never been released, but had been present on master between releases.

ERR_ENTRY_TYPE_MISMATCH#

Stability: 1 - Experimental

The --entry-type=commonjs flag was used to attempt to execute an .mjs file or a .js file where the nearest parent package.json contains "type": "module"; or the --entry-type=module flag was used to attempt to execute a .cjs file or a .js file where the nearest parent package.json either lacks a "type" field or contains "type": "commonjs".

ERR_FS_WATCHER_ALREADY_STARTED#

An attempt was made to start a watcher returned by fs.watch() that has already been started.

ERR_FS_WATCHER_NOT_STARTED#

An attempt was made to initiate operations on a watcher returned by fs.watch() that has not yet been started.

ERR_HTTP2_ALREADY_SHUTDOWN#

Occurs with multiple attempts to shutdown an HTTP/2 session.

ERR_HTTP2_ERROR#

A non-specific HTTP/2 error has occurred.

ERR_INVALID_REPL_HISTORY#

Used in the repl in case the old history file is used and an error occurred while trying to read and parse it.

ERR_INVALID_REPL_TYPE#

Stability: 1 - Experimental

The --entry-type=... flag is not compatible with the Node.js REPL.

ERR_MISSING_DYNAMIC_INSTANTIATE_HOOK#

Used when an ES Module loader hook specifies format: 'dynamic' but does not provide a dynamicInstantiate hook.

ERR_FEATURE_UNAVAILABLE_ON_PLATFORM#

Used when a feature that is not available to the current platform which is running Node.js is used.

ERR_STREAM_HAS_STRINGDECODER#

Used to prevent an abort if a string decoder was set on the Socket.

const Socket = require('net').Socket;
const instance = new Socket();

instance.setEncoding('utf8');

ERR_STRING_TOO_LARGE#

An attempt has been made to create a string larger than the maximum allowed size.

ERR_TTY_WRITABLE_NOT_READABLE#

This Error is thrown when a read is attempted on a TTY WriteStream, such as process.stdout.on('data').

Events#

Stability: 2 - Stable

Much of the Node.js core API is built around an idiomatic asynchronous event-driven architecture in which certain kinds of objects (called "emitters") emit named events that cause Function objects ("listeners") to be called.

For instance: a net.Server object emits an event each time a peer connects to it; a fs.ReadStream emits an event when the file is opened; a stream emits an event whenever data is available to be read.

All objects that emit events are instances of the EventEmitter class. These objects expose an eventEmitter.on() function that allows one or more functions to be attached to named events emitted by the object. Typically, event names are camel-cased strings but any valid JavaScript property key can be used.

When the EventEmitter object emits an event, all of the functions attached to that specific event are called synchronously. Any values returned by the called listeners are ignored and will be discarded.

The following example shows a simple EventEmitter instance with a single listener. The eventEmitter.on() method is used to register listeners, while the eventEmitter.emit() method is used to trigger the event.

const EventEmitter = require('events');

class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();
myEmitter.on('event', () => {
  console.log('an event occurred!');
});
myEmitter.emit('event');

Passing arguments and this to listeners#

The eventEmitter.emit() method allows an arbitrary set of arguments to be passed to the listener functions. Keep in mind that when an ordinary listener function is called, the standard this keyword is intentionally set to reference the EventEmitter instance to which the listener is attached.

const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
  console.log(a, b, this, this === myEmitter);
  // Prints:
  //   a b MyEmitter {
  //     domain: null,
  //     _events: { event: [Function] },
  //     _eventsCount: 1,
  //     _maxListeners: undefined } true
});
myEmitter.emit('event', 'a', 'b');

It is possible to use ES6 Arrow Functions as listeners, however, when doing so, the this keyword will no longer reference the EventEmitter instance:

const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  console.log(a, b, this);
  // Prints: a b {}
});
myEmitter.emit('event', 'a', 'b');

Asynchronous vs. Synchronous#

The EventEmitter calls all listeners synchronously in the order in which they were registered. This ensures the proper sequencing of events and helps avoid race conditions and logic errors. When appropriate, listener functions can switch to an asynchronous mode of operation using the setImmediate() or process.nextTick() methods:

const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  setImmediate(() => {
    console.log('this happens asynchronously');
  });
});
myEmitter.emit('event', 'a', 'b');

Handling events only once#

When a listener is registered using the eventEmitter.on() method, that listener will be invoked every time the named event is emitted.

const myEmitter = new MyEmitter();
let m = 0;
myEmitter.on('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Prints: 2

Using the eventEmitter.once() method, it is possible to register a listener that is called at most once for a particular event. Once the event is emitted, the listener is unregistered and then called.

const myEmitter = new MyEmitter();
let m = 0;
myEmitter.once('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Ignored

Error events#

When an error occurs within an EventEmitter instance, the typical action is for an 'error' event to be emitted. These are treated as special cases within Node.js.

If an EventEmitter does not have at least one listener registered for the 'error' event, and an 'error' event is emitted, the error is thrown, a stack trace is printed, and the Node.js process exits.

const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));
// Throws and crashes Node.js

To guard against crashing the Node.js process the domain module can be used. (Note, however, that the domain module is deprecated.)

As a best practice, listeners should always be added for the 'error' events.

const myEmitter = new MyEmitter();
myEmitter.on('error', (err) => {
  console.error('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error

It is possible to monitor 'error' events without consuming the emitted error by installing a listener using the symbol errorMonitor.

const myEmitter = new MyEmitter();
myEmitter.on(EventEmitter.errorMonitor, (err) => {
  MyMonitoringTool.log(err);
});
myEmitter.emit('error', new Error('whoops!'));
// Still throws and crashes Node.js

Capture Rejections of Promises#

Stability: 1 - captureRejections is experimental.

Using async functions with event handlers is problematic, because it can lead to an unhandled rejection in case of a thrown exception:

const ee = new EventEmitter();
ee.on('something', async (value) => {
  throw new Error('kaboom');
});

The captureRejections option in the EventEmitter constructor or the global setting change this behavior, installing a .then(undefined, handler) handler on the Promise. This handler routes the exception asynchronously to the Symbol.for('nodejs.rejection') method if there is one, or to 'error' event handler if there is none.

const ee1 = new EventEmitter({ captureRejections: true });
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);

const ee2 = new EventEmitter({ captureRejections: true });
ee2.on('something', async (value) => {
  throw new Error('kaboom');
});

ee2[Symbol.for('nodejs.rejection')] = console.log;

Setting EventEmitter.captureRejections = true will change the default for all new instances of EventEmitter.

EventEmitter.captureRejections = true;
const ee1 = new EventEmitter();
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);

The 'error' events that are generated by the captureRejections behavior do not have a catch handler to avoid infinite error loops: the recommendation is to not use async functions as 'error' event handlers.

Class: EventEmitter#

History
VersionChanges
v13.4.0, v12.16.0

Added captureRejections option.

v0.1.26

Added in: v0.1.26

The EventEmitter class is defined and exposed by the events module:

const EventEmitter = require('events');

All EventEmitters emit the event 'newListener' when new listeners are added and 'removeListener' when existing listeners are removed.

It supports the following option:

Event: 'newListener'#

Added in: v0.1.26

The EventEmitter instance will emit its own 'newListener' event before a listener is added to its internal array of listeners.

Listeners registered for the 'newListener' event will be passed the event name and a reference to the listener being added.

The fact that the event is triggered before adding the listener has a subtle but important side effect: any additional listeners registered to the same name within the 'newListener' callback will be inserted before the listener that is in the process of being added.

const myEmitter = new MyEmitter();
// Only do this once so we don't loop forever
myEmitter.once('newListener', (event, listener) => {
  if (event === 'event') {
    // Insert a new listener in front
    myEmitter.on('event', () => {
      console.log('B');
    });
  }
});
myEmitter.on('event', () => {
  console.log('A');
});
myEmitter.emit('event');
// Prints:
//   B
//   A

Event: 'removeListener'#

History
VersionChanges
v6.1.0, v4.7.0

For listeners attached using .once(), the listener argument now yields the original listener function.

v0.9.3

Added in: v0.9.3

The 'removeListener' event is emitted after the listener is removed.

EventEmitter.listenerCount(emitter, eventName)#

Added in: v0.9.12Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use emitter.listenerCount() instead.

A class method that returns the number of listeners for the given eventName registered on the given emitter.

const myEmitter = new MyEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(EventEmitter.listenerCount(myEmitter, 'event'));
// Prints: 2

EventEmitter.defaultMaxListeners#

Added in: v0.11.2

By default, a maximum of 10 listeners can be registered for any single event. This limit can be changed for individual EventEmitter instances using the emitter.setMaxListeners(n) method. To change the default for all EventEmitter instances, the EventEmitter.defaultMaxListeners property can be used. If this value is not a positive number, a TypeError will be thrown.

Take caution when setting the EventEmitter.defaultMaxListeners because the change affects all EventEmitter instances, including those created before the change is made. However, calling emitter.setMaxListeners(n) still has precedence over EventEmitter.defaultMaxListeners.

This is not a hard limit. The EventEmitter instance will allow more listeners to be added but will output a trace warning to stderr indicating that a "possible EventEmitter memory leak" has been detected. For any single EventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners() methods can be used to temporarily avoid this warning:

emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
  // do stuff
  emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});

The --trace-warnings command line flag can be used to display the stack trace for such warnings.

The emitted warning can be inspected with process.on('warning') and will have the additional emitter, type and count properties, referring to the event emitter instance, the event’s name and the number of attached listeners, respectively. Its name property is set to 'MaxListenersExceededWarning'.

EventEmitter.errorMonitor#

Added in: v13.6.0, v12.16.0

This symbol shall be used to install a listener for only monitoring 'error' events. Listeners installed using this symbol are called before the regular 'error' listeners are called.

Installing a listener using this symbol does not change the behavior once an 'error' event is emitted, therefore the process will still crash if no regular 'error' listener is installed.

emitter.addListener(eventName, listener)#

Added in: v0.1.26

Alias for emitter.on(eventName, listener).

emitter.emit(eventName[, ...args])#

Added in: v0.1.26

Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.

Returns true if the event had listeners, false otherwise.

const EventEmitter = require('events');
const myEmitter = new EventEmitter();

// First listener
myEmitter.on('event', function firstListener() {
  console.log('Helloooo! first listener');
});
// Second listener
myEmitter.on('event', function secondListener(arg1, arg2) {
  console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
});
// Third listener
myEmitter.on('event', function thirdListener(...args) {
  const parameters = args.join(', ');
  console.log(`event with parameters ${parameters} in third listener`);
});

console.log(myEmitter.listeners('event'));

myEmitter.emit('event', 1, 2, 3, 4, 5);

// Prints:
// [
//   [Function: firstListener],
//   [Function: secondListener],
//   [Function: thirdListener]
// ]
// Helloooo! first listener
// event with parameters 1, 2 in second listener
// event with parameters 1, 2, 3, 4, 5 in third listener

emitter.eventNames()#

Added in: v6.0.0

Returns an array listing the events for which the emitter has registered listeners. The values in the array will be strings or Symbols.

const EventEmitter = require('events');
const myEE = new EventEmitter();
myEE.on('foo', () => {});
myEE.on('bar', () => {});

const sym = Symbol('symbol');
myEE.on(sym, () => {});

console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]

emitter.getMaxListeners()#

Added in: v1.0.0

Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to EventEmitter.defaultMaxListeners.

emitter.listenerCount(eventName)#

Added in: v3.2.0

Returns the number of listeners listening to the event named eventName.

emitter.listeners(eventName)#

History
VersionChanges
v7.0.0

For listeners attached using .once() this returns the original listeners instead of wrapper functions now.

v0.1.26

Added in: v0.1.26

Returns a copy of the array of listeners for the event named eventName.

server.on('connection', (stream) => {
  console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]

emitter.off(eventName, listener)#

Added in: v10.0.0

Alias for emitter.removeListener().

emitter.on(eventName, listener)#

Added in: v0.1.101

Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.on('connection', (stream) => {
  console.log('someone connected!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a

emitter.once(eventName, listener)#

Added in: v0.3.0

Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

server.once('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a

emitter.prependListener(eventName, listener)#

Added in: v6.0.0

Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.prependListener('connection', (stream) => {
  console.log('someone connected!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.prependOnceListener(eventName, listener)#

Added in: v6.0.0

Adds a one-time listener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

server.prependOnceListener('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.removeAllListeners([eventName])#

Added in: v0.1.26

Removes all listeners, or those of the specified eventName.

It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.removeListener(eventName, listener)#

Added in: v0.1.26

Removes the specified listener from the listener array for the event named eventName.

const callback = (stream) => {
  console.log('someone connected!');
};
server.on('connection', callback);
// ...
server.removeListener('connection', callback);

removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.

Once an event has been emitted, all listeners attached to it at the time of emitting will be called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them from emit() in progress. Subsequent events will behave as expected.

const myEmitter = new MyEmitter();

const callbackA = () => {
  console.log('A');
  myEmitter.removeListener('event', callbackB);
};

const callbackB = () => {
  console.log('B');
};

myEmitter.on('event', callbackA);

myEmitter.on('event', callbackB);

// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event');
// Prints:
//   A
//   B

// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event');
// Prints:
//   A

Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping') listener is removed:

const ee = new EventEmitter();

function pong() {
  console.log('pong');
}

ee.on('ping', pong);
ee.once('ping', pong);
ee.removeListener('ping', pong);

ee.emit('ping');
ee.emit('ping');

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.setMaxListeners(n)#

Added in: v0.3.5

By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.rawListeners(eventName)#

Added in: v9.4.0

Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).

const emitter = new EventEmitter();
emitter.once('log', () => console.log('log once'));

// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners('log');
const logFnWrapper = listeners[0];

// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener();

// Logs "log once" to the console and removes the listener
logFnWrapper();

emitter.on('log', () => console.log('log persistently'));
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners('log');

// Logs "log persistently" twice
newListeners[0]();
emitter.emit('log');

emitter[Symbol.for('nodejs.rejection')](err, eventName[, ...args])#

Added in: v13.4.0, v12.16.0

Stability: 1 - captureRejections is experimental.

The Symbol.for('nodejs.rejection') method is called in case a promise rejection happens when emitting an event and captureRejections is enabled on the emitter. It is possible to use events.captureRejectionSymbol in place of Symbol.for('nodejs.rejection').

const { EventEmitter, captureRejectionSymbol } = require('events');

class MyClass extends EventEmitter {
  constructor() {
    super({ captureRejections: true });
  }

  [captureRejectionSymbol](err, event, ...args) {
    console.log('rejection happened for', event, 'with', err, ...args);
    this.destroy(err);
  }

  destroy(err) {
    // Tear the resource down here.
  }
}

events.once(emitter, name)#

Added in: v11.13.0, v10.16.0

Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected when the EventEmitter emits 'error'. The Promise will resolve with an array of all the arguments emitted to the given event.

This method is intentionally generic and works with the web platform EventTarget interface, which has no special 'error' event semantics and does not listen to the 'error' event.

const { once, EventEmitter } = require('events');

async function run() {
  const ee = new EventEmitter();

  process.nextTick(() => {
    ee.emit('myevent', 42);
  });

  const [value] = await once(ee, 'myevent');
  console.log(value);

  const err = new Error('kaboom');
  process.nextTick(() => {
    ee.emit('error', err);
  });

  try {
    await once(ee, 'myevent');
  } catch (err) {
    console.log('error happened', err);
  }
}

run();

events.captureRejections#

Added in: v13.4.0, v12.16.0

Stability: 1 - captureRejections is experimental.

Value: <boolean>

Change the default captureRejections option on all new EventEmitter objects.

events.captureRejectionSymbol#

Added in: v13.4.0, v12.16.0

Stability: 1 - captureRejections is experimental.

Value: Symbol.for('nodejs.rejection')

See how to write a custom rejection handler.

events.on(emitter, eventName)#

Added in: v13.6.0, v12.16.0 
const { on, EventEmitter } = require('events');

(async () => {
  const ee = new EventEmitter();

  // Emit later on
  process.nextTick(() => {
    ee.emit('foo', 'bar');
    ee.emit('foo', 42);
  });

  for await (const event of on(ee, 'foo')) {
    // The execution of this inner block is synchronous and it
    // processes one event at a time (even with await). Do not use
    // if concurrent execution is required.
    console.log(event); // prints ['bar'] [42]
  }
  // Unreachable here
})();

Returns an AsyncIterator that iterates eventName events. It will throw if the EventEmitter emits 'error'. It removes all listeners when exiting the loop. The value returned by each iteration is an array composed of the emitted event arguments.

File System#

Stability: 2 - Stable

The fs module provides an API for interacting with the file system in a manner closely modeled around standard POSIX functions.

To use this module:

const fs = require('fs');

All file system operations have synchronous and asynchronous forms.

The asynchronous form always takes a completion callback as its last argument. The arguments passed to the completion callback depend on the method, but the first argument is always reserved for an exception. If the operation was completed successfully, then the first argument will be null or undefined.

const fs = require('fs');

fs.unlink('/tmp/hello', (err) => {
  if (err) throw err;
  console.log('successfully deleted /tmp/hello');
});

Exceptions that occur using synchronous operations are thrown immediately and may be handled using try…catch, or may be allowed to bubble up.

const fs = require('fs');

try {
  fs.unlinkSync('/tmp/hello');
  console.log('successfully deleted /tmp/hello');
} catch (err) {
  // handle the error
}

There is no guaranteed ordering when using asynchronous methods. So the following is prone to error because the fs.stat() operation may complete before the fs.rename() operation:

fs.rename('/tmp/hello', '/tmp/world', (err) => {
  if (err) throw err;
  console.log('renamed complete');
});
fs.stat('/tmp/world', (err, stats) => {
  if (err) throw err;
  console.log(`stats: ${JSON.stringify(stats)}`);
});

To correctly order the operations, move the fs.stat() call into the callback of the fs.rename() operation:

fs.rename('/tmp/hello', '/tmp/world', (err) => {
  if (err) throw err;
  fs.stat('/tmp/world', (err, stats) => {
    if (err) throw err;
    console.log(`stats: ${JSON.stringify(stats)}`);
  });
});

In busy processes, use the asynchronous versions of these calls. The synchronous versions will block the entire process until they complete, halting all connections.

While it is not recommended, most fs functions allow the callback argument to be omitted, in which case a default callback is used that rethrows errors. To get a trace to the original call site, set the NODE_DEBUG environment variable:

Omitting the callback function on asynchronous fs functions is deprecated and may result in an error being thrown in the future.

$ cat script.js
function bad() {
  require('fs').readFile('/');
}
bad();

$ env NODE_DEBUG=fs node script.js
fs.js:88
        throw backtrace;
        ^
Error: EISDIR: illegal operation on a directory, read
    <stack trace.>

File paths#

Most fs operations accept filepaths that may be specified in the form of a string, a Buffer, or a URL object using the file: protocol.

String form paths are interpreted as UTF-8 character sequences identifying the absolute or relative filename. Relative paths will be resolved relative to the current working directory as specified by process.cwd().

Example using an absolute path on POSIX:

const fs = require('fs');

fs.open('/open/some/file.txt', 'r', (err, fd) => {
  if (err) throw err;
  fs.close(fd, (err) => {
    if (err) throw err;
  });
});

Example using a relative path on POSIX (relative to process.cwd()):

fs.open('file.txt', 'r', (err, fd) => {
  if (err) throw err;
  fs.close(fd, (err) => {
    if (err) throw err;
  });
});

Paths specified using a Buffer are useful primarily on certain POSIX operating systems that treat file paths as opaque byte sequences. On such systems, it is possible for a single file path to contain sub-sequences that use multiple character encodings. As with string paths, Buffer paths may be relative or absolute:

Example using an absolute path on POSIX:

fs.open(Buffer.from('/open/some/file.txt'), 'r', (err, fd) => {
  if (err) throw err;
  fs.close(fd, (err) => {
    if (err) throw err;
  });
});

On Windows, Node.js follows the concept of per-drive working directory. This behavior can be observed when using a drive path without a backslash. For example fs.readdirSync('C:\\') can potentially return a different result than fs.readdirSync('C:'). For more information, see this MSDN page.

URL object support#

Added in: v7.6.0

For most fs module functions, the path or filename argument may be passed as a WHATWG URL object. Only URL objects using the file: protocol are supported.

const fs = require('fs');
const fileUrl = new URL('file:///tmp/hello');

fs.readFileSync(fileUrl);

file: URLs are always absolute paths.

Using WHATWG URL objects might introduce platform-specific behaviors.

On Windows, file: URLs with a host name convert to UNC paths, while file: URLs with drive letters convert to local absolute paths. file: URLs without a host name nor a drive letter will result in a throw:

// On Windows :

// - WHATWG file URLs with hostname convert to UNC path
// file://hostname/p/a/t/h/file => \\hostname\p\a\t\h\file
fs.readFileSync(new URL('file://hostname/p/a/t/h/file'));

// - WHATWG file URLs with drive letters convert to absolute path
// file:///C:/tmp/hello => C:\tmp\hello
fs.readFileSync(new URL('file:///C:/tmp/hello'));

// - WHATWG file URLs without hostname must have a drive letters
fs.readFileSync(new URL('file:///notdriveletter/p/a/t/h/file'));
fs.readFileSync(new URL('file:///c/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must be absolute

file: URLs with drive letters must use : as a separator just after the drive letter. Using another separator will result in a throw.

On all other platforms, file: URLs with a host name are unsupported and will result in a throw:

// On other platforms:

// - WHATWG file URLs with hostname are unsupported
// file://hostname/p/a/t/h/file => throw!
fs.readFileSync(new URL('file://hostname/p/a/t/h/file'));
// TypeError [ERR_INVALID_FILE_URL_PATH]: must be absolute

// - WHATWG file URLs convert to absolute path
// file:///tmp/hello => /tmp/hello
fs.readFileSync(new URL('file:///tmp/hello'));

A file: URL having encoded slash characters will result in a throw on all platforms:

// On Windows
fs.readFileSync(new URL('file:///C:/p/a/t/h/%2F'));
fs.readFileSync(new URL('file:///C:/p/a/t/h/%2f'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */

// On POSIX
fs.readFileSync(new URL('file:///p/a/t/h/%2F'));
fs.readFileSync(new URL('file:///p/a/t/h/%2f'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
/ characters */

On Windows, file: URLs having encoded backslash will result in a throw:

// On Windows
fs.readFileSync(new URL('file:///C:/path/%5C'));
fs.readFileSync(new URL('file:///C:/path/%5c'));
/* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded
\ or / characters */

File Descriptors#

On POSIX systems, for every process, the kernel maintains a table of currently open files and resources. Each open file is assigned a simple numeric identifier called a file descriptor. At the system-level, all file system operations use these file descriptors to identify and track each specific file. Windows systems use a different but conceptually similar mechanism for tracking resources. To simplify things for users, Node.js abstracts away the specific differences between operating systems and assigns all open files a numeric file descriptor.

The fs.open() method is used to allocate a new file descriptor. Once allocated, the file descriptor may be used to read data from, write data to, or request information about the file.

fs.open('/open/some/file.txt', 'r', (err, fd) => {
  if (err) throw err;
  fs.fstat(fd, (err, stat) => {
    if (err) throw err;
    // use stat

    // always close the file descriptor!
    fs.close(fd, (err) => {
      if (err) throw err;
    });
  });
});

Most operating systems limit the number of file descriptors that may be open at any given time so it is critical to close the descriptor when operations are completed. Failure to do so will result in a memory leak that will eventually cause an application to crash.

Threadpool Usage#

All file system APIs except fs.FSWatcher() and those that are explicitly synchronous use libuv's threadpool, which can have surprising and negative performance implications for some applications. See the UV_THREADPOOL_SIZE documentation for more information.

Class fs.Dir#

Added in: v12.12.0

A class representing a directory stream.

Created by fs.opendir(), fs.opendirSync(), or fsPromises.opendir().

const fs = require('fs');

async function print(path) {
  const dir = await fs.promises.opendir(path);
  for await (const dirent of dir) {
    console.log(dirent.name);
  }
}
print('./').catch(console.error);

dir.close()#

Added in: v12.12.0

Asynchronously close the directory's underlying resource handle. Subsequent reads will result in errors.

A Promise is returned that will be resolved after the resource has been closed.

dir.close(callback)#

Added in: v12.12.0

Asynchronously close the directory's underlying resource handle. Subsequent reads will result in errors.

The callback will be called after the resource handle has been closed.

dir.closeSync()#

Added in: v12.12.0

Synchronously close the directory's underlying resource handle. Subsequent reads will result in errors.

dir.path#

Added in: v12.12.0

The read-only path of this directory as was provided to fs.opendir(), fs.opendirSync(), or fsPromises.opendir().

dir.read()#

Added in: v12.12.0

Asynchronously read the next directory entry via readdir(3) as an fs.Dirent.

After the read is completed, a Promise is returned that will be resolved with an fs.Dirent, or null if there are no more directory entries to read.

Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory may or may not be included in the iteration results.

dir.read(callback)#

Added in: v12.12.0

Asynchronously read the next directory entry via readdir(3) as an fs.Dirent.

After the read is completed, the callback will be called with an fs.Dirent, or null if there are no more directory entries to read.

Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory may or may not be included in the iteration results.

dir.readSync()#

Added in: v12.12.0

Synchronously read the next directory entry via readdir(3) as an fs.Dirent.

If there are no more directory entries to read, null will be returned.

Directory entries returned by this function are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory may or may not be included in the iteration results.

dir[Symbol.asyncIterator]()#

Added in: v12.12.0

Asynchronously iterates over the directory via readdir(3) until all entries have been read.

Entries returned by the async iterator are always an fs.Dirent. The null case from dir.read() is handled internally.

See fs.Dir for an example.

Directory entries returned by this iterator are in no particular order as provided by the operating system's underlying directory mechanisms. Entries added or removed while iterating over the directory may or may not be included in the iteration results.

Class: fs.Dirent#

Added in: v10.10.0

A representation of a directory entry, which can be a file or a subdirectory within the directory, as returned by reading from an fs.Dir. The directory entry is a combination of the file name and file type pairs.

Additionally, when fs.readdir() or fs.readdirSync() is called with the withFileTypes option set to true, the resulting array is filled with fs.Dirent objects, rather than strings or Buffers.

dirent.isBlockDevice()#

Added in: v10.10.0

Returns true if the fs.Dirent object describes a block device.

dirent.isCharacterDevice()#

Added in: v10.10.0

Returns true if the fs.Dirent object describes a character device.

dirent.isDirectory()#

Added in: v10.10.0

Returns true if the fs.Dirent object describes a file system directory.

dirent.isFIFO()#

Added in: v10.10.0

Returns true if the fs.Dirent object describes a first-in-first-out (FIFO) pipe.

dirent.isFile()#

Added in: v10.10.0

Returns true if the fs.Dirent object describes a regular file.

dirent.isSocket()#

Added in: v10.10.0

Returns true if the fs.Dirent object describes a socket.

dirent.isSymbolicLink()#

Added in: v10.10.0

Returns true if the fs.Dirent object describes a symbolic link.

dirent.name#

Added in: v10.10.0

The file name that this fs.Dirent object refers to. The type of this value is determined by the options.encoding passed to fs.readdir() or fs.readdirSync().

Class: fs.FSWatcher#

Added in: v0.5.8

A successful call to fs.watch() method will return a new fs.FSWatcher object.

All fs.FSWatcher objects emit a 'change' event whenever a specific watched file is modified.

Event: 'change'#

Added in: v0.5.8
  • eventType <string> The type of change event that has occurred
  • filename <string> | <Buffer> The filename that changed (if relevant/available)

Emitted when something changes in a watched directory or file. See more details in fs.watch().

The filename argument may not be provided depending on operating system support. If filename is provided, it will be provided as a Buffer if fs.watch() is called with its encoding option set to 'buffer', otherwise filename will be a UTF-8 string.

// Example when handled through fs.watch() listener
fs.watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => {
  if (filename) {
    console.log(filename);
    // Prints: <Buffer ...>
  }
});

Event: 'close'#

Added in: v10.0.0

Emitted when the watcher stops watching for changes. The closed fs.FSWatcher object is no longer usable in the event handler.

Event: 'error'#

Added in: v0.5.8

Emitted when an error occurs while watching the file. The errored fs.FSWatcher object is no longer usable in the event handler.

watcher.close()#

Added in: v0.5.8

Stop watching for changes on the given fs.FSWatcher. Once stopped, the fs.FSWatcher object is no longer usable.

watcher.ref()#

Added in: v14.3.0

When called, requests that the Node.js event loop not exit so long as the FSWatcher is active. Calling watcher.ref() multiple times will have no effect.

By default, all FSWatcher objects are "ref'ed", making it normally unnecessary to call watcher.ref() unless watcher.unref() had been called previously.

watcher.unref()#

Added in: v14.3.0

When called, the active FSWatcher object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the FSWatcher object's callback is invoked. Calling watcher.unref() multiple times will have no effect.

Class: fs.StatWatcher#

Added in: v14.3.0

A successful call to fs.watchFile() method will return a new fs.StatWatcher object.

watcher.ref()#

Added in: v14.3.0

When called, requests that the Node.js event loop not exit so long as the StatWatcher is active. Calling watcher.ref() multiple times will have no effect.

By default, all StatWatcher objects are "ref'ed", making it normally unnecessary to call watcher.ref() unless watcher.unref() had been called previously.

watcher.unref()#

Added in: v14.3.0

When called, the active StatWatcher object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the StatWatcher object's callback is invoked. Calling watcher.unref() multiple times will have no effect.

Class: fs.ReadStream#

Added in: v0.1.93

A successful call to fs.createReadStream() will return a new fs.ReadStream object.

Event: 'close'#

Added in: v0.1.93

Emitted when the fs.ReadStream's underlying file descriptor has been closed.

Event: 'open'#

Added in: v0.1.93
  • fd <integer> Integer file descriptor used by the ReadStream.

Emitted when the fs.ReadStream's file descriptor has been opened.

Event: 'ready'#

Added in: v9.11.0

Emitted when the fs.ReadStream is ready to be used.

Fires immediately after 'open'.

readStream.bytesRead#

Added in: v6.4.0

The number of bytes that have been read so far.

readStream.path#

Added in: v0.1.93

The path to the file the stream is reading from as specified in the first argument to fs.createReadStream(). If path is passed as a string, then readStream.path will be a string. If path is passed as a Buffer, then readStream.path will be a Buffer.

readStream.pending#

Added in: v11.2.0, v10.16.0

This property is true if the underlying file has not been opened yet, i.e. before the 'ready' event is emitted.

Class: fs.Stats#

History
VersionChanges
v8.1.0

Added times as numbers.

v0.1.21

Added in: v0.1.21

A fs.Stats object provides information about a file.

Objects returned from fs.stat(), fs.lstat() and fs.fstat() and their synchronous counterparts are of this type. If bigint in the options passed to those methods is true, the numeric values will be bigint instead of number, and the object will contain additional nanosecond-precision properties suffixed with Ns.

Stats {
  dev: 2114,
  ino: 48064969,
  mode: 33188,
  nlink: 1,
  uid: 85,
  gid: 100,
  rdev: 0,
  size: 527,
  blksize: 4096,
  blocks: 8,
  atimeMs: 1318289051000.1,
  mtimeMs: 1318289051000.1,
  ctimeMs: 1318289051000.1,
  birthtimeMs: 1318289051000.1,
  atime: Mon, 10 Oct 2011 23:24:11 GMT,
  mtime: Mon, 10 Oct 2011 23:24:11 GMT,
  ctime: Mon, 10 Oct 2011 23:24:11 GMT,
  birthtime: Mon, 10 Oct 2011 23:24:11 GMT }

bigint version:

BigIntStats {
  dev: 2114n,
  ino: 48064969n,
  mode: 33188n,
  nlink: 1n,
  uid: 85n,
  gid: 100n,
  rdev: 0n,
  size: 527n,
  blksize: 4096n,
  blocks: 8n,
  atimeMs: 1318289051000n,
  mtimeMs: 1318289051000n,
  ctimeMs: 1318289051000n,
  birthtimeMs: 1318289051000n,
  atimeNs: 1318289051000000000n,
  mtimeNs: 1318289051000000000n,
  ctimeNs: 1318289051000000000n,
  birthtimeNs: 1318289051000000000n,
  atime: Mon, 10 Oct 2011 23:24:11 GMT,
  mtime: Mon, 10 Oct 2011 23:24:11 GMT,
  ctime: Mon, 10 Oct 2011 23:24:11 GMT,
  birthtime: Mon, 10 Oct 2011 23:24:11 GMT }

stats.isBlockDevice()#

Added in: v0.1.10

Returns true if the fs.Stats object describes a block device.

stats.isCharacterDevice()#

Added in: v0.1.10

Returns true if the fs.Stats object describes a character device.

stats.isDirectory()#

Added in: v0.1.10

Returns true if the fs.Stats object describes a file system directory.

stats.isFIFO()#

Added in: v0.1.10

Returns true if the fs.Stats object describes a first-in-first-out (FIFO) pipe.

stats.isFile()#

Added in: v0.1.10

Returns true if the fs.Stats object describes a regular file.

stats.isSocket()#

Added in: v0.1.10

Returns true if the fs.Stats object describes a socket.

stats.isSymbolicLink()#

Added in: v0.1.10

Returns true if the fs.Stats object describes a symbolic link.

This method is only valid when using fs.lstat().

stats.dev#

The numeric identifier of the device containing the file.

stats.ino#

The file system specific "Inode" number for the file.

stats.mode#

A bit-field describing the file type and mode.

stats.nlink#

The number of hard-links that exist for the file.

stats.uid#

The numeric user identifier of the user that owns the file (POSIX).

stats.gid#

The numeric group identifier of the group that owns the file (POSIX).

stats.rdev#

A numeric device identifier if the file is considered "special".

stats.size#

The size of the file in bytes.

stats.blksize#

The file system block size for i/o operations.

stats.blocks#

The number of blocks allocated for this file.

stats.atimeMs#

Added in: v8.1.0

The timestamp indicating the last time this file was accessed expressed in milliseconds since the POSIX Epoch.

stats.mtimeMs#

Added in: v8.1.0

The timestamp indicating the last time this file was modified expressed in milliseconds since the POSIX Epoch.

stats.ctimeMs#

Added in: v8.1.0

The timestamp indicating the last time the file status was changed expressed in milliseconds since the POSIX Epoch.

stats.birthtimeMs#

Added in: v8.1.0

The timestamp indicating the creation time of this file expressed in milliseconds since the POSIX Epoch.

stats.atimeNs#

Added in: v12.10.0

Only present when bigint: true is passed into the method that generates the object. The timestamp indicating the last time this file was accessed expressed in nanoseconds since the POSIX Epoch.

stats.mtimeNs#

Added in: v12.10.0

Only present when bigint: true is passed into the method that generates the object. The timestamp indicating the last time this file was modified expressed in nanoseconds since the POSIX Epoch.

stats.ctimeNs#

Added in: v12.10.0

Only present when bigint: true is passed into the method that generates the object. The timestamp indicating the last time the file status was changed expressed in nanoseconds since the POSIX Epoch.

stats.birthtimeNs#

Added in: v12.10.0

Only present when bigint: true is passed into the method that generates the object. The timestamp indicating the creation time of this file expressed in nanoseconds since the POSIX Epoch.

stats.atime#

Added in: v0.11.13

The timestamp indicating the last time this file was accessed.

stats.mtime#

Added in: v0.11.13

The timestamp indicating the last time this file was modified.

stats.ctime#

Added in: v0.11.13

The timestamp indicating the last time the file status was changed.

stats.birthtime#

Added in: v0.11.13

The timestamp indicating the creation time of this file.

Stat Time Values#

The atimeMs, mtimeMs, ctimeMs, birthtimeMs properties are numeric values that hold the corresponding times in milliseconds. Their precision is platform specific. When bigint: true is passed into the method that generates the object, the properties will be bigints, otherwise they will be numbers.

The atimeNs, mtimeNs, ctimeNs, birthtimeNs properties are bigints that hold the corresponding times in nanoseconds. They are only present when bigint: true is passed into the method that generates the object. Their precision is platform specific.

atime, mtime, ctime, and birthtime are Date object alternate representations of the various times. The Date and number values are not connected. Assigning a new number value, or mutating the Date value, will not be reflected in the corresponding alternate representation.

The times in the stat object have the following semantics:

  • atime "Access Time": Time when file data last accessed. Changed by the mknod(2), utimes(2), and read(2) system calls.
  • mtime "Modified Time": Time when file data last modified. Changed by the mknod(2), utimes(2), and write(2) system calls.
  • ctime "Change Time": Time when file status was last changed (inode data modification). Changed by the chmod(2), chown(2), link(2), mknod(2), rename(2), unlink(2), utimes(2), read(2), and write(2) system calls.
  • birthtime "Birth Time": Time of file creation. Set once when the file is created. On filesystems where birthtime is not available, this field may instead hold either the ctime or 1970-01-01T00:00Z (ie, Unix epoch timestamp 0). This value may be greater than atime or mtime in this case. On Darwin and other FreeBSD variants, also set if the atime is explicitly set to an earlier value than the current birthtime using the utimes(2) system call.

Prior to Node.js 0.12, the ctime held the birthtime on Windows systems. As of 0.12, ctime is not "creation time", and on Unix systems, it never was.

Class: fs.WriteStream#

Added in: v0.1.93

Event: 'close'#

Added in: v0.1.93

Emitted when the WriteStream's underlying file descriptor has been closed.

Event: 'open'#

Added in: v0.1.93
  • fd <integer> Integer file descriptor used by the WriteStream.

Emitted when the WriteStream's file is opened.

Event: 'ready'#

Added in: v9.11.0

Emitted when the fs.WriteStream is ready to be used.

Fires immediately after 'open'.

writeStream.bytesWritten#

Added in: v0.4.7

The number of bytes written so far. Does not include data that is still queued for writing.

writeStream.path#

Added in: v0.1.93

The path to the file the stream is writing to as specified in the first argument to fs.createWriteStream(). If path is passed as a string, then writeStream.path will be a string. If path is passed as a Buffer, then writeStream.path will be a Buffer.

writeStream.pending#

Added in: v11.2.0

This property is true if the underlying file has not been opened yet, i.e. before the 'ready' event is emitted.

fs.access(path[, mode], callback)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v6.3.0

The constants like fs.R_OK, etc which were present directly on fs were moved into fs.constants as a soft deprecation. Thus for Node.js < v6.3.0 use fs to access those constants, or do something like (fs.constants || fs).R_OK to work with all versions.

v0.11.15

Added in: v0.11.15

Tests a user's permissions for the file or directory specified by path. The mode argument is an optional integer that specifies the accessibility checks to be performed. Check File Access Constants for possible values of mode. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.W_OK | fs.constants.R_OK).

The final argument, callback, is a callback function that is invoked with a possible error argument. If any of the accessibility checks fail, the error argument will be an Error object. The following examples check if package.json exists, and if it is readable or writable.

const file = 'package.json';

// Check if the file exists in the current directory.
fs.access(file, fs.constants.F_OK, (err) => {
  console.log(`${file} ${err ? 'does not exist' : 'exists'}`);
});

// Check if the file is readable.
fs.access(file, fs.constants.R_OK, (err) => {
  console.log(`${file} ${err ? 'is not readable' : 'is readable'}`);
});

// Check if the file is writable.
fs.access(file, fs.constants.W_OK, (err) => {
  console.log(`${file} ${err ? 'is not writable' : 'is writable'}`);
});

// Check if the file exists in the current directory, and if it is writable.
fs.access(file, fs.constants.F_OK | fs.constants.W_OK, (err) => {
  if (err) {
    console.error(
      `${file} ${err.code === 'ENOENT' ? 'does not exist' : 'is read-only'}`);
  } else {
    console.log(`${file} exists, and it is writable`);
  }
});

Using fs.access() to check for the accessibility of a file before calling fs.open(), fs.readFile() or fs.writeFile() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file is not accessible.

write (NOT RECOMMENDED)

fs.access('myfile', (err) => {
  if (!err) {
    console.error('myfile already exists');
    return;
  }

  fs.open('myfile', 'wx', (err, fd) => {
    if (err) throw err;
    writeMyData(fd);
  });
});

write (RECOMMENDED)

fs.open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === 'EEXIST') {
      console.error('myfile already exists');
      return;
    }

    throw err;
  }

  writeMyData(fd);
});

read (NOT RECOMMENDED)

fs.access('myfile', (err) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  fs.open('myfile', 'r', (err, fd) => {
    if (err) throw err;
    readMyData(fd);
  });
});

read (RECOMMENDED)

fs.open('myfile', 'r', (err, fd) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  readMyData(fd);
});

The "not recommended" examples above check for accessibility and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.

In general, check for the accessibility of a file only if the file will not be used directly, for example when its accessibility is a signal from another process.

On Windows, access-control policies (ACLs) on a directory may limit access to a file or directory. The fs.access() function, however, does not check the ACL and therefore may report that a path is accessible even if the ACL restricts the user from reading or writing to it.

fs.accessSync(path[, mode])#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.11.15

Added in: v0.11.15

Synchronously tests a user's permissions for the file or directory specified by path. The mode argument is an optional integer that specifies the accessibility checks to be performed. Check File Access Constants for possible values of mode. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.W_OK | fs.constants.R_OK).

If any of the accessibility checks fail, an Error will be thrown. Otherwise, the method will return undefined.

try {
  fs.accessSync('etc/passwd', fs.constants.R_OK | fs.constants.W_OK);
  console.log('can read/write');
} catch (err) {
  console.error('no access!');
}

fs.appendFile(path, data[, options], callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v7.0.0

The passed options object will never be modified.

v5.0.0

The file parameter can be a file descriptor now.

v0.6.7

Added in: v0.6.7

Asynchronously append data to a file, creating the file if it does not yet exist. data can be a string or a Buffer.

fs.appendFile('message.txt', 'data to append', (err) => {
  if (err) throw err;
  console.log('The "data to append" was appended to file!');
});

If options is a string, then it specifies the encoding:

fs.appendFile('message.txt', 'data to append', 'utf8', callback);

The path may be specified as a numeric file descriptor that has been opened for appending (using fs.open() or fs.openSync()). The file descriptor will not be closed automatically.

fs.open('message.txt', 'a', (err, fd) => {
  if (err) throw err;
  fs.appendFile(fd, 'data to append', 'utf8', (err) => {
    fs.close(fd, (err) => {
      if (err) throw err;
    });
    if (err) throw err;
  });
});

fs.appendFileSync(path, data[, options])#

History
VersionChanges
v7.0.0

The passed options object will never be modified.

v5.0.0

The file parameter can be a file descriptor now.

v0.6.7

Added in: v0.6.7

Synchronously append data to a file, creating the file if it does not yet exist. data can be a string or a Buffer.

try {
  fs.appendFileSync('message.txt', 'data to append');
  console.log('The "data to append" was appended to file!');
} catch (err) {
  /* Handle the error */
}

If options is a string, then it specifies the encoding:

fs.appendFileSync('message.txt', 'data to append', 'utf8');

The path may be specified as a numeric file descriptor that has been opened for appending (using fs.open() or fs.openSync()). The file descriptor will not be closed automatically.

let fd;

try {
  fd = fs.openSync('message.txt', 'a');
  fs.appendFileSync(fd, 'data to append', 'utf8');
} catch (err) {
  /* Handle the error */
} finally {
  if (fd !== undefined)
    fs.closeSync(fd);
}

fs.chmod(path, mode, callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.30

Added in: v0.1.30

Asynchronously changes the permissions of a file. No arguments other than a possible exception are given to the completion callback.

See also: chmod(2).

fs.chmod('my_file.txt', 0o775, (err) => {
  if (err) throw err;
  console.log('The permissions for file "my_file.txt" have been changed!');
});

File modes#

The mode argument used in both the fs.chmod() and fs.chmodSync() methods is a numeric bitmask created using a logical OR of the following constants:

ConstantOctalDescription
fs.constants.S_IRUSR0o400read by owner
fs.constants.S_IWUSR0o200write by owner
fs.constants.S_IXUSR0o100execute/search by owner
fs.constants.S_IRGRP0o40read by group
fs.constants.S_IWGRP0o20write by group
fs.constants.S_IXGRP0o10execute/search by group
fs.constants.S_IROTH0o4read by others
fs.constants.S_IWOTH0o2write by others
fs.constants.S_IXOTH0o1execute/search by others

An easier method of constructing the mode is to use a sequence of three octal digits (e.g. 765). The left-most digit (7 in the example), specifies the permissions for the file owner. The middle digit (6 in the example), specifies permissions for the group. The right-most digit (5 in the example), specifies the permissions for others.

NumberDescription
7read, write, and execute
6read and write
5read and execute
4read only
3write and execute
2write only
1execute only
0no permission

For example, the octal value 0o765 means:

  • The owner may read, write and execute the file.
  • The group may read and write the file.
  • Others may read and execute the file.

When using raw numbers where file modes are expected, any value larger than 0o777 may result in platform-specific behaviors that are not supported to work consistently. Therefore constants like S_ISVTX, S_ISGID or S_ISUID are not exposed in fs.constants.

Caveats: on Windows only the write permission can be changed, and the distinction among the permissions of group, owner or others is not implemented.

fs.chmodSync(path, mode)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.6.7

Added in: v0.6.7

For detailed information, see the documentation of the asynchronous version of this API: fs.chmod().

See also: chmod(2).

fs.chown(path, uid, gid, callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.97

Added in: v0.1.97

Asynchronously changes owner and group of a file. No arguments other than a possible exception are given to the completion callback.

See also: chown(2).

fs.chownSync(path, uid, gid)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.97

Added in: v0.1.97

Synchronously changes owner and group of a file. Returns undefined. This is the synchronous version of fs.chown().

See also: chown(2).

fs.close(fd, callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.0.2

Added in: v0.0.2

Asynchronous close(2). No arguments other than a possible exception are given to the completion callback.

Calling fs.close() on any file descriptor (fd) that is currently in use through any other fs operation may lead to undefined behavior.

fs.closeSync(fd)#

Added in: v0.1.21

Synchronous close(2). Returns undefined.

Calling fs.closeSync() on any file descriptor (fd) that is currently in use through any other fs operation may lead to undefined behavior.

fs.constants#

Returns an object containing commonly used constants for file system operations. The specific constants currently defined are described in FS Constants.

fs.copyFile(src, dest[, mode], callback)#

Added in: v8.5.0

Asynchronously copies src to dest. By default, dest is overwritten if it already exists. No arguments other than a possible exception are given to the callback function. Node.js makes no guarantees about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, Node.js will attempt to remove the destination.

mode is an optional integer that specifies the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE).

  • fs.constants.COPYFILE_EXCL: The copy operation will fail if dest already exists.
  • fs.constants.COPYFILE_FICLONE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.
  • fs.constants.COPYFILE_FICLONE_FORCE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
const fs = require('fs');
const { COPYFILE_EXCL } = fs.constants;

function callback(err) {
  if (err) throw err;
  console.log('source.txt was copied to destination.txt');
}

// destination.txt will be created or overwritten by default.
fs.copyFile('source.txt', 'destination.txt', callback);

// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
fs.copyFile('source.txt', 'destination.txt', COPYFILE_EXCL, callback);

fs.copyFileSync(src, dest[, mode])#

Added in: v8.5.0

Synchronously copies src to dest. By default, dest is overwritten if it already exists. Returns undefined. Node.js makes no guarantees about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, Node.js will attempt to remove the destination.

mode is an optional integer that specifies the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE).

  • fs.constants.COPYFILE_EXCL: The copy operation will fail if dest already exists.
  • fs.constants.COPYFILE_FICLONE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.
  • fs.constants.COPYFILE_FICLONE_FORCE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
const fs = require('fs');
const { COPYFILE_EXCL } = fs.constants;

// destination.txt will be created or overwritten by default.
fs.copyFileSync('source.txt', 'destination.txt');
console.log('source.txt was copied to destination.txt');

// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
fs.copyFileSync('source.txt', 'destination.txt', COPYFILE_EXCL);

fs.createReadStream(path[, options])#

History
VersionChanges
v13.6.0

The fs options allow overriding the used fs implementation.

v12.10.0

Enable emitClose option.

v11.0.0

Impose new restrictions on start and end, throwing more appropriate errors in cases when we cannot reasonably handle the input values.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The passed options object will never be modified.

v2.3.0

The passed options object can be a string now.

v0.1.31

Added in: v0.1.31

Unlike the 16 kb default highWaterMark for a readable stream, the stream returned by this method has a default highWaterMark of 64 kb.

options can include start and end values to read a range of bytes from the file instead of the entire file. Both start and end are inclusive and start counting at 0, allowed values are in the [0, Number.MAX_SAFE_INTEGER] range. If fd is specified and start is omitted or undefined, fs.createReadStream() reads sequentially from the current file position. The encoding can be any one of those accepted by Buffer.

If fd is specified, ReadStream will ignore the path argument and will use the specified file descriptor. This means that no 'open' event will be emitted. fd should be blocking; non-blocking fds should be passed to net.Socket.

If fd points to a character device that only supports blocking reads (such as keyboard or sound card), read operations do not finish until data is available. This can prevent the process from exiting and the stream from closing naturally.

By default, the stream will not emit a 'close' event after it has been destroyed. This is the opposite of the default for other Readable streams. Set the emitClose option to true to change this behavior.

By providing the fs option, it is possible to override the corresponding fs implementations for open, read, and close. When providing the fs option, overrides for open, read, and close are required.

const fs = require('fs');
// Create a stream from some character device.
const stream = fs.createReadStream('/dev/input/event0');
setTimeout(() => {
  stream.close(); // This may not close the stream.
  // Artificially marking end-of-stream, as if the underlying resource had
  // indicated end-of-file by itself, allows the stream to close.
  // This does not cancel pending read operations, and if there is such an
  // operation, the process may still not be able to exit successfully
  // until it finishes.
  stream.push(null);
  stream.read(0);
}, 100);

If autoClose is false, then the file descriptor won't be closed, even if there's an error. It is the application's responsibility to close it and make sure there's no file descriptor leak. If autoClose is set to true (default behavior), on 'error' or 'end' the file descriptor will be closed automatically.

mode sets the file mode (permission and sticky bits), but only if the file was created.

An example to read the last 10 bytes of a file which is 100 bytes long:

fs.createReadStream('sample.txt', { start: 90, end: 99 });

If options is a string, then it specifies the encoding.

fs.createWriteStream(path[, options])#

History
VersionChanges
v13.6.0

The fs options allow overriding the used fs implementation.

v12.10.0

Enable emitClose option.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The passed options object will never be modified.

v5.5.0

The autoClose option is supported now.

v2.3.0

The passed options object can be a string now.

v0.1.31

Added in: v0.1.31

options may also include a start option to allow writing data at some position past the beginning of the file, allowed values are in the [0, Number.MAX_SAFE_INTEGER] range. Modifying a file rather than replacing it may require the flags option to be set to r+ rather than the default w. The encoding can be any one of those accepted by Buffer.

If autoClose is set to true (default behavior) on 'error' or 'finish' the file descriptor will be closed automatically. If autoClose is false, then the file descriptor won't be closed, even if there's an error. It is the application's responsibility to close it and make sure there's no file descriptor leak.

By default, the stream will not emit a 'close' event after it has been destroyed. This is the opposite of the default for other Writable streams. Set the emitClose option to true to change this behavior.

By providing the fs option it is possible to override the corresponding fs implementations for open, write, writev and close. Overriding write() without writev() can reduce performance as some optimizations (_writev()) will be disabled. When providing the fs option, overrides for open, close, and at least one of write and writev are required.

Like ReadStream, if fd is specified, WriteStream will ignore the path argument and will use the specified file descriptor. This means that no 'open' event will be emitted. fd should be blocking; non-blocking fds should be passed to net.Socket.

If options is a string, then it specifies the encoding.

fs.exists(path, callback)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v1.0.0

Deprecated since: v1.0.0

v0.0.2

Added in: v0.0.2

Stability: 0 - Deprecated: Use fs.stat() or fs.access() instead.

Test whether or not the given path exists by checking with the file system. Then call the callback argument with either true or false:

fs.exists('/etc/passwd', (exists) => {
  console.log(exists ? 'it\'s there' : 'no passwd!');
});

The parameters for this callback are not consistent with other Node.js callbacks. Normally, the first parameter to a Node.js callback is an err parameter, optionally followed by other parameters. The fs.exists() callback has only one boolean parameter. This is one reason fs.access() is recommended instead of fs.exists().

Using fs.exists() to check for the existence of a file before calling fs.open(), fs.readFile() or fs.writeFile() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file does not exist.

write (NOT RECOMMENDED)

fs.exists('myfile', (exists) => {
  if (exists) {
    console.error('myfile already exists');
  } else {
    fs.open('myfile', 'wx', (err, fd) => {
      if (err) throw err;
      writeMyData(fd);
    });
  }
});

write (RECOMMENDED)

fs.open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === 'EEXIST') {
      console.error('myfile already exists');
      return;
    }

    throw err;
  }

  writeMyData(fd);
});

read (NOT RECOMMENDED)

fs.exists('myfile', (exists) => {
  if (exists) {
    fs.open('myfile', 'r', (err, fd) => {
      if (err) throw err;
      readMyData(fd);
    });
  } else {
    console.error('myfile does not exist');
  }
});

read (RECOMMENDED)

fs.open('myfile', 'r', (err, fd) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  readMyData(fd);
});

The "not recommended" examples above check for existence and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.

In general, check for the existence of a file only if the file won’t be used directly, for example when its existence is a signal from another process.

fs.existsSync(path)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Returns true if the path exists, false otherwise.

For detailed information, see the documentation of the asynchronous version of this API: fs.exists().

fs.exists() is deprecated, but fs.existsSync() is not. The callback parameter to fs.exists() accepts parameters that are inconsistent with other Node.js callbacks. fs.existsSync() does not use a callback.

if (fs.existsSync('/etc/passwd')) {
  console.log('The path exists.');
}

fs.fchmod(fd, mode, callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.4.7

Added in: v0.4.7

Asynchronous fchmod(2). No arguments other than a possible exception are given to the completion callback.

fs.fchmodSync(fd, mode)#

Added in: v0.4.7

Synchronous fchmod(2). Returns undefined.

fs.fchown(fd, uid, gid, callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.4.7

Added in: v0.4.7

Asynchronous fchown(2). No arguments other than a possible exception are given to the completion callback.

fs.fchownSync(fd, uid, gid)#

Added in: v0.4.7

Synchronous fchown(2). Returns undefined.

fs.fdatasync(fd, callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.96

Added in: v0.1.96

Asynchronous fdatasync(2). No arguments other than a possible exception are given to the completion callback.

fs.fdatasyncSync(fd)#

Added in: v0.1.96

Synchronous fdatasync(2). Returns undefined.

fs.fstat(fd[, options], callback)#

History
VersionChanges
v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.95

Added in: v0.1.95

Asynchronous fstat(2). The callback gets two arguments (err, stats) where stats is an fs.Stats object. fstat() is identical to stat(), except that the file to be stat-ed is specified by the file descriptor fd.

fs.fstatSync(fd[, options])#

History
VersionChanges
v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v0.1.95

Added in: v0.1.95

Synchronous fstat(2).

fs.fsync(fd, callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.96

Added in: v0.1.96

Asynchronous fsync(2). No arguments other than a possible exception are given to the completion callback.

fs.fsyncSync(fd)#

Added in: v0.1.96

Synchronous fsync(2). Returns undefined.

fs.ftruncate(fd[, len], callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.8.6

Added in: v0.8.6

Asynchronous ftruncate(2). No arguments other than a possible exception are given to the completion callback.

If the file referred to by the file descriptor was larger than len bytes, only the first len bytes will be retained in the file.

For example, the following program retains only the first four bytes of the file:

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

// get the file descriptor of the file to be truncated
const fd = fs.openSync('temp.txt', 'r+');

// Truncate the file to first four bytes
fs.ftruncate(fd, 4, (err) => {
  assert.ifError(err);
  console.log(fs.readFileSync('temp.txt', 'utf8'));
});
// Prints: Node

If the file previously was shorter than len bytes, it is extended, and the extended part is filled with null bytes ('\0'):

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

// get the file descriptor of the file to be truncated
const fd = fs.openSync('temp.txt', 'r+');

// Truncate the file to 10 bytes, whereas the actual size is 7 bytes
fs.ftruncate(fd, 10, (err) => {
  assert.ifError(err);
  console.log(fs.readFileSync('temp.txt'));
});
// Prints: <Buffer 4e 6f 64 65 2e 6a 73 00 00 00>
// ('Node.js\0\0\0' in UTF8)

The last three bytes are null bytes ('\0'), to compensate the over-truncation.

fs.ftruncateSync(fd[, len])#

Added in: v0.8.6

Returns undefined.

For detailed information, see the documentation of the asynchronous version of this API: fs.ftruncate().

fs.futimes(fd, atime, mtime, callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v4.1.0

Numeric strings, NaN and Infinity are now allowed time specifiers.

v0.4.2

Added in: v0.4.2

Change the file system timestamps of the object referenced by the supplied file descriptor. See fs.utimes().

This function does not work on AIX versions before 7.1, it will return the error UV_ENOSYS.

fs.futimesSync(fd, atime, mtime)#

History
VersionChanges
v4.1.0

Numeric strings, NaN and Infinity are now allowed time specifiers.

v0.4.2

Added in: v0.4.2

Synchronous version of fs.futimes(). Returns undefined.

fs.lchmod(path, mode, callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.4.7

Deprecated since: v0.4.7

Asynchronous lchmod(2). No arguments other than a possible exception are given to the completion callback.

Only available on macOS.

fs.lchmodSync(path, mode)#

Deprecated since: v0.4.7

Synchronous lchmod(2). Returns undefined.

fs.lchown(path, uid, gid, callback)#

History
VersionChanges
v10.6.0

This API is no longer deprecated.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

Asynchronous lchown(2). No arguments other than a possible exception are given to the completion callback.

fs.lchownSync(path, uid, gid)#

History
VersionChanges
v10.6.0

This API is no longer deprecated.

Synchronous lchown(2). Returns undefined.

fs.link(existingPath, newPath, callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The existingPath and newPath parameters can be WHATWG URL objects using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.31

Added in: v0.1.31

Asynchronous link(2). No arguments other than a possible exception are given to the completion callback.

fs.linkSync(existingPath, newPath)#

History
VersionChanges
v7.6.0

The existingPath and newPath parameters can be WHATWG URL objects using file: protocol. Support is currently still experimental.

v0.1.31

Added in: v0.1.31

Synchronous link(2). Returns undefined.

fs.lstat(path[, options], callback)#

History
VersionChanges
v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.30

Added in: v0.1.30

Asynchronous lstat(2). The callback gets two arguments (err, stats) where stats is a fs.Stats object. lstat() is identical to stat(), except that if path is a symbolic link, then the link itself is stat-ed, not the file that it refers to.

fs.lstatSync(path[, options])#

History
VersionChanges
v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.30

Added in: v0.1.30

Synchronous lstat(2).

fs.mkdir(path[, options], callback)#

History
VersionChanges
v13.11.0

In recursive mode, the callback now receives the first created path as an argument.

v10.12.0

The second argument can now be an options object with recursive and mode properties.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.8

Added in: v0.1.8

Asynchronously creates a directory.

The callback is given a possible exception and, if recursive is true, the first directory path created, (err, [path]).

The optional options argument can be an integer specifying mode (permission and sticky bits), or an object with a mode property and a recursive property indicating whether parent directories should be created. Calling fs.mkdir() when path is a directory that exists results in an error only when recursive is false.

// Creates /tmp/a/apple, regardless of whether `/tmp` and /tmp/a exist.
fs.mkdir('/tmp/a/apple', { recursive: true }, (err) => {
  if (err) throw err;
});

On Windows, using fs.mkdir() on the root directory even with recursion will result in an error:

fs.mkdir('/', { recursive: true }, (err) => {
  // => [Error: EPERM: operation not permitted, mkdir 'C:\']
});

See also: mkdir(2).

fs.mkdirSync(path[, options])#

History
VersionChanges
v13.11.0

In recursive mode, the first created path is returned now.

v10.12.0

The second argument can now be an options object with recursive and mode properties.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Synchronously creates a directory. Returns undefined, or if recursive is true, the first directory path created. This is the synchronous version of fs.mkdir().

See also: mkdir(2).

fs.mkdtemp(prefix[, options], callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v6.2.1

The callback parameter is optional now.

v5.10.0

Added in: v5.10.0

Creates a unique temporary directory.

Generates six random characters to be appended behind a required prefix to create a unique temporary directory. Due to platform inconsistencies, avoid trailing X characters in prefix. Some platforms, notably the BSDs, can return more than six random characters, and replace trailing X characters in prefix with random characters.

The created directory path is passed as a string to the callback's second parameter.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use.

fs.mkdtemp(path.join(os.tmpdir(), 'foo-'), (err, directory) => {
  if (err) throw err;
  console.log(directory);
  // Prints: /tmp/foo-itXde2 or C:\Users\...\AppData\Local\Temp\foo-itXde2
});

The fs.mkdtemp() method will append the six randomly selected characters directly to the prefix string. For instance, given a directory /tmp, if the intention is to create a temporary directory within /tmp, the prefix must end with a trailing platform-specific path separator (require('path').sep).

// The parent directory for the new temporary directory
const tmpDir = os.tmpdir();

// This method is *INCORRECT*:
fs.mkdtemp(tmpDir, (err, directory) => {
  if (err) throw err;
  console.log(directory);
  // Will print something similar to `/tmpabc123`.
  // A new temporary directory is created at the file system root
  // rather than *within* the /tmp directory.
});

// This method is *CORRECT*:
const { sep } = require('path');
fs.mkdtemp(`${tmpDir}${sep}`, (err, directory) => {
  if (err) throw err;
  console.log(directory);
  // Will print something similar to `/tmp/abc123`.
  // A new temporary directory is created within
  // the /tmp directory.
});

fs.mkdtempSync(prefix[, options])#

Added in: v5.10.0

Returns the created directory path.

For detailed information, see the documentation of the asynchronous version of this API: fs.mkdtemp().

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use.

fs.open(path[, flags[, mode]], callback)#

History
VersionChanges
v11.1.0

The flags argument is now optional and defaults to 'r'.

v9.9.0

The as and as+ flags are supported now.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.0.2

Added in: v0.0.2

Asynchronous file open. See open(2).

mode sets the file mode (permission and sticky bits), but only if the file was created. On Windows, only the write permission can be manipulated; see fs.chmod().

The callback gets two arguments (err, fd).

Some characters (< > : " / \ | ? *) are reserved under Windows as documented by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains a colon, Node.js will open a file system stream, as described by this MSDN page.

Functions based on fs.open() exhibit this behavior as well: fs.writeFile(), fs.readFile(), etc.

fs.opendir(path[, options], callback)#

History
VersionChanges
v13.1.0, v12.16.0

The bufferSize option was introduced.

v12.12.0

Added in: v12.12.0

Asynchronously open a directory. See opendir(3).

Creates an fs.Dir, which contains all further functions for reading from and cleaning up the directory.

The encoding option sets the encoding for the path while opening the directory and subsequent read operations.

fs.opendirSync(path[, options])#

History
VersionChanges
v13.1.0, v12.16.0

The bufferSize option was introduced.

v12.12.0

Added in: v12.12.0

  • path <string> | <Buffer> | <URL>

  • options <Object>

    • encoding <string> | <null> Default: 'utf8'
    • bufferSize <number> Number of directory entries that are buffered internally when reading from the directory. Higher values lead to better performance but higher memory usage. Default: 32
  • Returns: <fs.Dir>

Synchronously open a directory. See opendir(3).

Creates an fs.Dir, which contains all further functions for reading from and cleaning up the directory.

The encoding option sets the encoding for the path while opening the directory and subsequent read operations.

fs.openSync(path[, flags, mode])#

History
VersionChanges
v11.1.0

The flags argument is now optional and defaults to 'r'.

v9.9.0

The as and as+ flags are supported now.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Returns an integer representing the file descriptor.

For detailed information, see the documentation of the asynchronous version of this API: fs.open().

fs.read(fd, buffer, offset, length, position, callback)#

History
VersionChanges
v10.10.0

The buffer parameter can now be any TypedArray, or a DataView.

v7.4.0

The buffer parameter can now be a Uint8Array.

v6.0.0

The length parameter can now be 0.

v0.0.2

Added in: v0.0.2

Read data from the file specified by fd.

buffer is the buffer that the data (read from the fd) will be written to.

offset is the offset in the buffer to start writing at.

length is an integer specifying the number of bytes to read.

position is an argument specifying where to begin reading from in the file. If position is null, data will be read from the current file position, and the file position will be updated. If position is an integer, the file position will remain unchanged.

The callback is given the three arguments, (err, bytesRead, buffer).

If this method is invoked as its util.promisify()ed version, it returns a Promise for an Object with bytesRead and buffer properties.

fs.read(fd, [options,] callback)#

History
VersionChanges
v13.11.0

Options object can be passed in to make Buffer, offset, length and position optional

v13.11.0

Added in: v13.11.0

Similar to the above fs.read function, this version takes an optional options object. If no options object is specified, it will default with the above values.

fs.readdir(path[, options], callback)#

History
VersionChanges
v10.10.0

New option withFileTypes was added.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v6.0.0

The options parameter was added.

v0.1.8

Added in: v0.1.8

Asynchronous readdir(3). Reads the contents of a directory. The callback gets two arguments (err, files) where files is an array of the names of the files in the directory excluding '.' and '..'.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames passed to the callback. If the encoding is set to 'buffer', the filenames returned will be passed as Buffer objects.

If options.withFileTypes is set to true, the files array will contain fs.Dirent objects.

fs.readdirSync(path[, options])#

History
VersionChanges
v10.10.0

New option withFileTypes was added.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Synchronous readdir(3).

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames returned. If the encoding is set to 'buffer', the filenames returned will be passed as Buffer objects.

If options.withFileTypes is set to true, the result will contain fs.Dirent objects.

fs.readFile(path[, options], callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v5.1.0

The callback will always be called with null as the error parameter in case of success.

v5.0.0

The path parameter can be a file descriptor now.

v0.1.29

Added in: v0.1.29

Asynchronously reads the entire contents of a file.

fs.readFile('/etc/passwd', (err, data) => {
  if (err) throw err;
  console.log(data);
});

The callback is passed two arguments (err, data), where data is the contents of the file.

If no encoding is specified, then the raw buffer is returned.

If options is a string, then it specifies the encoding:

fs.readFile('/etc/passwd', 'utf8', callback);

When the path is a directory, the behavior of fs.readFile() and fs.readFileSync() is platform-specific. On macOS, Linux, and Windows, an error will be returned. On FreeBSD, a representation of the directory's contents will be returned.

// macOS, Linux, and Windows
fs.readFile('<directory>', (err, data) => {
  // => [Error: EISDIR: illegal operation on a directory, read <directory>]
});

//  FreeBSD
fs.readFile('<directory>', (err, data) => {
  // => null, <data>
});

The fs.readFile() function buffers the entire file. To minimize memory costs, when possible prefer streaming via fs.createReadStream().

File Descriptors#

  1. Any specified file descriptor has to support reading.
  2. If a file descriptor is specified as the path, it will not be closed automatically.
  3. The reading will begin at the current position. For example, if the file already had 'Hello World' and six bytes are read with the file descriptor, the call to fs.readFile() with the same file descriptor, would give 'World', rather than 'Hello World'.

fs.readFileSync(path[, options])#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v5.0.0

The path parameter can be a file descriptor now.

v0.1.8

Added in: v0.1.8

Returns the contents of the path.

For detailed information, see the documentation of the asynchronous version of this API: fs.readFile().

If the encoding option is specified then this function returns a string. Otherwise it returns a buffer.

Similar to fs.readFile(), when the path is a directory, the behavior of fs.readFileSync() is platform-specific.

// macOS, Linux, and Windows
fs.readFileSync('<directory>');
// => [Error: EISDIR: illegal operation on a directory, read <directory>]

//  FreeBSD
fs.readFileSync('<directory>'); // => <data>

fs.readlink(path[, options], callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.1.31

Added in: v0.1.31

Asynchronous readlink(2). The callback gets two arguments (err, linkString).

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path passed to the callback. If the encoding is set to 'buffer', the link path returned will be passed as a Buffer object.

fs.readlinkSync(path[, options])#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.31

Added in: v0.1.31

Synchronous readlink(2). Returns the symbolic link's string value.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path returned. If the encoding is set to 'buffer', the link path returned will be passed as a Buffer object.

fs.readSync(fd, buffer, offset, length, position)#

History
VersionChanges
v10.10.0

The buffer parameter can now be any TypedArray or a DataView.

v6.0.0

The length parameter can now be 0.

v0.1.21

Added in: v0.1.21

Returns the number of bytesRead.

For detailed information, see the documentation of the asynchronous version of this API: fs.read().

fs.readSync(fd, buffer, [options])#

History
VersionChanges
v14.0.0

Options object can be passed in to make offset, length and position optional

v14.0.0

Added in: v14.0.0

Returns the number of bytesRead.

Similar to the above fs.readSync function, this version takes an optional options object. If no options object is specified, it will default with the above values.

For detailed information, see the documentation of the asynchronous version of this API: fs.read().

fs.readv(fd, buffers[, position], callback)#

Added in: v14.0.0

Read from a file specified by fd and write to an array of ArrayBufferViews using readv().

position is the offset from the beginning of the file from where data should be read. If typeof position !== 'number', the data will be read from the current position.

The callback will be given three arguments: err, bytesRead, and buffers. bytesRead is how many bytes were read from the file.

fs.readvSync(fd, buffers[, position])#

Added in: v14.0.0

For detailed information, see the documentation of the asynchronous version of this API: fs.readv().

fs.realpath(path[, options], callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v8.0.0

Pipe/Socket resolve support was added.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v6.4.0

Calling realpath now works again for various edge cases on Windows.

v6.0.0

The cache parameter was removed.

v0.1.31

Added in: v0.1.31

Asynchronously computes the canonical pathname by resolving ., .. and symbolic links.

A canonical pathname is not necessarily unique. Hard links and bind mounts can expose a file system entity through many pathnames.

This function behaves like realpath(3), with some exceptions:

  1. No case conversion is performed on case-insensitive file systems.

  2. The maximum number of symbolic links is platform-independent and generally (much) higher than what the native realpath(3) implementation supports.

The callback gets two arguments (err, resolvedPath). May use process.cwd to resolve relative paths.

Only paths that can be converted to UTF8 strings are supported.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path passed to the callback. If the encoding is set to 'buffer', the path returned will be passed as a Buffer object.

If path resolves to a socket or a pipe, the function will return a system dependent name for that object.

fs.realpath.native(path[, options], callback)#

Added in: v9.2.0

Asynchronous realpath(3).

The callback gets two arguments (err, resolvedPath).

Only paths that can be converted to UTF8 strings are supported.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path passed to the callback. If the encoding is set to 'buffer', the path returned will be passed as a Buffer object.

On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on /proc in order for this function to work. Glibc does not have this restriction.

fs.realpathSync(path[, options])#

History
VersionChanges
v8.0.0

Pipe/Socket resolve support was added.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v6.4.0

Calling realpathSync now works again for various edge cases on Windows.

v6.0.0

The cache parameter was removed.

v0.1.31

Added in: v0.1.31

Returns the resolved pathname.

For detailed information, see the documentation of the asynchronous version of this API: fs.realpath().

fs.realpathSync.native(path[, options])#

Added in: v9.2.0

Synchronous realpath(3).

Only paths that can be converted to UTF8 strings are supported.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path returned. If the encoding is set to 'buffer', the path returned will be passed as a Buffer object.

On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on /proc in order for this function to work. Glibc does not have this restriction.

fs.rename(oldPath, newPath, callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The oldPath and newPath parameters can be WHATWG URL objects using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.0.2

Added in: v0.0.2

Asynchronously rename file at oldPath to the pathname provided as newPath. In the case that newPath already exists, it will be overwritten. If there is a directory at newPath, an error will be raised instead. No arguments other than a possible exception are given to the completion callback.

See also: rename(2).

fs.rename('oldFile.txt', 'newFile.txt', (err) => {
  if (err) throw err;
  console.log('Rename complete!');
});

fs.renameSync(oldPath, newPath)#

History
VersionChanges
v7.6.0

The oldPath and newPath parameters can be WHATWG URL objects using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Synchronous rename(2). Returns undefined.

fs.rmdir(path[, options], callback)#

History
VersionChanges
v13.3.0, v12.16.0

The maxBusyTries option is renamed to maxRetries, and its default is 0. The emfileWait option has been removed, and EMFILE errors use the same retry logic as other errors. The retryDelay option is now supported. ENFILE errors are now retried.

v12.10.0

The recursive, maxBusyTries, and emfileWait options are now supported.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameters can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.0.2

Added in: v0.0.2

Stability: 1 - Recursive removal is experimental.

  • path <string> | <Buffer> | <URL>

  • options <Object>

    • maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js will retry the operation with a linear backoff wait of retryDelay ms longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.
    • recursive <boolean> If true, perform a recursive directory removal. In recursive mode, errors are not reported if path does not exist, and operations are retried on failure. Default: false.
    • retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.

  • callback <Function>

Asynchronous rmdir(2). No arguments other than a possible exception are given to the completion callback.

Using fs.rmdir() on a file (not a directory) results in an ENOENT error on Windows and an ENOTDIR error on POSIX.

fs.rmdirSync(path[, options])#

History
VersionChanges
v13.3.0, v12.16.0

The maxBusyTries option is renamed to maxRetries, and its default is 0. The emfileWait option has been removed, and EMFILE errors use the same retry logic as other errors. The retryDelay option is now supported. ENFILE errors are now retried.

v12.10.0

The recursive, maxBusyTries, and emfileWait options are now supported.

v7.6.0

The path parameters can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Stability: 1 - Recursive removal is experimental.

  • path <string> | <Buffer> | <URL>

  • options <Object>

    • maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js will retry the operation with a linear backoff wait of retryDelay ms longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.
    • recursive <boolean> If true, perform a recursive directory removal. In recursive mode, errors are not reported if path does not exist, and operations are retried on failure. Default: false.
    • retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.

Synchronous rmdir(2). Returns undefined.

Using fs.rmdirSync() on a file (not a directory) results in an ENOENT error on Windows and an ENOTDIR error on POSIX.

fs.stat(path[, options], callback)#

History
VersionChanges
v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.0.2

Added in: v0.0.2

Asynchronous stat(2). The callback gets two arguments (err, stats) where stats is an fs.Stats object.

In case of an error, the err.code will be one of Common System Errors.

Using fs.stat() to check for the existence of a file before calling fs.open(), fs.readFile() or fs.writeFile() is not recommended. Instead, user code should open/read/write the file directly and handle the error raised if the file is not available.

To check if a file exists without manipulating it afterwards, fs.access() is recommended.

For example, given the following directory structure:

- txtDir
-- file.txt
- app.js

The next program will check for the stats of the given paths:

const fs = require('fs');

const pathsToCheck = ['./txtDir', './txtDir/file.txt'];

for (let i = 0; i < pathsToCheck.length; i++) {
  fs.stat(pathsToCheck[i], function(err, stats) {
    console.log(stats.isDirectory());
    console.log(stats);
  });
}

The resulting output will resemble:

true
Stats {
  dev: 16777220,
  mode: 16877,
  nlink: 3,
  uid: 501,
  gid: 20,
  rdev: 0,
  blksize: 4096,
  ino: 14214262,
  size: 96,
  blocks: 0,
  atimeMs: 1561174653071.963,
  mtimeMs: 1561174614583.3518,
  ctimeMs: 1561174626623.5366,
  birthtimeMs: 1561174126937.2893,
  atime: 2019-06-22T03:37:33.072Z,
  mtime: 2019-06-22T03:36:54.583Z,
  ctime: 2019-06-22T03:37:06.624Z,
  birthtime: 2019-06-22T03:28:46.937Z
}
false
Stats {
  dev: 16777220,
  mode: 33188,
  nlink: 1,
  uid: 501,
  gid: 20,
  rdev: 0,
  blksize: 4096,
  ino: 14214074,
  size: 8,
  blocks: 8,
  atimeMs: 1561174616618.8555,
  mtimeMs: 1561174614584,
  ctimeMs: 1561174614583.8145,
  birthtimeMs: 1561174007710.7478,
  atime: 2019-06-22T03:36:56.619Z,
  mtime: 2019-06-22T03:36:54.584Z,
  ctime: 2019-06-22T03:36:54.584Z,
  birthtime: 2019-06-22T03:26:47.711Z
}

fs.statSync(path[, options])#

History
VersionChanges
v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Synchronous stat(2).

fs.symlink(target, path[, type], callback)#

History
VersionChanges
v12.0.0

If the type argument is left undefined, Node will autodetect target type and automatically select dir or file

v7.6.0

The target and path parameters can be WHATWG URL objects using file: protocol. Support is currently still experimental.

v0.1.31

Added in: v0.1.31

Asynchronous symlink(2) which creates the link called path pointing to target. No arguments other than a possible exception are given to the completion callback.

The type argument is only available on Windows and ignored on other platforms. It can be set to 'dir', 'file', or 'junction'. If the type argument is not set, Node.js will autodetect target type and use 'file' or 'dir'. If the target does not exist, 'file' will be used. Windows junction points require the destination path to be absolute. When using 'junction', the target argument will automatically be normalized to absolute path.

Relative targets are relative to the link’s parent directory.

fs.symlink('./mew', './example/mewtwo', callback);

The above example creates a symbolic link mewtwo in the example which points to mew in the same directory:

$ tree example/
example/
├── mew
└── mewtwo -> ./mew

fs.symlinkSync(target, path[, type])#

History
VersionChanges
v12.0.0

If the type argument is left undefined, Node will autodetect target type and automatically select dir or file

v7.6.0

The target and path parameters can be WHATWG URL objects using file: protocol. Support is currently still experimental.

v0.1.31

Added in: v0.1.31

Returns undefined.

For detailed information, see the documentation of the asynchronous version of this API: fs.symlink().

fs.truncate(path[, len], callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.8.6

Added in: v0.8.6

Asynchronous truncate(2). No arguments other than a possible exception are given to the completion callback. A file descriptor can also be passed as the first argument. In this case, fs.ftruncate() is called.

Passing a file descriptor is deprecated and may result in an error being thrown in the future.

fs.truncateSync(path[, len])#

Added in: v0.8.6

Synchronous truncate(2). Returns undefined. A file descriptor can also be passed as the first argument. In this case, fs.ftruncateSync() is called.

Passing a file descriptor is deprecated and may result in an error being thrown in the future.

fs.unlink(path, callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.0.2

Added in: v0.0.2

Asynchronously removes a file or symbolic link. No arguments other than a possible exception are given to the completion callback.

// Assuming that 'path/file.txt' is a regular file.
fs.unlink('path/file.txt', (err) => {
  if (err) throw err;
  console.log('path/file.txt was deleted');
});

fs.unlink() will not work on a directory, empty or otherwise. To remove a directory, use fs.rmdir().

See also: unlink(2).

fs.unlinkSync(path)#

History
VersionChanges
v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.21

Added in: v0.1.21

Synchronous unlink(2). Returns undefined.

fs.unwatchFile(filename[, listener])#

Added in: v0.1.31

Stop watching for changes on filename. If listener is specified, only that particular listener is removed. Otherwise, all listeners are removed, effectively stopping watching of filename.

Calling fs.unwatchFile() with a filename that is not being watched is a no-op, not an error.

Using fs.watch() is more efficient than fs.watchFile() and fs.unwatchFile(). fs.watch() should be used instead of fs.watchFile() and fs.unwatchFile() when possible.

fs.utimes(path, atime, mtime, callback)#

History
VersionChanges
v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v8.0.0

NaN, Infinity, and -Infinity are no longer valid time specifiers.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v4.1.0

Numeric strings, NaN and Infinity are now allowed time specifiers.

v0.4.2

Added in: v0.4.2

Change the file system timestamps of the object referenced by path.

The atime and mtime arguments follow these rules:

  • Values can be either numbers representing Unix epoch time in seconds, Dates, or a numeric string like '123456789.0'.
  • If the value can not be converted to a number, or is NaN, Infinity or -Infinity, an Error will be thrown.

fs.utimesSync(path, atime, mtime)#

History
VersionChanges
v8.0.0

NaN, Infinity, and -Infinity are no longer valid time specifiers.

v7.6.0

The path parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v4.1.0

Numeric strings, NaN and Infinity are now allowed time specifiers.

v0.4.2

Added in: v0.4.2

Returns undefined.

For detailed information, see the documentation of the asynchronous version of this API: fs.utimes().

fs.watch(filename[, options][, listener])#

History
VersionChanges
v7.6.0

The filename parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v7.0.0

The passed options object will never be modified.

v0.5.10

Added in: v0.5.10

  • filename <string> | <Buffer> | <URL>

  • options <string> | <Object>

    • persistent <boolean> Indicates whether the process should continue to run as long as files are being watched. Default: true.
    • recursive <boolean> Indicates whether all subdirectories should be watched, or only the current directory. This applies when a directory is specified, and only on supported platforms (See Caveats). Default: false.
    • encoding <string> Specifies the character encoding to be used for the filename passed to the listener. Default: 'utf8'.

  • listener <Function> | <undefined> Default: undefined

  • Returns: <fs.FSWatcher>

Watch for changes on filename, where filename is either a file or a directory.

The second argument is optional. If options is provided as a string, it specifies the encoding. Otherwise options should be passed as an object.

The listener callback gets two arguments (eventType, filename). eventType is either 'rename' or 'change', and filename is the name of the file which triggered the event.

On most platforms, 'rename' is emitted whenever a filename appears or disappears in the directory.

The listener callback is attached to the 'change' event fired by fs.FSWatcher, but it is not the same thing as the 'change' value of eventType.

Caveats#

The fs.watch API is not 100% consistent across platforms, and is unavailable in some situations.

The recursive option is only supported on macOS and Windows. An ERR_FEATURE_UNAVAILABLE_ON_PLATFORM exception will be thrown when the option is used on a platform that does not support it.

On Windows, no events will be emitted if the watched directory is moved or renamed. An EPERM error is reported when the watched directory is deleted.

Availability#

This feature depends on the underlying operating system providing a way to be notified of filesystem changes.

If the underlying functionality is not available for some reason, then fs.watch() will not be able to function and may thrown an exception. For example, watching files or directories can be unreliable, and in some cases impossible, on network file systems (NFS, SMB, etc) or host file systems when using virtualization software such as Vagrant or Docker.

It is still possible to use fs.watchFile(), which uses stat polling, but this method is slower and less reliable.

Inodes#

On Linux and macOS systems, fs.watch() resolves the path to an inode and watches the inode. If the watched path is deleted and recreated, it is assigned a new inode. The watch will emit an event for the delete but will continue watching the original inode. Events for the new inode will not be emitted. This is expected behavior.

AIX files retain the same inode for the lifetime of a file. Saving and closing a watched file on AIX will result in two notifications (one for adding new content, and one for truncation).

Filename Argument#

Providing filename argument in the callback is only supported on Linux, macOS, Windows, and AIX. Even on supported platforms, filename is not always guaranteed to be provided. Therefore, don't assume that filename argument is always provided in the callback, and have some fallback logic if it is null.

fs.watch('somedir', (eventType, filename) => {
  console.log(`event type is: ${eventType}`);
  if (filename) {
    console.log(`filename provided: ${filename}`);
  } else {
    console.log('filename not provided');
  }
});

fs.watchFile(filename[, options], listener)#

History
VersionChanges
v10.5.0

The bigint option is now supported.

v7.6.0

The filename parameter can be a WHATWG URL object using file: protocol. Support is currently still experimental.

v0.1.31

Added in: v0.1.31

Watch for changes on filename. The callback listener will be called each time the file is accessed.

The options argument may be omitted. If provided, it should be an object. The options object may contain a boolean named persistent that indicates whether the process should continue to run as long as files are being watched. The options object may specify an interval property indicating how often the target should be polled in milliseconds.

The listener gets two arguments the current stat object and the previous stat object:

fs.watchFile('message.text', (curr, prev) => {
  console.log(`the current mtime is: ${curr.mtime}`);
  console.log(`the previous mtime was: ${prev.mtime}`);
});

These stat objects are instances of fs.Stat. If the bigint option is true, the numeric values in these objects are specified as BigInts.

To be notified when the file was modified, not just accessed, it is necessary to compare curr.mtime and prev.mtime.

When an fs.watchFile operation results in an ENOENT error, it will invoke the listener once, with all the fields zeroed (or, for dates, the Unix Epoch). If the file is created later on, the listener will be called again, with the latest stat objects. This is a change in functionality since v0.10.

Using fs.watch() is more efficient than fs.watchFile and fs.unwatchFile. fs.watch should be used instead of fs.watchFile and fs.unwatchFile when possible.

When a file being watched by fs.watchFile() disappears and reappears, then the previousStat reported in the second callback event (the file's reappearance) will be the same as the previousStat of the first callback event (its disappearance).

This happens when:

  • the file is deleted, followed by a restore
  • the file is renamed and then renamed a second time back to its original name

fs.write(fd, buffer[, offset[, length[, position]]], callback)#

History
VersionChanges
v14.0.0

The buffer parameter won't coerce unsupported input to strings anymore.

v10.10.0

The buffer parameter can now be any TypedArray or a DataView

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.4.0

The buffer parameter can now be a Uint8Array.

v7.2.0

The offset and length parameters are optional now.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.0.2

Added in: v0.0.2

Write buffer to the file specified by fd.

offset determines the part of the buffer to be written, and length is an integer specifying the number of bytes to write.

position refers to the offset from the beginning of the file where this data should be written. If typeof position !== 'number', the data will be written at the current position. See pwrite(2).

The callback will be given three arguments (err, bytesWritten, buffer) where bytesWritten specifies how many bytes were written from buffer.

If this method is invoked as its util.promisify()ed version, it returns a Promise for an Object with bytesWritten and buffer properties.

It is unsafe to use fs.write() multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream() is recommended.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

fs.write(fd, string[, position[, encoding]], callback)#

History
VersionChanges
v14.0.0

The string parameter won't coerce unsupported input to strings anymore.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.2.0

The position parameter is optional now.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v0.11.5

Added in: v0.11.5

Write string to the file specified by fd. If string is not a string, then an exception will be thrown.

position refers to the offset from the beginning of the file where this data should be written. If typeof position !== 'number' the data will be written at the current position. See pwrite(2).

encoding is the expected string encoding.

The callback will receive the arguments (err, written, string) where written specifies how many bytes the passed string required to be written. Bytes written is not necessarily the same as string characters written. See Buffer.byteLength.

It is unsafe to use fs.write() multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream() is recommended.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

On Windows, if the file descriptor is connected to the console (e.g. fd == 1 or stdout) a string containing non-ASCII characters will not be rendered properly by default, regardless of the encoding used. It is possible to configure the console to render UTF-8 properly by changing the active codepage with the chcp 65001 command. See the chcp docs for more details.

fs.writeFile(file, data[, options], callback)#

History
VersionChanges
v14.0.0

The data parameter won't coerce unsupported input to strings anymore.

v10.10.0

The data parameter can now be any TypedArray or a DataView.

v10.0.0

The callback parameter is no longer optional. Not passing it will throw a TypeError at runtime.

v7.4.0

The data parameter can now be a Uint8Array.

v7.0.0

The callback parameter is no longer optional. Not passing it will emit a deprecation warning with id DEP0013.

v5.0.0

The file parameter can be a file descriptor now.

v0.1.29

Added in: v0.1.29

When file is a filename, asynchronously writes data to the file, replacing the file if it already exists. data can be a string or a buffer.

When file is a file descriptor, the behavior is similar to calling fs.write() directly (which is recommended). See the notes below on using a file descriptor.

The encoding option is ignored if data is a buffer.

const data = new Uint8Array(Buffer.from('Hello Node.js'));
fs.writeFile('message.txt', data, (err) => {
  if (err) throw err;
  console.log('The file has been saved!');
});

If options is a string, then it specifies the encoding:

fs.writeFile('message.txt', 'Hello Node.js', 'utf8', callback);

It is unsafe to use fs.writeFile() multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream() is recommended.

Using fs.writeFile() with File Descriptors#

When file is a file descriptor, the behavior is almost identical to directly calling fs.write() like:

fs.write(fd, Buffer.from(data, options.encoding), callback);

The difference from directly calling fs.write() is that under some unusual conditions, fs.write() may write only part of the buffer and will need to be retried to write the remaining data, whereas fs.writeFile() will retry until the data is entirely written (or an error occurs).

The implications of this are a common source of confusion. In the file descriptor case, the file is not replaced! The data is not necessarily written to the beginning of the file, and the file's original data may remain before and/or after the newly written data.

For example, if fs.writeFile() is called twice in a row, first to write the string 'Hello', then to write the string ', World', the file would contain 'Hello, World', and might contain some of the file's original data (depending on the size of the original file, and the position of the file descriptor). If a file name had been used instead of a descriptor, the file would be guaranteed to contain only ', World'.

fs.writeFileSync(file, data[, options])#

History
VersionChanges
v14.0.0

The data parameter won't coerce unsupported input to strings anymore.

v10.10.0

The data parameter can now be any TypedArray or a DataView.

v7.4.0

The data parameter can now be a Uint8Array.

v5.0.0

The file parameter can be a file descriptor now.

v0.1.29

Added in: v0.1.29

Returns undefined.

For detailed information, see the documentation of the asynchronous version of this API: fs.writeFile().

fs.writeSync(fd, buffer[, offset[, length[, position]]])#

History
VersionChanges
v14.0.0

The buffer parameter won't coerce unsupported input to strings anymore.

v10.10.0

The buffer parameter can now be any TypedArray or a DataView.

v7.4.0

The buffer parameter can now be a Uint8Array.

v7.2.0

The offset and length parameters are optional now.

v0.1.21

Added in: v0.1.21

For detailed information, see the documentation of the asynchronous version of this API: fs.write(fd, buffer...).

fs.writeSync(fd, string[, position[, encoding]])#

History
VersionChanges
v14.0.0

The string parameter won't coerce unsupported input to strings anymore.

v7.2.0

The position parameter is optional now.

v0.11.5

Added in: v0.11.5

For detailed information, see the documentation of the asynchronous version of this API: fs.write(fd, string...).

fs.writev(fd, buffers[, position], callback)#

Added in: v12.9.0

Write an array of ArrayBufferViews to the file specified by fd using writev().

position is the offset from the beginning of the file where this data should be written. If typeof position !== 'number', the data will be written at the current position.

The callback will be given three arguments: err, bytesWritten, and buffers. bytesWritten is how many bytes were written from buffers.

If this method is util.promisify()ed, it returns a Promise for an Object with bytesWritten and buffers properties.

It is unsafe to use fs.writev() multiple times on the same file without waiting for the callback. For this scenario, use fs.createWriteStream().

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

fs.writevSync(fd, buffers[, position])#

Added in: v12.9.0

For detailed information, see the documentation of the asynchronous version of this API: fs.writev().

fs Promises API#

The fs.promises API provides an alternative set of asynchronous file system methods that return Promise objects rather than using callbacks. The API is accessible via require('fs').promises or require('fs/promises').

class: FileHandle#

Added in: v10.0.0

A FileHandle object is a wrapper for a numeric file descriptor. Instances of FileHandle are distinct from numeric file descriptors in that they provide an object oriented API for working with files.

If a FileHandle is not closed using the filehandle.close() method, it might automatically close the file descriptor and will emit a process warning, thereby helping to prevent memory leaks. Please do not rely on this behavior because it is unreliable and the file may not be closed. Instead, always explicitly close FileHandles. Node.js may change this behavior in the future.

Instances of the FileHandle object are created internally by the fsPromises.open() method.

Unlike the callback-based API (fs.fstat(), fs.fchown(), fs.fchmod(), and so on), a numeric file descriptor is not used by the promise-based API. Instead, the promise-based API uses the FileHandle class in order to help avoid accidental leaking of unclosed file descriptors after a Promise is resolved or rejected.

filehandle.appendFile(data, options)#

Added in: v10.0.0

Alias of filehandle.writeFile().

When operating on file handles, the mode cannot be changed from what it was set to with fsPromises.open(). Therefore, this is equivalent to filehandle.writeFile().

filehandle.chmod(mode)#

Added in: v10.0.0

Modifies the permissions on the file. The Promise is resolved with no arguments upon success.

filehandle.chown(uid, gid)#

Added in: v10.0.0

Changes the ownership of the file then resolves the Promise with no arguments upon success.

filehandle.close()#

Added in: v10.0.0
  • Returns: <Promise> A Promise that will be resolved once the underlying file descriptor is closed, or will be rejected if an error occurs while closing.

Closes the file descriptor.

const fsPromises = require('fs').promises;
async function openAndClose() {
  let filehandle;
  try {
    filehandle = await fsPromises.open('thefile.txt', 'r');
  } finally {
    if (filehandle !== undefined)
      await filehandle.close();
  }
}

filehandle.datasync()#

Added in: v10.0.0

Asynchronous fdatasync(2). The Promise is resolved with no arguments upon success.

filehandle.fd#

Added in: v10.0.0
  • <number> The numeric file descriptor managed by the FileHandle object.

filehandle.read(buffer, offset, length, position)#

Added in: v10.0.0

Read data from the file.

buffer is the buffer that the data will be written to.

offset is the offset in the buffer to start writing at.

length is an integer specifying the number of bytes to read.

position is an argument specifying where to begin reading from in the file. If position is null, data will be read from the current file position, and the file position will be updated. If position is an integer, the file position will remain unchanged.

Following successful read, the Promise is resolved with an object with a bytesRead property specifying the number of bytes read, and a buffer property that is a reference to the passed in buffer argument.

filehandle.read(options)#

Added in: v13.11.0

filehandle.readFile(options)#

Added in: v10.0.0

Asynchronously reads the entire contents of a file.

The Promise is resolved with the contents of the file. If no encoding is specified (using options.encoding), the data is returned as a Buffer object. Otherwise, the data will be a string.

If options is a string, then it specifies the encoding.

The FileHandle has to support reading.

If one or more filehandle.read() calls are made on a file handle and then a filehandle.readFile() call is made, the data will be read from the current position till the end of the file. It doesn't always read from the beginning of the file.

filehandle.readv(buffers[, position])#

Added in: v14.0.0

Read from a file and write to an array of ArrayBufferViews

The Promise is resolved with an object containing a bytesRead property identifying the number of bytes read, and a buffers property containing a reference to the buffers input.

position is the offset from the beginning of the file where this data should be read from. If typeof position !== 'number', the data will be read from the current position.

filehandle.stat([options])#

History
VersionChanges
v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v10.0.0

Added in: v10.0.0

Retrieves the fs.Stats for the file.

filehandle.sync()#

Added in: v10.0.0

Asynchronous fsync(2). The Promise is resolved with no arguments upon success.

filehandle.truncate(len)#

Added in: v10.0.0

Truncates the file then resolves the Promise with no arguments upon success.

If the file was larger than len bytes, only the first len bytes will be retained in the file.

For example, the following program retains only the first four bytes of the file:

const fs = require('fs');
const fsPromises = fs.promises;

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

async function doTruncate() {
  let filehandle = null;
  try {
    filehandle = await fsPromises.open('temp.txt', 'r+');
    await filehandle.truncate(4);
  } finally {
    if (filehandle) {
      // Close the file if it is opened.
      await filehandle.close();
    }
  }
  console.log(fs.readFileSync('temp.txt', 'utf8'));  // Prints: Node
}

doTruncate().catch(console.error);

If the file previously was shorter than len bytes, it is extended, and the extended part is filled with null bytes ('\0'):

const fs = require('fs');
const fsPromises = fs.promises;

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

async function doTruncate() {
  let filehandle = null;
  try {
    filehandle = await fsPromises.open('temp.txt', 'r+');
    await filehandle.truncate(10);
  } finally {
    if (filehandle) {
      // Close the file if it is opened.
      await filehandle.close();
    }
  }
  console.log(fs.readFileSync('temp.txt', 'utf8'));  // Prints Node.js\0\0\0
}

doTruncate().catch(console.error);

The last three bytes are null bytes ('\0'), to compensate the over-truncation.

filehandle.utimes(atime, mtime)#

Added in: v10.0.0

Change the file system timestamps of the object referenced by the FileHandle then resolves the Promise with no arguments upon success.

This function does not work on AIX versions before 7.1, it will resolve the Promise with an error using code UV_ENOSYS.

filehandle.write(buffer[, offset[, length[, position]]])#

History
VersionChanges
v14.0.0

The buffer parameter won't coerce unsupported input to buffers anymore.

v10.0.0

Added in: v10.0.0

Write buffer to the file.

The Promise is resolved with an object containing a bytesWritten property identifying the number of bytes written, and a buffer property containing a reference to the buffer written.

offset determines the part of the buffer to be written, and length is an integer specifying the number of bytes to write.

position refers to the offset from the beginning of the file where this data should be written. If typeof position !== 'number', the data will be written at the current position. See pwrite(2).

It is unsafe to use filehandle.write() multiple times on the same file without waiting for the Promise to be resolved (or rejected). For this scenario, use fs.createWriteStream().

On Linux, positional writes do not work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

filehandle.write(string[, position[, encoding]])#

History
VersionChanges
v14.0.0

The string parameter won't coerce unsupported input to strings anymore.

v10.0.0

Added in: v10.0.0

Write string to the file. If string is not a string, then an exception will be thrown.

The Promise is resolved with an object containing a bytesWritten property identifying the number of bytes written, and a buffer property containing a reference to the string written.

position refers to the offset from the beginning of the file where this data should be written. If the type of position is not a number the data will be written at the current position. See pwrite(2).

encoding is the expected string encoding.

It is unsafe to use filehandle.write() multiple times on the same file without waiting for the Promise to be resolved (or rejected). For this scenario, use fs.createWriteStream().

On Linux, positional writes do not work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

filehandle.writeFile(data, options)#

History
VersionChanges
v14.0.0

The data parameter won't coerce unsupported input to strings anymore.

v10.0.0

Added in: v10.0.0

Asynchronously writes data to a file, replacing the file if it already exists. data can be a string or a buffer. The Promise will be resolved with no arguments upon success.

The encoding option is ignored if data is a buffer.

If options is a string, then it specifies the encoding.

The FileHandle has to support writing.

It is unsafe to use filehandle.writeFile() multiple times on the same file without waiting for the Promise to be resolved (or rejected).

If one or more filehandle.write() calls are made on a file handle and then a filehandle.writeFile() call is made, the data will be written from the current position till the end of the file. It doesn't always write from the beginning of the file.

filehandle.writev(buffers[, position])#

Added in: v12.9.0

Write an array of ArrayBufferViews to the file.

The Promise is resolved with an object containing a bytesWritten property identifying the number of bytes written, and a buffers property containing a reference to the buffers input.

position is the offset from the beginning of the file where this data should be written. If typeof position !== 'number', the data will be written at the current position.

It is unsafe to call writev() multiple times on the same file without waiting for the previous operation to complete.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

fsPromises.access(path[, mode])#

Added in: v10.0.0

Tests a user's permissions for the file or directory specified by path. The mode argument is an optional integer that specifies the accessibility checks to be performed. Check File Access Constants for possible values of mode. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.W_OK | fs.constants.R_OK).

If the accessibility check is successful, the Promise is resolved with no value. If any of the accessibility checks fail, the Promise is rejected with an Error object. The following example checks if the file /etc/passwd can be read and written by the current process.

const fs = require('fs');
const fsPromises = fs.promises;

fsPromises.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK)
  .then(() => console.log('can access'))
  .catch(() => console.error('cannot access'));

Using fsPromises.access() to check for the accessibility of a file before calling fsPromises.open() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file is not accessible.

fsPromises.appendFile(path, data[, options])#

Added in: v10.0.0

Asynchronously append data to a file, creating the file if it does not yet exist. data can be a string or a Buffer. The Promise will be resolved with no arguments upon success.

If options is a string, then it specifies the encoding.

The path may be specified as a FileHandle that has been opened for appending (using fsPromises.open()).

fsPromises.chmod(path, mode)#

Added in: v10.0.0

Changes the permissions of a file then resolves the Promise with no arguments upon succces.

fsPromises.chown(path, uid, gid)#

Added in: v10.0.0

Changes the ownership of a file then resolves the Promise with no arguments upon success.

fsPromises.copyFile(src, dest[, mode])#

Added in: v10.0.0

Asynchronously copies src to dest. By default, dest is overwritten if it already exists. The Promise will be resolved with no arguments upon success.

Node.js makes no guarantees about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, Node.js will attempt to remove the destination.

mode is an optional integer that specifies the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE).

  • fs.constants.COPYFILE_EXCL: The copy operation will fail if dest already exists.
  • fs.constants.COPYFILE_FICLONE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used.
  • fs.constants.COPYFILE_FICLONE_FORCE: The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail.
const {
  promises: fsPromises,
  constants: {
    COPYFILE_EXCL
  }
} = require('fs');

// destination.txt will be created or overwritten by default.
fsPromises.copyFile('source.txt', 'destination.txt')
  .then(() => console.log('source.txt was copied to destination.txt'))
  .catch(() => console.log('The file could not be copied'));

// By using COPYFILE_EXCL, the operation will fail if destination.txt exists.
fsPromises.copyFile('source.txt', 'destination.txt', COPYFILE_EXCL)
  .then(() => console.log('source.txt was copied to destination.txt'))
  .catch(() => console.log('The file could not be copied'));

fsPromises.lchmod(path, mode)#

Deprecated since: v10.0.0

Changes the permissions on a symbolic link then resolves the Promise with no arguments upon success. This method is only implemented on macOS.

fsPromises.lchown(path, uid, gid)#

History
VersionChanges
v10.6.0

This API is no longer deprecated.

v10.0.0

Added in: v10.0.0

Changes the ownership on a symbolic link then resolves the Promise with no arguments upon success.

fsPromises.link(existingPath, newPath)#

Added in: v10.0.0

Asynchronous link(2). The Promise is resolved with no arguments upon success.

fsPromises.lstat(path[, options])#

History
VersionChanges
v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v10.0.0

Added in: v10.0.0

Asynchronous lstat(2). The Promise is resolved with the fs.Stats object for the given symbolic link path.

fsPromises.mkdir(path[, options])#

Added in: v10.0.0

Asynchronously creates a directory then resolves the Promise with either no arguments, or the first directory path created if recursive is true.

The optional options argument can be an integer specifying mode (permission and sticky bits), or an object with a mode property and a recursive property indicating whether parent directories should be created. Calling fsPromises.mkdir() when path is a directory that exists results in a rejection only when recursive is false.

fsPromises.mkdtemp(prefix[, options])#

Added in: v10.0.0

Creates a unique temporary directory and resolves the Promise with the created directory path. A unique directory name is generated by appending six random characters to the end of the provided prefix. Due to platform inconsistencies, avoid trailing X characters in prefix. Some platforms, notably the BSDs, can return more than six random characters, and replace trailing X characters in prefix with random characters.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use.

fsPromises.mkdtemp(path.join(os.tmpdir(), 'foo-'))
  .catch(console.error);

The fsPromises.mkdtemp() method will append the six randomly selected characters directly to the prefix string. For instance, given a directory /tmp, if the intention is to create a temporary directory within /tmp, the prefix must end with a trailing platform-specific path separator (require('path').sep).

fsPromises.open(path, flags[, mode])#

History
VersionChanges
v11.1.0

The flags argument is now optional and defaults to 'r'.

v10.0.0

Added in: v10.0.0

Asynchronous file open that returns a Promise that, when resolved, yields a FileHandle object. See open(2).

mode sets the file mode (permission and sticky bits), but only if the file was created.

Some characters (< > : " / \ | ? *) are reserved under Windows as documented by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains a colon, Node.js will open a file system stream, as described by this MSDN page.

fsPromises.opendir(path[, options])#

History
VersionChanges
v13.1.0, v12.16.0

The bufferSize option was introduced.

v12.12.0

Added in: v12.12.0

Asynchronously open a directory. See opendir(3).

Creates an fs.Dir, which contains all further functions for reading from and cleaning up the directory.

The encoding option sets the encoding for the path while opening the directory and subsequent read operations.

Example using async iteration:

const fs = require('fs');

async function print(path) {
  const dir = await fs.promises.opendir(path);
  for await (const dirent of dir) {
    console.log(dirent.name);
  }
}
print('./').catch(console.error);

fsPromises.readdir(path[, options])#

History
VersionChanges
v10.11.0

New option withFileTypes was added.

v10.0.0

Added in: v10.0.0

Reads the contents of a directory then resolves the Promise with an array of the names of the files in the directory excluding '.' and '..'.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames. If the encoding is set to 'buffer', the filenames returned will be passed as Buffer objects.

If options.withFileTypes is set to true, the resolved array will contain fs.Dirent objects.

const fs = require('fs');

async function print(path) {
  const files = await fs.promises.readdir(path);
  for (const file of files) {
    console.log(file);
  }
}
print('./').catch(console.error);

fsPromises.readFile(path[, options])#

Added in: v10.0.0

Asynchronously reads the entire contents of a file.

The Promise is resolved with the contents of the file. If no encoding is specified (using options.encoding), the data is returned as a Buffer object. Otherwise, the data will be a string.

If options is a string, then it specifies the encoding.

When the path is a directory, the behavior of fsPromises.readFile() is platform-specific. On macOS, Linux, and Windows, the promise will be rejected with an error. On FreeBSD, a representation of the directory's contents will be returned.

Any specified FileHandle has to support reading.

fsPromises.readlink(path[, options])#

Added in: v10.0.0

Asynchronous readlink(2). The Promise is resolved with the linkString upon success.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path returned. If the encoding is set to 'buffer', the link path returned will be passed as a Buffer object.

fsPromises.realpath(path[, options])#

Added in: v10.0.0

Determines the actual location of path using the same semantics as the fs.realpath.native() function then resolves the Promise with the resolved path.

Only paths that can be converted to UTF8 strings are supported.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path. If the encoding is set to 'buffer', the path returned will be passed as a Buffer object.

On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on /proc in order for this function to work. Glibc does not have this restriction.

fsPromises.rename(oldPath, newPath)#

Added in: v10.0.0

Renames oldPath to newPath and resolves the Promise with no arguments upon success.

fsPromises.rmdir(path[, options])#

History
VersionChanges
v13.3.0, v12.16.0

The maxBusyTries option is renamed to maxRetries, and its default is 0. The emfileWait option has been removed, and EMFILE errors use the same retry logic as other errors. The retryDelay option is now supported. ENFILE errors are now retried.

v12.10.0

The recursive, maxBusyTries, and emfileWait options are now supported.

v10.0.0

Added in: v10.0.0

Stability: 1 - Recursive removal is experimental.

  • path <string> | <Buffer> | <URL>

  • options <Object>

    • maxRetries <integer> If an EBUSY, EMFILE, ENFILE, ENOTEMPTY, or EPERM error is encountered, Node.js will retry the operation with a linear backoff wait of retryDelay ms longer on each try. This option represents the number of retries. This option is ignored if the recursive option is not true. Default: 0.
    • recursive <boolean> If true, perform a recursive directory removal. In recursive mode, errors are not reported if path does not exist, and operations are retried on failure. Default: false.
    • retryDelay <integer> The amount of time in milliseconds to wait between retries. This option is ignored if the recursive option is not true. Default: 100.
  • Returns: <Promise>

Removes the directory identified by path then resolves the Promise with no arguments upon success.

Using fsPromises.rmdir() on a file (not a directory) results in the Promise being rejected with an ENOENT error on Windows and an ENOTDIR error on POSIX.

fsPromises.stat(path[, options])#

History
VersionChanges
v10.5.0

Accepts an additional options object to specify whether the numeric values returned should be bigint.

v10.0.0

Added in: v10.0.0

The Promise is resolved with the fs.Stats object for the given path.

fsPromises.symlink(target, path[, type])#

Added in: v10.0.0

Creates a symbolic link then resolves the Promise with no arguments upon success.

The type argument is only used on Windows platforms and can be one of 'dir', 'file', or 'junction'. Windows junction points require the destination path to be absolute. When using 'junction', the target argument will automatically be normalized to absolute path.

fsPromises.truncate(path[, len])#

Added in: v10.0.0

Truncates the path then resolves the Promise with no arguments upon success. The path must be a string or Buffer.

fsPromises.unlink(path)#

Added in: v10.0.0

Asynchronous unlink(2). The Promise is resolved with no arguments upon success.

fsPromises.utimes(path, atime, mtime)#

Added in: v10.0.0

Change the file system timestamps of the object referenced by path then resolves the Promise with no arguments upon success.

The atime and mtime arguments follow these rules:

  • Values can be either numbers representing Unix epoch time, Dates, or a numeric string like '123456789.0'.
  • If the value can not be converted to a number, or is NaN, Infinity or -Infinity, an Error will be thrown.

fsPromises.writeFile(file, data[, options])#

History
VersionChanges
v14.0.0

The data parameter won't coerce unsupported input to strings anymore.

v10.0.0

Added in: v10.0.0

Asynchronously writes data to a file, replacing the file if it already exists. data can be a string or a buffer. The Promise will be resolved with no arguments upon success.

The encoding option is ignored if data is a buffer.

If options is a string, then it specifies the encoding.

Any specified FileHandle has to support writing.

It is unsafe to use fsPromises.writeFile() multiple times on the same file without waiting for the Promise to be resolved (or rejected).

FS Constants#

The following constants are exported by fs.constants.

Not every constant will be available on every operating system.

File Access Constants#

The following constants are meant for use with fs.access().

Constant Description
F_OK Flag indicating that the file is visible to the calling process. This is useful for determining if a file exists, but says nothing about rwx permissions. Default if no mode is specified.
R_OK Flag indicating that the file can be read by the calling process.
W_OK Flag indicating that the file can be written by the calling process.
X_OK Flag indicating that the file can be executed by the calling process. This has no effect on Windows (will behave like fs.constants.F_OK).

File Copy Constants#

The following constants are meant for use with fs.copyFile().

Constant Description
COPYFILE_EXCL If present, the copy operation will fail with an error if the destination path already exists.
COPYFILE_FICLONE If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then a fallback copy mechanism is used.
COPYFILE_FICLONE_FORCE If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then the operation will fail with an error.

File Open Constants#

The following constants are meant for use with fs.open().

Constant Description
O_RDONLY Flag indicating to open a file for read-only access.
O_WRONLY Flag indicating to open a file for write-only access.
O_RDWR Flag indicating to open a file for read-write access.
O_CREAT Flag indicating to create the file if it does not already exist.
O_EXCL Flag indicating that opening a file should fail if the O_CREAT flag is set and the file already exists.
O_NOCTTY Flag indicating that if path identifies a terminal device, opening the path shall not cause that terminal to become the controlling terminal for the process (if the process does not already have one).
O_TRUNC Flag indicating that if the file exists and is a regular file, and the file is opened successfully for write access, its length shall be truncated to zero.
O_APPEND Flag indicating that data will be appended to the end of the file.
O_DIRECTORY Flag indicating that the open should fail if the path is not a directory.
O_NOATIME Flag indicating reading accesses to the file system will no longer result in an update to the atime information associated with the file. This flag is available on Linux operating systems only.
O_NOFOLLOW Flag indicating that the open should fail if the path is a symbolic link.
O_SYNC Flag indicating that the file is opened for synchronized I/O with write operations waiting for file integrity.
O_DSYNC Flag indicating that the file is opened for synchronized I/O with write operations waiting for data integrity.
O_SYMLINK Flag indicating to open the symbolic link itself rather than the resource it is pointing to.
O_DIRECT When set, an attempt will be made to minimize caching effects of file I/O.
O_NONBLOCK Flag indicating to open the file in nonblocking mode when possible.
UV_FS_O_FILEMAP When set, a memory file mapping is used to access the file. This flag is available on Windows operating systems only. On other operating systems, this flag is ignored.

File Type Constants#

The following constants are meant for use with the fs.Stats object's mode property for determining a file's type.

Constant Description
S_IFMT Bit mask used to extract the file type code.
S_IFREG File type constant for a regular file.
S_IFDIR File type constant for a directory.
S_IFCHR File type constant for a character-oriented device file.
S_IFBLK File type constant for a block-oriented device file.
S_IFIFO File type constant for a FIFO/pipe.
S_IFLNK File type constant for a symbolic link.
S_IFSOCK File type constant for a socket.

File Mode Constants#

The following constants are meant for use with the fs.Stats object's mode property for determining the access permissions for a file.

Constant Description
S_IRWXU File mode indicating readable, writable, and executable by owner.
S_IRUSR File mode indicating readable by owner.
S_IWUSR File mode indicating writable by owner.
S_IXUSR File mode indicating executable by owner.
S_IRWXG File mode indicating readable, writable, and executable by group.
S_IRGRP File mode indicating readable by group.
S_IWGRP File mode indicating writable by group.
S_IXGRP File mode indicating executable by group.
S_IRWXO File mode indicating readable, writable, and executable by others.
S_IROTH File mode indicating readable by others.
S_IWOTH File mode indicating writable by others.
S_IXOTH File mode indicating executable by others.

File System Flags#

The following flags are available wherever the flag option takes a string.

  • 'a': Open file for appending. The file is created if it does not exist.

  • 'ax': Like 'a' but fails if the path exists.

  • 'a+': Open file for reading and appending. The file is created if it does not exist.

  • 'ax+': Like 'a+' but fails if the path exists.

  • 'as': Open file for appending in synchronous mode. The file is created if it does not exist.

  • 'as+': Open file for reading and appending in synchronous mode. The file is created if it does not exist.

  • 'r': Open file for reading. An exception occurs if the file does not exist.

  • 'r+': Open file for reading and writing. An exception occurs if the file does not exist.

  • 'rs+': Open file for reading and writing in synchronous mode. Instructs the operating system to bypass the local file system cache.

    This is primarily useful for opening files on NFS mounts as it allows skipping the potentially stale local cache. It has a very real impact on I/O performance so using this flag is not recommended unless it is needed.

    This doesn't turn fs.open() or fsPromises.open() into a synchronous blocking call. If synchronous operation is desired, something like fs.openSync() should be used.

  • 'w': Open file for writing. The file is created (if it does not exist) or truncated (if it exists).

  • 'wx': Like 'w' but fails if the path exists.

  • 'w+': Open file for reading and writing. The file is created (if it does not exist) or truncated (if it exists).

  • 'wx+': Like 'w+' but fails if the path exists.

flag can also be a number as documented by open(2); commonly used constants are available from fs.constants. On Windows, flags are translated to their equivalent ones where applicable, e.g. O_WRONLY to FILE_GENERIC_WRITE, or O_EXCL|O_CREAT to CREATE_NEW, as accepted by CreateFileW.

The exclusive flag 'x' (O_EXCL flag in open(2)) ensures that path is newly created. On POSIX systems, path is considered to exist even if it is a symlink to a non-existent file. The exclusive flag may or may not work with network file systems.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

Modifying a file rather than replacing it may require the flag option to be set to 'r+' rather than the default 'w'.

The behavior of some flags are platform-specific. As such, opening a directory on macOS and Linux with the 'a+' flag, as in the example below, will return an error. In contrast, on Windows and FreeBSD, a file descriptor or a FileHandle will be returned.

// macOS and Linux
fs.open('<directory>', 'a+', (err, fd) => {
  // => [Error: EISDIR: illegal operation on a directory, open <directory>]
});

// Windows and FreeBSD
fs.open('<directory>', 'a+', (err, fd) => {
  // => null, <fd>
});

On Windows, opening an existing hidden file using the 'w' flag (either through fs.open() or fs.writeFile() or fsPromises.open()) will fail with EPERM. Existing hidden files can be opened for writing with the 'r+' flag.

A call to fs.ftruncate() or filehandle.truncate() can be used to reset the file contents.

Global Objects#

These objects are available in all modules. The following variables may appear to be global but are not. They exist only in the scope of modules, see the module system documentation:

The objects listed here are specific to Node.js. There are built-in objects that are part of the JavaScript language itself, which are also globally accessible.

Class: Buffer#

Added in: v0.1.103

Used to handle binary data. See the buffer section.

__dirname#

This variable may appear to be global but is not. See __dirname.

__filename#

This variable may appear to be global but is not. See __filename.

clearImmediate(immediateObject)#

Added in: v0.9.1

clearImmediate is described in the timers section.

clearInterval(intervalObject)#

Added in: v0.0.1

clearInterval is described in the timers section.

clearTimeout(timeoutObject)#

Added in: v0.0.1

clearTimeout is described in the timers section.

console#

Added in: v0.1.100

Used to print to stdout and stderr. See the console section.

exports#

This variable may appear to be global but is not. See exports.

global#

Added in: v0.1.27

In browsers, the top-level scope is the global scope. This means that within the browser var something will define a new global variable. In Node.js this is different. The top-level scope is not the global scope; var something inside a Node.js module will be local to that module.

module#

This variable may appear to be global but is not. See module.

process#

Added in: v0.1.7

The process object. See the process object section.

queueMicrotask(callback)#

Added in: v11.0.0

The queueMicrotask() method queues a microtask to invoke callback. If callback throws an exception, the process object 'uncaughtException' event will be emitted.

The microtask queue is managed by V8 and may be used in a similar manner to the process.nextTick() queue, which is managed by Node.js. The process.nextTick() queue is always processed before the microtask queue within each turn of the Node.js event loop.

// Here, `queueMicrotask()` is used to ensure the 'load' event is always
// emitted asynchronously, and therefore consistently. Using
// `process.nextTick()` here would result in the 'load' event always emitting
// before any other promise jobs.

DataHandler.prototype.load = async function load(key) {
  const hit = this._cache.get(url);
  if (hit !== undefined) {
    queueMicrotask(() => {
      this.emit('load', hit);
    });
    return;
  }

  const data = await fetchData(key);
  this._cache.set(url, data);
  this.emit('load', data);
};

require()#

This variable may appear to be global but is not. See require().

setImmediate(callback[, ...args])#

Added in: v0.9.1

setImmediate is described in the timers section.

setInterval(callback, delay[, ...args])#

Added in: v0.0.1

setInterval is described in the timers section.

setTimeout(callback, delay[, ...args])#

Added in: v0.0.1

setTimeout is described in the timers section.

TextDecoder#

Added in: v11.0.0

The WHATWG TextDecoder class. See the TextDecoder section.

TextEncoder#

Added in: v11.0.0

The WHATWG TextEncoder class. See the TextEncoder section.

URL#

Added in: v10.0.0

The WHATWG URL class. See the URL section.

URLSearchParams#

Added in: v10.0.0

The WHATWG URLSearchParams class. See the URLSearchParams section.

WebAssembly#

Added in: v8.0.0

The object that acts as the namespace for all W3C WebAssembly related functionality. See the Mozilla Developer Network for usage and compatibility.

HTTP#

Stability: 2 - Stable

To use the HTTP server and client one must require('http').

The HTTP interfaces in Node.js are designed to support many features of the protocol which have been traditionally difficult to use. In particular, large, possibly chunk-encoded, messages. The interface is careful to never buffer entire requests or responses, so the user is able to stream data.

HTTP message headers are represented by an object like this:

{ 'content-length': '123',
  'content-type': 'text/plain',
  'connection': 'keep-alive',
  'host': 'mysite.com',
  'accept': '*/*' }

Keys are lowercased. Values are not modified.

In order to support the full spectrum of possible HTTP applications, the Node.js HTTP API is very low-level. It deals with stream handling and message parsing only. It parses a message into headers and body but it does not parse the actual headers or the body.

See message.headers for details on how duplicate headers are handled.

The raw headers as they were received are retained in the rawHeaders property, which is an array of [key, value, key2, value2, ...]. For example, the previous message header object might have a rawHeaders list like the following:

[ 'ConTent-Length', '123456',
  'content-LENGTH', '123',
  'content-type', 'text/plain',
  'CONNECTION', 'keep-alive',
  'Host', 'mysite.com',
  'accepT', '*/*' ]

Class: http.Agent#

Added in: v0.3.4

An Agent is responsible for managing connection persistence and reuse for HTTP clients. It maintains a queue of pending requests for a given host and port, reusing a single socket connection for each until the queue is empty, at which time the socket is either destroyed or put into a pool where it is kept to be used again for requests to the same host and port. Whether it is destroyed or pooled depends on the keepAlive option.

Pooled connections have TCP Keep-Alive enabled for them, but servers may still close idle connections, in which case they will be removed from the pool and a new connection will be made when a new HTTP request is made for that host and port. Servers may also refuse to allow multiple requests over the same connection, in which case the connection will have to be remade for every request and cannot be pooled. The Agent will still make the requests to that server, but each one will occur over a new connection.

When a connection is closed by the client or the server, it is removed from the pool. Any unused sockets in the pool will be unrefed so as not to keep the Node.js process running when there are no outstanding requests. (see socket.unref()).

It is good practice, to destroy() an Agent instance when it is no longer in use, because unused sockets consume OS resources.

Sockets are removed from an agent when the socket emits either a 'close' event or an 'agentRemove' event. When intending to keep one HTTP request open for a long time without keeping it in the agent, something like the following may be done:

http.get(options, (res) => {
  // Do stuff
}).on('socket', (socket) => {
  socket.emit('agentRemove');
});

An agent may also be used for an individual request. By providing {agent: false} as an option to the http.get() or http.request() functions, a one-time use Agent with default options will be used for the client connection.

agent:false:

http.get({
  hostname: 'localhost',
  port: 80,
  path: '/',
  agent: false  // Create a new agent just for this one request
}, (res) => {
  // Do stuff with response
});

new Agent([options])#

Added in: v0.3.4

  • options <Object> Set of configurable options to set on the agent. Can have the following fields:

    • keepAlive <boolean> Keep sockets around even when there are no outstanding requests, so they can be used for future requests without having to reestablish a TCP connection. Not to be confused with the keep-alive value of the Connection header. The Connection: keep-alive header is always sent when using an agent except when the Connection header is explicitly specified or when the keepAlive and maxSockets options are respectively set to false and Infinity, in which case Connection: close will be used. Default: false.
    • keepAliveMsecs <number> When using the keepAlive option, specifies the initial delay for TCP Keep-Alive packets. Ignored when the keepAlive option is false or undefined. Default: 1000.
    • maxSockets <number> Maximum number of sockets to allow per host. Each request will use a new socket until the maximum is reached. Default: Infinity.
    • maxFreeSockets <number> Maximum number of sockets to leave open in a free state. Only relevant if keepAlive is set to true. Default: 256.
    • timeout <number> Socket timeout in milliseconds. This will set the timeout when the socket is created.

options in socket.connect() are also supported.

The default http.globalAgent that is used by http.request() has all of these values set to their respective defaults.

To configure any of them, a custom http.Agent instance must be created.

const http = require('http');
const keepAliveAgent = new http.Agent({ keepAlive: true });
options.agent = keepAliveAgent;
http.request(options, onResponseCallback);

agent.createConnection(options[, callback])#

Added in: v0.11.4

Produces a socket/stream to be used for HTTP requests.

By default, this function is the same as net.createConnection(). However, custom agents may override this method in case greater flexibility is desired.

A socket/stream can be supplied in one of two ways: by returning the socket/stream from this function, or by passing the socket/stream to callback.

This method is guaranteed to return an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

callback has a signature of (err, stream).

agent.keepSocketAlive(socket)#

Added in: v8.1.0

Called when socket is detached from a request and could be persisted by the Agent. Default behavior is to:

socket.setKeepAlive(true, this.keepAliveMsecs);
socket.unref();
return true;

This method can be overridden by a particular Agent subclass. If this method returns a falsy value, the socket will be destroyed instead of persisting it for use with the next request.

The socket argument can be an instance of <net.Socket>, a subclass of <stream.Duplex>.

agent.reuseSocket(socket, request)#

Added in: v8.1.0

Called when socket is attached to request after being persisted because of the keep-alive options. Default behavior is to:

socket.ref();

This method can be overridden by a particular Agent subclass.

The socket argument can be an instance of <net.Socket>, a subclass of <stream.Duplex>.

agent.destroy()#

Added in: v0.11.4

Destroy any sockets that are currently in use by the agent.

It is usually not necessary to do this. However, if using an agent with keepAlive enabled, then it is best to explicitly shut down the agent when it will no longer be used. Otherwise, sockets may hang open for quite a long time before the server terminates them.

agent.freeSockets#

Added in: v0.11.4

An object which contains arrays of sockets currently awaiting use by the agent when keepAlive is enabled. Do not modify.

Sockets in the freeSockets list will be automatically destroyed and removed from the array on 'timeout'.

agent.getName(options)#

Added in: v0.11.4

  • options <Object> A set of options providing information for name generation

    • host <string> A domain name or IP address of the server to issue the request to
    • port <number> Port of remote server
    • localAddress <string> Local interface to bind for network connections when issuing the request
    • family <integer> Must be 4 or 6 if this doesn't equal undefined.
  • Returns: <string>

Get a unique name for a set of request options, to determine whether a connection can be reused. For an HTTP agent, this returns host:port:localAddress or host:port:localAddress:family. For an HTTPS agent, the name includes the CA, cert, ciphers, and other HTTPS/TLS-specific options that determine socket reusability.

agent.maxFreeSockets#

Added in: v0.11.7

By default set to 256. For agents with keepAlive enabled, this sets the maximum number of sockets that will be left open in the free state.

agent.maxSockets#

Added in: v0.3.6

By default set to Infinity. Determines how many concurrent sockets the agent can have open per origin. Origin is the returned value of agent.getName().

agent.requests#

Added in: v0.5.9

An object which contains queues of requests that have not yet been assigned to sockets. Do not modify.

agent.sockets#

Added in: v0.3.6

An object which contains arrays of sockets currently in use by the agent. Do not modify.

Class: http.ClientRequest#

Added in: v0.1.17

This object is created internally and returned from http.request(). It represents an in-progress request whose header has already been queued. The header is still mutable using the setHeader(name, value), getHeader(name), removeHeader(name) API. The actual header will be sent along with the first data chunk or when calling request.end().

To get the response, add a listener for 'response' to the request object. 'response' will be emitted from the request object when the response headers have been received. The 'response' event is executed with one argument which is an instance of http.IncomingMessage.

During the 'response' event, one can add listeners to the response object; particularly to listen for the 'data' event.

If no 'response' handler is added, then the response will be entirely discarded. However, if a 'response' event handler is added, then the data from the response object must be consumed, either by calling response.read() whenever there is a 'readable' event, or by adding a 'data' handler, or by calling the .resume() method. Until the data is consumed, the 'end' event will not fire. Also, until the data is read it will consume memory that can eventually lead to a 'process out of memory' error.

Unlike the request object, if the response closes prematurely, the response object does not emit an 'error' event but instead emits the 'aborted' event.

Node.js does not check whether Content-Length and the length of the body which has been transmitted are equal or not.

Event: 'abort'#

Added in: v1.4.1

Emitted when the request has been aborted by the client. This event is only emitted on the first call to abort().

Event: 'connect'#

Added in: v0.7.0

Emitted each time a server responds to a request with a CONNECT method. If this event is not being listened for, clients receiving a CONNECT method will have their connections closed.

This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

A client and server pair demonstrating how to listen for the 'connect' event:

const http = require('http');
const net = require('net');
const { URL } = require('url');

// Create an HTTP tunneling proxy
const proxy = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('okay');
});
proxy.on('connect', (req, clientSocket, head) => {
  // Connect to an origin server
  const { port, hostname } = new URL(`http://${req.url}`);
  const serverSocket = net.connect(port || 80, hostname, () => {
    clientSocket.write('HTTP/1.1 200 Connection Established\r\n' +
                    'Proxy-agent: Node.js-Proxy\r\n' +
                    '\r\n');
    serverSocket.write(head);
    serverSocket.pipe(clientSocket);
    clientSocket.pipe(serverSocket);
  });
});

// Now that proxy is running
proxy.listen(1337, '127.0.0.1', () => {

  // Make a request to a tunneling proxy
  const options = {
    port: 1337,
    host: '127.0.0.1',
    method: 'CONNECT',
    path: 'www.google.com:80'
  };

  const req = http.request(options);
  req.end();

  req.on('connect', (res, socket, head) => {
    console.log('got connected!');

    // Make a request over an HTTP tunnel
    socket.write('GET / HTTP/1.1\r\n' +
                 'Host: www.google.com:80\r\n' +
                 'Connection: close\r\n' +
                 '\r\n');
    socket.on('data', (chunk) => {
      console.log(chunk.toString());
    });
    socket.on('end', () => {
      proxy.close();
    });
  });
});

Event: 'continue'#

Added in: v0.3.2

Emitted when the server sends a '100 Continue' HTTP response, usually because the request contained 'Expect: 100-continue'. This is an instruction that the client should send the request body.

Event: 'information'#

Added in: v10.0.0

Emitted when the server sends a 1xx intermediate response (excluding 101 Upgrade). The listeners of this event will receive an object containing the HTTP version, status code, status message, key-value headers object, and array with the raw header names followed by their respective values.

const http = require('http');

const options = {
  host: '127.0.0.1',
  port: 8080,
  path: '/length_request'
};

// Make a request
const req = http.request(options);
req.end();

req.on('information', (info) => {
  console.log(`Got information prior to main response: ${info.statusCode}`);
});

101 Upgrade statuses do not fire this event due to their break from the traditional HTTP request/response chain, such as web sockets, in-place TLS upgrades, or HTTP 2.0. To be notified of 101 Upgrade notices, listen for the 'upgrade' event instead.

Event: 'response'#

Added in: v0.1.0

Emitted when a response is received to this request. This event is emitted only once.

Event: 'socket'#

Added in: v0.5.3

This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

Event: 'timeout'#

Added in: v0.7.8

Emitted when the underlying socket times out from inactivity. This only notifies that the socket has been idle. The request must be aborted manually.

See also: request.setTimeout().

Event: 'upgrade'#

Added in: v0.1.94

Emitted each time a server responds to a request with an upgrade. If this event is not being listened for and the response status code is 101 Switching Protocols, clients receiving an upgrade header will have their connections closed.

This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

A client server pair demonstrating how to listen for the 'upgrade' event.

const http = require('http');

// Create an HTTP server
const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('okay');
});
server.on('upgrade', (req, socket, head) => {
  socket.write('HTTP/1.1 101 Web Socket Protocol Handshake\r\n' +
               'Upgrade: WebSocket\r\n' +
               'Connection: Upgrade\r\n' +
               '\r\n');

  socket.pipe(socket); // echo back
});

// Now that server is running
server.listen(1337, '127.0.0.1', () => {

  // make a request
  const options = {
    port: 1337,
    host: '127.0.0.1',
    headers: {
      'Connection': 'Upgrade',
      'Upgrade': 'websocket'
    }
  };

  const req = http.request(options);
  req.end();

  req.on('upgrade', (res, socket, upgradeHead) => {
    console.log('got upgraded!');
    socket.end();
    process.exit(0);
  });
});

request.abort()#

Added in: v0.3.8Deprecated since: v14.1.0

Stability: 0 - Deprecated: Use request.destroy() instead.

Marks the request as aborting. Calling this will cause remaining data in the response to be dropped and the socket to be destroyed.

request.aborted#

History
VersionChanges
v11.0.0

The aborted property is no longer a timestamp number.

v0.11.14

Added in: v0.11.14

The request.aborted property will be true if the request has been aborted.

request.connection#

Added in: v0.3.0Deprecated since: v13.0.0

Stability: 0 - Deprecated. Use request.socket.

See request.socket.

request.end([data[, encoding]][, callback])#

History
VersionChanges
v10.0.0

This method now returns a reference to ClientRequest.

v0.1.90

Added in: v0.1.90

Finishes sending the request. If any parts of the body are unsent, it will flush them to the stream. If the request is chunked, this will send the terminating '0\r\n\r\n'.

If data is specified, it is equivalent to calling request.write(data, encoding) followed by request.end(callback).

If callback is specified, it will be called when the request stream is finished.

request.destroy([error])#

Added in: v0.3.0
  • error <Error> Optional, an error to emit with 'error' event.
  • Returns: <this>

Destroy the request. Optionally emit an 'error' event, and emit a 'close' event. Calling this will cause remaining data in the response to be dropped and the socket to be destroyed.

See writable.destroy() for further details.

request.destroyed#

Added in: v14.1.0

Is true after request.destroy() has been called.

See writable.destroyed for further details.

request.finished#

Added in: v0.0.1Deprecated since: v13.4.0, v12.16.0

Stability: 0 - Deprecated. Use request.writableEnded.

The request.finished property will be true if request.end() has been called. request.end() will automatically be called if the request was initiated via http.get().

request.flushHeaders()#

Added in: v1.6.0

Flushes the request headers.

For efficiency reasons, Node.js normally buffers the request headers until request.end() is called or the first chunk of request data is written. It then tries to pack the request headers and data into a single TCP packet.

That's usually desired (it saves a TCP round-trip), but not when the first data is not sent until possibly much later. request.flushHeaders() bypasses the optimization and kickstarts the request.

request.getHeader(name)#

Added in: v1.6.0

Reads out a header on the request. The name is case-insensitive. The type of the return value depends on the arguments provided to request.setHeader().

request.setHeader('content-type', 'text/html');
request.setHeader('Content-Length', Buffer.byteLength(body));
request.setHeader('Cookie', ['type=ninja', 'language=javascript']);
const contentType = request.getHeader('Content-Type');
// 'contentType' is 'text/html'
const contentLength = request.getHeader('Content-Length');
// 'contentLength' is of type number
const cookie = request.getHeader('Cookie');
// 'cookie' is of type string[]

request.maxHeadersCount#

Limits maximum response headers count. If set to 0, no limit will be applied.

request.path#

Added in: v0.4.0

request.removeHeader(name)#

Added in: v1.6.0

Removes a header that's already defined into headers object.

request.removeHeader('Content-Type');

request.reusedSocket#

Added in: v13.0.0, v12.16.0
  • <boolean> Whether the request is send through a reused socket.

When sending request through a keep-alive enabled agent, the underlying socket might be reused. But if server closes connection at unfortunate time, client may run into a 'ECONNRESET' error.

const http = require('http');

// Server has a 5 seconds keep-alive timeout by default
http
  .createServer((req, res) => {
    res.write('hello\n');
    res.end();
  })
  .listen(3000);

setInterval(() => {
  // Adapting a keep-alive agent
  http.get('http://localhost:3000', { agent }, (res) => {
    res.on('data', (data) => {
      // Do nothing
    });
  });
}, 5000); // Sending request on 5s interval so it's easy to hit idle timeout

By marking a request whether it reused socket or not, we can do automatic error retry base on it.

const http = require('http');
const agent = new http.Agent({ keepAlive: true });

function retriableRequest() {
  const req = http
    .get('http://localhost:3000', { agent }, (res) => {
      // ...
    })
    .on('error', (err) => {
      // Check if retry is needed
      if (req.reusedSocket && err.code === 'ECONNRESET') {
        retriableRequest();
      }
    });
}

retriableRequest();

request.setHeader(name, value)#

Added in: v1.6.0

Sets a single header value for headers object. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here to send multiple headers with the same name. Non-string values will be stored without modification. Therefore, request.getHeader() may return non-string values. However, the non-string values will be converted to strings for network transmission.

request.setHeader('Content-Type', 'application/json');

or

request.setHeader('Cookie', ['type=ninja', 'language=javascript']);

request.setNoDelay([noDelay])#

Added in: v0.5.9

Once a socket is assigned to this request and is connected socket.setNoDelay() will be called.

request.setSocketKeepAlive([enable][, initialDelay])#

Added in: v0.5.9

Once a socket is assigned to this request and is connected socket.setKeepAlive() will be called.

request.setTimeout(timeout[, callback])#

History
VersionChanges
v9.0.0

Consistently set socket timeout only when the socket connects.

v0.5.9

Added in: v0.5.9

  • timeout <number> Milliseconds before a request times out.
  • callback <Function> Optional function to be called when a timeout occurs. Same as binding to the 'timeout' event.
  • Returns: <http.ClientRequest>

Once a socket is assigned to this request and is connected socket.setTimeout() will be called.

request.socket#

Added in: v0.3.0

Reference to the underlying socket. Usually users will not want to access this property. In particular, the socket will not emit 'readable' events because of how the protocol parser attaches to the socket. The socket may also be accessed via request.connection.

const http = require('http');
const options = {
  host: 'www.google.com',
};
const req = http.get(options);
req.end();
req.once('response', (res) => {
  const ip = req.socket.localAddress;
  const port = req.socket.localPort;
  console.log(`Your IP address is ${ip} and your source port is ${port}.`);
  // Consume response object
});

This property is guaranteed to be an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specified a socket type other than <net.Socket>.

request.writableEnded#

Added in: v12.9.0

Is true after request.end() has been called. This property does not indicate whether the data has been flushed, for this use request.writableFinished instead.

request.writableFinished#

Added in: v12.7.0

Is true if all data has been flushed to the underlying system, immediately before the 'finish' event is emitted.

request.write(chunk[, encoding][, callback])#

Added in: v0.1.29

Sends a chunk of the body. By calling this method many times, a request body can be sent to a server. In that case, it is suggested to use the ['Transfer-Encoding', 'chunked'] header line when creating the request.

The encoding argument is optional and only applies when chunk is a string. Defaults to 'utf8'.

The callback argument is optional and will be called when this chunk of data is flushed, but only if the chunk is non-empty.

Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain' will be emitted when the buffer is free again.

When write function is called with empty string or buffer, it does nothing and waits for more input.

Class: http.Server#

Added in: v0.1.17

Event: 'checkContinue'#

Added in: v0.3.0

Emitted each time a request with an HTTP Expect: 100-continue is received. If this event is not listened for, the server will automatically respond with a 100 Continue as appropriate.

Handling this event involves calling response.writeContinue() if the client should continue to send the request body, or generating an appropriate HTTP response (e.g. 400 Bad Request) if the client should not continue to send the request body.

When this event is emitted and handled, the 'request' event will not be emitted.

Event: 'checkExpectation'#

Added in: v5.5.0

Emitted each time a request with an HTTP Expect header is received, where the value is not 100-continue. If this event is not listened for, the server will automatically respond with a 417 Expectation Failed as appropriate.

When this event is emitted and handled, the 'request' event will not be emitted.

Event: 'clientError'#

History
VersionChanges
v12.0.0

The default behavior will return a 431 Request Header Fields Too Large if a HPE_HEADER_OVERFLOW error occurs.

v9.4.0

The rawPacket is the current buffer that just parsed. Adding this buffer to the error object of 'clientError' event is to make it possible that developers can log the broken packet.

v6.0.0

The default action of calling .destroy() on the socket will no longer take place if there are listeners attached for 'clientError'.

v0.1.94

Added in: v0.1.94

If a client connection emits an 'error' event, it will be forwarded here. Listener of this event is responsible for closing/destroying the underlying socket. For example, one may wish to more gracefully close the socket with a custom HTTP response instead of abruptly severing the connection.

This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

Default behavior is to try close the socket with a HTTP '400 Bad Request', or a HTTP '431 Request Header Fields Too Large' in the case of a HPE_HEADER_OVERFLOW error. If the socket is not writable it is immediately destroyed.

socket is the net.Socket object that the error originated from.

const http = require('http');

const server = http.createServer((req, res) => {
  res.end();
});
server.on('clientError', (err, socket) => {
  socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
});
server.listen(8000);

When the 'clientError' event occurs, there is no request or response object, so any HTTP response sent, including response headers and payload, must be written directly to the socket object. Care must be taken to ensure the response is a properly formatted HTTP response message.

err is an instance of Error with two extra columns:

  • bytesParsed: the bytes count of request packet that Node.js may have parsed correctly;
  • rawPacket: the raw packet of current request.

In some cases, the client has already received the response and/or the socket has already been destroyed, like in case of ECONNRESET errors. Before trying to send data to the socket, it is better to check that it is still writable.

server.on('clientError', (err, socket) => {
  if (err.code === 'ECONNRESET' || !socket.writable) {
    return;
  }

  socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
});

Event: 'close'#

Added in: v0.1.4

Emitted when the server closes.

Event: 'connect'#

Added in: v0.7.0

Emitted each time a client requests an HTTP CONNECT method. If this event is not listened for, then clients requesting a CONNECT method will have their connections closed.

This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

After this event is emitted, the request's socket will not have a 'data' event listener, meaning it will need to be bound in order to handle data sent to the server on that socket.

Event: 'connection'#

Added in: v0.1.0

This event is emitted when a new TCP stream is established. socket is typically an object of type net.Socket. Usually users will not want to access this event. In particular, the socket will not emit 'readable' events because of how the protocol parser attaches to the socket. The socket can also be accessed at request.connection.

This event can also be explicitly emitted by users to inject connections into the HTTP server. In that case, any Duplex stream can be passed.

If socket.setTimeout() is called here, the timeout will be replaced with server.keepAliveTimeout when the socket has served a request (if server.keepAliveTimeout is non-zero).

This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

Event: 'request'#

Added in: v0.1.0

Emitted each time there is a request. There may be multiple requests per connection (in the case of HTTP Keep-Alive connections).

Event: 'upgrade'#

History
VersionChanges
v10.0.0

Not listening to this event no longer causes the socket to be destroyed if a client sends an Upgrade header.

v0.1.94

Added in: v0.1.94

Emitted each time a client requests an HTTP upgrade. Listening to this event is optional and clients cannot insist on a protocol change.

After this event is emitted, the request's socket will not have a 'data' event listener, meaning it will need to be bound in order to handle data sent to the server on that socket.

This event is guaranteed to be passed an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specifies a socket type other than <net.Socket>.

server.close([callback])#

Added in: v0.1.90

Stops the server from accepting new connections. See net.Server.close().

server.headersTimeout#

Added in: v11.3.0, v10.14.0

Limit the amount of time the parser will wait to receive the complete HTTP headers.

In case of inactivity, the rules defined in server.timeout apply. However, that inactivity based timeout would still allow the connection to be kept open if the headers are being sent very slowly (by default, up to a byte per 2 minutes). In order to prevent this, whenever header data arrives an additional check is made that more than server.headersTimeout milliseconds has not passed since the connection was established. If the check fails, a 'timeout' event is emitted on the server object, and (by default) the socket is destroyed. See server.timeout for more information on how timeout behavior can be customized.

server.listen()#

Starts the HTTP server listening for connections. This method is identical to server.listen() from net.Server.

server.listening#

Added in: v5.7.0
  • <boolean> Indicates whether or not the server is listening for connections.

server.maxHeadersCount#

Added in: v0.7.0

Limits maximum incoming headers count. If set to 0, no limit will be applied.

server.setTimeout([msecs][, callback])#

History
VersionChanges
v13.0.0

The default timeout changed from 120s to 0 (no timeout).

v0.9.12

Added in: v0.9.12

Sets the timeout value for sockets, and emits a 'timeout' event on the Server object, passing the socket as an argument, if a timeout occurs.

If there is a 'timeout' event listener on the Server object, then it will be called with the timed-out socket as an argument.

By default, the Server does not timeout sockets. However, if a callback is assigned to the Server's 'timeout' event, timeouts must be handled explicitly.

server.timeout#

History
VersionChanges
v13.0.0

The default timeout changed from 120s to 0 (no timeout).

v0.9.12

Added in: v0.9.12

  • <number> Timeout in milliseconds. Default: 0 (no timeout)

The number of milliseconds of inactivity before a socket is presumed to have timed out.

A value of 0 will disable the timeout behavior on incoming connections.

The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.

server.keepAliveTimeout#

Added in: v8.0.0
  • <number> Timeout in milliseconds. Default: 5000 (5 seconds).

The number of milliseconds of inactivity a server needs to wait for additional incoming data, after it has finished writing the last response, before a socket will be destroyed. If the server receives new data before the keep-alive timeout has fired, it will reset the regular inactivity timeout, i.e., server.timeout.

A value of 0 will disable the keep-alive timeout behavior on incoming connections. A value of 0 makes the http server behave similarly to Node.js versions prior to 8.0.0, which did not have a keep-alive timeout.

The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.

Class: http.ServerResponse#

Added in: v0.1.17

This object is created internally by an HTTP server, not by the user. It is passed as the second parameter to the 'request' event.

Event: 'close'#

Added in: v0.6.7

Indicates that the underlying connection was terminated.

Event: 'finish'#

Added in: v0.3.6

Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the operating system for transmission over the network. It does not imply that the client has received anything yet.

response.addTrailers(headers)#

Added in: v0.3.0

This method adds HTTP trailing headers (a header but at the end of the message) to the response.

Trailers will only be emitted if chunked encoding is used for the response; if it is not (e.g. if the request was HTTP/1.0), they will be silently discarded.

HTTP requires the Trailer header to be sent in order to emit trailers, with a list of the header fields in its value. E.g.,

response.writeHead(200, { 'Content-Type': 'text/plain',
                          'Trailer': 'Content-MD5' });
response.write(fileData);
response.addTrailers({ 'Content-MD5': '7895bf4b8828b55ceaf47747b4bca667' });
response.end();

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

response.connection#

Added in: v0.3.0Deprecated since: v13.0.0

Stability: 0 - Deprecated. Use response.socket.

See response.socket.

response.cork()#

Added in: v13.2.0, v12.16.0

See writable.cork().

response.end([data[, encoding]][, callback])#

History
VersionChanges
v10.0.0

This method now returns a reference to ServerResponse.

v0.1.90

Added in: v0.1.90

This method signals to the server that all of the response headers and body have been sent; that server should consider this message complete. The method, response.end(), MUST be called on each response.

If data is specified, it is similar in effect to calling response.write(data, encoding) followed by response.end(callback).

If callback is specified, it will be called when the response stream is finished.

response.finished#

Added in: v0.0.2Deprecated since: v13.4.0, v12.16.0

Stability: 0 - Deprecated. Use response.writableEnded.

The response.finished property will be true if response.end() has been called.

response.flushHeaders()#

Added in: v1.6.0

Flushes the response headers. See also: request.flushHeaders().

response.getHeader(name)#

Added in: v0.4.0

Reads out a header that's already been queued but not sent to the client. The name is case-insensitive. The type of the return value depends on the arguments provided to response.setHeader().

response.setHeader('Content-Type', 'text/html');
response.setHeader('Content-Length', Buffer.byteLength(body));
response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);
const contentType = response.getHeader('content-type');
// contentType is 'text/html'
const contentLength = response.getHeader('Content-Length');
// contentLength is of type number
const setCookie = response.getHeader('set-cookie');
// setCookie is of type string[]

response.getHeaderNames()#

Added in: v7.7.0

Returns an array containing the unique names of the current outgoing headers. All header names are lowercase.

response.setHeader('Foo', 'bar');
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

const headerNames = response.getHeaderNames();
// headerNames === ['foo', 'set-cookie']

response.getHeaders()#

Added in: v7.7.0

Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related http module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.

The object returned by the response.getHeaders() method does not prototypically inherit from the JavaScript Object. This means that typical Object methods such as obj.toString(), obj.hasOwnProperty(), and others are not defined and will not work.

response.setHeader('Foo', 'bar');
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

const headers = response.getHeaders();
// headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] }

response.hasHeader(name)#

Added in: v7.7.0

Returns true if the header identified by name is currently set in the outgoing headers. The header name matching is case-insensitive.

const hasContentType = response.hasHeader('content-type');

response.headersSent#

Added in: v0.9.3

Boolean (read-only). True if headers were sent, false otherwise.

response.removeHeader(name)#

Added in: v0.4.0

Removes a header that's queued for implicit sending.

response.removeHeader('Content-Encoding');

response.sendDate#

Added in: v0.7.5

When true, the Date header will be automatically generated and sent in the response if it is not already present in the headers. Defaults to true.

This should only be disabled for testing; HTTP requires the Date header in responses.

response.setHeader(name, value)#

Added in: v0.4.0

Sets a single header value for implicit headers. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here to send multiple headers with the same name. Non-string values will be stored without modification. Therefore, response.getHeader() may return non-string values. However, the non-string values will be converted to strings for network transmission.

response.setHeader('Content-Type', 'text/html');

or

response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

When headers have been set with response.setHeader(), they will be merged with any headers passed to response.writeHead(), with the headers passed to response.writeHead() given precedence.

// Returns content-type = text/plain
const server = http.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('ok');
});

If response.writeHead() method is called and this method has not been called, it will directly write the supplied header values onto the network channel without caching internally, and the response.getHeader() on the header will not yield the expected result. If progressive population of headers is desired with potential future retrieval and modification, use response.setHeader() instead of response.writeHead().

response.setTimeout(msecs[, callback])#

Added in: v0.9.12

Sets the Socket's timeout value to msecs. If a callback is provided, then it is added as a listener on the 'timeout' event on the response object.

If no 'timeout' listener is added to the request, the response, or the server, then sockets are destroyed when they time out. If a handler is assigned to the request, the response, or the server's 'timeout' events, timed out sockets must be handled explicitly.

response.socket#

Added in: v0.3.0

Reference to the underlying socket. Usually users will not want to access this property. In particular, the socket will not emit 'readable' events because of how the protocol parser attaches to the socket. After response.end(), the property is nulled. The socket may also be accessed via response.connection.

const http = require('http');
const server = http.createServer((req, res) => {
  const ip = res.socket.remoteAddress;
  const port = res.socket.remotePort;
  res.end(`Your IP address is ${ip} and your source port is ${port}.`);
}).listen(3000);

This property is guaranteed to be an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specified a socket type other than <net.Socket>.

response.statusCode#

Added in: v0.4.0

When using implicit headers (not calling response.writeHead() explicitly), this property controls the status code that will be sent to the client when the headers get flushed.

response.statusCode = 404;

After response header was sent to the client, this property indicates the status code which was sent out.

response.statusMessage#

Added in: v0.11.8

When using implicit headers (not calling response.writeHead() explicitly), this property controls the status message that will be sent to the client when the headers get flushed. If this is left as undefined then the standard message for the status code will be used.

response.statusMessage = 'Not found';

After response header was sent to the client, this property indicates the status message which was sent out.

response.uncork()#

Added in: v13.2.0, v12.16.0

See writable.uncork().

response.writableEnded#

Added in: v12.9.0

Is true after response.end() has been called. This property does not indicate whether the data has been flushed, for this use response.writableFinished instead.

response.writableFinished#

Added in: v12.7.0

Is true if all data has been flushed to the underlying system, immediately before the 'finish' event is emitted.

response.write(chunk[, encoding][, callback])#

Added in: v0.1.29

If this method is called and response.writeHead() has not been called, it will switch to implicit header mode and flush the implicit headers.

This sends a chunk of the response body. This method may be called multiple times to provide successive parts of the body.

In the http module, the response body is omitted when the request is a HEAD request. Similarly, the 204 and 304 responses must not include a message body.

chunk can be a string or a buffer. If chunk is a string, the second parameter specifies how to encode it into a byte stream. callback will be called when this chunk of data is flushed.

This is the raw HTTP body and has nothing to do with higher-level multi-part body encodings that may be used.

The first time response.write() is called, it will send the buffered header information and the first chunk of the body to the client. The second time response.write() is called, Node.js assumes data will be streamed, and sends the new data separately. That is, the response is buffered up to the first chunk of the body.

Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain' will be emitted when the buffer is free again.

response.writeContinue()#

Added in: v0.3.0

Sends a HTTP/1.1 100 Continue message to the client, indicating that the request body should be sent. See the 'checkContinue' event on Server.

response.writeHead(statusCode[, statusMessage][, headers])#

History
VersionChanges
v11.10.0, v10.17.0

Return this from writeHead() to allow chaining with end().

v5.11.0, v4.4.5

A RangeError is thrown if statusCode is not a number in the range [100, 999].

v0.1.30

Added in: v0.1.30

Sends a response header to the request. The status code is a 3-digit HTTP status code, like 404. The last argument, headers, are the response headers. Optionally one can give a human-readable statusMessage as the second argument.

Returns a reference to the ServerResponse, so that calls can be chained.

const body = 'hello world';
response
  .writeHead(200, {
    'Content-Length': Buffer.byteLength(body),
    'Content-Type': 'text/plain'
  })
  .end(body);

This method must only be called once on a message and it must be called before response.end() is called.

If response.write() or response.end() are called before calling this, the implicit/mutable headers will be calculated and call this function.

When headers have been set with response.setHeader(), they will be merged with any headers passed to response.writeHead(), with the headers passed to response.writeHead() given precedence.

If this method is called and response.setHeader() has not been called, it will directly write the supplied header values onto the network channel without caching internally, and the response.getHeader() on the header will not yield the expected result. If progressive population of headers is desired with potential future retrieval and modification, use response.setHeader() instead.

// Returns content-type = text/plain
const server = http.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('ok');
});

Content-Length is given in bytes, not characters. Use Buffer.byteLength() to determine the length of the body in bytes. Node.js does not check whether Content-Length and the length of the body which has been transmitted are equal or not.

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

response.writeProcessing()#

Added in: v10.0.0

Sends a HTTP/1.1 102 Processing message to the client, indicating that the request body should be sent.

Class: http.IncomingMessage#

History
VersionChanges
v13.1.0, v12.16.0

The readableHighWaterMark value mirrors that of the socket.

v0.1.17

Added in: v0.1.17

An IncomingMessage object is created by http.Server or http.ClientRequest and passed as the first argument to the 'request' and 'response' event respectively. It may be used to access response status, headers and data.

Event: 'aborted'#

Added in: v0.3.8

Emitted when the request has been aborted.

Event: 'close'#

Added in: v0.4.2

Indicates that the underlying connection was closed.

message.aborted#

Added in: v10.1.0

The message.aborted property will be true if the request has been aborted.

message.complete#

Added in: v0.3.0

The message.complete property will be true if a complete HTTP message has been received and successfully parsed.

This property is particularly useful as a means of determining if a client or server fully transmitted a message before a connection was terminated:

const req = http.request({
  host: '127.0.0.1',
  port: 8080,
  method: 'POST'
}, (res) => {
  res.resume();
  res.on('end', () => {
    if (!res.complete)
      console.error(
        'The connection was terminated while the message was still being sent');
  });
});

message.destroy([error])#

Added in: v0.3.0

Calls destroy() on the socket that received the IncomingMessage. If error is provided, an 'error' event is emitted on the socket and error is passed as an argument to any listeners on the event.

message.headers#

Added in: v0.1.5

The request/response headers object.

Key-value pairs of header names and values. Header names are lower-cased.

// Prints something like:
//
// { 'user-agent': 'curl/7.22.0',
//   host: '127.0.0.1:8000',
//   accept: '*/*' }
console.log(request.headers);

Duplicates in raw headers are handled in the following ways, depending on the header name:

  • Duplicates of age, authorization, content-length, content-type, etag, expires, from, host, if-modified-since, if-unmodified-since, last-modified, location, max-forwards, proxy-authorization, referer, retry-after, server, or user-agent are discarded.
  • set-cookie is always an array. Duplicates are added to the array.
  • For duplicate cookie headers, the values are joined together with '; '.
  • For all other headers, the values are joined together with ', '.

message.httpVersion#

Added in: v0.1.1

In case of server request, the HTTP version sent by the client. In the case of client response, the HTTP version of the connected-to server. Probably either '1.1' or '1.0'.

Also message.httpVersionMajor is the first integer and message.httpVersionMinor is the second.

message.method#

Added in: v0.1.1

Only valid for request obtained from http.Server.

The request method as a string. Read only. Examples: 'GET', 'DELETE'.

message.rawHeaders#

Added in: v0.11.6

The raw request/response headers list exactly as they were received.

The keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values.

Header names are not lowercased, and duplicates are not merged.

// Prints something like:
//
// [ 'user-agent',
//   'this is invalid because there can be only one',
//   'User-Agent',
//   'curl/7.22.0',
//   'Host',
//   '127.0.0.1:8000',
//   'ACCEPT',
//   '*/*' ]
console.log(request.rawHeaders);

message.rawTrailers#

Added in: v0.11.6

The raw request/response trailer keys and values exactly as they were received. Only populated at the 'end' event.

message.setTimeout(msecs[, callback])#

Added in: v0.5.9

Calls message.connection.setTimeout(msecs, callback).

message.socket#

Added in: v0.3.0

The net.Socket object associated with the connection.

With HTTPS support, use request.socket.getPeerCertificate() to obtain the client's authentication details.

This property is guaranteed to be an instance of the <net.Socket> class, a subclass of <stream.Duplex>, unless the user specified a socket type other than <net.Socket>.

message.statusCode#

Added in: v0.1.1

Only valid for response obtained from http.ClientRequest.

The 3-digit HTTP response status code. E.G. 404.

message.statusMessage#

Added in: v0.11.10

Only valid for response obtained from http.ClientRequest.

The HTTP response status message (reason phrase). E.G. OK or Internal Server Error.

message.trailers#

Added in: v0.3.0

The request/response trailers object. Only populated at the 'end' event.

message.url#

Added in: v0.1.90

Only valid for request obtained from http.Server.

Request URL string. This contains only the URL that is present in the actual HTTP request. If the request is:

GET /status?name=ryan HTTP/1.1\r\n
Accept: text/plain\r\n
\r\n

To parse the URL into its parts:

new URL(request.url, `http://${request.headers.host}`);

When request.url is '/status?name=ryan' and request.headers.host is 'localhost:3000':

$ node
> new URL(request.url, `http://${request.headers.host}`)
URL {
  href: 'http://localhost:3000/status?name=ryan',
  origin: 'http://localhost:3000',
  protocol: 'http:',
  username: '',
  password: '',
  host: 'localhost:3000',
  hostname: 'localhost',
  port: '3000',
  pathname: '/status',
  search: '?name=ryan',
  searchParams: URLSearchParams { 'name' => 'ryan' },
  hash: ''
}

http.METHODS#

Added in: v0.11.8

A list of the HTTP methods that are supported by the parser.

http.STATUS_CODES#

Added in: v0.1.22

A collection of all the standard HTTP response status codes, and the short description of each. For example, http.STATUS_CODES[404] === 'Not Found'.

http.createServer([options][, requestListener])#

History
VersionChanges
v13.3.0

The maxHeaderSize option is supported now.

v13.8.0, v12.15.0, v10.19.0

The insecureHTTPParser option is supported now.

v9.6.0, v8.12.0

The options argument is supported now.

v0.1.13

Added in: v0.1.13

  • options <Object>

    • IncomingMessage <http.IncomingMessage> Specifies the IncomingMessage class to be used. Useful for extending the original IncomingMessage. Default: IncomingMessage.
    • ServerResponse <http.ServerResponse> Specifies the ServerResponse class to be used. Useful for extending the original ServerResponse. Default: ServerResponse.
    • insecureHTTPParser <boolean> Use an insecure HTTP parser that accepts invalid HTTP headers when true. Using the insecure parser should be avoided. See --insecure-http-parser for more information. Default: false
    • maxHeaderSize <number> Optionally overrides the value of --max-http-header-size for requests received by this server, i.e. the maximum length of request headers in bytes. Default: 16384 (16KB).

  • requestListener <Function>

  • Returns: <http.Server>

Returns a new instance of http.Server.

The requestListener is a function which is automatically added to the 'request' event.

http.get(options[, callback])#

http.get(url[, options][, callback])#

History
VersionChanges
v10.9.0

The url parameter can now be passed along with a separate options object.

v7.5.0

The options parameter can be a WHATWG URL object.

v0.3.6

Added in: v0.3.6

Since most requests are GET requests without bodies, Node.js provides this convenience method. The only difference between this method and http.request() is that it sets the method to GET and calls req.end() automatically. The callback must take care to consume the response data for reasons stated in http.ClientRequest section.

The callback is invoked with a single argument that is an instance of http.IncomingMessage.

JSON fetching example:

http.get('http://nodejs.org/dist/index.json', (res) => {
  const { statusCode } = res;
  const contentType = res.headers['content-type'];

  let error;
  if (statusCode !== 200) {
    error = new Error('Request Failed.\n' +
                      `Status Code: ${statusCode}`);
  } else if (!/^application\/json/.test(contentType)) {
    error = new Error('Invalid content-type.\n' +
                      `Expected application/json but received ${contentType}`);
  }
  if (error) {
    console.error(error.message);
    // Consume response data to free up memory
    res.resume();
    return;
  }

  res.setEncoding('utf8');
  let rawData = '';
  res.on('data', (chunk) => { rawData += chunk; });
  res.on('end', () => {
    try {
      const parsedData = JSON.parse(rawData);
      console.log(parsedData);
    } catch (e) {
      console.error(e.message);
    }
  });
}).on('error', (e) => {
  console.error(`Got error: ${e.message}`);
});

http.globalAgent#

Added in: v0.5.9

Global instance of Agent which is used as the default for all HTTP client requests.

http.maxHeaderSize#

Added in: v11.6.0, v10.15.0

Read-only property specifying the maximum allowed size of HTTP headers in bytes. Defaults to 8KB. Configurable using the --max-http-header-size CLI option.

This can be overridden for servers and client requests by passing the maxHeaderSize option.

http.request(options[, callback])#

http.request(url[, options][, callback])#

History
VersionChanges
v13.3.0

The maxHeaderSize option is supported now.

v13.8.0, v12.15.0, v10.19.0

The insecureHTTPParser option is supported now.

v10.9.0

The url parameter can now be passed along with a separate options object.

v7.5.0

The options parameter can be a WHATWG URL object.

v0.3.6

Added in: v0.3.6

  • url <string> | <URL>

  • options <Object>

    • agent <http.Agent> | <boolean> Controls Agent behavior. Possible values:

      • undefined (default): use http.globalAgent for this host and port.
      • Agent object: explicitly use the passed in Agent.
      • false: causes a new Agent with default values to be used.
    • auth <string> Basic authentication i.e. 'user:password' to compute an Authorization header.
    • createConnection <Function> A function that produces a socket/stream to use for the request when the agent option is not used. This can be used to avoid creating a custom Agent class just to override the default createConnection function. See agent.createConnection() for more details. Any Duplex stream is a valid return value.
    • defaultPort <number> Default port for the protocol. Default: agent.defaultPort if an Agent is used, else undefined.
    • family <number> IP address family to use when resolving host or hostname. Valid values are 4 or 6. When unspecified, both IP v4 and v6 will be used.
    • headers <Object> An object containing request headers.
    • host <string> A domain name or IP address of the server to issue the request to. Default: 'localhost'.
    • hostname <string> Alias for host. To support url.parse(), hostname will be used if both host and hostname are specified.
    • insecureHTTPParser <boolean> Use an insecure HTTP parser that accepts invalid HTTP headers when true. Using the insecure parser should be avoided. See --insecure-http-parser for more information. Default: false
    • localAddress <string> Local interface to bind for network connections.
    • lookup <Function> Custom lookup function. Default: dns.lookup().
    • maxHeaderSize <number> Optionally overrides the value of --max-http-header-size for requests received from the server, i.e. the maximum length of response headers in bytes. Default: 16384 (16KB).
    • method <string> A string specifying the HTTP request method. Default: 'GET'.
    • path <string> Request path. Should include query string if any. E.G. '/index.html?page=12'. An exception is thrown when the request path contains illegal characters. Currently, only spaces are rejected but that may change in the future. Default: '/'.
    • port <number> Port of remote server. Default: defaultPort if set, else 80.
    • protocol <string> Protocol to use. Default: 'http:'.
    • setHost <boolean>: Specifies whether or not to automatically add the Host header. Defaults to true.
    • socketPath <string> Unix Domain Socket (cannot be used if one of host or port is specified, those specify a TCP Socket).
    • timeout <number>: A number specifying the socket timeout in milliseconds. This will set the timeout before the socket is connected.
  • callback <Function>
  • Returns: <http.ClientRequest>

Node.js maintains several connections per server to make HTTP requests. This function allows one to transparently issue requests.

url can be a string or a URL object. If url is a string, it is automatically parsed with new URL(). If it is a URL object, it will be automatically converted to an ordinary options object.

If both url and options are specified, the objects are merged, with the options properties taking precedence.

The optional callback parameter will be added as a one-time listener for the 'response' event.

http.request() returns an instance of the http.ClientRequest class. The ClientRequest instance is a writable stream. If one needs to upload a file with a POST request, then write to the ClientRequest object.

const postData = querystring.stringify({
  'msg': 'Hello World!'
});

const options = {
  hostname: 'www.google.com',
  port: 80,
  path: '/upload',
  method: 'POST',
  headers: {
    'Content-Type': 'application/x-www-form-urlencoded',
    'Content-Length': Buffer.byteLength(postData)
  }
};

const req = http.request(options, (res) => {
  console.log(`STATUS: ${res.statusCode}`);
  console.log(`HEADERS: ${JSON.stringify(res.headers)}`);
  res.setEncoding('utf8');
  res.on('data', (chunk) => {
    console.log(`BODY: ${chunk}`);
  });
  res.on('end', () => {
    console.log('No more data in response.');
  });
});

req.on('error', (e) => {
  console.error(`problem with request: ${e.message}`);
});

// Write data to request body
req.write(postData);
req.end();

In the example req.end() was called. With http.request() one must always call req.end() to signify the end of the request - even if there is no data being written to the request body.

If any error is encountered during the request (be that with DNS resolution, TCP level errors, or actual HTTP parse errors) an 'error' event is emitted on the returned request object. As with all 'error' events, if no listeners are registered the error will be thrown.

There are a few special headers that should be noted.

  • Sending a 'Connection: keep-alive' will notify Node.js that the connection to the server should be persisted until the next request.

  • Sending a 'Content-Length' header will disable the default chunked encoding.

  • Sending an 'Expect' header will immediately send the request headers. Usually, when sending 'Expect: 100-continue', both a timeout and a listener for the 'continue' event should be set. See RFC 2616 Section 8.2.3 for more information.

  • Sending an Authorization header will override using the auth option to compute basic authentication.

Example using a URL as options:

const options = new URL('http://abc:xyz@example.com');

const req = http.request(options, (res) => {
  // ...
});

In a successful request, the following events will be emitted in the following order:

  • 'socket'

  • 'response'

    • 'data' any number of times, on the res object ('data' will not be emitted at all if the response body is empty, for instance, in most redirects)
    • 'end' on the res object
  • 'close'

In the case of a connection error, the following events will be emitted:

  • 'socket'
  • 'error'
  • 'close'

In the case of a premature connection close before the response is received, the following events will be emitted in the following order:

  • 'socket'
  • 'error' with an error with message 'Error: socket hang up' and code 'ECONNRESET'
  • 'close'

In the case of a premature connection close after the response is received, the following events will be emitted in the following order:

  • 'socket'

  • 'response'

    • 'data' any number of times, on the res object
  • (connection closed here)
  • 'aborted' on the res object
  • 'close'
  • 'close' on the res object

If req.destroy() is called before a socket is assigned, the following events will be emitted in the following order:

  • (req.destroy() called here)
  • 'error' with an error with message 'Error: socket hang up' and code 'ECONNRESET'
  • 'close'

If req.destroy() is called before the connection succeeds, the following events will be emitted in the following order:

  • 'socket'
  • (req.destroy() called here)
  • 'error' with an error with message 'Error: socket hang up' and code 'ECONNRESET'
  • 'close'

If req.destroy() is called after the response is received, the following events will be emitted in the following order:

  • 'socket'

  • 'response'

    • 'data' any number of times, on the res object
  • (req.destroy() called here)
  • 'aborted' on the res object
  • 'close'
  • 'close' on the res object

If req.abort() is called before a socket is assigned, the following events will be emitted in the following order:

  • (req.abort() called here)
  • 'abort'
  • 'close'

If req.abort() is called before the connection succeeds, the following events will be emitted in the following order:

  • 'socket'
  • (req.abort() called here)
  • 'abort'
  • 'error' with an error with message 'Error: socket hang up' and code 'ECONNRESET'
  • 'close'

If req.abort() is called after the response is received, the following events will be emitted in the following order:

  • 'socket'

  • 'response'

    • 'data' any number of times, on the res object
  • (req.abort() called here)
  • 'abort'
  • 'aborted' on the res object
  • 'close'
  • 'close' on the res object

Setting the timeout option or using the setTimeout() function will not abort the request or do anything besides add a 'timeout' event.

http.validateHeaderName(name)#

Added in: v14.3.0

Performs the low-level validations on the provided name that are done when res.setHeader(name, value) is called.

Passing illegal value as name will result in a TypeError being thrown, identified by code: 'ERR_INVALID_HTTP_TOKEN'.

It is not necessary to use this method before passing headers to an HTTP request or response. The HTTP module will automatically validate such headers. Examples:

Example:

const { validateHeaderName } = require('http');

try {
  validateHeaderName('');
} catch (err) {
  err instanceof TypeError; // --> true
  err.code; // --> 'ERR_INVALID_HTTP_TOKEN'
  err.message; // --> 'Header name must be a valid HTTP token [""]'
}

http.validateHeaderValue(name, value)#

Added in: v14.3.0

Performs the low-level validations on the provided value that are done when res.setHeader(name, value) is called.

Passing illegal value as value will result in a TypeError being thrown.

  • Undefined value error is identified by code: 'ERR_HTTP_INVALID_HEADER_VALUE'.
  • Invalid value character error is identified by code: 'ERR_INVALID_CHAR'.

It is not necessary to use this method before passing headers to an HTTP request or response. The HTTP module will automatically validate such headers.

Examples:

const { validateHeaderValue } = require('http');

try {
  validateHeaderValue('x-my-header', undefined);
} catch (err) {
  err instanceof TypeError; // --> true
  err.code === 'ERR_HTTP_INVALID_HEADER_VALUE'; // --> true
  err.message; // --> 'Invalid value "undefined" for header "x-my-header"'
}

try {
  validateHeaderValue('x-my-header', 'oʊmɪɡə');
} catch (err) {
  err instanceof TypeError; // --> true
  err.code === 'ERR_INVALID_CHAR'; // --> true
  err.message; // --> 'Invalid character in header content ["x-my-header"]'
}

HTTP/2#

History
VersionChanges
v10.10.0

HTTP/2 is now Stable. Previously, it had been Experimental.

v8.4.0

Added in: v8.4.0

Stability: 2 - Stable

The http2 module provides an implementation of the HTTP/2 protocol. It can be accessed using:

const http2 = require('http2');

Core API#

The Core API provides a low-level interface designed specifically around support for HTTP/2 protocol features. It is specifically not designed for compatibility with the existing HTTP/1 module API. However, the Compatibility API is.

The http2 Core API is much more symmetric between client and server than the http API. For instance, most events, like 'error', 'connect' and 'stream', can be emitted either by client-side code or server-side code.

Server-side example#

The following illustrates a simple HTTP/2 server using the Core API. Since there are no browsers known that support unencrypted HTTP/2, the use of http2.createSecureServer() is necessary when communicating with browser clients.

const http2 = require('http2');
const fs = require('fs');

const server = http2.createSecureServer({
  key: fs.readFileSync('localhost-privkey.pem'),
  cert: fs.readFileSync('localhost-cert.pem')
});
server.on('error', (err) => console.error(err));

server.on('stream', (stream, headers) => {
  // stream is a Duplex
  stream.respond({
    'content-type': 'text/html',
    ':status': 200
  });
  stream.end('<h1>Hello World</h1>');
});

server.listen(8443);

To generate the certificate and key for this example, run:

openssl req -x509 -newkey rsa:2048 -nodes -sha256 -subj '/CN=localhost' \
  -keyout localhost-privkey.pem -out localhost-cert.pem

Client-side example#

The following illustrates an HTTP/2 client:

const http2 = require('http2');
const fs = require('fs');
const client = http2.connect('https://localhost:8443', {
  ca: fs.readFileSync('localhost-cert.pem')
});
client.on('error', (err) => console.error(err));

const req = client.request({ ':path': '/' });

req.on('response', (headers, flags) => {
  for (const name in headers) {
    console.log(`${name}: ${headers[name]}`);
  }
});

req.setEncoding('utf8');
let data = '';
req.on('data', (chunk) => { data += chunk; });
req.on('end', () => {
  console.log(`\n${data}`);
  client.close();
});
req.end();

Class: Http2Session#

Added in: v8.4.0

Instances of the http2.Http2Session class represent an active communications session between an HTTP/2 client and server. Instances of this class are not intended to be constructed directly by user code.

Each Http2Session instance will exhibit slightly different behaviors depending on whether it is operating as a server or a client. The http2session.type property can be used to determine the mode in which an Http2Session is operating. On the server side, user code should rarely have occasion to work with the Http2Session object directly, with most actions typically taken through interactions with either the Http2Server or Http2Stream objects.

User code will not create Http2Session instances directly. Server-side Http2Session instances are created by the Http2Server instance when a new HTTP/2 connection is received. Client-side Http2Session instances are created using the http2.connect() method.

Http2Session and Sockets#

Every Http2Session instance is associated with exactly one net.Socket or tls.TLSSocket when it is created. When either the Socket or the Http2Session are destroyed, both will be destroyed.

Because of the specific serialization and processing requirements imposed by the HTTP/2 protocol, it is not recommended for user code to read data from or write data to a Socket instance bound to a Http2Session. Doing so can put the HTTP/2 session into an indeterminate state causing the session and the socket to become unusable.

Once a Socket has been bound to an Http2Session, user code should rely solely on the API of the Http2Session.

Event: 'close'#

Added in: v8.4.0

The 'close' event is emitted once the Http2Session has been destroyed. Its listener does not expect any arguments.

Event: 'connect'#

Added in: v8.4.0

The 'connect' event is emitted once the Http2Session has been successfully connected to the remote peer and communication may begin.

User code will typically not listen for this event directly.

Event: 'error'#

Added in: v8.4.0

The 'error' event is emitted when an error occurs during the processing of an Http2Session.

Event: 'frameError'#

Added in: v8.4.0
  • type <integer> The frame type.
  • code <integer> The error code.
  • id <integer> The stream id (or 0 if the frame isn't associated with a stream).

The 'frameError' event is emitted when an error occurs while attempting to send a frame on the session. If the frame that could not be sent is associated with a specific Http2Stream, an attempt to emit a 'frameError' event on the Http2Stream is made.

If the 'frameError' event is associated with a stream, the stream will be closed and destroyed immediately following the 'frameError' event. If the event is not associated with a stream, the Http2Session will be shut down immediately following the 'frameError' event.

Event: 'goaway'#

Added in: v8.4.0
  • errorCode <number> The HTTP/2 error code specified in the GOAWAY frame.
  • lastStreamID <number> The ID of the last stream the remote peer successfully processed (or 0 if no ID is specified).
  • opaqueData <Buffer> If additional opaque data was included in the GOAWAY frame, a Buffer instance will be passed containing that data.

The 'goaway' event is emitted when a GOAWAY frame is received.

The Http2Session instance will be shut down automatically when the 'goaway' event is emitted.

Event: 'localSettings'#

Added in: v8.4.0

The 'localSettings' event is emitted when an acknowledgment SETTINGS frame has been received.

When using http2session.settings() to submit new settings, the modified settings do not take effect until the 'localSettings' event is emitted.

session.settings({ enablePush: false });

session.on('localSettings', (settings) => {
  /* Use the new settings */
});

Event: 'ping'#

Added in: v10.12.0
  • payload <Buffer> The PING frame 8-byte payload

The 'ping' event is emitted whenever a PING frame is received from the connected peer.

Event: 'remoteSettings'#

Added in: v8.4.0

The 'remoteSettings' event is emitted when a new SETTINGS frame is received from the connected peer.

session.on('remoteSettings', (settings) => {
  /* Use the new settings */
});

Event: 'stream'#

Added in: v8.4.0

The 'stream' event is emitted when a new Http2Stream is created.

const http2 = require('http2');
session.on('stream', (stream, headers, flags) => {
  const method = headers[':method'];
  const path = headers[':path'];
  // ...
  stream.respond({
    ':status': 200,
    'content-type': 'text/plain'
  });
  stream.write('hello ');
  stream.end('world');
});

On the server side, user code will typically not listen for this event directly, and would instead register a handler for the 'stream' event emitted by the net.Server or tls.Server instances returned by http2.createServer() and http2.createSecureServer(), respectively, as in the example below:

const http2 = require('http2');

// Create an unencrypted HTTP/2 server
const server = http2.createServer();

server.on('stream', (stream, headers) => {
  stream.respond({
    'content-type': 'text/html',
    ':status': 200
  });
  stream.on('error', (error) => console.error(error));
  stream.end('<h1>Hello World</h1>');
});

server.listen(80);

Even though HTTP/2 streams and network sockets are not in a 1:1 correspondence, a network error will destroy each individual stream and must be handled on the stream level, as shown above.

Event: 'timeout'#

Added in: v8.4.0

After the http2session.setTimeout() method is used to set the timeout period for this Http2Session, the 'timeout' event is emitted if there is no activity on the Http2Session after the configured number of milliseconds. Its listener does not expect any arguments.

session.setTimeout(2000);
session.on('timeout', () => { /* .. */ });

http2session.alpnProtocol#

Added in: v9.4.0

Value will be undefined if the Http2Session is not yet connected to a socket, h2c if the Http2Session is not connected to a TLSSocket, or will return the value of the connected TLSSocket's own alpnProtocol property.

http2session.close([callback])#

Added in: v9.4.0

Gracefully closes the Http2Session, allowing any existing streams to complete on their own and preventing new Http2Stream instances from being created. Once closed, http2session.destroy() might be called if there are no open Http2Stream instances.

If specified, the callback function is registered as a handler for the 'close' event.

http2session.closed#

Added in: v9.4.0

Will be true if this Http2Session instance has been closed, otherwise false.

http2session.connecting#

Added in: v10.0.0

Will be true if this Http2Session instance is still connecting, will be set to false before emitting connect event and/or calling the http2.connect callback.

http2session.destroy([error][, code])#

Added in: v8.4.0
  • error <Error> An Error object if the Http2Session is being destroyed due to an error.
  • code <number> The HTTP/2 error code to send in the final GOAWAY frame. If unspecified, and error is not undefined, the default is INTERNAL_ERROR, otherwise defaults to NO_ERROR.

Immediately terminates the Http2Session and the associated net.Socket or tls.TLSSocket.

Once destroyed, the Http2Session will emit the 'close' event. If error is not undefined, an 'error' event will be emitted immediately before the 'close' event.

If there are any remaining open Http2Streams associated with the Http2Session, those will also be destroyed.

http2session.destroyed#

Added in: v8.4.0

Will be true if this Http2Session instance has been destroyed and must no longer be used, otherwise false.

http2session.encrypted#

Added in: v9.4.0

Value is undefined if the Http2Session session socket has not yet been connected, true if the Http2Session is connected with a TLSSocket, and false if the Http2Session is connected to any other kind of socket or stream.

http2session.goaway([code[, lastStreamID[, opaqueData]]])#

Added in: v9.4.0
  • code <number> An HTTP/2 error code
  • lastStreamID <number> The numeric ID of the last processed Http2Stream
  • opaqueData <Buffer> | <TypedArray> | <DataView> A TypedArray or DataView instance containing additional data to be carried within the GOAWAY frame.

Transmits a GOAWAY frame to the connected peer without shutting down the Http2Session.

http2session.localSettings#

Added in: v8.4.0

A prototype-less object describing the current local settings of this Http2Session. The local settings are local to this Http2Session instance.

http2session.originSet#

Added in: v9.4.0

If the Http2Session is connected to a TLSSocket, the originSet property will return an Array of origins for which the Http2Session may be considered authoritative.

The originSet property is only available when using a secure TLS connection.

http2session.pendingSettingsAck#

Added in: v8.4.0

Indicates whether the Http2Session is currently waiting for acknowledgment of a sent SETTINGS frame. Will be true after calling the http2session.settings() method. Will be false once all sent SETTINGS frames have been acknowledged.

http2session.ping([payload, ]callback)#

Added in: v8.9.3

Sends a PING frame to the connected HTTP/2 peer. A callback function must be provided. The method will return true if the PING was sent, false otherwise.

The maximum number of outstanding (unacknowledged) pings is determined by the maxOutstandingPings configuration option. The default maximum is 10.

If provided, the payload must be a Buffer, TypedArray, or DataView containing 8 bytes of data that will be transmitted with the PING and returned with the ping acknowledgment.

The callback will be invoked with three arguments: an error argument that will be null if the PING was successfully acknowledged, a duration argument that reports the number of milliseconds elapsed since the ping was sent and the acknowledgment was received, and a Buffer containing the 8-byte PING payload.

session.ping(Buffer.from('abcdefgh'), (err, duration, payload) => {
  if (!err) {
    console.log(`Ping acknowledged in ${duration} milliseconds`);
    console.log(`With payload '${payload.toString()}'`);
  }
});

If the payload argument is not specified, the default payload will be the 64-bit timestamp (little endian) marking the start of the PING duration.

http2session.ref()#

Added in: v9.4.0

Calls ref() on this Http2Session instance's underlying net.Socket.

http2session.remoteSettings#

Added in: v8.4.0

A prototype-less object describing the current remote settings of this Http2Session. The remote settings are set by the connected HTTP/2 peer.

http2session.setTimeout(msecs, callback)#

Added in: v8.4.0

Used to set a callback function that is called when there is no activity on the Http2Session after msecs milliseconds. The given callback is registered as a listener on the 'timeout' event.

http2session.socket#

Added in: v8.4.0

Returns a Proxy object that acts as a net.Socket (or tls.TLSSocket) but limits available methods to ones safe to use with HTTP/2.

destroy, emit, end, pause, read, resume, and write will throw an error with code ERR_HTTP2_NO_SOCKET_MANIPULATION. See Http2Session and Sockets for more information.

setTimeout method will be called on this Http2Session.

All other interactions will be routed directly to the socket.

http2session.state#

Added in: v8.4.0

Provides miscellaneous information about the current state of the Http2Session.

  • <Object>

    • effectiveLocalWindowSize <number> The current local (receive) flow control window size for the Http2Session.
    • effectiveRecvDataLength <number> The current number of bytes that have been received since the last flow control WINDOW_UPDATE.
    • nextStreamID <number> The numeric identifier to be used the next time a new Http2Stream is created by this Http2Session.
    • localWindowSize <number> The number of bytes that the remote peer can send without receiving a WINDOW_UPDATE.
    • lastProcStreamID <number> The numeric id of the Http2Stream for which a HEADERS or DATA frame was most recently received.
    • remoteWindowSize <number> The number of bytes that this Http2Session may send without receiving a WINDOW_UPDATE.
    • outboundQueueSize <number> The number of frames currently within the outbound queue for this Http2Session.
    • deflateDynamicTableSize <number> The current size in bytes of the outbound header compression state table.
    • inflateDynamicTableSize <number> The current size in bytes of the inbound header compression state table.

An object describing the current status of this Http2Session.

http2session.settings([settings][, callback])#

Added in: v8.4.0

Updates the current local settings for this Http2Session and sends a new SETTINGS frame to the connected HTTP/2 peer.

Once called, the http2session.pendingSettingsAck property will be true while the session is waiting for the remote peer to acknowledge the new settings.

The new settings will not become effective until the SETTINGS acknowledgment is received and the 'localSettings' event is emitted. It is possible to send multiple SETTINGS frames while acknowledgment is still pending.

http2session.type#

Added in: v8.4.0

The http2session.type will be equal to http2.constants.NGHTTP2_SESSION_SERVER if this Http2Session instance is a server, and http2.constants.NGHTTP2_SESSION_CLIENT if the instance is a client.

http2session.unref()#

Added in: v9.4.0

Calls unref() on this Http2Session instance's underlying net.Socket.

Class: ServerHttp2Session#

Added in: v8.4.0

serverhttp2session.altsvc(alt, originOrStream)#

Added in: v9.4.0
  • alt <string> A description of the alternative service configuration as defined by RFC 7838.
  • originOrStream <number> | <string> | <URL> | <Object> Either a URL string specifying the origin (or an Object with an origin property) or the numeric identifier of an active Http2Stream as given by the http2stream.id property.

Submits an ALTSVC frame (as defined by RFC 7838) to the connected client.

const http2 = require('http2');

const server = http2.createServer();
server.on('session', (session) => {
  // Set altsvc for origin https://example.org:80
  session.altsvc('h2=":8000"', 'https://example.org:80');
});

server.on('stream', (stream) => {
  // Set altsvc for a specific stream
  stream.session.altsvc('h2=":8000"', stream.id);
});

Sending an ALTSVC frame with a specific stream ID indicates that the alternate service is associated with the origin of the given Http2Stream.

The alt and origin string must contain only ASCII bytes and are strictly interpreted as a sequence of ASCII bytes. The special value 'clear' may be passed to clear any previously set alternative service for a given domain.

When a string is passed for the originOrStream argument, it will be parsed as a URL and the origin will be derived. For instance, the origin for the HTTP URL 'https://example.org/foo/bar' is the ASCII string 'https://example.org'. An error will be thrown if either the given string cannot be parsed as a URL or if a valid origin cannot be derived.

A URL object, or any object with an origin property, may be passed as originOrStream, in which case the value of the origin property will be used. The value of the origin property must be a properly serialized ASCII origin.

Specifying alternative services#

The format of the alt parameter is strictly defined by RFC 7838 as an ASCII string containing a comma-delimited list of "alternative" protocols associated with a specific host and port.

For example, the value 'h2="example.org:81"' indicates that the HTTP/2 protocol is available on the host 'example.org' on TCP/IP port 81. The host and port must be contained within the quote (") characters.

Multiple alternatives may be specified, for instance: 'h2="example.org:81", h2=":82"'.

The protocol identifier ('h2' in the examples) may be any valid ALPN Protocol ID.

The syntax of these values is not validated by the Node.js implementation and are passed through as provided by the user or received from the peer.

serverhttp2session.origin(...origins)#

Added in: v10.12.0

Submits an ORIGIN frame (as defined by RFC 8336) to the connected client to advertise the set of origins for which the server is capable of providing authoritative responses.

const http2 = require('http2');
const options = getSecureOptionsSomehow();
const server = http2.createSecureServer(options);
server.on('stream', (stream) => {
  stream.respond();
  stream.end('ok');
});
server.on('session', (session) => {
  session.origin('https://example.com', 'https://example.org');
});

When a string is passed as an origin, it will be parsed as a URL and the origin will be derived. For instance, the origin for the HTTP URL 'https://example.org/foo/bar' is the ASCII string 'https://example.org'. An error will be thrown if either the given string cannot be parsed as a URL or if a valid origin cannot be derived.

A URL object, or any object with an origin property, may be passed as an origin, in which case the value of the origin property will be used. The value of the origin property must be a properly serialized ASCII origin.

Alternatively, the origins option may be used when creating a new HTTP/2 server using the http2.createSecureServer() method:

const http2 = require('http2');
const options = getSecureOptionsSomehow();
options.origins = ['https://example.com', 'https://example.org'];
const server = http2.createSecureServer(options);
server.on('stream', (stream) => {
  stream.respond();
  stream.end('ok');
});

Class: ClientHttp2Session#

Added in: v8.4.0

Event: 'altsvc'#

Added in: v9.4.0

The 'altsvc' event is emitted whenever an ALTSVC frame is received by the client. The event is emitted with the ALTSVC value, origin, and stream ID. If no origin is provided in the ALTSVC frame, origin will be an empty string.

const http2 = require('http2');
const client = http2.connect('https://example.org');

client.on('altsvc', (alt, origin, streamId) => {
  console.log(alt);
  console.log(origin);
  console.log(streamId);
});

Event: 'origin'#

Added in: v10.12.0

The 'origin' event is emitted whenever an ORIGIN frame is received by the client. The event is emitted with an array of origin strings. The http2session.originSet will be updated to include the received origins.

const http2 = require('http2');
const client = http2.connect('https://example.org');

client.on('origin', (origins) => {
  for (let n = 0; n < origins.length; n++)
    console.log(origins[n]);
});

The 'origin' event is only emitted when using a secure TLS connection.

clienthttp2session.request(headers[, options])#

Added in: v8.4.0

  • headers <HTTP/2 Headers Object>

  • options <Object>

    • endStream <boolean> true if the Http2Stream writable side should be closed initially, such as when sending a GET request that should not expect a payload body.
    • exclusive <boolean> When true and parent identifies a parent Stream, the created stream is made the sole direct dependency of the parent, with all other existing dependents made a dependent of the newly created stream. Default: false.
    • parent <number> Specifies the numeric identifier of a stream the newly created stream is dependent on.
    • weight <number> Specifies the relative dependency of a stream in relation to other streams with the same parent. The value is a number between 1 and 256 (inclusive).
    • waitForTrailers <boolean> When true, the Http2Stream will emit the 'wantTrailers' event after the final DATA frame has been sent.

  • Returns: <ClientHttp2Stream>

For HTTP/2 Client Http2Session instances only, the http2session.request() creates and returns an Http2Stream instance that can be used to send an HTTP/2 request to the connected server.

This method is only available if http2session.type is equal to http2.constants.NGHTTP2_SESSION_CLIENT.

const http2 = require('http2');
const clientSession = http2.connect('https://localhost:1234');
const {
  HTTP2_HEADER_PATH,
  HTTP2_HEADER_STATUS
} = http2.constants;

const req = clientSession.request({ [HTTP2_HEADER_PATH]: '/' });
req.on('response', (headers) => {
  console.log(headers[HTTP2_HEADER_STATUS]);
  req.on('data', (chunk) => { /* .. */ });
  req.on('end', () => { /* .. */ });
});

When the options.waitForTrailers option is set, the 'wantTrailers' event is emitted immediately after queuing the last chunk of payload data to be sent. The http2stream.sendTrailers() method can then be called to send trailing headers to the peer.

When options.waitForTrailers is set, the Http2Stream will not automatically close when the final DATA frame is transmitted. User code must call either http2stream.sendTrailers() or http2stream.close() to close the Http2Stream.

The :method and :path pseudo-headers are not specified within headers, they respectively default to:

  • :method = 'GET'
  • :path = /

Class: Http2Stream#

Added in: v8.4.0

Each instance of the Http2Stream class represents a bidirectional HTTP/2 communications stream over an Http2Session instance. Any single Http2Session may have up to 231-1 Http2Stream instances over its lifetime.

User code will not construct Http2Stream instances directly. Rather, these are created, managed, and provided to user code through the Http2Session instance. On the server, Http2Stream instances are created either in response to an incoming HTTP request (and handed off to user code via the 'stream' event), or in response to a call to the http2stream.pushStream() method. On the client, Http2Stream instances are created and returned when either the http2session.request() method is called, or in response to an incoming 'push' event.

The Http2Stream class is a base for the ServerHttp2Stream and ClientHttp2Stream classes, each of which is used specifically by either the Server or Client side, respectively.

All Http2Stream instances are Duplex streams. The Writable side of the Duplex is used to send data to the connected peer, while the Readable side is used to receive data sent by the connected peer.

Http2Stream Lifecycle#

Creation#

On the server side, instances of ServerHttp2Stream are created either when:

  • A new HTTP/2 HEADERS frame with a previously unused stream ID is received;
  • The http2stream.pushStream() method is called.

On the client side, instances of ClientHttp2Stream are created when the http2session.request() method is called.

On the client, the Http2Stream instance returned by http2session.request() may not be immediately ready for use if the parent Http2Session has not yet been fully established. In such cases, operations called on the Http2Stream will be buffered until the 'ready' event is emitted. User code should rarely, if ever, need to handle the 'ready' event directly. The ready status of an Http2Stream can be determined by checking the value of http2stream.id. If the value is undefined, the stream is not yet ready for use.

Destruction#

All Http2Stream instances are destroyed either when:

  • An RST_STREAM frame for the stream is received by the connected peer, and (for client streams only) pending data has been read.
  • The http2stream.close() method is called, and (for client streams only) pending data has been read.
  • The http2stream.destroy() or http2session.destroy() methods are called.

When an Http2Stream instance is destroyed, an attempt will be made to send an RST_STREAM frame to the connected peer.

When the Http2Stream instance is destroyed, the 'close' event will be emitted. Because Http2Stream is an instance of stream.Duplex, the 'end' event will also be emitted if the stream data is currently flowing. The 'error' event may also be emitted if http2stream.destroy() was called with an Error passed as the first argument.

After the Http2Stream has been destroyed, the http2stream.destroyed property will be true and the http2stream.rstCode property will specify the RST_STREAM error code. The Http2Stream instance is no longer usable once destroyed.

Event: 'aborted'#

Added in: v8.4.0

The 'aborted' event is emitted whenever a Http2Stream instance is abnormally aborted in mid-communication. Its listener does not expect any arguments.

The 'aborted' event will only be emitted if the Http2Stream writable side has not been ended.

Event: 'close'#

Added in: v8.4.0

The 'close' event is emitted when the Http2Stream is destroyed. Once this event is emitted, the Http2Stream instance is no longer usable.

The HTTP/2 error code used when closing the stream can be retrieved using the http2stream.rstCode property. If the code is any value other than NGHTTP2_NO_ERROR (0), an 'error' event will have also been emitted.

Event: 'error'#

Added in: v8.4.0

The 'error' event is emitted when an error occurs during the processing of an Http2Stream.

Event: 'frameError'#

Added in: v8.4.0
  • type <integer> The frame type.
  • code <integer> The error code.
  • id <integer> The stream id (or 0 if the frame isn't associated with a stream).

The 'frameError' event is emitted when an error occurs while attempting to send a frame. When invoked, the handler function will receive an integer argument identifying the frame type, and an integer argument identifying the error code. The Http2Stream instance will be destroyed immediately after the 'frameError' event is emitted.

Event: 'timeout'#

Added in: v8.4.0

The 'timeout' event is emitted after no activity is received for this Http2Stream within the number of milliseconds set using http2stream.setTimeout(). Its listener does not expect any arguments.

Event: 'trailers'#

Added in: v8.4.0

The 'trailers' event is emitted when a block of headers associated with trailing header fields is received. The listener callback is passed the HTTP/2 Headers Object and flags associated with the headers.

This event might not be emitted if http2stream.end() is called before trailers are received and the incoming data is not being read or listened for.

stream.on('trailers', (headers, flags) => {
  console.log(headers);
});

Event: 'wantTrailers'#

Added in: v10.0.0

The 'wantTrailers' event is emitted when the Http2Stream has queued the final DATA frame to be sent on a frame and the Http2Stream is ready to send trailing headers. When initiating a request or response, the waitForTrailers option must be set for this event to be emitted.

http2stream.aborted#

Added in: v8.4.0

Set to true if the Http2Stream instance was aborted abnormally. When set, the 'aborted' event will have been emitted.

http2stream.bufferSize#

Added in: v11.2.0, v10.16.0

This property shows the number of characters currently buffered to be written. See net.Socket.bufferSize for details.

http2stream.close(code[, callback])#

Added in: v8.4.0
  • code <number> Unsigned 32-bit integer identifying the error code. Default: http2.constants.NGHTTP2_NO_ERROR (0x00).
  • callback <Function> An optional function registered to listen for the 'close' event.

Closes the Http2Stream instance by sending an RST_STREAM frame to the connected HTTP/2 peer.

http2stream.closed#

Added in: v9.4.0

Set to true if the Http2Stream instance has been closed.

http2stream.destroyed#

Added in: v8.4.0

Set to true if the Http2Stream instance has been destroyed and is no longer usable.

http2stream.endAfterHeaders#

Added in: v10.11.0

Set the true if the END_STREAM flag was set in the request or response HEADERS frame received, indicating that no additional data should be received and the readable side of the Http2Stream will be closed.

http2stream.id#

Added in: v8.4.0

The numeric stream identifier of this Http2Stream instance. Set to undefined if the stream identifier has not yet been assigned.

http2stream.pending#

Added in: v9.4.0

Set to true if the Http2Stream instance has not yet been assigned a numeric stream identifier.

http2stream.priority(options)#

Added in: v8.4.0

  • options <Object>

    • exclusive <boolean> When true and parent identifies a parent Stream, this stream is made the sole direct dependency of the parent, with all other existing dependents made a dependent of this stream. Default: false.
    • parent <number> Specifies the numeric identifier of a stream this stream is dependent on.
    • weight <number> Specifies the relative dependency of a stream in relation to other streams with the same parent. The value is a number between 1 and 256 (inclusive).
    • silent <boolean> When true, changes the priority locally without sending a PRIORITY frame to the connected peer.

Updates the priority for this Http2Stream instance.

http2stream.rstCode#

Added in: v8.4.0

Set to the RST_STREAM error code reported when the Http2Stream is destroyed after either receiving an RST_STREAM frame from the connected peer, calling http2stream.close(), or http2stream.destroy(). Will be undefined if the Http2Stream has not been closed.

http2stream.sentHeaders#

Added in: v9.5.0

An object containing the outbound headers sent for this Http2Stream.

http2stream.sentInfoHeaders#

Added in: v9.5.0

An array of objects containing the outbound informational (additional) headers sent for this Http2Stream.

http2stream.sentTrailers#

Added in: v9.5.0

An object containing the outbound trailers sent for this HttpStream.

http2stream.session#

Added in: v8.4.0

A reference to the Http2Session instance that owns this Http2Stream. The value will be undefined after the Http2Stream instance is destroyed.

http2stream.setTimeout(msecs, callback)#

Added in: v8.4.0 
const http2 = require('http2');
const client = http2.connect('http://example.org:8000');
const { NGHTTP2_CANCEL } = http2.constants;
const req = client.request({ ':path': '/' });

// Cancel the stream if there's no activity after 5 seconds
req.setTimeout(5000, () => req.close(NGHTTP2_CANCEL));

http2stream.state#

Added in: v8.4.0

Provides miscellaneous information about the current state of the Http2Stream.

  • <Object>

    • localWindowSize <number> The number of bytes the connected peer may send for this Http2Stream without receiving a WINDOW_UPDATE.
    • state <number> A flag indicating the low-level current state of the Http2Stream as determined by nghttp2.
    • localClose <number> 1 if this Http2Stream has been closed locally.
    • remoteClose <number> 1 if this Http2Stream has been closed remotely.
    • sumDependencyWeight <number> The sum weight of all Http2Stream instances that depend on this Http2Stream as specified using PRIORITY frames.
    • weight <number> The priority weight of this Http2Stream.

A current state of this Http2Stream.

http2stream.sendTrailers(headers)#

Added in: v10.0.0

Sends a trailing HEADERS frame to the connected HTTP/2 peer. This method will cause the Http2Stream to be immediately closed and must only be called after the 'wantTrailers' event has been emitted. When sending a request or sending a response, the options.waitForTrailers option must be set in order to keep the Http2Stream open after the final DATA frame so that trailers can be sent.

const http2 = require('http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  stream.respond(undefined, { waitForTrailers: true });
  stream.on('wantTrailers', () => {
    stream.sendTrailers({ xyz: 'abc' });
  });
  stream.end('Hello World');
});

The HTTP/1 specification forbids trailers from containing HTTP/2 pseudo-header fields (e.g. ':method', ':path', etc).

Class: ClientHttp2Stream#

Added in: v8.4.0

The ClientHttp2Stream class is an extension of Http2Stream that is used exclusively on HTTP/2 Clients. Http2Stream instances on the client provide events such as 'response' and 'push' that are only relevant on the client.

Event: 'continue'#

Added in: v8.5.0

Emitted when the server sends a 100 Continue status, usually because the request contained Expect: 100-continue. This is an instruction that the client should send the request body.

Event: 'headers'#

Added in: v8.4.0

The 'headers' event is emitted when an additional block of headers is received for a stream, such as when a block of 1xx informational headers is received. The listener callback is passed the HTTP/2 Headers Object and flags associated with the headers.

stream.on('headers', (headers, flags) => {
  console.log(headers);
});

Event: 'push'#

Added in: v8.4.0

The 'push' event is emitted when response headers for a Server Push stream are received. The listener callback is passed the HTTP/2 Headers Object and flags associated with the headers.

stream.on('push', (headers, flags) => {
  console.log(headers);
});

Event: 'response'#

Added in: v8.4.0

The 'response' event is emitted when a response HEADERS frame has been received for this stream from the connected HTTP/2 server. The listener is invoked with two arguments: an Object containing the received HTTP/2 Headers Object, and flags associated with the headers.

const http2 = require('http2');
const client = http2.connect('https://localhost');
const req = client.request({ ':path': '/' });
req.on('response', (headers, flags) => {
  console.log(headers[':status']);
});

Class: ServerHttp2Stream#

Added in: v8.4.0

The ServerHttp2Stream class is an extension of Http2Stream that is used exclusively on HTTP/2 Servers. Http2Stream instances on the server provide additional methods such as http2stream.pushStream() and http2stream.respond() that are only relevant on the server.

http2stream.additionalHeaders(headers)#

Added in: v8.4.0

Sends an additional informational HEADERS frame to the connected HTTP/2 peer.

http2stream.headersSent#

Added in: v8.4.0

True if headers were sent, false otherwise (read-only).

http2stream.pushAllowed#

Added in: v8.4.0

Read-only property mapped to the SETTINGS_ENABLE_PUSH flag of the remote client's most recent SETTINGS frame. Will be true if the remote peer accepts push streams, false otherwise. Settings are the same for every Http2Stream in the same Http2Session.

http2stream.pushStream(headers[, options], callback)#

Added in: v8.4.0
  • headers <HTTP/2 Headers Object>

  • options <Object>

    • exclusive <boolean> When true and parent identifies a parent Stream, the created stream is made the sole direct dependency of the parent, with all other existing dependents made a dependent of the newly created stream. Default: false.
    • parent <number> Specifies the numeric identifier of a stream the newly created stream is dependent on.

  • callback <Function> Callback that is called once the push stream has been initiated.

Initiates a push stream. The callback is invoked with the new Http2Stream instance created for the push stream passed as the second argument, or an Error passed as the first argument.

const http2 = require('http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  stream.respond({ ':status': 200 });
  stream.pushStream({ ':path': '/' }, (err, pushStream, headers) => {
    if (err) throw err;
    pushStream.respond({ ':status': 200 });
    pushStream.end('some pushed data');
  });
  stream.end('some data');
});

Setting the weight of a push stream is not allowed in the HEADERS frame. Pass a weight value to http2stream.priority with the silent option set to true to enable server-side bandwidth balancing between concurrent streams.

Calling http2stream.pushStream() from within a pushed stream is not permitted and will throw an error.

http2stream.respond([headers[, options]])#

Added in: v8.4.0
  • headers <HTTP/2 Headers Object>

  • options <Object>

    • endStream <boolean> Set to true to indicate that the response will not include payload data.
    • waitForTrailers <boolean> When true, the Http2Stream will emit the 'wantTrailers' event after the final DATA frame has been sent.
const http2 = require('http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  stream.respond({ ':status': 200 });
  stream.end('some data');
});

When the options.waitForTrailers option is set, the 'wantTrailers' event will be emitted immediately after queuing the last chunk of payload data to be sent. The http2stream.sendTrailers() method can then be used to sent trailing header fields to the peer.

When options.waitForTrailers is set, the Http2Stream will not automatically close when the final DATA frame is transmitted. User code must call either http2stream.sendTrailers() or http2stream.close() to close the Http2Stream.

const http2 = require('http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  stream.respond({ ':status': 200 }, { waitForTrailers: true });
  stream.on('wantTrailers', () => {
    stream.sendTrailers({ ABC: 'some value to send' });
  });
  stream.end('some data');
});

http2stream.respondWithFD(fd[, headers[, options]])#

History
VersionChanges
v12.12.0

The fd option may now be a FileHandle.

v10.0.0

Any readable file descriptor, not necessarily for a regular file, is supported now.

v8.4.0

Added in: v8.4.0

Initiates a response whose data is read from the given file descriptor. No validation is performed on the given file descriptor. If an error occurs while attempting to read data using the file descriptor, the Http2Stream will be closed using an RST_STREAM frame using the standard INTERNAL_ERROR code.

When used, the Http2Stream object's Duplex interface will be closed automatically.

const http2 = require('http2');
const fs = require('fs');

const server = http2.createServer();
server.on('stream', (stream) => {
  const fd = fs.openSync('/some/file', 'r');

  const stat = fs.fstatSync(fd);
  const headers = {
    'content-length': stat.size,
    'last-modified': stat.mtime.toUTCString(),
    'content-type': 'text/plain'
  };
  stream.respondWithFD(fd, headers);
  stream.on('close', () => fs.closeSync(fd));
});

The optional options.statCheck function may be specified to give user code an opportunity to set additional content headers based on the fs.Stat details of the given fd. If the statCheck function is provided, the http2stream.respondWithFD() method will perform an fs.fstat() call to collect details on the provided file descriptor.

The offset and length options may be used to limit the response to a specific range subset. This can be used, for instance, to support HTTP Range requests.

The file descriptor or FileHandle is not closed when the stream is closed, so it will need to be closed manually once it is no longer needed. Using the same file descriptor concurrently for multiple streams is not supported and may result in data loss. Re-using a file descriptor after a stream has finished is supported.

When the options.waitForTrailers option is set, the 'wantTrailers' event will be emitted immediately after queuing the last chunk of payload data to be sent. The http2stream.sendTrailers() method can then be used to sent trailing header fields to the peer.

When options.waitForTrailers is set, the Http2Stream will not automatically close when the final DATA frame is transmitted. User code must call either http2stream.sendTrailers() or http2stream.close() to close the Http2Stream.

const http2 = require('http2');
const fs = require('fs');

const server = http2.createServer();
server.on('stream', (stream) => {
  const fd = fs.openSync('/some/file', 'r');

  const stat = fs.fstatSync(fd);
  const headers = {
    'content-length': stat.size,
    'last-modified': stat.mtime.toUTCString(),
    'content-type': 'text/plain'
  };
  stream.respondWithFD(fd, headers, { waitForTrailers: true });
  stream.on('wantTrailers', () => {
    stream.sendTrailers({ ABC: 'some value to send' });
  });

  stream.on('close', () => fs.closeSync(fd));
});

http2stream.respondWithFile(path[, headers[, options]])#

History
VersionChanges
v10.0.0

Any readable file, not necessarily a regular file, is supported now.

v8.4.0

Added in: v8.4.0

Sends a regular file as the response. The path must specify a regular file or an 'error' event will be emitted on the Http2Stream object.

When used, the Http2Stream object's Duplex interface will be closed automatically.

The optional options.statCheck function may be specified to give user code an opportunity to set additional content headers based on the fs.Stat details of the given file:

If an error occurs while attempting to read the file data, the Http2Stream will be closed using an RST_STREAM frame using the standard INTERNAL_ERROR code. If the onError callback is defined, then it will be called. Otherwise the stream will be destroyed.

Example using a file path:

const http2 = require('http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  function statCheck(stat, headers) {
    headers['last-modified'] = stat.mtime.toUTCString();
  }

  function onError(err) {
    if (err.code === 'ENOENT') {
      stream.respond({ ':status': 404 });
    } else {
      stream.respond({ ':status': 500 });
    }
    stream.end();
  }

  stream.respondWithFile('/some/file',
                         { 'content-type': 'text/plain' },
                         { statCheck, onError });
});

The options.statCheck function may also be used to cancel the send operation by returning false. For instance, a conditional request may check the stat results to determine if the file has been modified to return an appropriate 304 response:

const http2 = require('http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  function statCheck(stat, headers) {
    // Check the stat here...
    stream.respond({ ':status': 304 });
    return false; // Cancel the send operation
  }
  stream.respondWithFile('/some/file',
                         { 'content-type': 'text/plain' },
                         { statCheck });
});

The content-length header field will be automatically set.

The offset and length options may be used to limit the response to a specific range subset. This can be used, for instance, to support HTTP Range requests.

The options.onError function may also be used to handle all the errors that could happen before the delivery of the file is initiated. The default behavior is to destroy the stream.

When the options.waitForTrailers option is set, the 'wantTrailers' event will be emitted immediately after queuing the last chunk of payload data to be sent. The http2stream.sendTrailers() method can then be used to sent trailing header fields to the peer.

When options.waitForTrailers is set, the Http2Stream will not automatically close when the final DATA frame is transmitted. User code must call either http2stream.sendTrailers() or http2stream.close() to close the Http2Stream.

const http2 = require('http2');
const server = http2.createServer();
server.on('stream', (stream) => {
  stream.respondWithFile('/some/file',
                         { 'content-type': 'text/plain' },
                         { waitForTrailers: true });
  stream.on('wantTrailers', () => {
    stream.sendTrailers({ ABC: 'some value to send' });
  });
});

Class: Http2Server#

Added in: v8.4.0

Instances of Http2Server are created using the http2.createServer() function. The Http2Server class is not exported directly by the http2 module.

Event: 'checkContinue'#

Added in: v8.5.0

If a 'request' listener is registered or http2.createServer() is supplied a callback function, the 'checkContinue' event is emitted each time a request with an HTTP Expect: 100-continue is received. If this event is not listened for, the server will automatically respond with a status 100 Continue as appropriate.

Handling this event involves calling response.writeContinue() if the client should continue to send the request body, or generating an appropriate HTTP response (e.g. 400 Bad Request) if the client should not continue to send the request body.

When this event is emitted and handled, the 'request' event will not be emitted.

Event: 'request'#

Added in: v8.4.0

Emitted each time there is a request. There may be multiple requests per session. See the Compatibility API.

Event: 'session'#

Added in: v8.4.0

The 'session' event is emitted when a new Http2Session is created by the Http2Server.

Event: 'sessionError'#

Added in: v8.4.0

The 'sessionError' event is emitted when an 'error' event is emitted by an Http2Session object associated with the Http2Server.

Event: 'stream'#

Added in: v8.4.0

The 'stream' event is emitted when a 'stream' event has been emitted by an Http2Session associated with the server.

const http2 = require('http2');
const {
  HTTP2_HEADER_METHOD,
  HTTP2_HEADER_PATH,
  HTTP2_HEADER_STATUS,
  HTTP2_HEADER_CONTENT_TYPE
} = http2.constants;

const server = http2.createServer();
server.on('stream', (stream, headers, flags) => {
  const method = headers[HTTP2_HEADER_METHOD];
  const path = headers[HTTP2_HEADER_PATH];
  // ...
  stream.respond({
    [HTTP2_HEADER_STATUS]: 200,
    [HTTP2_HEADER_CONTENT_TYPE]: 'text/plain'
  });
  stream.write('hello ');
  stream.end('world');
});

Event: 'timeout'#

History
VersionChanges
v13.0.0

The default timeout changed from 120s to 0 (no timeout).

v8.4.0

Added in: v8.4.0

The 'timeout' event is emitted when there is no activity on the Server for a given number of milliseconds set using http2server.setTimeout(). Default: 0 (no timeout)

server.close([callback])#

Added in: v8.4.0

Stops the server from establishing new sessions. This does not prevent new request streams from being created due to the persistent nature of HTTP/2 sessions. To gracefully shut down the server, call http2session.close() on all active sessions.

If callback is provided, it is not invoked until all active sessions have been closed, although the server has already stopped allowing new sessions. See net.Server.close() for more details.

server.setTimeout([msecs][, callback])#

History
VersionChanges
v13.0.0

The default timeout changed from 120s to 0 (no timeout).

v8.4.0

Added in: v8.4.0

Used to set the timeout value for http2 server requests, and sets a callback function that is called when there is no activity on the Http2Server after msecs milliseconds.

The given callback is registered as a listener on the 'timeout' event.

In case if callback is not a function, a new ERR_INVALID_CALLBACK error will be thrown.

server.timeout#

History
VersionChanges
v13.0.0

The default timeout changed from 120s to 0 (no timeout).

v8.4.0

Added in: v8.4.0

  • <number> Timeout in milliseconds. Default: 0 (no timeout)

The number of milliseconds of inactivity before a socket is presumed to have timed out.

A value of 0 will disable the timeout behavior on incoming connections.

The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.

Class: Http2SecureServer#

Added in: v8.4.0

Instances of Http2SecureServer are created using the http2.createSecureServer() function. The Http2SecureServer class is not exported directly by the http2 module.

Event: 'checkContinue'#

Added in: v8.5.0

If a 'request' listener is registered or http2.createSecureServer() is supplied a callback function, the 'checkContinue' event is emitted each time a request with an HTTP Expect: 100-continue is received. If this event is not listened for, the server will automatically respond with a status 100 Continue as appropriate.

Handling this event involves calling response.writeContinue() if the client should continue to send the request body, or generating an appropriate HTTP response (e.g. 400 Bad Request) if the client should not continue to send the request body.

When this event is emitted and handled, the 'request' event will not be emitted.

Event: 'request'#

Added in: v8.4.0

Emitted each time there is a request. There may be multiple requests per session. See the Compatibility API.

Event: 'session'#

Added in: v8.4.0

The 'session' event is emitted when a new Http2Session is created by the Http2SecureServer.

Event: 'sessionError'#

Added in: v8.4.0

The 'sessionError' event is emitted when an 'error' event is emitted by an Http2Session object associated with the Http2SecureServer.

Event: 'stream'#

Added in: v8.4.0

The 'stream' event is emitted when a 'stream' event has been emitted by an Http2Session associated with the server.

const http2 = require('http2');
const {
  HTTP2_HEADER_METHOD,
  HTTP2_HEADER_PATH,
  HTTP2_HEADER_STATUS,
  HTTP2_HEADER_CONTENT_TYPE
} = http2.constants;

const options = getOptionsSomehow();

const server = http2.createSecureServer(options);
server.on('stream', (stream, headers, flags) => {
  const method = headers[HTTP2_HEADER_METHOD];
  const path = headers[HTTP2_HEADER_PATH];
  // ...
  stream.respond({
    [HTTP2_HEADER_STATUS]: 200,
    [HTTP2_HEADER_CONTENT_TYPE]: 'text/plain'
  });
  stream.write('hello ');
  stream.end('world');
});

Event: 'timeout'#

Added in: v8.4.0

The 'timeout' event is emitted when there is no activity on the Server for a given number of milliseconds set using http2secureServer.setTimeout(). Default: 2 minutes.

Event: 'unknownProtocol'#

Added in: v8.4.0

The 'unknownProtocol' event is emitted when a connecting client fails to negotiate an allowed protocol (i.e. HTTP/2 or HTTP/1.1). The event handler receives the socket for handling. If no listener is registered for this event, the connection is terminated. See the Compatibility API.

server.close([callback])#

Added in: v8.4.0

Stops the server from establishing new sessions. This does not prevent new request streams from being created due to the persistent nature of HTTP/2 sessions. To gracefully shut down the server, call http2session.close() on all active sessions.

If callback is provided, it is not invoked until all active sessions have been closed, although the server has already stopped allowing new sessions. See tls.Server.close() for more details.

server.setTimeout([msecs][, callback])#

Added in: v8.4.0

Used to set the timeout value for http2 secure server requests, and sets a callback function that is called when there is no activity on the Http2SecureServer after msecs milliseconds.

The given callback is registered as a listener on the 'timeout' event.

In case if callback is not a function, a new ERR_INVALID_CALLBACK error will be thrown.

server.timeout#

History
VersionChanges
v13.0.0

The default timeout changed from 120s to 0 (no timeout).

v8.4.0

Added in: v8.4.0

  • <number> Timeout in milliseconds. Default: 0 (no timeout)

The number of milliseconds of inactivity before a socket is presumed to have timed out.

A value of 0 will disable the timeout behavior on incoming connections.

The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.

http2.createServer(options[, onRequestHandler])#

History
VersionChanges
v14.4.0

Added maxSettings option with a default of 32.

v13.0.0

The PADDING_STRATEGY_CALLBACK has been made equivalent to providing PADDING_STRATEGY_ALIGNED and selectPadding has been removed.

v13.3.0, v12.16.0

Added maxSessionRejectedStreams option with a default of 100.

v13.3.0, v12.16.0

Added maxSessionInvalidFrames option with a default of 1000.

v12.4.0

The options parameter now supports net.createServer() options.

v9.6.0

Added the Http1IncomingMessage and Http1ServerResponse option.

v8.9.3

Added the maxOutstandingPings option with a default limit of 10.

v8.9.3

Added the maxHeaderListPairs option with a default limit of 128 header pairs.

v8.4.0

Added in: v8.4.0

  • options <Object>

    • maxDeflateDynamicTableSize <number> Sets the maximum dynamic table size for deflating header fields. Default: 4Kib.
    • maxSettings <number> Sets the maximum number of settings entries per SETTINGS frame. The minimum value allowed is 1. Default: 32.
    • maxSessionMemory<number> Sets the maximum memory that the Http2Session is permitted to use. The value is expressed in terms of number of megabytes, e.g. 1 equal 1 megabyte. The minimum value allowed is 1. This is a credit based limit, existing Http2Streams may cause this limit to be exceeded, but new Http2Stream instances will be rejected while this limit is exceeded. The current number of Http2Stream sessions, the current memory use of the header compression tables, current data queued to be sent, and unacknowledged PING and SETTINGS frames are all counted towards the current limit. Default: 10.
    • maxHeaderListPairs <number> Sets the maximum number of header entries. The minimum value is 4. Default: 128.
    • maxOutstandingPings <number> Sets the maximum number of outstanding, unacknowledged pings. Default: 10.
    • maxSendHeaderBlockLength <number> Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a 'frameError' event being emitted and the stream being closed and destroyed.

    • paddingStrategy <number> The strategy used for determining the amount of padding to use for HEADERS and DATA frames. Default: http2.constants.PADDING_STRATEGY_NONE. Value may be one of:

      • http2.constants.PADDING_STRATEGY_NONE: No padding is applied.
      • http2.constants.PADDING_STRATEGY_MAX: The maximum amount of padding, determined by the internal implementation, is applied.
      • http2.constants.PADDING_STRATEGY_ALIGNED: Attempts to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum is used and the total frame length is not necessarily aligned at 8 bytes.
    • peerMaxConcurrentStreams <number> Sets the maximum number of concurrent streams for the remote peer as if a SETTINGS frame had been received. Will be overridden if the remote peer sets its own value for maxConcurrentStreams. Default: 100.
    • maxSessionInvalidFrames <integer> Sets the maximum number of invalid frames that will be tolerated before the session is closed. Default: 1000.
    • maxSessionRejectedStreams <integer> Sets the maximum number of rejected upon creation streams that will be tolerated before the session is closed. Each rejection is associated with an NGHTTP2_ENHANCE_YOUR_CALM error that should tell the peer to not open any more streams, continuing to open streams is therefore regarded as a sign of a misbehaving peer. Default: 100.
    • settings <HTTP/2 Settings Object> The initial settings to send to the remote peer upon connection.
    • Http1IncomingMessage <http.IncomingMessage> Specifies the IncomingMessage class to used for HTTP/1 fallback. Useful for extending the original http.IncomingMessage. Default: http.IncomingMessage.
    • Http1ServerResponse <http.ServerResponse> Specifies the ServerResponse class to used for HTTP/1 fallback. Useful for extending the original http.ServerResponse. Default: http.ServerResponse.
    • Http2ServerRequest <http2.Http2ServerRequest> Specifies the Http2ServerRequest class to use. Useful for extending the original Http2ServerRequest. Default: Http2ServerRequest.
    • Http2ServerResponse <http2.Http2ServerResponse> Specifies the Http2ServerResponse class to use. Useful for extending the original Http2ServerResponse. Default: Http2ServerResponse.
    • ...: Any net.createServer() option can be provided.
  • onRequestHandler <Function> See Compatibility API
  • Returns: <Http2Server>

Returns a net.Server instance that creates and manages Http2Session instances.

Since there are no browsers known that support unencrypted HTTP/2, the use of http2.createSecureServer() is necessary when communicating with browser clients.

const http2 = require('http2');

// Create an unencrypted HTTP/2 server.
// Since there are no browsers known that support
// unencrypted HTTP/2, the use of `http2.createSecureServer()`
// is necessary when communicating with browser clients.
const server = http2.createServer();

server.on('stream', (stream, headers) => {
  stream.respond({
    'content-type': 'text/html',
    ':status': 200
  });
  stream.end('<h1>Hello World</h1>');
});

server.listen(80);

http2.createSecureServer(options[, onRequestHandler])#

History
VersionChanges
v14.4.0

Added maxSettings option with a default of 32.

v13.0.0

The PADDING_STRATEGY_CALLBACK has been made equivalent to providing PADDING_STRATEGY_ALIGNED and selectPadding has been removed.

v13.3.0, v12.16.0

Added maxSessionRejectedStreams option with a default of 100.

v13.3.0, v12.16.0

Added maxSessionInvalidFrames option with a default of 1000.

v10.12.0

Added the origins option to automatically send an ORIGIN frame on Http2Session startup.

v8.9.3

Added the maxOutstandingPings option with a default limit of 10.

v8.9.3

Added the maxHeaderListPairs option with a default limit of 128 header pairs.

v8.4.0

Added in: v8.4.0

  • options <Object>

    • allowHTTP1 <boolean> Incoming client connections that do not support HTTP/2 will be downgraded to HTTP/1.x when set to true. See the 'unknownProtocol' event. See ALPN negotiation. Default: false.
    • maxDeflateDynamicTableSize <number> Sets the maximum dynamic table size for deflating header fields. Default: 4Kib.
    • maxSettings <number> Sets the maximum number of settings entries per SETTINGS frame. The minimum value allowed is 1. Default: 32.
    • maxSessionMemory<number> Sets the maximum memory that the Http2Session is permitted to use. The value is expressed in terms of number of megabytes, e.g. 1 equal 1 megabyte. The minimum value allowed is 1. This is a credit based limit, existing Http2Streams may cause this limit to be exceeded, but new Http2Stream instances will be rejected while this limit is exceeded. The current number of Http2Stream sessions, the current memory use of the header compression tables, current data queued to be sent, and unacknowledged PING and SETTINGS frames are all counted towards the current limit. Default: 10.
    • maxHeaderListPairs <number> Sets the maximum number of header entries. The minimum value is 4. Default: 128.
    • maxOutstandingPings <number> Sets the maximum number of outstanding, unacknowledged pings. Default: 10.
    • maxSendHeaderBlockLength <number> Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a 'frameError' event being emitted and the stream being closed and destroyed.

    • paddingStrategy <number> Strategy used for determining the amount of padding to use for HEADERS and DATA frames. Default: http2.constants.PADDING_STRATEGY_NONE. Value may be one of:

      • http2.constants.PADDING_STRATEGY_NONE: No padding is applied.
      • http2.constants.PADDING_STRATEGY_MAX: The maximum amount of padding, determined by the internal implementation, is applied.
      • http2.constants.PADDING_STRATEGY_ALIGNED: Attempts to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum is used and the total frame length is not necessarily aligned at 8 bytes.
    • peerMaxConcurrentStreams <number> Sets the maximum number of concurrent streams for the remote peer as if a SETTINGS frame had been received. Will be overridden if the remote peer sets its own value for maxConcurrentStreams. Default: 100.
    • maxSessionInvalidFrames <integer> Sets the maximum number of invalid frames that will be tolerated before the session is closed. Default: 1000.
    • maxSessionRejectedStreams <integer> Sets the maximum number of rejected upon creation streams that will be tolerated before the session is closed. Each rejection is associated with an NGHTTP2_ENHANCE_YOUR_CALM error that should tell the peer to not open any more streams, continuing to open streams is therefore regarded as a sign of a misbehaving peer. Default: 100.
    • settings <HTTP/2 Settings Object> The initial settings to send to the remote peer upon connection.
    • ...: Any tls.createServer() options can be provided. For servers, the identity options (pfx or key/cert) are usually required.
    • origins <string[]> An array of origin strings to send within an ORIGIN frame immediately following creation of a new server Http2Session.
  • onRequestHandler <Function> See Compatibility API
  • Returns: <Http2SecureServer>

Returns a tls.Server instance that creates and manages Http2Session instances.

const http2 = require('http2');
const fs = require('fs');

const options = {
  key: fs.readFileSync('server-key.pem'),
  cert: fs.readFileSync('server-cert.pem')
};

// Create a secure HTTP/2 server
const server = http2.createSecureServer(options);

server.on('stream', (stream, headers) => {
  stream.respond({
    'content-type': 'text/html',
    ':status': 200
  });
  stream.end('<h1>Hello World</h1>');
});

server.listen(80);

http2.connect(authority[, options][, listener])#

History
VersionChanges
v14.4.0

Added maxSettings option with a default of 32.

v13.0.0

The PADDING_STRATEGY_CALLBACK has been made equivalent to providing PADDING_STRATEGY_ALIGNED and selectPadding has been removed.

v8.9.3

Added the maxOutstandingPings option with a default limit of 10.

v8.9.3

Added the maxHeaderListPairs option with a default limit of 128 header pairs.

v8.4.0

Added in: v8.4.0

  • authority <string> | <URL> The remote HTTP/2 server to connect to. This must be in the form of a minimal, valid URL with the http:// or https:// prefix, host name, and IP port (if a non-default port is used). Userinfo (user ID and password), path, querystring, and fragment details in the URL will be ignored.

  • options <Object>

    • maxDeflateDynamicTableSize <number> Sets the maximum dynamic table size for deflating header fields. Default: 4Kib.
    • maxSettings <number> Sets the maximum number of settings entries per SETTINGS frame. The minimum value allowed is 1. Default: 32.
    • maxSessionMemory<number> Sets the maximum memory that the Http2Session is permitted to use. The value is expressed in terms of number of megabytes, e.g. 1 equal 1 megabyte. The minimum value allowed is 1. This is a credit based limit, existing Http2Streams may cause this limit to be exceeded, but new Http2Stream instances will be rejected while this limit is exceeded. The current number of Http2Stream sessions, the current memory use of the header compression tables, current data queued to be sent, and unacknowledged PING and SETTINGS frames are all counted towards the current limit. Default: 10.
    • maxHeaderListPairs <number> Sets the maximum number of header entries. The minimum value is 1. Default: 128.
    • maxOutstandingPings <number> Sets the maximum number of outstanding, unacknowledged pings. Default: 10.
    • maxReservedRemoteStreams <number> Sets the maximum number of reserved push streams the client will accept at any given time. Once the current number of currently reserved push streams exceeds reaches this limit, new push streams sent by the server will be automatically rejected. The minimum allowed value is 0. The maximum allowed value is 232-1. A negative value sets this option to the maximum allowed value. Default: 200.
    • maxSendHeaderBlockLength <number> Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a 'frameError' event being emitted and the stream being closed and destroyed.

    • paddingStrategy <number> Strategy used for determining the amount of padding to use for HEADERS and DATA frames. Default: http2.constants.PADDING_STRATEGY_NONE. Value may be one of:

      • http2.constants.PADDING_STRATEGY_NONE: No padding is applied.
      • http2.constants.PADDING_STRATEGY_MAX: The maximum amount of padding, determined by the internal implementation, is applied.
      • http2.constants.PADDING_STRATEGY_ALIGNED: Attempts to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum is used and the total frame length is not necessarily aligned at 8 bytes.
    • peerMaxConcurrentStreams <number> Sets the maximum number of concurrent streams for the remote peer as if a SETTINGS frame had been received. Will be overridden if the remote peer sets its own value for maxConcurrentStreams. Default: 100.
    • protocol <string> The protocol to connect with, if not set in the authority. Value may be either 'http:' or 'https:'. Default: 'https:'
    • settings <HTTP/2 Settings Object> The initial settings to send to the remote peer upon connection.
    • createConnection <Function> An optional callback that receives the URL instance passed to connect and the options object, and returns any Duplex stream that is to be used as the connection for this session.
    • ...: Any net.connect() or tls.connect() options can be provided.
  • listener <Function> Will be registered as a one-time listener of the 'connect' event.
  • Returns: <ClientHttp2Session>

Returns a ClientHttp2Session instance.

const http2 = require('http2');
const client = http2.connect('https://localhost:1234');

/* Use the client */

client.close();

http2.constants#

Added in: v8.4.0

Error Codes for RST_STREAM and GOAWAY#

ValueNameConstant
0x00No Errorhttp2.constants.NGHTTP2_NO_ERROR
0x01Protocol Errorhttp2.constants.NGHTTP2_PROTOCOL_ERROR
0x02Internal Errorhttp2.constants.NGHTTP2_INTERNAL_ERROR
0x03Flow Control Errorhttp2.constants.NGHTTP2_FLOW_CONTROL_ERROR
0x04Settings Timeouthttp2.constants.NGHTTP2_SETTINGS_TIMEOUT
0x05Stream Closedhttp2.constants.NGHTTP2_STREAM_CLOSED
0x06Frame Size Errorhttp2.constants.NGHTTP2_FRAME_SIZE_ERROR
0x07Refused Streamhttp2.constants.NGHTTP2_REFUSED_STREAM
0x08Cancelhttp2.constants.NGHTTP2_CANCEL
0x09Compression Errorhttp2.constants.NGHTTP2_COMPRESSION_ERROR
0x0aConnect Errorhttp2.constants.NGHTTP2_CONNECT_ERROR
0x0bEnhance Your Calmhttp2.constants.NGHTTP2_ENHANCE_YOUR_CALM
0x0cInadequate Securityhttp2.constants.NGHTTP2_INADEQUATE_SECURITY
0x0dHTTP/1.1 Requiredhttp2.constants.NGHTTP2_HTTP_1_1_REQUIRED

The 'timeout' event is emitted when there is no activity on the Server for a given number of milliseconds set using http2server.setTimeout().

http2.getDefaultSettings()#

Added in: v8.4.0

Returns an object containing the default settings for an Http2Session instance. This method returns a new object instance every time it is called so instances returned may be safely modified for use.

http2.getPackedSettings([settings])#

Added in: v8.4.0

Returns a Buffer instance containing serialized representation of the given HTTP/2 settings as specified in the HTTP/2 specification. This is intended for use with the HTTP2-Settings header field.

const http2 = require('http2');

const packed = http2.getPackedSettings({ enablePush: false });

console.log(packed.toString('base64'));
// Prints: AAIAAAAA

http2.getUnpackedSettings(buf)#

Added in: v8.4.0

Returns a HTTP/2 Settings Object containing the deserialized settings from the given Buffer as generated by http2.getPackedSettings().

Headers Object#

Headers are represented as own-properties on JavaScript objects. The property keys will be serialized to lower-case. Property values should be strings (if they are not they will be coerced to strings) or an Array of strings (in order to send more than one value per header field).

const headers = {
  ':status': '200',
  'content-type': 'text-plain',
  'ABC': ['has', 'more', 'than', 'one', 'value']
};

stream.respond(headers);

Header objects passed to callback functions will have a null prototype. This means that normal JavaScript object methods such as Object.prototype.toString() and Object.prototype.hasOwnProperty() will not work.

For incoming headers:

  • The :status header is converted to number.
  • Duplicates of :status, :method, :authority, :scheme, :path, :protocol, age, authorization, access-control-allow-credentials, access-control-max-age, access-control-request-method, content-encoding, content-language, content-length, content-location, content-md5, content-range, content-type, date, dnt, etag, expires, from, if-match, if-modified-since, if-none-match, if-range, if-unmodified-since, last-modified, location, max-forwards, proxy-authorization, range, referer,retry-after, tk, upgrade-insecure-requests, user-agent or x-content-type-options are discarded.
  • set-cookie is always an array. Duplicates are added to the array.
  • For duplicate cookie headers, the values are joined together with '; '.
  • For all other headers, the values are joined together with ', '.
const http2 = require('http2');
const server = http2.createServer();
server.on('stream', (stream, headers) => {
  console.log(headers[':path']);
  console.log(headers.ABC);
});

Settings Object#

History
VersionChanges
v12.12.0

The maxConcurrentStreams setting is stricter.

v8.9.3

The maxHeaderListSize setting is now strictly enforced.

v8.4.0

Added in: v8.4.0

The http2.getDefaultSettings(), http2.getPackedSettings(), http2.createServer(), http2.createSecureServer(), http2session.settings(), http2session.localSettings, and http2session.remoteSettings APIs either return or receive as input an object that defines configuration settings for an Http2Session object. These objects are ordinary JavaScript objects containing the following properties.

  • headerTableSize <number> Specifies the maximum number of bytes used for header compression. The minimum allowed value is 0. The maximum allowed value is 232-1. Default: 4,096 octets.
  • enablePush <boolean> Specifies true if HTTP/2 Push Streams are to be permitted on the Http2Session instances. Default: true.
  • initialWindowSize <number> Specifies the senders initial window size for stream-level flow control. The minimum allowed value is 0. The maximum allowed value is 232-1. Default: 65,535 bytes.
  • maxFrameSize <number> Specifies the size of the largest frame payload. The minimum allowed value is 16,384. The maximum allowed value is 224-1. Default: 16,384 bytes.
  • maxConcurrentStreams <number> Specifies the maximum number of concurrent streams permitted on an Http2Session. There is no default value which implies, at least theoretically, 232-1 streams may be open concurrently at any given time in an Http2Session. The minimum value is 0. The maximum allowed value is 232-1. Default: 4294967295.
  • maxHeaderListSize <number> Specifies the maximum size (uncompressed octets) of header list that will be accepted. The minimum allowed value is 0. The maximum allowed value is 232-1. Default: 65535.
  • enableConnectProtocol<boolean> Specifies true if the "Extended Connect Protocol" defined by RFC 8441 is to be enabled. This setting is only meaningful if sent by the server. Once the enableConnectProtocol setting has been enabled for a given Http2Session, it cannot be disabled. Default: false.

All additional properties on the settings object are ignored.

Error Handling#

There are several types of error conditions that may arise when using the http2 module:

Validation errors occur when an incorrect argument, option, or setting value is passed in. These will always be reported by a synchronous throw.

State errors occur when an action is attempted at an incorrect time (for instance, attempting to send data on a stream after it has closed). These will be reported using either a synchronous throw or via an 'error' event on the Http2Stream, Http2Session or HTTP/2 Server objects, depending on where and when the error occurs.

Internal errors occur when an HTTP/2 session fails unexpectedly. These will be reported via an 'error' event on the Http2Session or HTTP/2 Server objects.

Protocol errors occur when various HTTP/2 protocol constraints are violated. These will be reported using either a synchronous throw or via an 'error' event on the Http2Stream, Http2Session or HTTP/2 Server objects, depending on where and when the error occurs.

Invalid character handling in header names and values#

The HTTP/2 implementation applies stricter handling of invalid characters in HTTP header names and values than the HTTP/1 implementation.

Header field names are case-insensitive and are transmitted over the wire strictly as lower-case strings. The API provided by Node.js allows header names to be set as mixed-case strings (e.g. Content-Type) but will convert those to lower-case (e.g. content-type) upon transmission.

Header field-names must only contain one or more of the following ASCII characters: a-z, A-Z, 0-9, !, #, $, %, &, ', *, +, -, ., ^, _, ` (backtick), |, and ~.

Using invalid characters within an HTTP header field name will cause the stream to be closed with a protocol error being reported.

Header field values are handled with more leniency but should not contain new-line or carriage return characters and should be limited to US-ASCII characters, per the requirements of the HTTP specification.

Push streams on the client#

To receive pushed streams on the client, set a listener for the 'stream' event on the ClientHttp2Session:

const http2 = require('http2');

const client = http2.connect('http://localhost');

client.on('stream', (pushedStream, requestHeaders) => {
  pushedStream.on('push', (responseHeaders) => {
    // Process response headers
  });
  pushedStream.on('data', (chunk) => { /* handle pushed data */ });
});

const req = client.request({ ':path': '/' });

Supporting the CONNECT method#

The CONNECT method is used to allow an HTTP/2 server to be used as a proxy for TCP/IP connections.

A simple TCP Server:

const net = require('net');

const server = net.createServer((socket) => {
  let name = '';
  socket.setEncoding('utf8');
  socket.on('data', (chunk) => name += chunk);
  socket.on('end', () => socket.end(`hello ${name}`));
});

server.listen(8000);

An HTTP/2 CONNECT proxy:

const http2 = require('http2');
const { NGHTTP2_REFUSED_STREAM } = http2.constants;
const net = require('net');

const proxy = http2.createServer();
proxy.on('stream', (stream, headers) => {
  if (headers[':method'] !== 'CONNECT') {
    // Only accept CONNECT requests
    stream.close(NGHTTP2_REFUSED_STREAM);
    return;
  }
  const auth = new URL(`tcp://${headers[':authority']}`);
  // It's a very good idea to verify that hostname and port are
  // things this proxy should be connecting to.
  const socket = net.connect(auth.port, auth.hostname, () => {
    stream.respond();
    socket.pipe(stream);
    stream.pipe(socket);
  });
  socket.on('error', (error) => {
    stream.close(http2.constants.NGHTTP2_CONNECT_ERROR);
  });
});

proxy.listen(8001);

An HTTP/2 CONNECT client:

const http2 = require('http2');

const client = http2.connect('http://localhost:8001');

// Must not specify the ':path' and ':scheme' headers
// for CONNECT requests or an error will be thrown.
const req = client.request({
  ':method': 'CONNECT',
  ':authority': `localhost:${port}`
});

req.on('response', (headers) => {
  console.log(headers[http2.constants.HTTP2_HEADER_STATUS]);
});
let data = '';
req.setEncoding('utf8');
req.on('data', (chunk) => data += chunk);
req.on('end', () => {
  console.log(`The server says: ${data}`);
  client.close();
});
req.end('Jane');

The Extended CONNECT Protocol#

RFC 8441 defines an "Extended CONNECT Protocol" extension to HTTP/2 that may be used to bootstrap the use of an Http2Stream using the CONNECT method as a tunnel for other communication protocols (such as WebSockets).

The use of the Extended CONNECT Protocol is enabled by HTTP/2 servers by using the enableConnectProtocol setting:

const http2 = require('http2');
const settings = { enableConnectProtocol: true };
const server = http2.createServer({ settings });

Once the client receives the SETTINGS frame from the server indicating that the extended CONNECT may be used, it may send CONNECT requests that use the ':protocol' HTTP/2 pseudo-header:

const http2 = require('http2');
const client = http2.connect('http://localhost:8080');
client.on('remoteSettings', (settings) => {
  if (settings.enableConnectProtocol) {
    const req = client.request({ ':method': 'CONNECT', ':protocol': 'foo' });
    // ...
  }
});

Compatibility API#

The Compatibility API has the goal of providing a similar developer experience of HTTP/1 when using HTTP/2, making it possible to develop applications that support both HTTP/1 and HTTP/2. This API targets only the public API of the HTTP/1. However many modules use internal methods or state, and those are not supported as it is a completely different implementation.

The following example creates an HTTP/2 server using the compatibility API:

const http2 = require('http2');
const server = http2.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('ok');
});

In order to create a mixed HTTPS and HTTP/2 server, refer to the ALPN negotiation section. Upgrading from non-tls HTTP/1 servers is not supported.

The HTTP/2 compatibility API is composed of Http2ServerRequest and Http2ServerResponse. They aim at API compatibility with HTTP/1, but they do not hide the differences between the protocols. As an example, the status message for HTTP codes is ignored.

ALPN negotiation#

ALPN negotiation allows supporting both HTTPS and HTTP/2 over the same socket. The req and res objects can be either HTTP/1 or HTTP/2, and an application must restrict itself to the public API of HTTP/1, and detect if it is possible to use the more advanced features of HTTP/2.

The following example creates a server that supports both protocols:

const { createSecureServer } = require('http2');
const { readFileSync } = require('fs');

const cert = readFileSync('./cert.pem');
const key = readFileSync('./key.pem');

const server = createSecureServer(
  { cert, key, allowHTTP1: true },
  onRequest
).listen(4443);

function onRequest(req, res) {
  // Detects if it is a HTTPS request or HTTP/2
  const { socket: { alpnProtocol } } = req.httpVersion === '2.0' ?
    req.stream.session : req;
  res.writeHead(200, { 'content-type': 'application/json' });
  res.end(JSON.stringify({
    alpnProtocol,
    httpVersion: req.httpVersion
  }));
}

The 'request' event works identically on both HTTPS and HTTP/2.

Class: http2.Http2ServerRequest#

Added in: v8.4.0

A Http2ServerRequest object is created by http2.Server or http2.SecureServer and passed as the first argument to the 'request' event. It may be used to access a request status, headers, and data.

Event: 'aborted'#

Added in: v8.4.0

The 'aborted' event is emitted whenever a Http2ServerRequest instance is abnormally aborted in mid-communication.

The 'aborted' event will only be emitted if the Http2ServerRequest writable side has not been ended.

Event: 'close'#

Added in: v8.4.0

Indicates that the underlying Http2Stream was closed. Just like 'end', this event occurs only once per response.

request.aborted#

Added in: v10.1.0

The request.aborted property will be true if the request has been aborted.

request.authority#

Added in: v8.4.0

The request authority pseudo header field. It can also be accessed via req.headers[':authority'].

request.complete#

Added in: v12.10.0

The request.complete property will be true if the request has been completed, aborted, or destroyed.

request.connection#

Added in: v8.4.0Deprecated since: v13.0.0

Stability: 0 - Deprecated. Use request.socket.

See request.socket.

request.destroy([error])#

Added in: v8.4.0

Calls destroy() on the Http2Stream that received the Http2ServerRequest. If error is provided, an 'error' event is emitted and error is passed as an argument to any listeners on the event.

It does nothing if the stream was already destroyed.

request.headers#

Added in: v8.4.0

The request/response headers object.

Key-value pairs of header names and values. Header names are lower-cased.

// Prints something like:
//
// { 'user-agent': 'curl/7.22.0',
//   host: '127.0.0.1:8000',
//   accept: '*/*' }
console.log(request.headers);

See HTTP/2 Headers Object.

In HTTP/2, the request path, host name, protocol, and method are represented as special headers prefixed with the : character (e.g. ':path'). These special headers will be included in the request.headers object. Care must be taken not to inadvertently modify these special headers or errors may occur. For instance, removing all headers from the request will cause errors to occur:

removeAllHeaders(request.headers);
assert(request.url);   // Fails because the :path header has been removed

request.httpVersion#

Added in: v8.4.0

In case of server request, the HTTP version sent by the client. In the case of client response, the HTTP version of the connected-to server. Returns '2.0'.

Also message.httpVersionMajor is the first integer and message.httpVersionMinor is the second.

request.method#

Added in: v8.4.0

The request method as a string. Read-only. Examples: 'GET', 'DELETE'.

request.rawHeaders#

Added in: v8.4.0

The raw request/response headers list exactly as they were received.

The keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values.

Header names are not lowercased, and duplicates are not merged.

// Prints something like:
//
// [ 'user-agent',
//   'this is invalid because there can be only one',
//   'User-Agent',
//   'curl/7.22.0',
//   'Host',
//   '127.0.0.1:8000',
//   'ACCEPT',
//   '*/*' ]
console.log(request.rawHeaders);

request.rawTrailers#

Added in: v8.4.0

The raw request/response trailer keys and values exactly as they were received. Only populated at the 'end' event.

request.scheme#

Added in: v8.4.0

The request scheme pseudo header field indicating the scheme portion of the target URL.

request.setTimeout(msecs, callback)#

Added in: v8.4.0

Sets the Http2Stream's timeout value to msecs. If a callback is provided, then it is added as a listener on the 'timeout' event on the response object.

If no 'timeout' listener is added to the request, the response, or the server, then Http2Streams are destroyed when they time out. If a handler is assigned to the request, the response, or the server's 'timeout' events, timed out sockets must be handled explicitly.

request.socket#

Added in: v8.4.0

Returns a Proxy object that acts as a net.Socket (or tls.TLSSocket) but applies getters, setters, and methods based on HTTP/2 logic.

destroyed, readable, and writable properties will be retrieved from and set on request.stream.

destroy, emit, end, on and once methods will be called on request.stream.

setTimeout method will be called on request.stream.session.

pause, read, resume, and write will throw an error with code ERR_HTTP2_NO_SOCKET_MANIPULATION. See Http2Session and Sockets for more information.

All other interactions will be routed directly to the socket. With TLS support, use request.socket.getPeerCertificate() to obtain the client's authentication details.

request.stream#

Added in: v8.4.0

The Http2Stream object backing the request.

request.trailers#

Added in: v8.4.0

The request/response trailers object. Only populated at the 'end' event.

request.url#

Added in: v8.4.0

Request URL string. This contains only the URL that is present in the actual HTTP request. If the request is:

GET /status?name=ryan HTTP/1.1\r\n
Accept: text/plain\r\n
\r\n

Then request.url will be:

'/status?name=ryan'

To parse the url into its parts require('url').parse(request.url) can be used:

$ node
> require('url').parse('/status?name=ryan')
Url {
  protocol: null,
  slashes: null,
  auth: null,
  host: null,
  port: null,
  hostname: null,
  hash: null,
  search: '?name=ryan',
  query: 'name=ryan',
  pathname: '/status',
  path: '/status?name=ryan',
  href: '/status?name=ryan' }

To extract the parameters from the query string, the require('querystring').parse function can be used, or true can be passed as the second argument to require('url').parse.

$ node
> require('url').parse('/status?name=ryan', true)
Url {
  protocol: null,
  slashes: null,
  auth: null,
  host: null,
  port: null,
  hostname: null,
  hash: null,
  search: '?name=ryan',
  query: { name: 'ryan' },
  pathname: '/status',
  path: '/status?name=ryan',
  href: '/status?name=ryan' }

Class: http2.Http2ServerResponse#

Added in: v8.4.0

This object is created internally by an HTTP server, not by the user. It is passed as the second parameter to the 'request' event.

Event: 'close'#

Added in: v8.4.0

Indicates that the underlying Http2Stream was terminated before response.end() was called or able to flush.

Event: 'finish'#

Added in: v8.4.0

Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the HTTP/2 multiplexing for transmission over the network. It does not imply that the client has received anything yet.

After this event, no more events will be emitted on the response object.

response.addTrailers(headers)#

Added in: v8.4.0

This method adds HTTP trailing headers (a header but at the end of the message) to the response.

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

response.connection#

Added in: v8.4.0Deprecated since: v13.0.0

Stability: 0 - Deprecated. Use response.socket.

See response.socket.

response.end([data[, encoding]][, callback])#

History
VersionChanges
v10.0.0

This method now returns a reference to ServerResponse.

v8.4.0

Added in: v8.4.0

This method signals to the server that all of the response headers and body have been sent; that server should consider this message complete. The method, response.end(), MUST be called on each response.

If data is specified, it is equivalent to calling response.write(data, encoding) followed by response.end(callback).

If callback is specified, it will be called when the response stream is finished.

response.finished#

Added in: v8.4.0Deprecated since: v13.4.0, v12.16.0

Stability: 0 - Deprecated. Use response.writableEnded.

Boolean value that indicates whether the response has completed. Starts as false. After response.end() executes, the value will be true.

response.getHeader(name)#

Added in: v8.4.0

Reads out a header that has already been queued but not sent to the client. The name is case-insensitive.

const contentType = response.getHeader('content-type');

response.getHeaderNames()#

Added in: v8.4.0

Returns an array containing the unique names of the current outgoing headers. All header names are lowercase.

response.setHeader('Foo', 'bar');
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

const headerNames = response.getHeaderNames();
// headerNames === ['foo', 'set-cookie']

response.getHeaders()#

Added in: v8.4.0

Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related http module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase.

The object returned by the response.getHeaders() method does not prototypically inherit from the JavaScript Object. This means that typical Object methods such as obj.toString(), obj.hasOwnProperty(), and others are not defined and will not work.

response.setHeader('Foo', 'bar');
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']);

const headers = response.getHeaders();
// headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] }

response.hasHeader(name)#

Added in: v8.4.0

Returns true if the header identified by name is currently set in the outgoing headers. The header name matching is case-insensitive.

const hasContentType = response.hasHeader('content-type');

response.headersSent#

Added in: v8.4.0

True if headers were sent, false otherwise (read-only).

response.removeHeader(name)#

Added in: v8.4.0

Removes a header that has been queued for implicit sending.

response.removeHeader('Content-Encoding');

response.sendDate#

Added in: v8.4.0

When true, the Date header will be automatically generated and sent in the response if it is not already present in the headers. Defaults to true.

This should only be disabled for testing; HTTP requires the Date header in responses.

response.setHeader(name, value)#

Added in: v8.4.0

Sets a single header value for implicit headers. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here to send multiple headers with the same name.

response.setHeader('Content-Type', 'text/html');

or

response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

When headers have been set with response.setHeader(), they will be merged with any headers passed to response.writeHead(), with the headers passed to response.writeHead() given precedence.

// Returns content-type = text/plain
const server = http2.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('ok');
});

response.setTimeout(msecs[, callback])#

Added in: v8.4.0

Sets the Http2Stream's timeout value to msecs. If a callback is provided, then it is added as a listener on the 'timeout' event on the response object.

If no 'timeout' listener is added to the request, the response, or the server, then Http2Streams are destroyed when they time out. If a handler is assigned to the request, the response, or the server's 'timeout' events, timed out sockets must be handled explicitly.

response.socket#

Added in: v8.4.0

Returns a Proxy object that acts as a net.Socket (or tls.TLSSocket) but applies getters, setters, and methods based on HTTP/2 logic.

destroyed, readable, and writable properties will be retrieved from and set on response.stream.

destroy, emit, end, on and once methods will be called on response.stream.

setTimeout method will be called on response.stream.session.

pause, read, resume, and write will throw an error with code ERR_HTTP2_NO_SOCKET_MANIPULATION. See Http2Session and Sockets for more information.

All other interactions will be routed directly to the socket.

const http2 = require('http2');
const server = http2.createServer((req, res) => {
  const ip = req.socket.remoteAddress;
  const port = req.socket.remotePort;
  res.end(`Your IP address is ${ip} and your source port is ${port}.`);
}).listen(3000);

response.statusCode#

Added in: v8.4.0

When using implicit headers (not calling response.writeHead() explicitly), this property controls the status code that will be sent to the client when the headers get flushed.

response.statusCode = 404;

After response header was sent to the client, this property indicates the status code which was sent out.

response.statusMessage#

Added in: v8.4.0

Status message is not supported by HTTP/2 (RFC 7540 8.1.2.4). It returns an empty string.

response.stream#

Added in: v8.4.0

The Http2Stream object backing the response.

response.writableEnded#

Added in: v12.9.0

Is true after response.end() has been called. This property does not indicate whether the data has been flushed, for this use writable.writableFinished instead.

response.write(chunk[, encoding][, callback])#

Added in: v8.4.0

If this method is called and response.writeHead() has not been called, it will switch to implicit header mode and flush the implicit headers.

This sends a chunk of the response body. This method may be called multiple times to provide successive parts of the body.

In the http module, the response body is omitted when the request is a HEAD request. Similarly, the 204 and 304 responses must not include a message body.

chunk can be a string or a buffer. If chunk is a string, the second parameter specifies how to encode it into a byte stream. By default the encoding is 'utf8'. callback will be called when this chunk of data is flushed.

This is the raw HTTP body and has nothing to do with higher-level multi-part body encodings that may be used.

The first time response.write() is called, it will send the buffered header information and the first chunk of the body to the client. The second time response.write() is called, Node.js assumes data will be streamed, and sends the new data separately. That is, the response is buffered up to the first chunk of the body.

Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain' will be emitted when the buffer is free again.

response.writeContinue()#

Added in: v8.4.0

Sends a status 100 Continue to the client, indicating that the request body should be sent. See the 'checkContinue' event on Http2Server and Http2SecureServer.

response.writeHead(statusCode[, statusMessage][, headers])#

History
VersionChanges
v11.10.0, v10.17.0

Return this from writeHead() to allow chaining with end().

v8.4.0

Added in: v8.4.0

Sends a response header to the request. The status code is a 3-digit HTTP status code, like 404. The last argument, headers, are the response headers.

Returns a reference to the Http2ServerResponse, so that calls can be chained.

For compatibility with HTTP/1, a human-readable statusMessage may be passed as the second argument. However, because the statusMessage has no meaning within HTTP/2, the argument will have no effect and a process warning will be emitted.

const body = 'hello world';
response.writeHead(200, {
  'Content-Length': Buffer.byteLength(body),
  'Content-Type': 'text/plain' });

Content-Length is given in bytes not characters. The Buffer.byteLength() API may be used to determine the number of bytes in a given encoding. On outbound messages, Node.js does not check if Content-Length and the length of the body being transmitted are equal or not. However, when receiving messages, Node.js will automatically reject messages when the Content-Length does not match the actual payload size.

This method may be called at most one time on a message before response.end() is called.

If response.write() or response.end() are called before calling this, the implicit/mutable headers will be calculated and call this function.

When headers have been set with response.setHeader(), they will be merged with any headers passed to response.writeHead(), with the headers passed to response.writeHead() given precedence.

// Returns content-type = text/plain
const server = http2.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('ok');
});

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

response.createPushResponse(headers, callback)#

Added in: v8.4.0
  • headers <HTTP/2 Headers Object> An object describing the headers

  • callback <Function> Called once http2stream.pushStream() is finished, or either when the attempt to create the pushed Http2Stream has failed or has been rejected, or the state of Http2ServerRequest is closed prior to calling the http2stream.pushStream() method

Call http2stream.pushStream() with the given headers, and wrap the given Http2Stream on a newly created Http2ServerResponse as the callback parameter if successful. When Http2ServerRequest is closed, the callback is called with an error ERR_HTTP2_INVALID_STREAM.

Collecting HTTP/2 Performance Metrics#

The Performance Observer API can be used to collect basic performance metrics for each Http2Session and Http2Stream instance.

const { PerformanceObserver } = require('perf_hooks');

const obs = new PerformanceObserver((items) => {
  const entry = items.getEntries()[0];
  console.log(entry.entryType);  // prints 'http2'
  if (entry.name === 'Http2Session') {
    // Entry contains statistics about the Http2Session
  } else if (entry.name === 'Http2Stream') {
    // Entry contains statistics about the Http2Stream
  }
});
obs.observe({ entryTypes: ['http2'] });

The entryType property of the PerformanceEntry will be equal to 'http2'.

The name property of the PerformanceEntry will be equal to either 'Http2Stream' or 'Http2Session'.

If name is equal to Http2Stream, the PerformanceEntry will contain the following additional properties:

  • bytesRead <number> The number of DATA frame bytes received for this Http2Stream.
  • bytesWritten <number> The number of DATA frame bytes sent for this Http2Stream.
  • id <number> The identifier of the associated Http2Stream
  • timeToFirstByte <number> The number of milliseconds elapsed between the PerformanceEntry startTime and the reception of the first DATA frame.
  • timeToFirstByteSent <number> The number of milliseconds elapsed between the PerformanceEntry startTime and sending of the first DATA frame.
  • timeToFirstHeader <number> The number of milliseconds elapsed between the PerformanceEntry startTime and the reception of the first header.

If name is equal to Http2Session, the PerformanceEntry will contain the following additional properties:

  • bytesRead <number> The number of bytes received for this Http2Session.
  • bytesWritten <number> The number of bytes sent for this Http2Session.
  • framesReceived <number> The number of HTTP/2 frames received by the Http2Session.
  • framesSent <number> The number of HTTP/2 frames sent by the Http2Session.
  • maxConcurrentStreams <number> The maximum number of streams concurrently open during the lifetime of the Http2Session.
  • pingRTT <number> The number of milliseconds elapsed since the transmission of a PING frame and the reception of its acknowledgment. Only present if a PING frame has been sent on the Http2Session.
  • streamAverageDuration <number> The average duration (in milliseconds) for all Http2Stream instances.
  • streamCount <number> The number of Http2Stream instances processed by the Http2Session.
  • type <string> Either 'server' or 'client' to identify the type of Http2Session.

HTTPS#

Stability: 2 - Stable

HTTPS is the HTTP protocol over TLS/SSL. In Node.js this is implemented as a separate module.

Class: https.Agent#

History
VersionChanges
v5.3.0

support 0 maxCachedSessions to disable TLS session caching.

v2.5.0

parameter maxCachedSessions added to options for TLS sessions reuse.

v0.4.5

Added in: v0.4.5

An Agent object for HTTPS similar to http.Agent. See https.request() for more information.

new Agent([options])#

History
VersionChanges
v12.5.0

do not automatically set servername if the target host was specified using an IP address.

  • options <Object> Set of configurable options to set on the agent. Can have the same fields as for http.Agent(options), and

    • maxCachedSessions <number> maximum number of TLS cached sessions. Use 0 to disable TLS session caching. Default: 100.

    • servername <string> the value of Server Name Indication extension to be sent to the server. Use empty string '' to disable sending the extension. Default: host name of the target server, unless the target server is specified using an IP address, in which case the default is '' (no extension).

      See Session Resumption for information about TLS session reuse.

Event: 'keylog'#

Added in: v13.2.0, v12.16.0
  • line <Buffer> Line of ASCII text, in NSS SSLKEYLOGFILE format.
  • tlsSocket <tls.TLSSocket> The tls.TLSSocket instance on which it was generated.

The keylog event is emitted when key material is generated or received by a connection managed by this agent (typically before handshake has completed, but not necessarily). This keying material can be stored for debugging, as it allows captured TLS traffic to be decrypted. It may be emitted multiple times for each socket.

A typical use case is to append received lines to a common text file, which is later used by software (such as Wireshark) to decrypt the traffic:

// ...
https.globalAgent.on('keylog', (line, tlsSocket) => {
  fs.appendFileSync('/tmp/ssl-keys.log', line, { mode: 0o600 });
});

Class: https.Server#

Added in: v0.3.4

See http.Server for more information.

server.close([callback])#

Added in: v0.1.90

See server.close() from the HTTP module for details.

server.headersTimeout#

Added in: v11.3.0

See http.Server#headersTimeout.

server.listen()#

Starts the HTTPS server listening for encrypted connections. This method is identical to server.listen() from net.Server.

server.maxHeadersCount#

See http.Server#maxHeadersCount.

server.setTimeout([msecs][, callback])#

Added in: v0.11.2

See http.Server#setTimeout().

server.timeout#

History
VersionChanges
v13.0.0

The default timeout changed from 120s to 0 (no timeout).

v0.11.2

Added in: v0.11.2

See http.Server#timeout.

server.keepAliveTimeout#

Added in: v8.0.0

See http.Server#keepAliveTimeout.

https.createServer([options][, requestListener])#

Added in: v0.3.4 
// curl -k https://localhost:8000/
const https = require('https');
const fs = require('fs');

const options = {
  key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
  cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem')
};

https.createServer(options, (req, res) => {
  res.writeHead(200);
  res.end('hello world\n');
}).listen(8000);

Or

const https = require('https');
const fs = require('fs');

const options = {
  pfx: fs.readFileSync('test/fixtures/test_cert.pfx'),
  passphrase: 'sample'
};

https.createServer(options, (req, res) => {
  res.writeHead(200);
  res.end('hello world\n');
}).listen(8000);

https.get(options[, callback])#

https.get(url[, options][, callback])#

History
VersionChanges
v10.9.0

The url parameter can now be passed along with a separate options object.

v7.5.0

The options parameter can be a WHATWG URL object.

v0.3.6

Added in: v0.3.6

Like http.get() but for HTTPS.

options can be an object, a string, or a URL object. If options is a string, it is automatically parsed with new URL(). If it is a URL object, it will be automatically converted to an ordinary options object.

const https = require('https');

https.get('https://encrypted.google.com/', (res) => {
  console.log('statusCode:', res.statusCode);
  console.log('headers:', res.headers);

  res.on('data', (d) => {
    process.stdout.write(d);
  });

}).on('error', (e) => {
  console.error(e);
});

https.globalAgent#

Added in: v0.5.9

Global instance of https.Agent for all HTTPS client requests.

https.request(options[, callback])#

https.request(url[, options][, callback])#

History
VersionChanges
v14.1.0

The highWaterMark option is accepted now.

v10.9.0

The url parameter can now be passed along with a separate options object.

v9.3.0

The options parameter can now include clientCertEngine.

v7.5.0

The options parameter can be a WHATWG URL object.

v0.3.6

Added in: v0.3.6

Makes a request to a secure web server.

The following additional options from tls.connect() are also accepted: ca, cert, ciphers, clientCertEngine, crl, dhparam, ecdhCurve, honorCipherOrder, key, passphrase, pfx, rejectUnauthorized, secureOptions, secureProtocol, servername, sessionIdContext, highWaterMark.

options can be an object, a string, or a URL object. If options is a string, it is automatically parsed with new URL(). If it is a URL object, it will be automatically converted to an ordinary options object.

const https = require('https');

const options = {
  hostname: 'encrypted.google.com',
  port: 443,
  path: '/',
  method: 'GET'
};

const req = https.request(options, (res) => {
  console.log('statusCode:', res.statusCode);
  console.log('headers:', res.headers);

  res.on('data', (d) => {
    process.stdout.write(d);
  });
});

req.on('error', (e) => {
  console.error(e);
});
req.end();

Example using options from tls.connect():

const options = {
  hostname: 'encrypted.google.com',
  port: 443,
  path: '/',
  method: 'GET',
  key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
  cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem')
};
options.agent = new https.Agent(options);

const req = https.request(options, (res) => {
  // ...
});

Alternatively, opt out of connection pooling by not using an Agent.

const options = {
  hostname: 'encrypted.google.com',
  port: 443,
  path: '/',
  method: 'GET',
  key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
  cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem'),
  agent: false
};

const req = https.request(options, (res) => {
  // ...
});

Example using a URL as options:

const options = new URL('https://abc:xyz@example.com');

const req = https.request(options, (res) => {
  // ...
});

Example pinning on certificate fingerprint, or the public key (similar to pin-sha256):

const tls = require('tls');
const https = require('https');
const crypto = require('crypto');

function sha256(s) {
  return crypto.createHash('sha256').update(s).digest('base64');
}
const options = {
  hostname: 'github.com',
  port: 443,
  path: '/',
  method: 'GET',
  checkServerIdentity: function(host, cert) {
    // Make sure the certificate is issued to the host we are connected to
    const err = tls.checkServerIdentity(host, cert);
    if (err) {
      return err;
    }

    // Pin the public key, similar to HPKP pin-sha25 pinning
    const pubkey256 = 'pL1+qb9HTMRZJmuC/bB/ZI9d302BYrrqiVuRyW+DGrU=';
    if (sha256(cert.pubkey) !== pubkey256) {
      const msg = 'Certificate verification error: ' +
        `The public key of '${cert.subject.CN}' ` +
        'does not match our pinned fingerprint';
      return new Error(msg);
    }

    // Pin the exact certificate, rather than the pub key
    const cert256 = '25:FE:39:32:D9:63:8C:8A:FC:A1:9A:29:87:' +
      'D8:3E:4C:1D:98:DB:71:E4:1A:48:03:98:EA:22:6A:BD:8B:93:16';
    if (cert.fingerprint256 !== cert256) {
      const msg = 'Certificate verification error: ' +
        `The certificate of '${cert.subject.CN}' ` +
        'does not match our pinned fingerprint';
      return new Error(msg);
    }

    // This loop is informational only.
    // Print the certificate and public key fingerprints of all certs in the
    // chain. Its common to pin the public key of the issuer on the public
    // internet, while pinning the public key of the service in sensitive
    // environments.
    do {
      console.log('Subject Common Name:', cert.subject.CN);
      console.log('  Certificate SHA256 fingerprint:', cert.fingerprint256);

      hash = crypto.createHash('sha256');
      console.log('  Public key ping-sha256:', sha256(cert.pubkey));

      lastprint256 = cert.fingerprint256;
      cert = cert.issuerCertificate;
    } while (cert.fingerprint256 !== lastprint256);

  },
};

options.agent = new https.Agent(options);
const req = https.request(options, (res) => {
  console.log('All OK. Server matched our pinned cert or public key');
  console.log('statusCode:', res.statusCode);
  // Print the HPKP values
  console.log('headers:', res.headers['public-key-pins']);

  res.on('data', (d) => {});
});

req.on('error', (e) => {
  console.error(e.message);
});
req.end();

Outputs for example:

Subject Common Name: github.com
  Certificate SHA256 fingerprint: 25:FE:39:32:D9:63:8C:8A:FC:A1:9A:29:87:D8:3E:4C:1D:98:DB:71:E4:1A:48:03:98:EA:22:6A:BD:8B:93:16
  Public key ping-sha256: pL1+qb9HTMRZJmuC/bB/ZI9d302BYrrqiVuRyW+DGrU=
Subject Common Name: DigiCert SHA2 Extended Validation Server CA
  Certificate SHA256 fingerprint: 40:3E:06:2A:26:53:05:91:13:28:5B:AF:80:A0:D4:AE:42:2C:84:8C:9F:78:FA:D0:1F:C9:4B:C5:B8:7F:EF:1A
  Public key ping-sha256: RRM1dGqnDFsCJXBTHky16vi1obOlCgFFn/yOhI/y+ho=
Subject Common Name: DigiCert High Assurance EV Root CA
  Certificate SHA256 fingerprint: 74:31:E5:F4:C3:C1:CE:46:90:77:4F:0B:61:E0:54:40:88:3B:A9:A0:1E:D0:0B:A6:AB:D7:80:6E:D3:B1:18:CF
  Public key ping-sha256: WoiWRyIOVNa9ihaBciRSC7XHjliYS9VwUGOIud4PB18=
All OK. Server matched our pinned cert or public key
statusCode: 200
headers: max-age=0; pin-sha256="WoiWRyIOVNa9ihaBciRSC7XHjliYS9VwUGOIud4PB18="; pin-sha256="RRM1dGqnDFsCJXBTHky16vi1obOlCgFFn/yOhI/y+ho="; pin-sha256="k2v657xBsOVe1PQRwOsHsw3bsGT2VzIqz5K+59sNQws="; pin-sha256="K87oWBWM9UZfyddvDfoxL+8lpNyoUB2ptGtn0fv6G2Q="; pin-sha256="IQBnNBEiFuhj+8x6X8XLgh01V9Ic5/V3IRQLNFFc7v4="; pin-sha256="iie1VXtL7HzAMF+/PVPR9xzT80kQxdZeJ+zduCB3uj0="; pin-sha256="LvRiGEjRqfzurezaWuj8Wie2gyHMrW5Q06LspMnox7A="; includeSubDomains

Inspector#

Stability: 1 - Experimental

The inspector module provides an API for interacting with the V8 inspector.

It can be accessed using:

const inspector = require('inspector');

inspector.close()#

Deactivate the inspector. Blocks until there are no active connections.

inspector.console#

  • <Object> An object to send messages to the remote inspector console.
require('inspector').console.log('a message');

The inspector console does not have API parity with Node.js console.

inspector.open([port[, host[, wait]]])#

  • port <number> Port to listen on for inspector connections. Optional. Default: what was specified on the CLI.
  • host <string> Host to listen on for inspector connections. Optional. Default: what was specified on the CLI.
  • wait <boolean> Block until a client has connected. Optional. Default: false.

Activate inspector on host and port. Equivalent to node --inspect=[[host:]port], but can be done programmatically after node has started.

If wait is true, will block until a client has connected to the inspect port and flow control has been passed to the debugger client.

See the security warning regarding the host parameter usage.

inspector.url()#

Return the URL of the active inspector, or undefined if there is none.

$ node --inspect -p 'inspector.url()'
Debugger listening on ws://127.0.0.1:9229/166e272e-7a30-4d09-97ce-f1c012b43c34
For help see https://nodejs.org/en/docs/inspector
ws://127.0.0.1:9229/166e272e-7a30-4d09-97ce-f1c012b43c34

$ node --inspect=localhost:3000 -p 'inspector.url()'
Debugger listening on ws://localhost:3000/51cf8d0e-3c36-4c59-8efd-54519839e56a
For help see https://nodejs.org/en/docs/inspector
ws://localhost:3000/51cf8d0e-3c36-4c59-8efd-54519839e56a

$ node -p 'inspector.url()'
undefined

inspector.waitForDebugger()#

Added in: v12.7.0

Blocks until a client (existing or connected later) has sent Runtime.runIfWaitingForDebugger command.

An exception will be thrown if there is no active inspector.

Class: inspector.Session#

The inspector.Session is used for dispatching messages to the V8 inspector back-end and receiving message responses and notifications.

Constructor: new inspector.Session()#

Added in: v8.0.0

Create a new instance of the inspector.Session class. The inspector session needs to be connected through session.connect() before the messages can be dispatched to the inspector backend.

Event: 'inspectorNotification'#

Added in: v8.0.0
  • <Object> The notification message object

Emitted when any notification from the V8 Inspector is received.

session.on('inspectorNotification', (message) => console.log(message.method));
// Debugger.paused
// Debugger.resumed

It is also possible to subscribe only to notifications with specific method:

Event: <inspector-protocol-method>;#

Added in: v8.0.0
  • <Object> The notification message object

Emitted when an inspector notification is received that has its method field set to the <inspector-protocol-method> value.

The following snippet installs a listener on the 'Debugger.paused' event, and prints the reason for program suspension whenever program execution is suspended (through breakpoints, for example):

session.on('Debugger.paused', ({ params }) => {
  console.log(params.hitBreakpoints);
});
// [ '/the/file/that/has/the/breakpoint.js:11:0' ]

session.connect()#

Added in: v8.0.0

Connects a session to the inspector back-end.

session.connectToMainThread()#

Added in: v12.11.0

Connects a session to the main thread inspector back-end. An exception will be thrown if this API was not called on a Worker thread.

session.disconnect()#

Added in: v8.0.0

Immediately close the session. All pending message callbacks will be called with an error. session.connect() will need to be called to be able to send messages again. Reconnected session will lose all inspector state, such as enabled agents or configured breakpoints.

session.post(method[, params][, callback])#

Added in: v8.0.0

Posts a message to the inspector back-end. callback will be notified when a response is received. callback is a function that accepts two optional arguments: error and message-specific result.

session.post('Runtime.evaluate', { expression: '2 + 2' },
             (error, { result }) => console.log(result));
// Output: { type: 'number', value: 4, description: '4' }

The latest version of the V8 inspector protocol is published on the Chrome DevTools Protocol Viewer.

Node.js inspector supports all the Chrome DevTools Protocol domains declared by V8. Chrome DevTools Protocol domain provides an interface for interacting with one of the runtime agents used to inspect the application state and listen to the run-time events.

Example usage#

Apart from the debugger, various V8 Profilers are available through the DevTools protocol.

CPU Profiler#

Here's an example showing how to use the CPU Profiler:

const inspector = require('inspector');
const fs = require('fs');
const session = new inspector.Session();
session.connect();

session.post('Profiler.enable', () => {
  session.post('Profiler.start', () => {
    // Invoke business logic under measurement here...

    // some time later...
    session.post('Profiler.stop', (err, { profile }) => {
      // Write profile to disk, upload, etc.
      if (!err) {
        fs.writeFileSync('./profile.cpuprofile', JSON.stringify(profile));
      }
    });
  });
});

Heap Profiler#

Here's an example showing how to use the Heap Profiler:

const inspector = require('inspector');
const fs = require('fs');
const session = new inspector.Session();

const fd = fs.openSync('profile.heapsnapshot', 'w');

session.connect();

session.on('HeapProfiler.addHeapSnapshotChunk', (m) => {
  fs.writeSync(fd, m.params.chunk);
});

session.post('HeapProfiler.takeHeapSnapshot', null, (err, r) => {
  console.log('HeapProfiler.takeHeapSnapshot done:', err, r);
  session.disconnect();
  fs.closeSync(fd);
});

Internationalization Support#

Node.js has many features that make it easier to write internationalized programs. Some of them are:

Node.js (and its underlying V8 engine) uses ICU to implement these features in native C/C++ code. The full ICU data set is provided by Node.js by default. However, due to the size of the ICU data file, several options are provided for customizing the ICU data set either when building or running Node.js.

Options for building Node.js#

To control how ICU is used in Node.js, four configure options are available during compilation. Additional details on how to compile Node.js are documented in BUILDING.md.

  • --with-intl=none/--without-intl
  • --with-intl=system-icu
  • --with-intl=small-icu
  • --with-intl=full-icu (default)

An overview of available Node.js and JavaScript features for each configure option:

nonesystem-icusmall-icufull-icu
String.prototype.normalize()none (function is no-op)fullfullfull
String.prototype.to*Case()fullfullfullfull
Intlnone (object does not exist)partial/full (depends on OS)partial (English-only)full
String.prototype.localeCompare()partial (not locale-aware)fullfullfull
String.prototype.toLocale*Case()partial (not locale-aware)fullfullfull
Number.prototype.toLocaleString()partial (not locale-aware)partial/full (depends on OS)partial (English-only)full
Date.prototype.toLocale*String()partial (not locale-aware)partial/full (depends on OS)partial (English-only)full
WHATWG URL Parserpartial (no IDN support)fullfullfull
require('buffer').transcode()none (function does not exist)fullfullfull
REPLpartial (inaccurate line editing)fullfullfull
require('util').TextDecoderpartial (basic encodings support)partial/full (depends on OS)partial (Unicode-only)full
RegExp Unicode Property Escapesnone (invalid RegExp error)fullfullfull

The "(not locale-aware)" designation denotes that the function carries out its operation just like the non-Locale version of the function, if one exists. For example, under none mode, Date.prototype.toLocaleString()'s operation is identical to that of Date.prototype.toString().

Disable all internationalization features (none)#

If this option is chosen, ICU is disabled and most internationalization features mentioned above will be unavailable in the resulting node binary.

Build with a pre-installed ICU (system-icu)#

Node.js can link against an ICU build already installed on the system. In fact, most Linux distributions already come with ICU installed, and this option would make it possible to reuse the same set of data used by other components in the OS.

Functionalities that only require the ICU library itself, such as String.prototype.normalize() and the WHATWG URL parser, are fully supported under system-icu. Features that require ICU locale data in addition, such as Intl.DateTimeFormat may be fully or partially supported, depending on the completeness of the ICU data installed on the system.

Embed a limited set of ICU data (small-icu)#

This option makes the resulting binary link against the ICU library statically, and includes a subset of ICU data (typically only the English locale) within the node executable.

Functionalities that only require the ICU library itself, such as String.prototype.normalize() and the WHATWG URL parser, are fully supported under small-icu. Features that require ICU locale data in addition, such as Intl.DateTimeFormat, generally only work with the English locale:

const january = new Date(9e8);
const english = new Intl.DateTimeFormat('en', { month: 'long' });
const spanish = new Intl.DateTimeFormat('es', { month: 'long' });

console.log(english.format(january));
// Prints "January"
console.log(spanish.format(january));
// Prints "M01" on small-icu
// Should print "enero"

This mode provides a balance between features and binary size.

Providing ICU data at runtime#

If the small-icu option is used, one can still provide additional locale data at runtime so that the JS methods would work for all ICU locales. Assuming the data file is stored at /some/directory, it can be made available to ICU through either:

  • The NODE_ICU_DATA environment variable:

    env NODE_ICU_DATA=/some/directory node

  • The --icu-data-dir CLI parameter:

    node --icu-data-dir=/some/directory

(If both are specified, the --icu-data-dir CLI parameter takes precedence.)

ICU is able to automatically find and load a variety of data formats, but the data must be appropriate for the ICU version, and the file correctly named. The most common name for the data file is icudt6X[bl].dat, where 6X denotes the intended ICU version, and b or l indicates the system's endianness. Check "ICU Data" article in the ICU User Guide for other supported formats and more details on ICU data in general.

The full-icu npm module can greatly simplify ICU data installation by detecting the ICU version of the running node executable and downloading the appropriate data file. After installing the module through npm i full-icu, the data file will be available at ./node_modules/full-icu. This path can be then passed either to NODE_ICU_DATA or --icu-data-dir as shown above to enable full Intl support.

Embed the entire ICU (full-icu)#

This option makes the resulting binary link against ICU statically and include a full set of ICU data. A binary created this way has no further external dependencies and supports all locales, but might be rather large. This is the default behavior if no --with-intl flag is passed. The official binaries are also built in this mode.

Detecting internationalization support#

To verify that ICU is enabled at all (system-icu, small-icu, or full-icu), simply checking the existence of Intl should suffice:

const hasICU = typeof Intl === 'object';

Alternatively, checking for process.versions.icu, a property defined only when ICU is enabled, works too:

const hasICU = typeof process.versions.icu === 'string';

To check for support for a non-English locale (i.e. full-icu or system-icu), Intl.DateTimeFormat can be a good distinguishing factor:

const hasFullICU = (() => {
  try {
    const january = new Date(9e8);
    const spanish = new Intl.DateTimeFormat('es', { month: 'long' });
    return spanish.format(january) === 'enero';
  } catch (err) {
    return false;
  }
})();

For more verbose tests for Intl support, the following resources may be found to be helpful:

  • btest402: Generally used to check whether Node.js with Intl support is built correctly.
  • Test262: ECMAScript's official conformance test suite includes a section dedicated to ECMA-402.

Modules#

Stability: 2 - Stable

In the Node.js module system, each file is treated as a separate module. For example, consider a file named foo.js:

const circle = require('./circle.js');
console.log(`The area of a circle of radius 4 is ${circle.area(4)}`);

On the first line, foo.js loads the module circle.js that is in the same directory as foo.js.

Here are the contents of circle.js:

const { PI } = Math;

exports.area = (r) => PI * r ** 2;

exports.circumference = (r) => 2 * PI * r;

The module circle.js has exported the functions area() and circumference(). Functions and objects are added to the root of a module by specifying additional properties on the special exports object.

Variables local to the module will be private, because the module is wrapped in a function by Node.js (see module wrapper). In this example, the variable PI is private to circle.js.

The module.exports property can be assigned a new value (such as a function or object).

Below, bar.js makes use of the square module, which exports a Square class:

const Square = require('./square.js');
const mySquare = new Square(2);
console.log(`The area of mySquare is ${mySquare.area()}`);

The square module is defined in square.js:

// Assigning to exports will not modify module, must use module.exports
module.exports = class Square {
  constructor(width) {
    this.width = width;
  }

  area() {
    return this.width ** 2;
  }
};

The module system is implemented in the require('module') module.

Accessing the main module#

When a file is run directly from Node.js, require.main is set to its module. That means that it is possible to determine whether a file has been run directly by testing require.main === module.

For a file foo.js, this will be true if run via node foo.js, but false if run by require('./foo').

Because module provides a filename property (normally equivalent to __filename), the entry point of the current application can be obtained by checking require.main.filename.

Addenda: Package Manager Tips#

The semantics of the Node.js require() function were designed to be general enough to support reasonable directory structures. Package manager programs such as dpkg, rpm, and npm will hopefully find it possible to build native packages from Node.js modules without modification.

Below we give a suggested directory structure that could work:

Let's say that we wanted to have the folder at /usr/lib/node/<some-package>/<some-version> hold the contents of a specific version of a package.

Packages can depend on one another. In order to install package foo, it may be necessary to install a specific version of package bar. The bar package may itself have dependencies, and in some cases, these may even collide or form cyclic dependencies.

Since Node.js looks up the realpath of any modules it loads (that is, resolves symlinks), and then looks for their dependencies in the node_modules folders as described here, this situation is very simple to resolve with the following architecture:

  • /usr/lib/node/foo/1.2.3/: Contents of the foo package, version 1.2.3.
  • /usr/lib/node/bar/4.3.2/: Contents of the bar package that foo depends on.
  • /usr/lib/node/foo/1.2.3/node_modules/bar: Symbolic link to /usr/lib/node/bar/4.3.2/.
  • /usr/lib/node/bar/4.3.2/node_modules/*: Symbolic links to the packages that bar depends on.

Thus, even if a cycle is encountered, or if there are dependency conflicts, every module will be able to get a version of its dependency that it can use.

When the code in the foo package does require('bar'), it will get the version that is symlinked into /usr/lib/node/foo/1.2.3/node_modules/bar. Then, when the code in the bar package calls require('quux'), it'll get the version that is symlinked into /usr/lib/node/bar/4.3.2/node_modules/quux.

Furthermore, to make the module lookup process even more optimal, rather than putting packages directly in /usr/lib/node, we could put them in /usr/lib/node_modules/<name>/<version>. Then Node.js will not bother looking for missing dependencies in /usr/node_modules or /node_modules.

In order to make modules available to the Node.js REPL, it might be useful to also add the /usr/lib/node_modules folder to the $NODE_PATH environment variable. Since the module lookups using node_modules folders are all relative, and based on the real path of the files making the calls to require(), the packages themselves can be anywhere.

Addenda: The .mjs extension#

It is not possible to require() files that have the .mjs extension. Attempting to do so will throw an error. The .mjs extension is reserved for ECMAScript Modules which cannot be loaded via require(). See ECMAScript Modules for more details.

All Together...#

To get the exact filename that will be loaded when require() is called, use the require.resolve() function.

Putting together all of the above, here is the high-level algorithm in pseudocode of what require() does:

require(X) from module at path Y
1. If X is a core module,
   a. return the core module
   b. STOP
2. If X begins with '/'
   a. set Y to be the filesystem root
3. If X begins with './' or '/' or '../'
   a. LOAD_AS_FILE(Y + X)
   b. LOAD_AS_DIRECTORY(Y + X)
   c. THROW "not found"
4. LOAD_SELF_REFERENCE(X, dirname(Y))
5. LOAD_NODE_MODULES(X, dirname(Y))
6. THROW "not found"

LOAD_AS_FILE(X)
1. If X is a file, load X as its file extension format.  STOP
2. If X.js is a file, load X.js as JavaScript text.  STOP
3. If X.json is a file, parse X.json to a JavaScript Object.  STOP
4. If X.node is a file, load X.node as binary addon.  STOP

LOAD_INDEX(X)
1. If X/index.js is a file, load X/index.js as JavaScript text.  STOP
2. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP
3. If X/index.node is a file, load X/index.node as binary addon.  STOP

LOAD_AS_DIRECTORY(X)
1. If X/package.json is a file,
   a. Parse X/package.json, and look for "main" field.
   b. If "main" is a falsy value, GOTO 2.
   c. let M = X + (json main field)
   d. LOAD_AS_FILE(M)
   e. LOAD_INDEX(M)
   f. LOAD_INDEX(X) DEPRECATED
   g. THROW "not found"
2. LOAD_INDEX(X)

LOAD_NODE_MODULES(X, START)
1. let DIRS = NODE_MODULES_PATHS(START)
2. for each DIR in DIRS:
   a. LOAD_PACKAGE_EXPORTS(DIR, X)
   b. LOAD_AS_FILE(DIR/X)
   c. LOAD_AS_DIRECTORY(DIR/X)

NODE_MODULES_PATHS(START)
1. let PARTS = path split(START)
2. let I = count of PARTS - 1
3. let DIRS = [GLOBAL_FOLDERS]
4. while I >= 0,
   a. if PARTS[I] = "node_modules" CONTINUE
   b. DIR = path join(PARTS[0 .. I] + "node_modules")
   c. DIRS = DIRS + DIR
   d. let I = I - 1
5. return DIRS

LOAD_SELF_REFERENCE(X, START)
1. Find the closest package scope to START.
2. If no scope was found, return.
3. If the `package.json` has no "exports", return.
4. If the name in `package.json` isn't a prefix of X, throw "not found".
5. Otherwise, load the remainder of X relative to this package as if it
  was loaded via `LOAD_NODE_MODULES` with a name in `package.json`.

LOAD_PACKAGE_EXPORTS(DIR, X)
1. Try to interpret X as a combination of name and subpath where the name
   may have a @scope/ prefix and the subpath begins with a slash (`/`).
2. If X does not match this pattern or DIR/name/package.json is not a file,
   return.
3. Parse DIR/name/package.json, and look for "exports" field.
4. If "exports" is null or undefined, return.
5. If "exports" is an object with some keys starting with "." and some keys
  not starting with ".", throw "invalid config".
6. If "exports" is a string, or object with no keys starting with ".", treat
  it as having that value as its "." object property.
7. If subpath is "." and "exports" does not have a "." entry, return.
8. Find the longest key in "exports" that the subpath starts with.
9. If no such key can be found, throw "not found".
10. let RESOLVED =
    fileURLToPath(PACKAGE_EXPORTS_TARGET_RESOLVE(pathToFileURL(DIR/name),
    exports[key], subpath.slice(key.length), ["node", "require"])), as defined
    in the ESM resolver.
11. If key ends with "/":
    a. LOAD_AS_FILE(RESOLVED)
    b. LOAD_AS_DIRECTORY(RESOLVED)
12. Otherwise
   a. If RESOLVED is a file, load it as its file extension format.  STOP
13. Throw "not found"

Caching#

Modules are cached after the first time they are loaded. This means (among other things) that every call to require('foo') will get exactly the same object returned, if it would resolve to the same file.

Provided require.cache is not modified, multiple calls to require('foo') will not cause the module code to be executed multiple times. This is an important feature. With it, "partially done" objects can be returned, thus allowing transitive dependencies to be loaded even when they would cause cycles.

To have a module execute code multiple times, export a function, and call that function.

Module Caching Caveats#

Modules are cached based on their resolved filename. Since modules may resolve to a different filename based on the location of the calling module (loading from node_modules folders), it is not a guarantee that require('foo') will always return the exact same object, if it would resolve to different files.

Additionally, on case-insensitive file systems or operating systems, different resolved filenames can point to the same file, but the cache will still treat them as different modules and will reload the file multiple times. For example, require('./foo') and require('./FOO') return two different objects, irrespective of whether or not ./foo and ./FOO are the same file.

Core Modules#

Node.js has several modules compiled into the binary. These modules are described in greater detail elsewhere in this documentation.

The core modules are defined within the Node.js source and are located in the lib/ folder.

Core modules are always preferentially loaded if their identifier is passed to require(). For instance, require('http') will always return the built in HTTP module, even if there is a file by that name.

Cycles#

When there are circular require() calls, a module might not have finished executing when it is returned.

Consider this situation:

a.js:

console.log('a starting');
exports.done = false;
const b = require('./b.js');
console.log('in a, b.done = %j', b.done);
exports.done = true;
console.log('a done');

b.js:

console.log('b starting');
exports.done = false;
const a = require('./a.js');
console.log('in b, a.done = %j', a.done);
exports.done = true;
console.log('b done');

main.js:

console.log('main starting');
const a = require('./a.js');
const b = require('./b.js');
console.log('in main, a.done = %j, b.done = %j', a.done, b.done);

When main.js loads a.js, then a.js in turn loads b.js. At that point, b.js tries to load a.js. In order to prevent an infinite loop, an unfinished copy of the a.js exports object is returned to the b.js module. b.js then finishes loading, and its exports object is provided to the a.js module.

By the time main.js has loaded both modules, they're both finished. The output of this program would thus be:

$ node main.js
main starting
a starting
b starting
in b, a.done = false
b done
in a, b.done = true
a done
in main, a.done = true, b.done = true

Careful planning is required to allow cyclic module dependencies to work correctly within an application.

File Modules#

If the exact filename is not found, then Node.js will attempt to load the required filename with the added extensions: .js, .json, and finally .node.

.js files are interpreted as JavaScript text files, and .json files are parsed as JSON text files. .node files are interpreted as compiled addon modules loaded with process.dlopen().

A required module prefixed with '/' is an absolute path to the file. For example, require('/home/marco/foo.js') will load the file at /home/marco/foo.js.

A required module prefixed with './' is relative to the file calling require(). That is, circle.js must be in the same directory as foo.js for require('./circle') to find it.

Without a leading '/', './', or '../' to indicate a file, the module must either be a core module or is loaded from a node_modules folder.

If the given path does not exist, require() will throw an Error with its code property set to 'MODULE_NOT_FOUND'.

Folders as Modules#

It is convenient to organize programs and libraries into self-contained directories, and then provide a single entry point to those directories. There are three ways in which a folder may be passed to require() as an argument.

The first is to create a package.json file in the root of the folder, which specifies a main module. An example package.json file might look like this:

{ "name" : "some-library",
  "main" : "./lib/some-library.js" }

If this was in a folder at ./some-library, then require('./some-library') would attempt to load ./some-library/lib/some-library.js.

This is the extent of the awareness of package.json files within Node.js.

If there is no package.json file present in the directory, or if the 'main' entry is missing or cannot be resolved, then Node.js will attempt to load an index.js or index.node file out of that directory. For example, if there was no package.json file in the above example, then require('./some-library') would attempt to load:

  • ./some-library/index.js
  • ./some-library/index.node

If these attempts fail, then Node.js will report the entire module as missing with the default error:

Error: Cannot find module 'some-library'

Loading from node_modules Folders#

If the module identifier passed to require() is not a core module, and does not begin with '/', '../', or './', then Node.js starts at the parent directory of the current module, and adds /node_modules, and attempts to load the module from that location. Node.js will not append node_modules to a path already ending in node_modules.

If it is not found there, then it moves to the parent directory, and so on, until the root of the file system is reached.

For example, if the file at '/home/ry/projects/foo.js' called require('bar.js'), then Node.js would look in the following locations, in this order:

  • /home/ry/projects/node_modules/bar.js
  • /home/ry/node_modules/bar.js
  • /home/node_modules/bar.js
  • /node_modules/bar.js

This allows programs to localize their dependencies, so that they do not clash.

It is possible to require specific files or sub modules distributed with a module by including a path suffix after the module name. For instance require('example-module/path/to/file') would resolve path/to/file relative to where example-module is located. The suffixed path follows the same module resolution semantics.

Loading from the global folders#

If the NODE_PATH environment variable is set to a colon-delimited list of absolute paths, then Node.js will search those paths for modules if they are not found elsewhere.

On Windows, NODE_PATH is delimited by semicolons (;) instead of colons.

NODE_PATH was originally created to support loading modules from varying paths before the current module resolution algorithm was defined.

NODE_PATH is still supported, but is less necessary now that the Node.js ecosystem has settled on a convention for locating dependent modules. Sometimes deployments that rely on NODE_PATH show surprising behavior when people are unaware that NODE_PATH must be set. Sometimes a module's dependencies change, causing a different version (or even a different module) to be loaded as the NODE_PATH is searched.

Additionally, Node.js will search in the following list of GLOBAL_FOLDERS:

  • 1: $HOME/.node_modules
  • 2: $HOME/.node_libraries
  • 3: $PREFIX/lib/node

Where $HOME is the user's home directory, and $PREFIX is the Node.js configured node_prefix.

These are mostly for historic reasons.

It is strongly encouraged to place dependencies in the local node_modules folder. These will be loaded faster, and more reliably.

The module wrapper#

Before a module's code is executed, Node.js will wrap it with a function wrapper that looks like the following:

(function(exports, require, module, __filename, __dirname) {
// Module code actually lives in here
});

By doing this, Node.js achieves a few things:

  • It keeps top-level variables (defined with var, const or let) scoped to the module rather than the global object.

  • It helps to provide some global-looking variables that are actually specific to the module, such as:

    • The module and exports objects that the implementor can use to export values from the module.
    • The convenience variables __filename and __dirname, containing the module's absolute filename and directory path.

The module scope#

__dirname#

Added in: v0.1.27

The directory name of the current module. This is the same as the path.dirname() of the __filename.

Example: running node example.js from /Users/mjr

console.log(__dirname);
// Prints: /Users/mjr
console.log(path.dirname(__filename));
// Prints: /Users/mjr

__filename#

Added in: v0.0.1

The file name of the current module. This is the current module file's absolute path with symlinks resolved.

For a main program this is not necessarily the same as the file name used in the command line.

See __dirname for the directory name of the current module.

Examples:

Running node example.js from /Users/mjr

console.log(__filename);
// Prints: /Users/mjr/example.js
console.log(__dirname);
// Prints: /Users/mjr

Given two modules: a and b, where b is a dependency of a and there is a directory structure of:

  • /Users/mjr/app/a.js
  • /Users/mjr/app/node_modules/b/b.js

References to __filename within b.js will return /Users/mjr/app/node_modules/b/b.js while references to __filename within a.js will return /Users/mjr/app/a.js.

exports#

Added in: v0.1.12

A reference to the module.exports that is shorter to type. See the section about the exports shortcut for details on when to use exports and when to use module.exports.

module#

Added in: v0.1.16

A reference to the current module, see the section about the module object. In particular, module.exports is used for defining what a module exports and makes available through require().

require(id)#

Added in: v0.1.13
  • id <string> module name or path
  • Returns: <any> exported module content

Used to import modules, JSON, and local files. Modules can be imported from node_modules. Local modules and JSON files can be imported using a relative path (e.g. ./, ./foo, ./bar/baz, ../foo) that will be resolved against the directory named by __dirname (if defined) or the current working directory. The relative paths of POSIX style are resolved in an OS independent fashion, meaning that the examples above will work on Windows in the same way they would on Unix systems.

// Importing a local module with a path relative to the `__dirname` or current
// working directory. (On Windows, this would resolve to .\path\myLocalModule.)
const myLocalModule = require('./path/myLocalModule');

// Importing a JSON file:
const jsonData = require('./path/filename.json');

// Importing a module from node_modules or Node.js built-in module:
const crypto = require('crypto');

require.cache#

Added in: v0.3.0

Modules are cached in this object when they are required. By deleting a key value from this object, the next require will reload the module. This does not apply to native addons, for which reloading will result in an error.

Adding or replacing entries is also possible. This cache is checked before native modules and if a name matching a native module is added to the cache, no require call is going to receive the native module anymore. Use with care!

require.extensions#

Added in: v0.3.0Deprecated since: v0.10.6

Stability: 0 - Deprecated

Instruct require on how to handle certain file extensions.

Process files with the extension .sjs as .js:

require.extensions['.sjs'] = require.extensions['.js'];

Deprecated. In the past, this list has been used to load non-JavaScript modules into Node.js by compiling them on-demand. However, in practice, there are much better ways to do this, such as loading modules via some other Node.js program, or compiling them to JavaScript ahead of time.

Avoid using require.extensions. Use could cause subtle bugs and resolving the extensions gets slower with each registered extension.

require.main#

Added in: v0.1.17

The Module object representing the entry script loaded when the Node.js process launched. See "Accessing the main module".

In entry.js script:

console.log(require.main);

node entry.js

Module {
  id: '.',
  exports: {},
  parent: null,
  filename: '/absolute/path/to/entry.js',
  loaded: false,
  children: [],
  paths:
   [ '/absolute/path/to/node_modules',
     '/absolute/path/node_modules',
     '/absolute/node_modules',
     '/node_modules' ] }

require.resolve(request[, options])#

History
VersionChanges
v8.9.0

The paths option is now supported.

v0.3.0

Added in: v0.3.0

  • request <string> The module path to resolve.

  • options <Object>

    • paths <string[]> Paths to resolve module location from. If present, these paths are used instead of the default resolution paths, with the exception of GLOBAL_FOLDERS like $HOME/.node_modules, which are always included. Each of these paths is used as a starting point for the module resolution algorithm, meaning that the node_modules hierarchy is checked from this location.
  • Returns: <string>

Use the internal require() machinery to look up the location of a module, but rather than loading the module, just return the resolved filename.

If the module can not be found, a MODULE_NOT_FOUND error is thrown.

require.resolve.paths(request)#
Added in: v8.9.0

Returns an array containing the paths searched during resolution of request or null if the request string references a core module, for example http or fs.

The module Object#

Added in: v0.1.16

In each module, the module free variable is a reference to the object representing the current module. For convenience, module.exports is also accessible via the exports module-global. module is not actually a global but rather local to each module.

module.children#

Added in: v0.1.16

The module objects required for the first time by this one.

module.exports#

Added in: v0.1.16

The module.exports object is created by the Module system. Sometimes this is not acceptable; many want their module to be an instance of some class. To do this, assign the desired export object to module.exports. Assigning the desired object to exports will simply rebind the local exports variable, which is probably not what is desired.

For example, suppose we were making a module called a.js:

const EventEmitter = require('events');

module.exports = new EventEmitter();

// Do some work, and after some time emit
// the 'ready' event from the module itself.
setTimeout(() => {
  module.exports.emit('ready');
}, 1000);

Then in another file we could do:

const a = require('./a');
a.on('ready', () => {
  console.log('module "a" is ready');
});

Assignment to module.exports must be done immediately. It cannot be done in any callbacks. This does not work:

x.js:

setTimeout(() => {
  module.exports = { a: 'hello' };
}, 0);

y.js:

const x = require('./x');
console.log(x.a);

exports shortcut#

Added in: v0.1.16

The exports variable is available within a module's file-level scope, and is assigned the value of module.exports before the module is evaluated.

It allows a shortcut, so that module.exports.f = ... can be written more succinctly as exports.f = .... However, be aware that like any variable, if a new value is assigned to exports, it is no longer bound to module.exports:

module.exports.hello = true; // Exported from require of module
exports = { hello: false };  // Not exported, only available in the module

When the module.exports property is being completely replaced by a new object, it is common to also reassign exports:

module.exports = exports = function Constructor() {
  // ... etc.
};

To illustrate the behavior, imagine this hypothetical implementation of require(), which is quite similar to what is actually done by require():

function require(/* ... */) {
  const module = { exports: {} };
  ((module, exports) => {
    // Module code here. In this example, define a function.
    function someFunc() {}
    exports = someFunc;
    // At this point, exports is no longer a shortcut to module.exports, and
    // this module will still export an empty default object.
    module.exports = someFunc;
    // At this point, the module will now export someFunc, instead of the
    // default object.
  })(module, module.exports);
  return module.exports;
}

module.filename#

Added in: v0.1.16

The fully resolved filename of the module.

module.id#

Added in: v0.1.16

The identifier for the module. Typically this is the fully resolved filename.

module.loaded#

Added in: v0.1.16

Whether or not the module is done loading, or is in the process of loading.

module.parent#

Added in: v0.1.16

The module that first required this one.

module.paths#

Added in: v0.4.0

The search paths for the module.

module.require(id)#

Added in: v0.5.1

The module.require() method provides a way to load a module as if require() was called from the original module.

In order to do this, it is necessary to get a reference to the module object. Since require() returns the module.exports, and the module is typically only available within a specific module's code, it must be explicitly exported in order to be used.

The Module Object#

Added in: v0.3.7

Provides general utility methods when interacting with instances of Module, the module variable often seen in file modules. Accessed via require('module').

module.builtinModules#

Added in: v9.3.0, v8.10.0, v6.13.0

A list of the names of all modules provided by Node.js. Can be used to verify if a module is maintained by a third party or not.

module in this context isn't the same object that's provided by the module wrapper. To access it, require the Module module:

const builtin = require('module').builtinModules;

module.createRequire(filename)#

Added in: v12.2.0
  • filename <string> | <URL> Filename to be used to construct the require function. Must be a file URL object, file URL string, or absolute path string.
  • Returns: <require> Require function
import { createRequire } from 'module';
const require = createRequire(import.meta.url);

// sibling-module.js is a CommonJS module.
const siblingModule = require('./sibling-module');

module.createRequireFromPath(filename)#

Added in: v10.12.0Deprecated since: v12.2.0

Stability: 0 - Deprecated: Please use createRequire() instead.

  • filename <string> Filename to be used to construct the relative require function.
  • Returns: <require> Require function
const { createRequireFromPath } = require('module');
const requireUtil = createRequireFromPath('../src/utils/');

// Require `../src/utils/some-tool`
requireUtil('./some-tool');

module.syncBuiltinESMExports()#

Added in: v12.12.0

The module.syncBuiltinESMExports() method updates all the live bindings for builtin ES Modules to match the properties of the CommonJS exports. It does not add or remove exported names from the ES Modules.

const fs = require('fs');
const { syncBuiltinESMExports } = require('module');

fs.readFile = null;

delete fs.readFileSync;

fs.newAPI = function newAPI() {
  // ...
};

syncBuiltinESMExports();

import('fs').then((esmFS) => {
  assert.strictEqual(esmFS.readFile, null);
  assert.strictEqual('readFileSync' in fs, true);
  assert.strictEqual(esmFS.newAPI, undefined);
});

Source Map V3 Support#

Added in: v13.7.0

Stability: 1 - Experimental

Helpers for interacting with the source map cache. This cache is populated when source map parsing is enabled and source map include directives are found in a modules' footer.

To enable source map parsing, Node.js must be run with the flag --enable-source-maps, or with code coverage enabled by setting NODE_V8_COVERAGE=dir.

const { findSourceMap, SourceMap } = require('module');

module.findSourceMap(path[, error])#

Added in: v13.7.0

path is the resolved path for the file for which a corresponding source map should be fetched.

The error instance should be passed as the second parameter to findSourceMap in exceptional flows, e.g., when an overridden Error.prepareStackTrace(error, trace) is invoked. Modules are not added to the module cache until they are successfully loaded, in these cases source maps will be associated with the error instance along with the path.

Class: module.SourceMap#

Added in: v13.7.0

new SourceMap(payload)#

Creates a new sourceMap instance.

payload is an object with keys matching the Source Map V3 format:

sourceMap.payload#

Getter for the payload used to construct the SourceMap instance.

sourceMap.findEntry(lineNumber, columnNumber)#

Given a line number and column number in the generated source file, returns an object representing the position in the original file. The object returned consists of the following keys:

Net#

Stability: 2 - Stable

The net module provides an asynchronous network API for creating stream-based TCP or IPC servers (net.createServer()) and clients (net.createConnection()).

It can be accessed using:

const net = require('net');

IPC Support#

The net module supports IPC with named pipes on Windows, and Unix domain sockets on other operating systems.

Identifying paths for IPC connections#

net.connect(), net.createConnection(), server.listen() and socket.connect() take a path parameter to identify IPC endpoints.

On Unix, the local domain is also known as the Unix domain. The path is a filesystem pathname. It gets truncated to an OS-dependent length of sizeof(sockaddr_un.sun_path) - 1. Typical values are 107 bytes on Linux and 103 bytes on macOS. If a Node.js API abstraction creates the Unix domain socket, it will unlink the Unix domain socket as well. For example, net.createServer() may create a Unix domain socket and server.close() will unlink it. But if a user creates the Unix domain socket outside of these abstractions, the user will need to remove it. The same applies when a Node.js API creates a Unix domain socket but the program then crashes. In short, a Unix domain socket will be visible in the filesystem and will persist until unlinked.

On Windows, the local domain is implemented using a named pipe. The path must refer to an entry in \\?\pipe\ or \\.\pipe\. Any characters are permitted, but the latter may do some processing of pipe names, such as resolving .. sequences. Despite how it might look, the pipe namespace is flat. Pipes will not persist. They are removed when the last reference to them is closed. Unlike Unix domain sockets, Windows will close and remove the pipe when the owning process exits.

JavaScript string escaping requires paths to be specified with extra backslash escaping such as:

net.createServer().listen(
  path.join('\\\\?\\pipe', process.cwd(), 'myctl'));

Class: net.Server#

Added in: v0.1.90

This class is used to create a TCP or IPC server.

new net.Server([options][, connectionListener])#

net.Server is an EventEmitter with the following events:

Event: 'close'#

Added in: v0.5.0

Emitted when the server closes. If connections exist, this event is not emitted until all connections are ended.

Event: 'connection'#

Added in: v0.1.90

Emitted when a new connection is made. socket is an instance of net.Socket.

Event: 'error'#

Added in: v0.1.90

Emitted when an error occurs. Unlike net.Socket, the 'close' event will not be emitted directly following this event unless server.close() is manually called. See the example in discussion of server.listen().

Event: 'listening'#

Added in: v0.1.90

Emitted when the server has been bound after calling server.listen().

server.address()#

Added in: v0.1.90

Returns the bound address, the address family name, and port of the server as reported by the operating system if listening on an IP socket (useful to find which port was assigned when getting an OS-assigned address): { port: 12346, family: 'IPv4', address: '127.0.0.1' }.

For a server listening on a pipe or Unix domain socket, the name is returned as a string.

const server = net.createServer((socket) => {
  socket.end('goodbye\n');
}).on('error', (err) => {
  // Handle errors here.
  throw err;
});

// Grab an arbitrary unused port.
server.listen(() => {
  console.log('opened server on', server.address());
});

server.address() returns null before the 'listening' event has been emitted or after calling server.close().

server.close([callback])#

Added in: v0.1.90

Stops the server from accepting new connections and keeps existing connections. This function is asynchronous, the server is finally closed when all connections are ended and the server emits a 'close' event. The optional callback will be called once the 'close' event occurs. Unlike that event, it will be called with an Error as its only argument if the server was not open when it was closed.

server.connections#

Added in: v0.2.0Deprecated since: v0.9.7

Stability: 0 - Deprecated: Use server.getConnections() instead.

The number of concurrent connections on the server.

This becomes null when sending a socket to a child with child_process.fork(). To poll forks and get current number of active connections, use asynchronous server.getConnections() instead.

server.getConnections(callback)#

Added in: v0.9.7

Asynchronously get the number of concurrent connections on the server. Works when sockets were sent to forks.

Callback should take two arguments err and count.

server.listen()#

Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.

Possible signatures:

This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callback will be added as a listener for the 'listening' event.

All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).

All net.Socket are set to SO_REUSEADDR (see socket(7) for details).

The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.

One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requested port/path/handle. One way to handle this would be to retry after a certain amount of time:

server.on('error', (e) => {
  if (e.code === 'EADDRINUSE') {
    console.log('Address in use, retrying...');
    setTimeout(() => {
      server.close();
      server.listen(PORT, HOST);
    }, 1000);
  }
});

server.listen(handle[, backlog][, callback])#

Added in: v0.5.10

Start a server listening for connections on a given handle that has already been bound to a port, a Unix domain socket, or a Windows named pipe.

The handle object can be either a server, a socket (anything with an underlying _handle member), or an object with an fd member that is a valid file descriptor.

Listening on a file descriptor is not supported on Windows.

server.listen(options[, callback])#

History
VersionChanges
v11.4.0

The ipv6Only option is supported.

v0.11.14

Added in: v0.11.14

If port is specified, it behaves the same as server.listen([port[, host[, backlog]]][, callback]). Otherwise, if path is specified, it behaves the same as server.listen(path[, backlog][, callback]). If none of them is specified, an error will be thrown.

If exclusive is false (default), then cluster workers will use the same underlying handle, allowing connection handling duties to be shared. When exclusive is true, the handle is not shared, and attempted port sharing results in an error. An example which listens on an exclusive port is shown below.

server.listen({
  host: 'localhost',
  port: 80,
  exclusive: true
});

Starting an IPC server as root may cause the server path to be inaccessible for unprivileged users. Using readableAll and writableAll will make the server accessible for all users.

server.listen(path[, backlog][, callback])#

Added in: v0.1.90

Start an IPC server listening for connections on the given path.

server.listen([port[, host[, backlog]]][, callback])#

Added in: v0.1.90

Start a TCP server listening for connections on the given port and host.

If port is omitted or is 0, the operating system will assign an arbitrary unused port, which can be retrieved by using server.address().port after the 'listening' event has been emitted.

If host is omitted, the server will accept connections on the unspecified IPv6 address (::) when IPv6 is available, or the unspecified IPv4 address (0.0.0.0) otherwise.

In most operating systems, listening to the unspecified IPv6 address (::) may cause the net.Server to also listen on the unspecified IPv4 address (0.0.0.0).

server.listening#

Added in: v5.7.0
  • <boolean> Indicates whether or not the server is listening for connections.

server.maxConnections#

Added in: v0.2.0

Set this property to reject connections when the server's connection count gets high.

It is not recommended to use this option once a socket has been sent to a child with child_process.fork().

server.ref()#

Added in: v0.9.1

Opposite of unref(), calling ref() on a previously unrefed server will not let the program exit if it's the only server left (the default behavior). If the server is refed calling ref() again will have no effect.

server.unref()#

Added in: v0.9.1

Calling unref() on a server will allow the program to exit if this is the only active server in the event system. If the server is already unrefed calling unref() again will have no effect.

Class: net.Socket#

Added in: v0.3.4

This class is an abstraction of a TCP socket or a streaming IPC endpoint (uses named pipes on Windows, and Unix domain sockets otherwise). It is also an EventEmitter.

A net.Socket can be created by the user and used directly to interact with a server. For example, it is returned by net.createConnection(), so the user can use it to talk to the server.

It can also be created by Node.js and passed to the user when a connection is received. For example, it is passed to the listeners of a 'connection' event emitted on a net.Server, so the user can use it to interact with the client.

new net.Socket([options])#

Added in: v0.3.4

  • options <Object> Available options are:

    • fd <number> If specified, wrap around an existing socket with the given file descriptor, otherwise a new socket will be created.
    • allowHalfOpen <boolean> Indicates whether half-opened TCP connections are allowed. See net.createServer() and the 'end' event for details. Default: false.
    • readable <boolean> Allow reads on the socket when an fd is passed, otherwise ignored. Default: false.
    • writable <boolean> Allow writes on the socket when an fd is passed, otherwise ignored. Default: false.
  • Returns: <net.Socket>

Creates a new socket object.

The newly created socket can be either a TCP socket or a streaming IPC endpoint, depending on what it connect() to.

Event: 'close'#

Added in: v0.1.90
  • hadError <boolean> true if the socket had a transmission error.

Emitted once the socket is fully closed. The argument hadError is a boolean which says if the socket was closed due to a transmission error.

Event: 'connect'#

Added in: v0.1.90

Emitted when a socket connection is successfully established. See net.createConnection().

Event: 'data'#

Added in: v0.1.90

Emitted when data is received. The argument data will be a Buffer or String. Encoding of data is set by socket.setEncoding().

The data will be lost if there is no listener when a Socket emits a 'data' event.

Event: 'drain'#

Added in: v0.1.90

Emitted when the write buffer becomes empty. Can be used to throttle uploads.

See also: the return values of socket.write().

Event: 'end'#

Added in: v0.1.90

Emitted when the other end of the socket sends a FIN packet, thus ending the readable side of the socket.

By default (allowHalfOpen is false) the socket will send a FIN packet back and destroy its file descriptor once it has written out its pending write queue. However, if allowHalfOpen is set to true, the socket will not automatically end() its writable side, allowing the user to write arbitrary amounts of data. The user must call end() explicitly to close the connection (i.e. sending a FIN packet back).

Event: 'error'#

Added in: v0.1.90

Emitted when an error occurs. The 'close' event will be called directly following this event.

Event: 'lookup'#

History
VersionChanges
v5.10.0

The host parameter is supported now.

v0.11.3

Added in: v0.11.3

Emitted after resolving the host name but before connecting. Not applicable to Unix sockets.

Event: 'ready'#

Added in: v9.11.0

Emitted when a socket is ready to be used.

Triggered immediately after 'connect'.

Event: 'timeout'#

Added in: v0.1.90

Emitted if the socket times out from inactivity. This is only to notify that the socket has been idle. The user must manually close the connection.

See also: socket.setTimeout().

socket.address()#

Added in: v0.1.90

Returns the bound address, the address family name and port of the socket as reported by the operating system: { port: 12346, family: 'IPv4', address: '127.0.0.1' }

socket.bufferSize#

Added in: v0.3.8

This property shows the number of characters buffered for writing. The buffer may contain strings whose length after encoding is not yet known. So this number is only an approximation of the number of bytes in the buffer.

net.Socket has the property that socket.write() always works. This is to help users get up and running quickly. The computer cannot always keep up with the amount of data that is written to a socket. The network connection simply might be too slow. Node.js will internally queue up the data written to a socket and send it out over the wire when it is possible.

The consequence of this internal buffering is that memory may grow. Users who experience large or growing bufferSize should attempt to "throttle" the data flows in their program with socket.pause() and socket.resume().

socket.bytesRead#

Added in: v0.5.3

The amount of received bytes.

socket.bytesWritten#

Added in: v0.5.3

The amount of bytes sent.

socket.connect()#

Initiate a connection on a given socket.

Possible signatures:

This function is asynchronous. When the connection is established, the 'connect' event will be emitted. If there is a problem connecting, instead of a 'connect' event, an 'error' event will be emitted with the error passed to the 'error' listener. The last parameter connectListener, if supplied, will be added as a listener for the 'connect' event once.

This function should only be used for reconnecting a socket after 'close' has been emitted or otherwise it may lead to undefined behavior.

socket.connect(options[, connectListener])#

History
VersionChanges
v12.10.0

Added onread option.

v6.0.0

The hints option defaults to 0 in all cases now. Previously, in the absence of the family option it would default to dns.ADDRCONFIG | dns.V4MAPPED.

v5.11.0

The hints option is supported now.

v0.1.90

Added in: v0.1.90

Initiate a connection on a given socket. Normally this method is not needed, the socket should be created and opened with net.createConnection(). Use this only when implementing a custom Socket.

For TCP connections, available options are:

  • port <number> Required. Port the socket should connect to.
  • host <string> Host the socket should connect to. Default: 'localhost'.
  • localAddress <string> Local address the socket should connect from.
  • localPort <number> Local port the socket should connect from.
  • family <number>: Version of IP stack. Must be 4, 6, or 0. The value 0 indicates that both IPv4 and IPv6 addresses are allowed. Default: 0.
  • hints <number> Optional dns.lookup() hints.
  • lookup <Function> Custom lookup function. Default: dns.lookup().

For IPC connections, available options are:

For both types, available options include:

  • onread <Object> If specified, incoming data is stored in a single buffer and passed to the supplied callback when data arrives on the socket. This will cause the streaming functionality to not provide any data. The socket will emit events like 'error', 'end', and 'close' as usual. Methods like pause() and resume() will also behave as expected.

    • buffer <Buffer> | <Uint8Array> | <Function> Either a reusable chunk of memory to use for storing incoming data or a function that returns such.
    • callback <Function> This function is called for every chunk of incoming data. Two arguments are passed to it: the number of bytes written to buffer and a reference to buffer. Return false from this function to implicitly pause() the socket. This function will be executed in the global context.

Following is an example of a client using the onread option:

const net = require('net');
net.connect({
  port: 80,
  onread: {
    // Reuses a 4KiB Buffer for every read from the socket.
    buffer: Buffer.alloc(4 * 1024),
    callback: function(nread, buf) {
      // Received data is available in `buf` from 0 to `nread`.
      console.log(buf.toString('utf8', 0, nread));
    }
  }
});

socket.connect(path[, connectListener])#

Initiate an IPC connection on the given socket.

Alias to socket.connect(options[, connectListener]) called with { path: path } as options.

socket.connect(port[, host][, connectListener])#

Added in: v0.1.90

Initiate a TCP connection on the given socket.

Alias to socket.connect(options[, connectListener]) called with {port: port, host: host} as options.

socket.connecting#

Added in: v6.1.0

If true, socket.connect(options[, connectListener]) was called and has not yet finished. It will stay true until the socket becomes connected, then it is set to false and the 'connect' event is emitted. Note that the socket.connect(options[, connectListener]) callback is a listener for the 'connect' event.

socket.destroy([error])#

Added in: v0.1.90

Ensures that no more I/O activity happens on this socket. Destroys the stream and closes the connection.

See writable.destroy() for further details.

socket.destroyed#

  • <boolean> Indicates if the connection is destroyed or not. Once a connection is destroyed no further data can be transferred using it.

See writable.destroyed for further details.

socket.end([data[, encoding]][, callback])#

Added in: v0.1.90

Half-closes the socket. i.e., it sends a FIN packet. It is possible the server will still send some data.

See writable.end() for further details.

socket.localAddress#

Added in: v0.9.6

The string representation of the local IP address the remote client is connecting on. For example, in a server listening on '0.0.0.0', if a client connects on '192.168.1.1', the value of socket.localAddress would be '192.168.1.1'.

socket.localPort#

Added in: v0.9.6

The numeric representation of the local port. For example, 80 or 21.

socket.pause()#

Pauses the reading of data. That is, 'data' events will not be emitted. Useful to throttle back an upload.

socket.pending#

Added in: v11.2.0, v10.16.0

This is true if the socket is not connected yet, either because .connect() has not yet been called or because it is still in the process of connecting (see socket.connecting).

socket.ref()#

Added in: v0.9.1

Opposite of unref(), calling ref() on a previously unrefed socket will not let the program exit if it's the only socket left (the default behavior). If the socket is refed calling ref again will have no effect.

socket.remoteAddress#

Added in: v0.5.10

The string representation of the remote IP address. For example, '74.125.127.100' or '2001:4860:a005::68'. Value may be undefined if the socket is destroyed (for example, if the client disconnected).

socket.remoteFamily#

Added in: v0.11.14

The string representation of the remote IP family. 'IPv4' or 'IPv6'.

socket.remotePort#

Added in: v0.5.10

The numeric representation of the remote port. For example, 80 or 21.

socket.resume()#

Resumes reading after a call to socket.pause().

socket.setEncoding([encoding])#

Added in: v0.1.90

Set the encoding for the socket as a Readable Stream. See readable.setEncoding() for more information.

socket.setKeepAlive([enable][, initialDelay])#

Added in: v0.1.92

Enable/disable keep-alive functionality, and optionally set the initial delay before the first keepalive probe is sent on an idle socket.

Set initialDelay (in milliseconds) to set the delay between the last data packet received and the first keepalive probe. Setting 0 for initialDelay will leave the value unchanged from the default (or previous) setting.

socket.setNoDelay([noDelay])#

Added in: v0.1.90

Enable/disable the use of Nagle's algorithm.

When a TCP connection is created, it will have Nagle's algorithm enabled.

Nagle's algorithm delays data before it is sent via the network. It attempts to optimize throughput at the expense of latency.

Passing true for noDelay or not passing an argument will disable Nagle's algorithm for the socket. Passing false for noDelay will enable Nagle's algorithm.

socket.setTimeout(timeout[, callback])#

Added in: v0.1.90

Sets the socket to timeout after timeout milliseconds of inactivity on the socket. By default net.Socket do not have a timeout.

When an idle timeout is triggered the socket will receive a 'timeout' event but the connection will not be severed. The user must manually call socket.end() or socket.destroy() to end the connection.

socket.setTimeout(3000);
socket.on('timeout', () => {
  console.log('socket timeout');
  socket.end();
});

If timeout is 0, then the existing idle timeout is disabled.

The optional callback parameter will be added as a one-time listener for the 'timeout' event.

socket.unref()#

Added in: v0.9.1

Calling unref() on a socket will allow the program to exit if this is the only active socket in the event system. If the socket is already unrefed calling unref() again will have no effect.

socket.write(data[, encoding][, callback])#

Added in: v0.1.90

Sends data on the socket. The second parameter specifies the encoding in the case of a string. It defaults to UTF8 encoding.

Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain' will be emitted when the buffer is again free.

The optional callback parameter will be executed when the data is finally written out, which may not be immediately.

See Writable stream write() method for more information.

net.connect()#

Aliases to net.createConnection().

Possible signatures:

net.connect(options[, connectListener])#

Added in: v0.7.0

Alias to net.createConnection(options[, connectListener]).

net.connect(path[, connectListener])#

Added in: v0.1.90

Alias to net.createConnection(path[, connectListener]).

net.connect(port[, host][, connectListener])#

Added in: v0.1.90

Alias to net.createConnection(port[, host][, connectListener]).

net.createConnection()#

A factory function, which creates a new net.Socket, immediately initiates connection with socket.connect(), then returns the net.Socket that starts the connection.

When the connection is established, a 'connect' event will be emitted on the returned socket. The last parameter connectListener, if supplied, will be added as a listener for the 'connect' event once.

Possible signatures:

The net.connect() function is an alias to this function.

net.createConnection(options[, connectListener])#

Added in: v0.1.90

For available options, see new net.Socket([options]) and socket.connect(options[, connectListener]).

Additional options:

Following is an example of a client of the echo server described in the net.createServer() section:

const net = require('net');
const client = net.createConnection({ port: 8124 }, () => {
  // 'connect' listener.
  console.log('connected to server!');
  client.write('world!\r\n');
});
client.on('data', (data) => {
  console.log(data.toString());
  client.end();
});
client.on('end', () => {
  console.log('disconnected from server');
});

To connect on the socket /tmp/echo.sock:

const client = net.createConnection({ path: '/tmp/echo.sock' });

net.createConnection(path[, connectListener])#

Added in: v0.1.90

Initiates an IPC connection.

This function creates a new net.Socket with all options set to default, immediately initiates connection with socket.connect(path[, connectListener]), then returns the net.Socket that starts the connection.

net.createConnection(port[, host][, connectListener])#

Added in: v0.1.90

Initiates a TCP connection.

This function creates a new net.Socket with all options set to default, immediately initiates connection with socket.connect(port[, host][, connectListener]), then returns the net.Socket that starts the connection.

net.createServer([options][, connectionListener])#

Added in: v0.5.0

  • options <Object>

    • allowHalfOpen <boolean> Indicates whether half-opened TCP connections are allowed. Default: false.
    • pauseOnConnect <boolean> Indicates whether the socket should be paused on incoming connections. Default: false.
  • connectionListener <Function> Automatically set as a listener for the 'connection' event.
  • Returns: <net.Server>

Creates a new TCP or IPC server.

If allowHalfOpen is set to true, when the other end of the socket sends a FIN packet, the server will only send a FIN packet back when socket.end() is explicitly called, until then the connection is half-closed (non-readable but still writable). See 'end' event and RFC 1122 (section 4.2.2.13) for more information.

If pauseOnConnect is set to true, then the socket associated with each incoming connection will be paused, and no data will be read from its handle. This allows connections to be passed between processes without any data being read by the original process. To begin reading data from a paused socket, call socket.resume().

The server can be a TCP server or an IPC server, depending on what it listen() to.

Here is an example of an TCP echo server which listens for connections on port 8124:

const net = require('net');
const server = net.createServer((c) => {
  // 'connection' listener.
  console.log('client connected');
  c.on('end', () => {
    console.log('client disconnected');
  });
  c.write('hello\r\n');
  c.pipe(c);
});
server.on('error', (err) => {
  throw err;
});
server.listen(8124, () => {
  console.log('server bound');
});

Test this by using telnet:

$ telnet localhost 8124

To listen on the socket /tmp/echo.sock:

server.listen('/tmp/echo.sock', () => {
  console.log('server bound');
});

Use nc to connect to a Unix domain socket server:

$ nc -U /tmp/echo.sock

net.isIP(input)#

Added in: v0.3.0

Tests if input is an IP address. Returns 0 for invalid strings, returns 4 for IP version 4 addresses, and returns 6 for IP version 6 addresses.

net.isIPv4(input)#

Added in: v0.3.0

Returns true if input is a version 4 IP address, otherwise returns false.

net.isIPv6(input)#

Added in: v0.3.0

Returns true if input is a version 6 IP address, otherwise returns false.

OS#

Stability: 2 - Stable

The os module provides operating system-related utility methods and properties. It can be accessed using:

const os = require('os');

os.EOL#

Added in: v0.7.8

The operating system-specific end-of-line marker.

  • \n on POSIX
  • \r\n on Windows

os.arch()#

Added in: v0.5.0

Returns the operating system CPU architecture for which the Node.js binary was compiled. Possible values are 'arm', 'arm64', 'ia32', 'mips', 'mipsel', 'ppc', 'ppc64', 's390', 's390x', 'x32', and 'x64'.

The return value is equivalent to process.arch.

os.constants#

Added in: v6.3.0

Contains commonly used operating system-specific constants for error codes, process signals, and so on. The specific constants defined are described in OS Constants.

os.cpus()#

Added in: v0.3.3

Returns an array of objects containing information about each logical CPU core.

The properties included on each object include:

  • model <string>
  • speed <number> (in MHz)

  • times <Object>

    • user <number> The number of milliseconds the CPU has spent in user mode.
    • nice <number> The number of milliseconds the CPU has spent in nice mode.
    • sys <number> The number of milliseconds the CPU has spent in sys mode.
    • idle <number> The number of milliseconds the CPU has spent in idle mode.
    • irq <number> The number of milliseconds the CPU has spent in irq mode.
[
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 252020,
      nice: 0,
      sys: 30340,
      idle: 1070356870,
      irq: 0
    }
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 306960,
      nice: 0,
      sys: 26980,
      idle: 1071569080,
      irq: 0
    }
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 248450,
      nice: 0,
      sys: 21750,
      idle: 1070919370,
      irq: 0
    }
  },
  {
    model: 'Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz',
    speed: 2926,
    times: {
      user: 256880,
      nice: 0,
      sys: 19430,
      idle: 1070905480,
      irq: 20
    }
  }
]

nice values are POSIX-only. On Windows, the nice values of all processors are always 0.

os.endianness()#

Added in: v0.9.4

Returns a string identifying the endianness of the CPU for which the Node.js binary was compiled.

Possible values are 'BE' for big endian and 'LE' for little endian.

os.freemem()#

Added in: v0.3.3

Returns the amount of free system memory in bytes as an integer.

os.getPriority([pid])#

Added in: v10.10.0
  • pid <integer> The process ID to retrieve scheduling priority for. Default 0.
  • Returns: <integer>

Returns the scheduling priority for the process specified by pid. If pid is not provided or is 0, the priority of the current process is returned.

os.homedir()#

Added in: v2.3.0

Returns the string path of the current user's home directory.

On POSIX, it uses the $HOME environment variable if defined. Otherwise it uses the effective UID to look up the user's home directory.

On Windows, it uses the USERPROFILE environment variable if defined. Otherwise it uses the path to the profile directory of the current user.

os.hostname()#

Added in: v0.3.3

Returns the host name of the operating system as a string.

os.loadavg()#

Added in: v0.3.3

Returns an array containing the 1, 5, and 15 minute load averages.

The load average is a measure of system activity calculated by the operating system and expressed as a fractional number.

The load average is a Unix-specific concept. On Windows, the return value is always [0, 0, 0].

os.networkInterfaces()#

Added in: v0.6.0

Returns an object containing network interfaces that have been assigned a network address.

Each key on the returned object identifies a network interface. The associated value is an array of objects that each describe an assigned network address.

The properties available on the assigned network address object include:

  • address <string> The assigned IPv4 or IPv6 address
  • netmask <string> The IPv4 or IPv6 network mask
  • family <string> Either IPv4 or IPv6
  • mac <string> The MAC address of the network interface
  • internal <boolean> true if the network interface is a loopback or similar interface that is not remotely accessible; otherwise false
  • scopeid <number> The numeric IPv6 scope ID (only specified when family is IPv6)
  • cidr <string> The assigned IPv4 or IPv6 address with the routing prefix in CIDR notation. If the netmask is invalid, this property is set to null.
{
  lo: [
    {
      address: '127.0.0.1',
      netmask: '255.0.0.0',
      family: 'IPv4',
      mac: '00:00:00:00:00:00',
      internal: true,
      cidr: '127.0.0.1/8'
    },
    {
      address: '::1',
      netmask: 'ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff',
      family: 'IPv6',
      mac: '00:00:00:00:00:00',
      scopeid: 0,
      internal: true,
      cidr: '::1/128'
    }
  ],
  eth0: [
    {
      address: '192.168.1.108',
      netmask: '255.255.255.0',
      family: 'IPv4',
      mac: '01:02:03:0a:0b:0c',
      internal: false,
      cidr: '192.168.1.108/24'
    },
    {
      address: 'fe80::a00:27ff:fe4e:66a1',
      netmask: 'ffff:ffff:ffff:ffff::',
      family: 'IPv6',
      mac: '01:02:03:0a:0b:0c',
      scopeid: 1,
      internal: false,
      cidr: 'fe80::a00:27ff:fe4e:66a1/64'
    }
  ]
}

os.platform()#

Added in: v0.5.0

Returns a string identifying the operating system platform. The value is set at compile time. Possible values are 'aix', 'darwin', 'freebsd', 'linux', 'openbsd', 'sunos', and 'win32'.

The return value is equivalent to process.platform.

The value 'android' may also be returned if Node.js is built on the Android operating system. Android support is experimental.

os.release()#

Added in: v0.3.3

Returns the operating system as a string.

On POSIX systems, the operating system release is determined by calling uname(3). On Windows, GetVersionExW() is used. See https://en.wikipedia.org/wiki/Uname#Examples for more information.

os.setPriority([pid, ]priority)#

Added in: v10.10.0
  • pid <integer> The process ID to set scheduling priority for. Default 0.
  • priority <integer> The scheduling priority to assign to the process.

Attempts to set the scheduling priority for the process specified by pid. If pid is not provided or is 0, the process ID of the current process is used.

The priority input must be an integer between -20 (high priority) and 19 (low priority). Due to differences between Unix priority levels and Windows priority classes, priority is mapped to one of six priority constants in os.constants.priority. When retrieving a process priority level, this range mapping may cause the return value to be slightly different on Windows. To avoid confusion, set priority to one of the priority constants.

On Windows, setting priority to PRIORITY_HIGHEST requires elevated user privileges. Otherwise the set priority will be silently reduced to PRIORITY_HIGH.

os.tmpdir()#

History
VersionChanges
v2.0.0

This function is now cross-platform consistent and no longer returns a path with a trailing slash on any platform

v0.9.9

Added in: v0.9.9

Returns the operating system's default directory for temporary files as a string.

os.totalmem()#

Added in: v0.3.3

Returns the total amount of system memory in bytes as an integer.

os.type()#

Added in: v0.3.3

Returns the operating system name as returned by uname(3). For example, it returns 'Linux' on Linux, 'Darwin' on macOS, and 'Windows_NT' on Windows.

See https://en.wikipedia.org/wiki/Uname#Examples for additional information about the output of running uname(3) on various operating systems.

os.uptime()#

History
VersionChanges
v10.0.0

The result of this function no longer contains a fraction component on Windows.

v0.3.3

Added in: v0.3.3

Returns the system uptime in number of seconds.

os.userInfo([options])#

Added in: v6.0.0

  • options <Object>

    • encoding <string> Character encoding used to interpret resulting strings. If encoding is set to 'buffer', the username, shell, and homedir values will be Buffer instances. Default: 'utf8'.
  • Returns: <Object>

Returns information about the currently effective user. On POSIX platforms, this is typically a subset of the password file. The returned object includes the username, uid, gid, shell, and homedir. On Windows, the uid and gid fields are -1, and shell is null.

The value of homedir returned by os.userInfo() is provided by the operating system. This differs from the result of os.homedir(), which queries environment variables for the home directory before falling back to the operating system response.

Throws a SystemError if a user has no username or homedir.

os.version()#

Added in: v13.11.0

Returns a string identifying the kernel version.

On POSIX systems, the operating system release is determined by calling uname(3). On Windows, RtlGetVersion() is used, and if it is not available, GetVersionExW() will be used. See https://en.wikipedia.org/wiki/Uname#Examples for more information.

OS Constants#

The following constants are exported by os.constants.

Not all constants will be available on every operating system.

Signal Constants#

History
VersionChanges
v5.11.0

Added support for SIGINFO.

The following signal constants are exported by os.constants.signals.

Constant Description
SIGHUP Sent to indicate when a controlling terminal is closed or a parent process exits.
SIGINT Sent to indicate when a user wishes to interrupt a process ((Ctrl+C)).
SIGQUIT Sent to indicate when a user wishes to terminate a process and perform a core dump.
SIGILL Sent to a process to notify that it has attempted to perform an illegal, malformed, unknown, or privileged instruction.
SIGTRAP Sent to a process when an exception has occurred.
SIGABRT Sent to a process to request that it abort.
SIGIOT Synonym for SIGABRT
SIGBUS Sent to a process to notify that it has caused a bus error.
SIGFPE Sent to a process to notify that it has performed an illegal arithmetic operation.
SIGKILL Sent to a process to terminate it immediately.
SIGUSR1 SIGUSR2 Sent to a process to identify user-defined conditions.
SIGSEGV Sent to a process to notify of a segmentation fault.
SIGPIPE Sent to a process when it has attempted to write to a disconnected pipe.
SIGALRM Sent to a process when a system timer elapses.
SIGTERM Sent to a process to request termination.
SIGCHLD Sent to a process when a child process terminates.
SIGSTKFLT Sent to a process to indicate a stack fault on a coprocessor.
SIGCONT Sent to instruct the operating system to continue a paused process.
SIGSTOP Sent to instruct the operating system to halt a process.
SIGTSTP Sent to a process to request it to stop.
SIGBREAK Sent to indicate when a user wishes to interrupt a process.
SIGTTIN Sent to a process when it reads from the TTY while in the background.
SIGTTOU Sent to a process when it writes to the TTY while in the background.
SIGURG Sent to a process when a socket has urgent data to read.
SIGXCPU Sent to a process when it has exceeded its limit on CPU usage.
SIGXFSZ Sent to a process when it grows a file larger than the maximum allowed.
SIGVTALRM Sent to a process when a virtual timer has elapsed.
SIGPROF Sent to a process when a system timer has elapsed.
SIGWINCH Sent to a process when the controlling terminal has changed its size.
SIGIO Sent to a process when I/O is available.
SIGPOLL Synonym for SIGIO
SIGLOST Sent to a process when a file lock has been lost.
SIGPWR Sent to a process to notify of a power failure.
SIGINFO Synonym for SIGPWR
SIGSYS Sent to a process to notify of a bad argument.
SIGUNUSED Synonym for SIGSYS

Error Constants#

The following error constants are exported by os.constants.errno.

POSIX Error Constants#

Constant Description
E2BIG Indicates that the list of arguments is longer than expected.
EACCES Indicates that the operation did not have sufficient permissions.
EADDRINUSE Indicates that the network address is already in use.
EADDRNOTAVAIL Indicates that the network address is currently unavailable for use.
EAFNOSUPPORT Indicates that the network address family is not supported.
EAGAIN Indicates that there is no data available and to try the operation again later.
EALREADY Indicates that the socket already has a pending connection in progress.
EBADF Indicates that a file descriptor is not valid.
EBADMSG Indicates an invalid data message.
EBUSY Indicates that a device or resource is busy.
ECANCELED Indicates that an operation was canceled.
ECHILD Indicates that there are no child processes.
ECONNABORTED Indicates that the network connection has been aborted.
ECONNREFUSED Indicates that the network connection has been refused.
ECONNRESET Indicates that the network connection has been reset.
EDEADLK Indicates that a resource deadlock has been avoided.
EDESTADDRREQ Indicates that a destination address is required.
EDOM Indicates that an argument is out of the domain of the function.
EDQUOT Indicates that the disk quota has been exceeded.
EEXIST Indicates that the file already exists.
EFAULT Indicates an invalid pointer address.
EFBIG Indicates that the file is too large.
EHOSTUNREACH Indicates that the host is unreachable.
EIDRM Indicates that the identifier has been removed.
EILSEQ Indicates an illegal byte sequence.
EINPROGRESS Indicates that an operation is already in progress.
EINTR Indicates that a function call was interrupted.
EINVAL Indicates that an invalid argument was provided.
EIO Indicates an otherwise unspecified I/O error.
EISCONN Indicates that the socket is connected.
EISDIR Indicates that the path is a directory.
ELOOP Indicates too many levels of symbolic links in a path.
EMFILE Indicates that there are too many open files.
EMLINK Indicates that there are too many hard links to a file.
EMSGSIZE Indicates that the provided message is too long.
EMULTIHOP Indicates that a multihop was attempted.
ENAMETOOLONG Indicates that the filename is too long.
ENETDOWN Indicates that the network is down.
ENETRESET Indicates that the connection has been aborted by the network.
ENETUNREACH Indicates that the network is unreachable.
ENFILE Indicates too many open files in the system.
ENOBUFS Indicates that no buffer space is available.
ENODATA Indicates that no message is available on the stream head read queue.
ENODEV Indicates that there is no such device.
ENOENT Indicates that there is no such file or directory.
ENOEXEC Indicates an exec format error.
ENOLCK Indicates that there are no locks available.
ENOLINK Indications that a link has been severed.
ENOMEM Indicates that there is not enough space.
ENOMSG Indicates that there is no message of the desired type.
ENOPROTOOPT Indicates that a given protocol is not available.
ENOSPC Indicates that there is no space available on the device.
ENOSR Indicates that there are no stream resources available.
ENOSTR Indicates that a given resource is not a stream.
ENOSYS Indicates that a function has not been implemented.
ENOTCONN Indicates that the socket is not connected.
ENOTDIR Indicates that the path is not a directory.
ENOTEMPTY Indicates that the directory is not empty.
ENOTSOCK Indicates that the given item is not a socket.
ENOTSUP Indicates that a given operation is not supported.
ENOTTY Indicates an inappropriate I/O control operation.
ENXIO Indicates no such device or address.
EOPNOTSUPP Indicates that an operation is not supported on the socket. Although ENOTSUP and EOPNOTSUPP have the same value on Linux, according to POSIX.1 these error values should be distinct.)
EOVERFLOW Indicates that a value is too large to be stored in a given data type.
EPERM Indicates that the operation is not permitted.
EPIPE Indicates a broken pipe.
EPROTO Indicates a protocol error.
EPROTONOSUPPORT Indicates that a protocol is not supported.
EPROTOTYPE Indicates the wrong type of protocol for a socket.
ERANGE Indicates that the results are too large.
EROFS Indicates that the file system is read only.
ESPIPE Indicates an invalid seek operation.
ESRCH Indicates that there is no such process.
ESTALE Indicates that the file handle is stale.
ETIME Indicates an expired timer.
ETIMEDOUT Indicates that the connection timed out.
ETXTBSY Indicates that a text file is busy.
EWOULDBLOCK Indicates that the operation would block.
EXDEV Indicates an improper link.

Windows Specific Error Constants#

The following error codes are specific to the Windows operating system.

Constant Description
WSAEINTR Indicates an interrupted function call.
WSAEBADF Indicates an invalid file handle.
WSAEACCES Indicates insufficient permissions to complete the operation.
WSAEFAULT Indicates an invalid pointer address.
WSAEINVAL Indicates that an invalid argument was passed.
WSAEMFILE Indicates that there are too many open files.
WSAEWOULDBLOCK Indicates that a resource is temporarily unavailable.
WSAEINPROGRESS Indicates that an operation is currently in progress.
WSAEALREADY Indicates that an operation is already in progress.
WSAENOTSOCK Indicates that the resource is not a socket.
WSAEDESTADDRREQ Indicates that a destination address is required.
WSAEMSGSIZE Indicates that the message size is too long.
WSAEPROTOTYPE Indicates the wrong protocol type for the socket.
WSAENOPROTOOPT Indicates a bad protocol option.
WSAEPROTONOSUPPORT Indicates that the protocol is not supported.
WSAESOCKTNOSUPPORT Indicates that the socket type is not supported.
WSAEOPNOTSUPP Indicates that the operation is not supported.
WSAEPFNOSUPPORT Indicates that the protocol family is not supported.
WSAEAFNOSUPPORT Indicates that the address family is not supported.
WSAEADDRINUSE Indicates that the network address is already in use.
WSAEADDRNOTAVAIL Indicates that the network address is not available.
WSAENETDOWN Indicates that the network is down.
WSAENETUNREACH Indicates that the network is unreachable.
WSAENETRESET Indicates that the network connection has been reset.
WSAECONNABORTED Indicates that the connection has been aborted.
WSAECONNRESET Indicates that the connection has been reset by the peer.
WSAENOBUFS Indicates that there is no buffer space available.
WSAEISCONN Indicates that the socket is already connected.
WSAENOTCONN Indicates that the socket is not connected.
WSAESHUTDOWN Indicates that data cannot be sent after the socket has been shutdown.
WSAETOOMANYREFS Indicates that there are too many references.
WSAETIMEDOUT Indicates that the connection has timed out.
WSAECONNREFUSED Indicates that the connection has been refused.
WSAELOOP Indicates that a name cannot be translated.
WSAENAMETOOLONG Indicates that a name was too long.
WSAEHOSTDOWN Indicates that a network host is down.
WSAEHOSTUNREACH Indicates that there is no route to a network host.
WSAENOTEMPTY Indicates that the directory is not empty.
WSAEPROCLIM Indicates that there are too many processes.
WSAEUSERS Indicates that the user quota has been exceeded.
WSAEDQUOT Indicates that the disk quota has been exceeded.
WSAESTALE Indicates a stale file handle reference.
WSAEREMOTE Indicates that the item is remote.
WSASYSNOTREADY Indicates that the network subsystem is not ready.
WSAVERNOTSUPPORTED Indicates that the winsock.dll version is out of range.
WSANOTINITIALISED Indicates that successful WSAStartup has not yet been performed.
WSAEDISCON Indicates that a graceful shutdown is in progress.
WSAENOMORE Indicates that there are no more results.
WSAECANCELLED Indicates that an operation has been canceled.
WSAEINVALIDPROCTABLE Indicates that the procedure call table is invalid.
WSAEINVALIDPROVIDER Indicates an invalid service provider.
WSAEPROVIDERFAILEDINIT Indicates that the service provider failed to initialized.
WSASYSCALLFAILURE Indicates a system call failure.
WSASERVICE_NOT_FOUND Indicates that a service was not found.
WSATYPE_NOT_FOUND Indicates that a class type was not found.
WSA_E_NO_MORE Indicates that there are no more results.
WSA_E_CANCELLED Indicates that the call was canceled.
WSAEREFUSED Indicates that a database query was refused.

dlopen Constants#

If available on the operating system, the following constants are exported in os.constants.dlopen. See dlopen(3) for detailed information.

Constant Description
RTLD_LAZY Perform lazy binding. Node.js sets this flag by default.
RTLD_NOW Resolve all undefined symbols in the library before dlopen(3) returns.
RTLD_GLOBAL Symbols defined by the library will be made available for symbol resolution of subsequently loaded libraries.
RTLD_LOCAL The converse of RTLD_GLOBAL. This is the default behavior if neither flag is specified.
RTLD_DEEPBIND Make a self-contained library use its own symbols in preference to symbols from previously loaded libraries.

Priority Constants#

Added in: v10.10.0

The following process scheduling constants are exported by os.constants.priority.

Constant Description
PRIORITY_LOW The lowest process scheduling priority. This corresponds to IDLE_PRIORITY_CLASS on Windows, and a nice value of 19 on all other platforms.
PRIORITY_BELOW_NORMAL The process scheduling priority above PRIORITY_LOW and below PRIORITY_NORMAL. This corresponds to BELOW_NORMAL_PRIORITY_CLASS on Windows, and a nice value of 10 on all other platforms.
PRIORITY_NORMAL The default process scheduling priority. This corresponds to NORMAL_PRIORITY_CLASS on Windows, and a nice value of 0 on all other platforms.
PRIORITY_ABOVE_NORMAL The process scheduling priority above PRIORITY_NORMAL and below PRIORITY_HIGH. This corresponds to ABOVE_NORMAL_PRIORITY_CLASS on Windows, and a nice value of -7 on all other platforms.
PRIORITY_HIGH The process scheduling priority above PRIORITY_ABOVE_NORMAL and below PRIORITY_HIGHEST. This corresponds to HIGH_PRIORITY_CLASS on Windows, and a nice value of -14 on all other platforms.
PRIORITY_HIGHEST The highest process scheduling priority. This corresponds to REALTIME_PRIORITY_CLASS on Windows, and a nice value of -20 on all other platforms.

libuv Constants#

Constant Description
UV_UDP_REUSEADDR

Path#

Stability: 2 - Stable

The path module provides utilities for working with file and directory paths. It can be accessed using:

const path = require('path');

Windows vs. POSIX#

The default operation of the path module varies based on the operating system on which a Node.js application is running. Specifically, when running on a Windows operating system, the path module will assume that Windows-style paths are being used.

So using path.basename() might yield different results on POSIX and Windows:

On POSIX:

path.basename('C:\\temp\\myfile.html');
// Returns: 'C:\\temp\\myfile.html'

On Windows:

path.basename('C:\\temp\\myfile.html');
// Returns: 'myfile.html'

To achieve consistent results when working with Windows file paths on any operating system, use path.win32:

On POSIX and Windows:

path.win32.basename('C:\\temp\\myfile.html');
// Returns: 'myfile.html'

To achieve consistent results when working with POSIX file paths on any operating system, use path.posix:

On POSIX and Windows:

path.posix.basename('/tmp/myfile.html');
// Returns: 'myfile.html'

On Windows Node.js follows the concept of per-drive working directory. This behavior can be observed when using a drive path without a backslash. For example, path.resolve('C:\\') can potentially return a different result than path.resolve('C:'). For more information, see this MSDN page.

path.basename(path[, ext])#

History
VersionChanges
v6.0.0

Passing a non-string as the path argument will throw now.

v0.1.25

Added in: v0.1.25

The path.basename() methods returns the last portion of a path, similar to the Unix basename command. Trailing directory separators are ignored, see path.sep.

path.basename('/foo/bar/baz/asdf/quux.html');
// Returns: 'quux.html'

path.basename('/foo/bar/baz/asdf/quux.html', '.html');
// Returns: 'quux'

A TypeError is thrown if path is not a string or if ext is given and is not a string.

path.delimiter#

Added in: v0.9.3

Provides the platform-specific path delimiter:

  • ; for Windows
  • : for POSIX

For example, on POSIX:

console.log(process.env.PATH);
// Prints: '/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin'

process.env.PATH.split(path.delimiter);
// Returns: ['/usr/bin', '/bin', '/usr/sbin', '/sbin', '/usr/local/bin']

On Windows:

console.log(process.env.PATH);
// Prints: 'C:\Windows\system32;C:\Windows;C:\Program Files\node\'

process.env.PATH.split(path.delimiter);
// Returns ['C:\\Windows\\system32', 'C:\\Windows', 'C:\\Program Files\\node\\']

path.dirname(path)#

History
VersionChanges
v6.0.0

Passing a non-string as the path argument will throw now.

v0.1.16

Added in: v0.1.16

The path.dirname() method returns the directory name of a path, similar to the Unix dirname command. Trailing directory separators are ignored, see path.sep.

path.dirname('/foo/bar/baz/asdf/quux');
// Returns: '/foo/bar/baz/asdf'

A TypeError is thrown if path is not a string.

path.extname(path)#

History
VersionChanges
v6.0.0

Passing a non-string as the path argument will throw now.

v0.1.25

Added in: v0.1.25

The path.extname() method returns the extension of the path, from the last occurrence of the . (period) character to end of string in the last portion of the path. If there is no . in the last portion of the path, or if there are no . characters other than the first character of the basename of path (see path.basename()) , an empty string is returned.

path.extname('index.html');
// Returns: '.html'

path.extname('index.coffee.md');
// Returns: '.md'

path.extname('index.');
// Returns: '.'

path.extname('index');
// Returns: ''

path.extname('.index');
// Returns: ''

path.extname('.index.md');
// Returns: '.md'

A TypeError is thrown if path is not a string.

path.format(pathObject)#

Added in: v0.11.15

The path.format() method returns a path string from an object. This is the opposite of path.parse().

When providing properties to the pathObject remember that there are combinations where one property has priority over another:

  • pathObject.root is ignored if pathObject.dir is provided
  • pathObject.ext and pathObject.name are ignored if pathObject.base exists

For example, on POSIX:

// If `dir`, `root` and `base` are provided,
// `${dir}${path.sep}${base}`
// will be returned. `root` is ignored.
path.format({
  root: '/ignored',
  dir: '/home/user/dir',
  base: 'file.txt'
});
// Returns: '/home/user/dir/file.txt'

// `root` will be used if `dir` is not specified.
// If only `root` is provided or `dir` is equal to `root` then the
// platform separator will not be included. `ext` will be ignored.
path.format({
  root: '/',
  base: 'file.txt',
  ext: 'ignored'
});
// Returns: '/file.txt'

// `name` + `ext` will be used if `base` is not specified.
path.format({
  root: '/',
  name: 'file',
  ext: '.txt'
});
// Returns: '/file.txt'

On Windows:

path.format({
  dir: 'C:\\path\\dir',
  base: 'file.txt'
});
// Returns: 'C:\\path\\dir\\file.txt'

path.isAbsolute(path)#

Added in: v0.11.2

The path.isAbsolute() method determines if path is an absolute path.

If the given path is a zero-length string, false will be returned.

For example, on POSIX:

path.isAbsolute('/foo/bar'); // true
path.isAbsolute('/baz/..');  // true
path.isAbsolute('qux/');     // false
path.isAbsolute('.');        // false

On Windows:

path.isAbsolute('//server');    // true
path.isAbsolute('\\\\server');  // true
path.isAbsolute('C:/foo/..');   // true
path.isAbsolute('C:\\foo\\..'); // true
path.isAbsolute('bar\\baz');    // false
path.isAbsolute('bar/baz');     // false
path.isAbsolute('.');           // false

A TypeError is thrown if path is not a string.

path.join([...paths])#

Added in: v0.1.16

The path.join() method joins all given path segments together using the platform-specific separator as a delimiter, then normalizes the resulting path.

Zero-length path segments are ignored. If the joined path string is a zero-length string then '.' will be returned, representing the current working directory.

path.join('/foo', 'bar', 'baz/asdf', 'quux', '..');
// Returns: '/foo/bar/baz/asdf'

path.join('foo', {}, 'bar');
// Throws 'TypeError: Path must be a string. Received {}'

A TypeError is thrown if any of the path segments is not a string.

path.normalize(path)#

Added in: v0.1.23

The path.normalize() method normalizes the given path, resolving '..' and '.' segments.

When multiple, sequential path segment separation characters are found (e.g. / on POSIX and either \ or / on Windows), they are replaced by a single instance of the platform-specific path segment separator (/ on POSIX and \ on Windows). Trailing separators are preserved.

If the path is a zero-length string, '.' is returned, representing the current working directory.

For example, on POSIX:

path.normalize('/foo/bar//baz/asdf/quux/..');
// Returns: '/foo/bar/baz/asdf'

On Windows:

path.normalize('C:\\temp\\\\foo\\bar\\..\\');
// Returns: 'C:\\temp\\foo\\'

Since Windows recognizes multiple path separators, both separators will be replaced by instances of the Windows preferred separator (\):

path.win32.normalize('C:////temp\\\\/\\/\\/foo/bar');
// Returns: 'C:\\temp\\foo\\bar'

A TypeError is thrown if path is not a string.

path.parse(path)#

Added in: v0.11.15

The path.parse() method returns an object whose properties represent significant elements of the path. Trailing directory separators are ignored, see path.sep.

The returned object will have the following properties:

For example, on POSIX:

path.parse('/home/user/dir/file.txt');
// Returns:
// { root: '/',
//   dir: '/home/user/dir',
//   base: 'file.txt',
//   ext: '.txt',
//   name: 'file' }

┌─────────────────────┬────────────┐
│          dir        │    base    │
├──────┬              ├──────┬─────┤
│ root │              │ name │ ext │
"  /    home/user/dir / file  .txt "
└──────┴──────────────┴──────┴─────┘
(All spaces in the "" line should be ignored. They are purely for formatting.)

On Windows:

path.parse('C:\\path\\dir\\file.txt');
// Returns:
// { root: 'C:\\',
//   dir: 'C:\\path\\dir',
//   base: 'file.txt',
//   ext: '.txt',
//   name: 'file' }

┌─────────────────────┬────────────┐
│          dir        │    base    │
├──────┬              ├──────┬─────┤
│ root │              │ name │ ext │
" C:\      path\dir   \ file  .txt "
└──────┴──────────────┴──────┴─────┘
(All spaces in the "" line should be ignored. They are purely for formatting.)

A TypeError is thrown if path is not a string.

path.posix#

Added in: v0.11.15

The path.posix property provides access to POSIX specific implementations of the path methods.

path.relative(from, to)#

History
VersionChanges
v6.8.0

On Windows, the leading slashes for UNC paths are now included in the return value.

v0.5.0

Added in: v0.5.0

The path.relative() method returns the relative path from from to to based on the current working directory. If from and to each resolve to the same path (after calling path.resolve() on each), a zero-length string is returned.

If a zero-length string is passed as from or to, the current working directory will be used instead of the zero-length strings.

For example, on POSIX:

path.relative('/data/orandea/test/aaa', '/data/orandea/impl/bbb');
// Returns: '../../impl/bbb'

On Windows:

path.relative('C:\\orandea\\test\\aaa', 'C:\\orandea\\impl\\bbb');
// Returns: '..\\..\\impl\\bbb'

A TypeError is thrown if either from or to is not a string.

path.resolve([...paths])#

Added in: v0.3.4

The path.resolve() method resolves a sequence of paths or path segments into an absolute path.

The given sequence of paths is processed from right to left, with each subsequent path prepended until an absolute path is constructed. For instance, given the sequence of path segments: /foo, /bar, baz, calling path.resolve('/foo', '/bar', 'baz') would return /bar/baz because 'baz' is not an absolute path but '/bar' + '/' + 'baz' is.

If after processing all given path segments an absolute path has not yet been generated, the current working directory is used.

The resulting path is normalized and trailing slashes are removed unless the path is resolved to the root directory.

Zero-length path segments are ignored.

If no path segments are passed, path.resolve() will return the absolute path of the current working directory.

path.resolve('/foo/bar', './baz');
// Returns: '/foo/bar/baz'

path.resolve('/foo/bar', '/tmp/file/');
// Returns: '/tmp/file'

path.resolve('wwwroot', 'static_files/png/', '../gif/image.gif');
// If the current working directory is /home/myself/node,
// this returns '/home/myself/node/wwwroot/static_files/gif/image.gif'

A TypeError is thrown if any of the arguments is not a string.

path.sep#

Added in: v0.7.9

Provides the platform-specific path segment separator:

  • \ on Windows
  • / on POSIX

For example, on POSIX:

'foo/bar/baz'.split(path.sep);
// Returns: ['foo', 'bar', 'baz']

On Windows:

'foo\\bar\\baz'.split(path.sep);
// Returns: ['foo', 'bar', 'baz']

On Windows, both the forward slash (/) and backward slash (\) are accepted as path segment separators; however, the path methods only add backward slashes (\).

path.toNamespacedPath(path)#

Added in: v9.0.0

On Windows systems only, returns an equivalent namespace-prefixed path for the given path. If path is not a string, path will be returned without modifications.

This method is meaningful only on Windows system. On POSIX systems, the method is non-operational and always returns path without modifications.

path.win32#

Added in: v0.11.15

The path.win32 property provides access to Windows-specific implementations of the path methods.

Performance Timing API#

Stability: 2 - Stable

The Performance Timing API provides an implementation of the W3C Performance Timeline specification. The purpose of the API is to support collection of high resolution performance metrics. This is the same Performance API as implemented in modern Web browsers.

const { PerformanceObserver, performance } = require('perf_hooks');

const obs = new PerformanceObserver((items) => {
  console.log(items.getEntries()[0].duration);
  performance.clearMarks();
});
obs.observe({ entryTypes: ['measure'] });
performance.measure('Start to Now');

performance.mark('A');
doSomeLongRunningProcess(() => {
  performance.measure('A to Now', 'A');

  performance.mark('B');
  performance.measure('A to B', 'A', 'B');
});

Class: Performance#

Added in: v8.5.0

performance.clearMarks([name])#

Added in: v8.5.0

If name is not provided, removes all PerformanceMark objects from the Performance Timeline. If name is provided, removes only the named mark.

performance.mark([name])#

Added in: v8.5.0

Creates a new PerformanceMark entry in the Performance Timeline. A PerformanceMark is a subclass of PerformanceEntry whose performanceEntry.entryType is always 'mark', and whose performanceEntry.duration is always 0. Performance marks are used to mark specific significant moments in the Performance Timeline.

performance.measure(name[, startMark[, endMark]])#

History
VersionChanges
v14.0.0

Make startMark and endMark parameters optional.

v8.5.0

Added in: v8.5.0

Creates a new PerformanceMeasure entry in the Performance Timeline. A PerformanceMeasure is a subclass of PerformanceEntry whose performanceEntry.entryType is always 'measure', and whose performanceEntry.duration measures the number of milliseconds elapsed since startMark and endMark.

The startMark argument may identify any existing PerformanceMark in the Performance Timeline, or may identify any of the timestamp properties provided by the PerformanceNodeTiming class. If the named startMark does not exist, then startMark is set to timeOrigin by default.

The optional endMark argument must identify any existing PerformanceMark in the Performance Timeline or any of the timestamp properties provided by the PerformanceNodeTiming class. endMark will be performance.now() if no parameter is passed, otherwise if the named endMark does not exist, an error will be thrown.

performance.nodeTiming#

Added in: v8.5.0

An instance of the PerformanceNodeTiming class that provides performance metrics for specific Node.js operational milestones.

performance.now()#

Added in: v8.5.0

Returns the current high resolution millisecond timestamp, where 0 represents the start of the current node process.

performance.timeOrigin#

Added in: v8.5.0

The timeOrigin specifies the high resolution millisecond timestamp at which the current node process began, measured in Unix time.

performance.timerify(fn)#

Added in: v8.5.0

Wraps a function within a new function that measures the running time of the wrapped function. A PerformanceObserver must be subscribed to the 'function' event type in order for the timing details to be accessed.

const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

function someFunction() {
  console.log('hello world');
}

const wrapped = performance.timerify(someFunction);

const obs = new PerformanceObserver((list) => {
  console.log(list.getEntries()[0].duration);
  obs.disconnect();
});
obs.observe({ entryTypes: ['function'] });

// A performance timeline entry will be created
wrapped();

Class: PerformanceEntry#

Added in: v8.5.0

performanceEntry.duration#

Added in: v8.5.0

The total number of milliseconds elapsed for this entry. This value will not be meaningful for all Performance Entry types.

performanceEntry.name#

Added in: v8.5.0

The name of the performance entry.

performanceEntry.startTime#

Added in: v8.5.0

The high resolution millisecond timestamp marking the starting time of the Performance Entry.

performanceEntry.entryType#

Added in: v8.5.0

The type of the performance entry. Currently it may be one of: 'node', 'mark', 'measure', 'gc', 'function', 'http2' or 'http'.

performanceEntry.kind#

Added in: v8.5.0

When performanceEntry.entryType is equal to 'gc', the performance.kind property identifies the type of garbage collection operation that occurred. The value may be one of:

  • perf_hooks.constants.NODE_PERFORMANCE_GC_MAJOR
  • perf_hooks.constants.NODE_PERFORMANCE_GC_MINOR
  • perf_hooks.constants.NODE_PERFORMANCE_GC_INCREMENTAL
  • perf_hooks.constants.NODE_PERFORMANCE_GC_WEAKCB

performanceEntry.flags#

Added in: v13.9.0

When performanceEntry.entryType is equal to 'gc', the performance.flags property contains additional information about garbage collection operation. The value may be one of:

  • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_NO
  • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_CONSTRUCT_RETAINED
  • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_FORCED
  • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SYNCHRONOUS_PHANTOM_PROCESSING
  • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_AVAILABLE_GARBAGE
  • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_EXTERNAL_MEMORY
  • perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SCHEDULE_IDLE

Class: PerformanceNodeTiming extends PerformanceEntry#

Added in: v8.5.0

Provides timing details for Node.js itself.

performanceNodeTiming.bootstrapComplete#

Added in: v8.5.0

The high resolution millisecond timestamp at which the Node.js process completed bootstrapping. If bootstrapping has not yet finished, the property has the value of -1.

performanceNodeTiming.environment#

Added in: v8.5.0

The high resolution millisecond timestamp at which the Node.js environment was initialized.

performanceNodeTiming.loopExit#

Added in: v8.5.0

The high resolution millisecond timestamp at which the Node.js event loop exited. If the event loop has not yet exited, the property has the value of -1. It can only have a value of not -1 in a handler of the 'exit' event.

performanceNodeTiming.loopStart#

Added in: v8.5.0

The high resolution millisecond timestamp at which the Node.js event loop started. If the event loop has not yet started (e.g., in the first tick of the main script), the property has the value of -1.

performanceNodeTiming.nodeStart#

Added in: v8.5.0

The high resolution millisecond timestamp at which the Node.js process was initialized.

performanceNodeTiming.v8Start#

Added in: v8.5.0

The high resolution millisecond timestamp at which the V8 platform was initialized.

Class: PerformanceObserver#

new PerformanceObserver(callback)#

Added in: v8.5.0

PerformanceObserver objects provide notifications when new PerformanceEntry instances have been added to the Performance Timeline.

const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  console.log(list.getEntries());
  observer.disconnect();
});
obs.observe({ entryTypes: ['mark'], buffered: true });

performance.mark('test');

Because PerformanceObserver instances introduce their own additional performance overhead, instances should not be left subscribed to notifications indefinitely. Users should disconnect observers as soon as they are no longer needed.

The callback is invoked when a PerformanceObserver is notified about new PerformanceEntry instances. The callback receives a PerformanceObserverEntryList instance and a reference to the PerformanceObserver.

performanceObserver.disconnect()#

Added in: v8.5.0

Disconnects the PerformanceObserver instance from all notifications.

performanceObserver.observe(options)#

Added in: v8.5.0

  • options <Object>

    • entryTypes <string[]> An array of strings identifying the types of PerformanceEntry instances the observer is interested in. If not provided an error will be thrown.
    • buffered <boolean> If true, the notification callback will be called using setImmediate() and multiple PerformanceEntry instance notifications will be buffered internally. If false, notifications will be immediate and synchronous. Default: false.

Subscribes the PerformanceObserver instance to notifications of new PerformanceEntry instances identified by options.entryTypes.

When options.buffered is false, the callback will be invoked once for every PerformanceEntry instance:

const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  // Called three times synchronously. `list` contains one item.
});
obs.observe({ entryTypes: ['mark'] });

for (let n = 0; n < 3; n++)
  performance.mark(`test${n}`);

const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  // Called once. `list` contains three items.
});
obs.observe({ entryTypes: ['mark'], buffered: true });

for (let n = 0; n < 3; n++)
  performance.mark(`test${n}`);

Class: PerformanceObserverEntryList#

Added in: v8.5.0

The PerformanceObserverEntryList class is used to provide access to the PerformanceEntry instances passed to a PerformanceObserver.

performanceObserverEntryList.getEntries()#

Added in: v8.5.0

Returns a list of PerformanceEntry objects in chronological order with respect to performanceEntry.startTime.

performanceObserverEntryList.getEntriesByName(name[, type])#

Added in: v8.5.0

Returns a list of PerformanceEntry objects in chronological order with respect to performanceEntry.startTime whose performanceEntry.name is equal to name, and optionally, whose performanceEntry.entryType is equal to type.

performanceObserverEntryList.getEntriesByType(type)#

Added in: v8.5.0

Returns a list of PerformanceEntry objects in chronological order with respect to performanceEntry.startTime whose performanceEntry.entryType is equal to type.

perf_hooks.monitorEventLoopDelay([options])#

Added in: v11.10.0

Creates a Histogram object that samples and reports the event loop delay over time. The delays will be reported in nanoseconds.

Using a timer to detect approximate event loop delay works because the execution of timers is tied specifically to the lifecycle of the libuv event loop. That is, a delay in the loop will cause a delay in the execution of the timer, and those delays are specifically what this API is intended to detect.

const { monitorEventLoopDelay } = require('perf_hooks');
const h = monitorEventLoopDelay({ resolution: 20 });
h.enable();
// Do something.
h.disable();
console.log(h.min);
console.log(h.max);
console.log(h.mean);
console.log(h.stddev);
console.log(h.percentiles);
console.log(h.percentile(50));
console.log(h.percentile(99));

Class: Histogram#

Added in: v11.10.0

Tracks the event loop delay at a given sampling rate.

histogram.disable()#

Added in: v11.10.0

Disables the event loop delay sample timer. Returns true if the timer was stopped, false if it was already stopped.

histogram.enable()#

Added in: v11.10.0

Enables the event loop delay sample timer. Returns true if the timer was started, false if it was already started.

histogram.exceeds#

Added in: v11.10.0

The number of times the event loop delay exceeded the maximum 1 hour event loop delay threshold.

histogram.max#

Added in: v11.10.0

The maximum recorded event loop delay.

histogram.mean#

Added in: v11.10.0

The mean of the recorded event loop delays.

histogram.min#

Added in: v11.10.0

The minimum recorded event loop delay.

histogram.percentile(percentile)#

Added in: v11.10.0

Returns the value at the given percentile.

histogram.percentiles#

Added in: v11.10.0

Returns a Map object detailing the accumulated percentile distribution.

histogram.reset()#

Added in: v11.10.0

Resets the collected histogram data.

histogram.stddev#

Added in: v11.10.0

The standard deviation of the recorded event loop delays.

Examples#

Measuring the duration of async operations#

The following example uses the Async Hooks and Performance APIs to measure the actual duration of a Timeout operation (including the amount of time it took to execute the callback).

'use strict';
const async_hooks = require('async_hooks');
const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const set = new Set();
const hook = async_hooks.createHook({
  init(id, type) {
    if (type === 'Timeout') {
      performance.mark(`Timeout-${id}-Init`);
      set.add(id);
    }
  },
  destroy(id) {
    if (set.has(id)) {
      set.delete(id);
      performance.mark(`Timeout-${id}-Destroy`);
      performance.measure(`Timeout-${id}`,
                          `Timeout-${id}-Init`,
                          `Timeout-${id}-Destroy`);
    }
  }
});
hook.enable();

const obs = new PerformanceObserver((list, observer) => {
  console.log(list.getEntries()[0]);
  performance.clearMarks();
  observer.disconnect();
});
obs.observe({ entryTypes: ['measure'], buffered: true });

setTimeout(() => {}, 1000);

Measuring how long it takes to load dependencies#

The following example measures the duration of require() operations to load dependencies:

'use strict';
const {
  performance,
  PerformanceObserver
} = require('perf_hooks');
const mod = require('module');

// Monkey patch the require function
mod.Module.prototype.require =
  performance.timerify(mod.Module.prototype.require);
require = performance.timerify(require);

// Activate the observer
const obs = new PerformanceObserver((list) => {
  const entries = list.getEntries();
  entries.forEach((entry) => {
    console.log(`require('${entry[0]}')`, entry.duration);
  });
  obs.disconnect();
});
obs.observe({ entryTypes: ['function'], buffered: true });

require('some-module');

Policies#

Stability: 1 - Experimental

Node.js contains experimental support for creating policies on loading code.

Policies are a security feature intended to allow guarantees about what code Node.js is able to load. The use of policies assumes safe practices for the policy files such as ensuring that policy files cannot be overwritten by the Node.js application by using file permissions.

A best practice would be to ensure that the policy manifest is read only for the running Node.js application, and that the file cannot be changed by the running Node.js application in any way. A typical setup would be to create the policy file as a different user id than the one running Node.js and granting read permissions to the user id running Node.js.

Enabling#

The --experimental-policy flag can be used to enable features for policies when loading modules.

Once this has been set, all modules must conform to a policy manifest file passed to the flag:

node --experimental-policy=policy.json app.js

The policy manifest will be used to enforce constraints on code loaded by Node.js.

To mitigate tampering with policy files on disk, an integrity for the policy file itself may be provided via --policy-integrity. This allows running node and asserting the policy file contents even if the file is changed on disk.

node --experimental-policy=policy.json --policy-integrity="sha384-SggXRQHwCG8g+DktYYzxkXRIkTiEYWBHqev0xnpCxYlqMBufKZHAHQM3/boDaI/0" app.js

Features#

Error Behavior#

When a policy check fails, Node.js by default will throw an error. It is possible to change the error behavior to one of a few possibilities by defining an "onerror" field in a policy manifest. The following values are available to change the behavior:

  • "exit": will exit the process immediately. No cleanup code will be allowed to run.
  • "log": will log the error at the site of the failure.
  • "throw": will throw a JS error at the site of the failure. This is the default.
{
  "onerror": "log",
  "resources": {
    "./app/checked.js": {
      "integrity": "sha384-SggXRQHwCG8g+DktYYzxkXRIkTiEYWBHqev0xnpCxYlqMBufKZHAHQM3/boDaI/0"
    }
  }
}

Integrity Checks#

Policy files must use integrity checks with Subresource Integrity strings compatible with the browser integrity attribute associated with absolute URLs.

When using require() all resources involved in loading are checked for integrity if a policy manifest has been specified. If a resource does not match the integrity listed in the manifest, an error will be thrown.

An example policy file that would allow loading a file checked.js:

{
  "resources": {
    "./app/checked.js": {
      "integrity": "sha384-SggXRQHwCG8g+DktYYzxkXRIkTiEYWBHqev0xnpCxYlqMBufKZHAHQM3/boDaI/0"
    }
  }
}

Each resource listed in the policy manifest can be of one the following formats to determine its location:

  1. A relative url string to a resource from the manifest such as ./resource.js, ../resource.js, or /resource.js.
  2. A complete url string to a resource such as file:///resource.js.

When loading resources the entire URL must match including search parameters and hash fragment. ./a.js?b will not be used when attempting to load ./a.js and vice versa.

To generate integrity strings, a script such as printf "sha384-$(cat checked.js | openssl dgst -sha384 -binary | base64)" can be used.

Integrity can be specified as the boolean value true to accept any body for the resource which can be useful for local development. It is not recommended in production since it would allow unexpected alteration of resources to be considered valid.

Dependency Redirection#

An application may need to ship patched versions of modules or to prevent modules from allowing all modules access to all other modules. Redirection can be used by intercepting attempts to load the modules wishing to be replaced.

{
  "builtins": [],
  "resources": {
    "./app/checked.js": {
      "dependencies": {
        "fs": true,
        "os": "./app/node_modules/alt-os"
      }
    }
  }
}

The dependencies are keyed by the requested string specifier and have values of either true or a string pointing to a module that will be resolved.

The specifier string does not perform any searching and must match exactly what is provided to the require(). Therefore, multiple specifiers may be needed in the policy if require() uses multiple different strings to point to the same module (such as excluding the extension).

If the value of the redirection is true the default searching algorithms will be used to find the module.

If the value of the redirection is a string, it will be resolved relative to the manifest and then immediately be used without searching.

Any specifier string that is require()ed and not listed in the dependencies will result in an error according to the policy.

Redirection will not prevent access to APIs through means such as direct access to require.cache and/or through module.constructor which allow access to loading modules. Policy redirection only affect specifiers to require(). Other means such as to prevent undesired access to APIs through variables are necessary to lock down that path of loading modules.

A boolean value of true for the dependencies map can be specified to allow a module to load any specifier without redirection. This can be useful for local development and may have some valid usage in production, but should be used only with care after auditing a module to ensure its behavior is valid.

Example: Patched Dependency#

Redirected dependencies can provide attenuated or modified functionality as fits the application. For example, log data about timing of function durations by wrapping the original:

const original = require('fn');
module.exports = function fn(...args) {
  console.time();
  try {
    return new.target ?
      Reflect.construct(original, args) :
      Reflect.apply(original, this, args);
  } finally {
    console.timeEnd();
  }
};

Process#

The process object is a global that provides information about, and control over, the current Node.js process. As a global, it is always available to Node.js applications without using require(). It can also be explicitly accessed using require():

const process = require('process');

Process Events#

The process object is an instance of EventEmitter.

Event: 'beforeExit'#

Added in: v0.11.12

The 'beforeExit' event is emitted when Node.js empties its event loop and has no additional work to schedule. Normally, the Node.js process will exit when there is no work scheduled, but a listener registered on the 'beforeExit' event can make asynchronous calls, and thereby cause the Node.js process to continue.

The listener callback function is invoked with the value of process.exitCode passed as the only argument.

The 'beforeExit' event is not emitted for conditions causing explicit termination, such as calling process.exit() or uncaught exceptions.

The 'beforeExit' should not be used as an alternative to the 'exit' event unless the intention is to schedule additional work.

process.on('beforeExit', (code) => {
  console.log('Process beforeExit event with code: ', code);
});

process.on('exit', (code) => {
  console.log('Process exit event with code: ', code);
});

console.log('This message is displayed first.');

// Prints:
// This message is displayed first.
// Process beforeExit event with code: 0
// Process exit event with code: 0

Event: 'disconnect'#

Added in: v0.7.7

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the 'disconnect' event will be emitted when the IPC channel is closed.

Event: 'exit'#

Added in: v0.1.7

The 'exit' event is emitted when the Node.js process is about to exit as a result of either:

  • The process.exit() method being called explicitly;
  • The Node.js event loop no longer having any additional work to perform.

There is no way to prevent the exiting of the event loop at this point, and once all 'exit' listeners have finished running the Node.js process will terminate.

The listener callback function is invoked with the exit code specified either by the process.exitCode property, or the exitCode argument passed to the process.exit() method.

process.on('exit', (code) => {
  console.log(`About to exit with code: ${code}`);
});

Listener functions must only perform synchronous operations. The Node.js process will exit immediately after calling the 'exit' event listeners causing any additional work still queued in the event loop to be abandoned. In the following example, for instance, the timeout will never occur:

process.on('exit', (code) => {
  setTimeout(() => {
    console.log('This will not run');
  }, 0);
});

Event: 'message'#

Added in: v0.5.10

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the 'message' event is emitted whenever a message sent by a parent process using childprocess.send() is received by the child process.

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

If the serialization option was set to advanced used when spawning the process, the message argument can contain data that JSON is not able to represent. See Advanced Serialization for child_process for more details.

Event: 'multipleResolves'#

Added in: v10.12.0
  • type <string> The resolution type. One of 'resolve' or 'reject'.
  • promise <Promise> The promise that resolved or rejected more than once.
  • value <any> The value with which the promise was either resolved or rejected after the original resolve.

The 'multipleResolves' event is emitted whenever a Promise has been either:

  • Resolved more than once.
  • Rejected more than once.
  • Rejected after resolve.
  • Resolved after reject.

This is useful for tracking potential errors in an application while using the Promise constructor, as multiple resolutions are silently swallowed. However, the occurrence of this event does not necessarily indicate an error. For example, Promise.race() can trigger a 'multipleResolves' event.

process.on('multipleResolves', (type, promise, reason) => {
  console.error(type, promise, reason);
  setImmediate(() => process.exit(1));
});

async function main() {
  try {
    return await new Promise((resolve, reject) => {
      resolve('First call');
      resolve('Swallowed resolve');
      reject(new Error('Swallowed reject'));
    });
  } catch {
    throw new Error('Failed');
  }
}

main().then(console.log);
// resolve: Promise { 'First call' } 'Swallowed resolve'
// reject: Promise { 'First call' } Error: Swallowed reject
//     at Promise (*)
//     at new Promise (<anonymous>)
//     at main (*)
// First call

Event: 'rejectionHandled'#

Added in: v1.4.1
  • promise <Promise> The late handled promise.

The 'rejectionHandled' event is emitted whenever a Promise has been rejected and an error handler was attached to it (using promise.catch(), for example) later than one turn of the Node.js event loop.

The Promise object would have previously been emitted in an 'unhandledRejection' event, but during the course of processing gained a rejection handler.

There is no notion of a top level for a Promise chain at which rejections can always be handled. Being inherently asynchronous in nature, a Promise rejection can be handled at a future point in time, possibly much later than the event loop turn it takes for the 'unhandledRejection' event to be emitted.

Another way of stating this is that, unlike in synchronous code where there is an ever-growing list of unhandled exceptions, with Promises there can be a growing-and-shrinking list of unhandled rejections.

In synchronous code, the 'uncaughtException' event is emitted when the list of unhandled exceptions grows.

In asynchronous code, the 'unhandledRejection' event is emitted when the list of unhandled rejections grows, and the 'rejectionHandled' event is emitted when the list of unhandled rejections shrinks.

const unhandledRejections = new Map();
process.on('unhandledRejection', (reason, promise) => {
  unhandledRejections.set(promise, reason);
});
process.on('rejectionHandled', (promise) => {
  unhandledRejections.delete(promise);
});

In this example, the unhandledRejections Map will grow and shrink over time, reflecting rejections that start unhandled and then become handled. It is possible to record such errors in an error log, either periodically (which is likely best for long-running application) or upon process exit (which is likely most convenient for scripts).

Event: 'uncaughtException'#

History
VersionChanges
v12.0.0, v10.17.0

Added the origin argument.

v0.1.18

Added in: v0.1.18

  • err <Error> The uncaught exception.
  • origin <string> Indicates if the exception originates from an unhandled rejection or from synchronous errors. Can either be 'uncaughtException' or 'unhandledRejection'.

The 'uncaughtException' event is emitted when an uncaught JavaScript exception bubbles all the way back to the event loop. By default, Node.js handles such exceptions by printing the stack trace to stderr and exiting with code 1, overriding any previously set process.exitCode. Adding a handler for the 'uncaughtException' event overrides this default behavior. Alternatively, change the process.exitCode in the 'uncaughtException' handler which will result in the process exiting with the provided exit code. Otherwise, in the presence of such handler the process will exit with 0.

process.on('uncaughtException', (err, origin) => {
  fs.writeSync(
    process.stderr.fd,
    `Caught exception: ${err}\n` +
    `Exception origin: ${origin}`
  );
});

setTimeout(() => {
  console.log('This will still run.');
}, 500);

// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
console.log('This will not run.');

It is possible to monitor 'uncaughtException' events without overriding the default behavior to exit the process by installing a 'uncaughtExceptionMonitor' listener.

Warning: Using 'uncaughtException' correctly#

'uncaughtException' is a crude mechanism for exception handling intended to be used only as a last resort. The event should not be used as an equivalent to On Error Resume Next. Unhandled exceptions inherently mean that an application is in an undefined state. Attempting to resume application code without properly recovering from the exception can cause additional unforeseen and unpredictable issues.

Exceptions thrown from within the event handler will not be caught. Instead the process will exit with a non-zero exit code and the stack trace will be printed. This is to avoid infinite recursion.

Attempting to resume normally after an uncaught exception can be similar to pulling out the power cord when upgrading a computer. Nine out of ten times, nothing happens. But the tenth time, the system becomes corrupted.

The correct use of 'uncaughtException' is to perform synchronous cleanup of allocated resources (e.g. file descriptors, handles, etc) before shutting down the process. It is not safe to resume normal operation after 'uncaughtException'.

To restart a crashed application in a more reliable way, whether 'uncaughtException' is emitted or not, an external monitor should be employed in a separate process to detect application failures and recover or restart as needed.

Event: 'uncaughtExceptionMonitor'#

Added in: v13.7.0
  • err <Error> The uncaught exception.
  • origin <string> Indicates if the exception originates from an unhandled rejection or from synchronous errors. Can either be 'uncaughtException' or 'unhandledRejection'.

The 'uncaughtExceptionMonitor' event is emitted before an 'uncaughtException' event is emitted or a hook installed via process.setUncaughtExceptionCaptureCallback() is called.

Installing an 'uncaughtExceptionMonitor' listener does not change the behavior once an 'uncaughtException' event is emitted. The process will still crash if no 'uncaughtException' listener is installed.

process.on('uncaughtExceptionMonitor', (err, origin) => {
  MyMonitoringTool.logSync(err, origin);
});

// Intentionally cause an exception, but don't catch it.
nonexistentFunc();
// Still crashes Node.js

Event: 'unhandledRejection'#

History
VersionChanges
v7.0.0

Not handling Promise rejections is deprecated.

v6.6.0

Unhandled Promise rejections will now emit a process warning.

v1.4.1

Added in: v1.4.1

  • reason <Error> | <any> The object with which the promise was rejected (typically an Error object).
  • promise <Promise> The rejected promise.

The 'unhandledRejection' event is emitted whenever a Promise is rejected and no error handler is attached to the promise within a turn of the event loop. When programming with Promises, exceptions are encapsulated as "rejected promises". Rejections can be caught and handled using promise.catch() and are propagated through a Promise chain. The 'unhandledRejection' event is useful for detecting and keeping track of promises that were rejected whose rejections have not yet been handled.

process.on('unhandledRejection', (reason, promise) => {
  console.log('Unhandled Rejection at:', promise, 'reason:', reason);
  // Application specific logging, throwing an error, or other logic here
});

somePromise.then((res) => {
  return reportToUser(JSON.pasre(res)); // Note the typo (`pasre`)
}); // No `.catch()` or `.then()`

The following will also trigger the 'unhandledRejection' event to be emitted:

function SomeResource() {
  // Initially set the loaded status to a rejected promise
  this.loaded = Promise.reject(new Error('Resource not yet loaded!'));
}

const resource = new SomeResource();
// no .catch or .then on resource.loaded for at least a turn

In this example case, it is possible to track the rejection as a developer error as would typically be the case for other 'unhandledRejection' events. To address such failures, a non-operational .catch(() => { }) handler may be attached to resource.loaded, which would prevent the 'unhandledRejection' event from being emitted.

Event: 'warning'#

Added in: v6.0.0

  • warning <Error> Key properties of the warning are:

    • name <string> The name of the warning. Default: 'Warning'.
    • message <string> A system-provided description of the warning.
    • stack <string> A stack trace to the location in the code where the warning was issued.

The 'warning' event is emitted whenever Node.js emits a process warning.

A process warning is similar to an error in that it describes exceptional conditions that are being brought to the user's attention. However, warnings are not part of the normal Node.js and JavaScript error handling flow. Node.js can emit warnings whenever it detects bad coding practices that could lead to sub-optimal application performance, bugs, or security vulnerabilities.

process.on('warning', (warning) => {
  console.warn(warning.name);    // Print the warning name
  console.warn(warning.message); // Print the warning message
  console.warn(warning.stack);   // Print the stack trace
});

By default, Node.js will print process warnings to stderr. The --no-warnings command-line option can be used to suppress the default console output but the 'warning' event will still be emitted by the process object.

The following example illustrates the warning that is printed to stderr when too many listeners have been added to an event:

$ node
> events.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> (node:38638) MaxListenersExceededWarning: Possible EventEmitter memory leak
detected. 2 foo listeners added. Use emitter.setMaxListeners() to increase limit

In contrast, the following example turns off the default warning output and adds a custom handler to the 'warning' event:

$ node --no-warnings
> const p = process.on('warning', (warning) => console.warn('Do not do that!'));
> events.defaultMaxListeners = 1;
> process.on('foo', () => {});
> process.on('foo', () => {});
> Do not do that!

The --trace-warnings command-line option can be used to have the default console output for warnings include the full stack trace of the warning.

Launching Node.js using the --throw-deprecation command line flag will cause custom deprecation warnings to be thrown as exceptions.

Using the --trace-deprecation command line flag will cause the custom deprecation to be printed to stderr along with the stack trace.

Using the --no-deprecation command line flag will suppress all reporting of the custom deprecation.

The *-deprecation command line flags only affect warnings that use the name 'DeprecationWarning'.

Emitting custom warnings#

See the process.emitWarning() method for issuing custom or application-specific warnings.

Signal Events#

Signal events will be emitted when the Node.js process receives a signal. Please refer to signal(7) for a listing of standard POSIX signal names such as 'SIGINT', 'SIGHUP', etc.

Signals are not available on Worker threads.

The signal handler will receive the signal's name ('SIGINT', 'SIGTERM', etc.) as the first argument.

The name of each event will be the uppercase common name for the signal (e.g. 'SIGINT' for SIGINT signals).

// Begin reading from stdin so the process does not exit.
process.stdin.resume();

process.on('SIGINT', () => {
  console.log('Received SIGINT. Press Control-D to exit.');
});

// Using a single function to handle multiple signals
function handle(signal) {
  console.log(`Received ${signal}`);
}

process.on('SIGINT', handle);
process.on('SIGTERM', handle);
  • 'SIGUSR1' is reserved by Node.js to start the debugger. It's possible to install a listener but doing so might interfere with the debugger.
  • 'SIGTERM' and 'SIGINT' have default handlers on non-Windows platforms that reset the terminal mode before exiting with code 128 + signal number. If one of these signals has a listener installed, its default behavior will be removed (Node.js will no longer exit).
  • 'SIGPIPE' is ignored by default. It can have a listener installed.
  • 'SIGHUP' is generated on Windows when the console window is closed, and on other platforms under various similar conditions. See signal(7). It can have a listener installed, however Node.js will be unconditionally terminated by Windows about 10 seconds later. On non-Windows platforms, the default behavior of SIGHUP is to terminate Node.js, but once a listener has been installed its default behavior will be removed.
  • 'SIGTERM' is not supported on Windows, it can be listened on.
  • 'SIGINT' from the terminal is supported on all platforms, and can usually be generated with <Ctrl>+C (though this may be configurable). It is not generated when terminal raw mode is enabled and <Ctrl>+C is used.
  • 'SIGBREAK' is delivered on Windows when <Ctrl>+<Break> is pressed, on non-Windows platforms it can be listened on, but there is no way to send or generate it.
  • 'SIGWINCH' is delivered when the console has been resized. On Windows, this will only happen on write to the console when the cursor is being moved, or when a readable tty is used in raw mode.
  • 'SIGKILL' cannot have a listener installed, it will unconditionally terminate Node.js on all platforms.
  • 'SIGSTOP' cannot have a listener installed.
  • 'SIGBUS', 'SIGFPE', 'SIGSEGV' and 'SIGILL', when not raised artificially using kill(2), inherently leave the process in a state from which it is not safe to attempt to call JS listeners. Doing so might lead to the process hanging in an endless loop, since listeners attached using process.on() are called asynchronously and therefore unable to correct the underlying problem.
  • 0 can be sent to test for the existence of a process, it has no effect if the process exists, but will throw an error if the process does not exist.

Windows does not support signals so has no equivalent to termination by signal, but Node.js offers some emulation with process.kill(), and subprocess.kill():

  • Sending SIGINT, SIGTERM, and SIGKILL will cause the unconditional termination of the target process, and afterwards, subprocess will report that the process was terminated by signal.
  • Sending signal 0 can be used as a platform independent way to test for the existence of a process.

process.abort()#

Added in: v0.7.0

The process.abort() method causes the Node.js process to exit immediately and generate a core file.

This feature is not available in Worker threads.

process.allowedNodeEnvironmentFlags#

Added in: v10.10.0

The process.allowedNodeEnvironmentFlags property is a special, read-only Set of flags allowable within the NODE_OPTIONS environment variable.

process.allowedNodeEnvironmentFlags extends Set, but overrides Set.prototype.has to recognize several different possible flag representations. process.allowedNodeEnvironmentFlags.has() will return true in the following cases:

  • Flags may omit leading single (-) or double (--) dashes; e.g., inspect-brk for --inspect-brk, or r for -r.
  • Flags passed through to V8 (as listed in --v8-options) may replace one or more non-leading dashes for an underscore, or vice-versa; e.g., --perf_basic_prof, --perf-basic-prof, --perf_basic-prof, etc.
  • Flags may contain one or more equals (=) characters; all characters after and including the first equals will be ignored; e.g., --stack-trace-limit=100.
  • Flags must be allowable within NODE_OPTIONS.

When iterating over process.allowedNodeEnvironmentFlags, flags will appear only once; each will begin with one or more dashes. Flags passed through to V8 will contain underscores instead of non-leading dashes:

process.allowedNodeEnvironmentFlags.forEach((flag) => {
  // -r
  // --inspect-brk
  // --abort_on_uncaught_exception
  // ...
});

The methods add(), clear(), and delete() of process.allowedNodeEnvironmentFlags do nothing, and will fail silently.

If Node.js was compiled without NODE_OPTIONS support (shown in process.config), process.allowedNodeEnvironmentFlags will contain what would have been allowable.

process.arch#

Added in: v0.5.0

The operating system CPU architecture for which the Node.js binary was compiled. Possible values are: 'arm', 'arm64', 'ia32', 'mips','mipsel', 'ppc', 'ppc64', 's390', 's390x', 'x32', and 'x64'.

console.log(`This processor architecture is ${process.arch}`);

process.argv#

Added in: v0.1.27

The process.argv property returns an array containing the command line arguments passed when the Node.js process was launched. The first element will be process.execPath. See process.argv0 if access to the original value of argv[0] is needed. The second element will be the path to the JavaScript file being executed. The remaining elements will be any additional command line arguments.

For example, assuming the following script for process-args.js:

// print process.argv
process.argv.forEach((val, index) => {
  console.log(`${index}: ${val}`);
});

Launching the Node.js process as:

$ node process-args.js one two=three four

Would generate the output:

0: /usr/local/bin/node
1: /Users/mjr/work/node/process-args.js
2: one
3: two=three
4: four

process.argv0#

Added in: v6.4.0

The process.argv0 property stores a read-only copy of the original value of argv[0] passed when Node.js starts.

$ bash -c 'exec -a customArgv0 ./node'
> process.argv[0]
'/Volumes/code/external/node/out/Release/node'
> process.argv0
'customArgv0'

process.channel#

History
VersionChanges
v14.0.0

The object no longer accidentally exposes native C++ bindings.

v7.1.0

Added in: v7.1.0

If the Node.js process was spawned with an IPC channel (see the Child Process documentation), the process.channel property is a reference to the IPC channel. If no IPC channel exists, this property is undefined.

process.channel.ref()#

Added in: v7.1.0

This method makes the IPC channel keep the event loop of the process running if .unref() has been called before.

Typically, this is managed through the number of 'disconnect' and 'message' listeners on the process object. However, this method can be used to explicitly request a specific behavior.

process.channel.unref()#

Added in: v7.1.0

This method makes the IPC channel not keep the event loop of the process running, and lets it finish even while the channel is open.

Typically, this is managed through the number of 'disconnect' and 'message' listeners on the process object. However, this method can be used to explicitly request a specific behavior.

process.chdir(directory)#

Added in: v0.1.17

The process.chdir() method changes the current working directory of the Node.js process or throws an exception if doing so fails (for instance, if the specified directory does not exist).

console.log(`Starting directory: ${process.cwd()}`);
try {
  process.chdir('/tmp');
  console.log(`New directory: ${process.cwd()}`);
} catch (err) {
  console.error(`chdir: ${err}`);
}

This feature is not available in Worker threads.

process.config#

Added in: v0.7.7

The process.config property returns an Object containing the JavaScript representation of the configure options used to compile the current Node.js executable. This is the same as the config.gypi file that was produced when running the ./configure script.

An example of the possible output looks like:

{
  target_defaults:
   { cflags: [],
     default_configuration: 'Release',
     defines: [],
     include_dirs: [],
     libraries: [] },
  variables:
   {
     host_arch: 'x64',
     napi_build_version: 5,
     node_install_npm: 'true',
     node_prefix: '',
     node_shared_cares: 'false',
     node_shared_http_parser: 'false',
     node_shared_libuv: 'false',
     node_shared_zlib: 'false',
     node_use_dtrace: 'false',
     node_use_openssl: 'true',
     node_shared_openssl: 'false',
     strict_aliasing: 'true',
     target_arch: 'x64',
     v8_use_snapshot: 1
   }
}

The process.config property is not read-only and there are existing modules in the ecosystem that are known to extend, modify, or entirely replace the value of process.config.

process.connected#

Added in: v0.7.2

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the process.connected property will return true so long as the IPC channel is connected and will return false after process.disconnect() is called.

Once process.connected is false, it is no longer possible to send messages over the IPC channel using process.send().

process.cpuUsage([previousValue])#

Added in: v6.1.0

The process.cpuUsage() method returns the user and system CPU time usage of the current process, in an object with properties user and system, whose values are microsecond values (millionth of a second). These values measure time spent in user and system code respectively, and may end up being greater than actual elapsed time if multiple CPU cores are performing work for this process.

The result of a previous call to process.cpuUsage() can be passed as the argument to the function, to get a diff reading.

const startUsage = process.cpuUsage();
// { user: 38579, system: 6986 }

// spin the CPU for 500 milliseconds
const now = Date.now();
while (Date.now() - now < 500);

console.log(process.cpuUsage(startUsage));
// { user: 514883, system: 11226 }

process.cwd()#

Added in: v0.1.8

The process.cwd() method returns the current working directory of the Node.js process.

console.log(`Current directory: ${process.cwd()}`);

process.debugPort#

Added in: v0.7.2

The port used by the Node.js debugger when enabled.

process.debugPort = 5858;

process.disconnect()#

Added in: v0.7.2

If the Node.js process is spawned with an IPC channel (see the Child Process and Cluster documentation), the process.disconnect() method will close the IPC channel to the parent process, allowing the child process to exit gracefully once there are no other connections keeping it alive.

The effect of calling process.disconnect() is the same as calling ChildProcess.disconnect() from the parent process.

If the Node.js process was not spawned with an IPC channel, process.disconnect() will be undefined.

process.dlopen(module, filename[, flags])#

History
VersionChanges
v9.0.0

Added support for the flags argument.

v0.1.16

Added in: v0.1.16

The process.dlopen() method allows to dynamically load shared objects. It is primarily used by require() to load C++ Addons, and should not be used directly, except in special cases. In other words, require() should be preferred over process.dlopen(), unless there are specific reasons.

The flags argument is an integer that allows to specify dlopen behavior. See the os.constants.dlopen documentation for details.

If there are specific reasons to use process.dlopen() (for instance, to specify dlopen flags), it's often useful to use require.resolve() to look up the module's path.

An important drawback when calling process.dlopen() is that the module instance must be passed. Functions exported by the C++ Addon will be accessible via module.exports.

The example below shows how to load a C++ Addon, named as binding, that exports a foo function. All the symbols will be loaded before the call returns, by passing the RTLD_NOW constant. In this example the constant is assumed to be available.

const os = require('os');
process.dlopen(module, require.resolve('binding'),
               os.constants.dlopen.RTLD_NOW);
module.exports.foo();

process.emitWarning(warning[, options])#

Added in: v8.0.0
  • warning <string> | <Error> The warning to emit.

  • options <Object>

    • type <string> When warning is a String, type is the name to use for the type of warning being emitted. Default: 'Warning'.
    • code <string> A unique identifier for the warning instance being emitted.
    • ctor <Function> When warning is a String, ctor is an optional function used to limit the generated stack trace. Default: process.emitWarning.
    • detail <string> Additional text to include with the error.

The process.emitWarning() method can be used to emit custom or application specific process warnings. These can be listened for by adding a handler to the 'warning' event.

// Emit a warning with a code and additional detail.
process.emitWarning('Something happened!', {
  code: 'MY_WARNING',
  detail: 'This is some additional information'
});
// Emits:
// (node:56338) [MY_WARNING] Warning: Something happened!
// This is some additional information

In this example, an Error object is generated internally by process.emitWarning() and passed through to the 'warning' handler.

process.on('warning', (warning) => {
  console.warn(warning.name);    // 'Warning'
  console.warn(warning.message); // 'Something happened!'
  console.warn(warning.code);    // 'MY_WARNING'
  console.warn(warning.stack);   // Stack trace
  console.warn(warning.detail);  // 'This is some additional information'
});

If warning is passed as an Error object, the options argument is ignored.

process.emitWarning(warning[, type[, code]][, ctor])#

Added in: v6.0.0
  • warning <string> | <Error> The warning to emit.
  • type <string> When warning is a String, type is the name to use for the type of warning being emitted. Default: 'Warning'.
  • code <string> A unique identifier for the warning instance being emitted.
  • ctor <Function> When warning is a String, ctor is an optional function used to limit the generated stack trace. Default: process.emitWarning.

The process.emitWarning() method can be used to emit custom or application specific process warnings. These can be listened for by adding a handler to the 'warning' event.

// Emit a warning using a string.
process.emitWarning('Something happened!');
// Emits: (node: 56338) Warning: Something happened!

// Emit a warning using a string and a type.
process.emitWarning('Something Happened!', 'CustomWarning');
// Emits: (node:56338) CustomWarning: Something Happened!

process.emitWarning('Something happened!', 'CustomWarning', 'WARN001');
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!

In each of the previous examples, an Error object is generated internally by process.emitWarning() and passed through to the 'warning' handler.

process.on('warning', (warning) => {
  console.warn(warning.name);
  console.warn(warning.message);
  console.warn(warning.code);
  console.warn(warning.stack);
});

If warning is passed as an Error object, it will be passed through to the 'warning' event handler unmodified (and the optional type, code and ctor arguments will be ignored):

// Emit a warning using an Error object.
const myWarning = new Error('Something happened!');
// Use the Error name property to specify the type name
myWarning.name = 'CustomWarning';
myWarning.code = 'WARN001';

process.emitWarning(myWarning);
// Emits: (node:56338) [WARN001] CustomWarning: Something happened!

A TypeError is thrown if warning is anything other than a string or Error object.

While process warnings use Error objects, the process warning mechanism is not a replacement for normal error handling mechanisms.

The following additional handling is implemented if the warning type is 'DeprecationWarning':

  • If the --throw-deprecation command-line flag is used, the deprecation warning is thrown as an exception rather than being emitted as an event.
  • If the --no-deprecation command-line flag is used, the deprecation warning is suppressed.
  • If the --trace-deprecation command-line flag is used, the deprecation warning is printed to stderr along with the full stack trace.

Avoiding duplicate warnings#

As a best practice, warnings should be emitted only once per process. To do so, it is recommended to place the emitWarning() behind a simple boolean flag as illustrated in the example below:

function emitMyWarning() {
  if (!emitMyWarning.warned) {
    emitMyWarning.warned = true;
    process.emitWarning('Only warn once!');
  }
}
emitMyWarning();
// Emits: (node: 56339) Warning: Only warn once!
emitMyWarning();
// Emits nothing

process.env#

History
VersionChanges
v11.14.0

Worker threads will now use a copy of the parent thread’s process.env by default, configurable through the env option of the Worker constructor.

v10.0.0

Implicit conversion of variable value to string is deprecated.

v0.1.27

Added in: v0.1.27

The process.env property returns an object containing the user environment. See environ(7).

An example of this object looks like:

{
  TERM: 'xterm-256color',
  SHELL: '/usr/local/bin/bash',
  USER: 'maciej',
  PATH: '~/.bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin',
  PWD: '/Users/maciej',
  EDITOR: 'vim',
  SHLVL: '1',
  HOME: '/Users/maciej',
  LOGNAME: 'maciej',
  _: '/usr/local/bin/node'
}

It is possible to modify this object, but such modifications will not be reflected outside the Node.js process, or (unless explicitly requested) to other Worker threads. In other words, the following example would not work:

$ node -e 'process.env.foo = "bar"' && echo $foo

While the following will:

process.env.foo = 'bar';
console.log(process.env.foo);

Assigning a property on process.env will implicitly convert the value to a string. This behavior is deprecated. Future versions of Node.js may throw an error when the value is not a string, number, or boolean.

process.env.test = null;
console.log(process.env.test);
// => 'null'
process.env.test = undefined;
console.log(process.env.test);
// => 'undefined'

Use delete to delete a property from process.env.

process.env.TEST = 1;
delete process.env.TEST;
console.log(process.env.TEST);
// => undefined

On Windows operating systems, environment variables are case-insensitive.

process.env.TEST = 1;
console.log(process.env.test);
// => 1

Unless explicitly specified when creating a Worker instance, each Worker thread has its own copy of process.env, based on its parent thread’s process.env, or whatever was specified as the env option to the Worker constructor. Changes to process.env will not be visible across Worker threads, and only the main thread can make changes that are visible to the operating system or to native add-ons.

process.execArgv#

Added in: v0.7.7

The process.execArgv property returns the set of Node.js-specific command-line options passed when the Node.js process was launched. These options do not appear in the array returned by the process.argv property, and do not include the Node.js executable, the name of the script, or any options following the script name. These options are useful in order to spawn child processes with the same execution environment as the parent.

$ node --harmony script.js --version

Results in process.execArgv:

['--harmony']

And process.argv:

['/usr/local/bin/node', 'script.js', '--version']

process.execPath#

Added in: v0.1.100

The process.execPath property returns the absolute pathname of the executable that started the Node.js process.

'/usr/local/bin/node'

process.exit([code])#

Added in: v0.1.13

The process.exit() method instructs Node.js to terminate the process synchronously with an exit status of code. If code is omitted, exit uses either the 'success' code 0 or the value of process.exitCode if it has been set. Node.js will not terminate until all the 'exit' event listeners are called.

To exit with a 'failure' code:

process.exit(1);

The shell that executed Node.js should see the exit code as 1.

Calling process.exit() will force the process to exit as quickly as possible even if there are still asynchronous operations pending that have not yet completed fully, including I/O operations to process.stdout and process.stderr.

In most situations, it is not actually necessary to call process.exit() explicitly. The Node.js process will exit on its own if there is no additional work pending in the event loop. The process.exitCode property can be set to tell the process which exit code to use when the process exits gracefully.

For instance, the following example illustrates a misuse of the process.exit() method that could lead to data printed to stdout being truncated and lost:

// This is an example of what *not* to do:
if (someConditionNotMet()) {
  printUsageToStdout();
  process.exit(1);
}

The reason this is problematic is because writes to process.stdout in Node.js are sometimes asynchronous and may occur over multiple ticks of the Node.js event loop. Calling process.exit(), however, forces the process to exit before those additional writes to stdout can be performed.

Rather than calling process.exit() directly, the code should set the process.exitCode and allow the process to exit naturally by avoiding scheduling any additional work for the event loop:

// How to properly set the exit code while letting
// the process exit gracefully.
if (someConditionNotMet()) {
  printUsageToStdout();
  process.exitCode = 1;
}

If it is necessary to terminate the Node.js process due to an error condition, throwing an uncaught error and allowing the process to terminate accordingly is safer than calling process.exit().

In Worker threads, this function stops the current thread rather than the current process.

process.exitCode#

Added in: v0.11.8

A number which will be the process exit code, when the process either exits gracefully, or is exited via process.exit() without specifying a code.

Specifying a code to process.exit(code) will override any previous setting of process.exitCode.

process.getegid()#

Added in: v2.0.0

The process.getegid() method returns the numerical effective group identity of the Node.js process. (See getegid(2).)

if (process.getegid) {
  console.log(`Current gid: ${process.getegid()}`);
}

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.geteuid()#

Added in: v2.0.0

The process.geteuid() method returns the numerical effective user identity of the process. (See geteuid(2).)

if (process.geteuid) {
  console.log(`Current uid: ${process.geteuid()}`);
}

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.getgid()#

Added in: v0.1.31

The process.getgid() method returns the numerical group identity of the process. (See getgid(2).)

if (process.getgid) {
  console.log(`Current gid: ${process.getgid()}`);
}

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.getgroups()#

Added in: v0.9.4

The process.getgroups() method returns an array with the supplementary group IDs. POSIX leaves it unspecified if the effective group ID is included but Node.js ensures it always is.

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.getuid()#

Added in: v0.1.28

The process.getuid() method returns the numeric user identity of the process. (See getuid(2).)

if (process.getuid) {
  console.log(`Current uid: ${process.getuid()}`);
}

This function is only available on POSIX platforms (i.e. not Windows or Android).

process.hasUncaughtExceptionCaptureCallback()#

Added in: v9.3.0

Indicates whether a callback has been set using process.setUncaughtExceptionCaptureCallback().

process.hrtime([time])#

Added in: v0.7.6

This is the legacy version of process.hrtime.bigint() before bigint was introduced in JavaScript.

The process.hrtime() method returns the current high-resolution real time in a [seconds, nanoseconds] tuple Array, where nanoseconds is the remaining part of the real time that can't be represented in second precision.

time is an optional parameter that must be the result of a previous process.hrtime() call to diff with the current time. If the parameter passed in is not a tuple Array, a TypeError will be thrown. Passing in a user-defined array instead of the result of a previous call to process.hrtime() will lead to undefined behavior.

These times are relative to an arbitrary time in the past, and not related to the time of day and therefore not subject to clock drift. The primary use is for measuring performance between intervals:

const NS_PER_SEC = 1e9;
const time = process.hrtime();
// [ 1800216, 25 ]

setTimeout(() => {
  const diff = process.hrtime(time);
  // [ 1, 552 ]

  console.log(`Benchmark took ${diff[0] * NS_PER_SEC + diff[1]} nanoseconds`);
  // Benchmark took 1000000552 nanoseconds
}, 1000);

process.hrtime.bigint()#

Added in: v10.7.0

The bigint version of the process.hrtime() method returning the current high-resolution real time in nanoseconds as a bigint.

Unlike process.hrtime(), it does not support an additional time argument since the difference can just be computed directly by subtraction of the two bigints.

const start = process.hrtime.bigint();
// 191051479007711n

setTimeout(() => {
  const end = process.hrtime.bigint();
  // 191052633396993n

  console.log(`Benchmark took ${end - start} nanoseconds`);
  // Benchmark took 1154389282 nanoseconds
}, 1000);

process.initgroups(user, extraGroup)#

Added in: v0.9.4

The process.initgroups() method reads the /etc/group file and initializes the group access list, using all groups of which the user is a member. This is a privileged operation that requires that the Node.js process either have root access or the CAP_SETGID capability.

Use care when dropping privileges:

console.log(process.getgroups());         // [ 0 ]
process.initgroups('bnoordhuis', 1000);   // switch user
console.log(process.getgroups());         // [ 27, 30, 46, 1000, 0 ]
process.setgid(1000);                     // drop root gid
console.log(process.getgroups());         // [ 27, 30, 46, 1000 ]

This function is only available on POSIX platforms (i.e. not Windows or Android). This feature is not available in Worker threads.

process.kill(pid[, signal])#

Added in: v0.0.6
  • pid <number> A process ID
  • signal <string> | <number> The signal to send, either as a string or number. Default: 'SIGTERM'.

The process.kill() method sends the signal to the process identified by pid.

Signal names are strings such as 'SIGINT' or 'SIGHUP'. See Signal Events and kill(2) for more information.

This method will throw an error if the target pid does not exist. As a special case, a signal of 0 can be used to test for the existence of a process. Windows platforms will throw an error if the pid is used to kill a process group.

Even though the name of this function is process.kill(), it is really just a signal sender, like the kill system call. The signal sent may do something other than kill the target process.

process.on('SIGHUP', () => {
  console.log('Got SIGHUP signal.');
});

setTimeout(() => {
  console.log('Exiting.');
  process.exit(0);
}, 100);

process.kill(process.pid, 'SIGHUP');

When SIGUSR1 is received by a Node.js process, Node.js will start the debugger. See Signal Events.

process.mainModule#

Added in: v0.1.17Deprecated since: v14.0.0

Stability: 0 - Deprecated: Use require.main instead.

The process.mainModule property provides an alternative way of retrieving require.main. The difference is that if the main module changes at runtime, require.main may still refer to the original main module in modules that were required before the change occurred. Generally, it's safe to assume that the two refer to the same module.

As with require.main, process.mainModule will be undefined if there is no entry script.

process.memoryUsage()#

History
VersionChanges
v13.9.0

Added arrayBuffers to the returned object.

v7.2.0

Added external to the returned object.

v0.1.16

Added in: v0.1.16

The process.memoryUsage() method returns an object describing the memory usage of the Node.js process measured in bytes.

For example, the code:

console.log(process.memoryUsage());

Will generate:

{
  rss: 4935680,
  heapTotal: 1826816,
  heapUsed: 650472,
  external: 49879,
  arrayBuffers: 9386
}
  • heapTotal and heapUsed refer to V8's memory usage.
  • external refers to the memory usage of C++ objects bound to JavaScript objects managed by V8.
  • rss, Resident Set Size, is the amount of space occupied in the main memory device (that is a subset of the total allocated memory) for the process, including all C++ and JavaScript objects and code.
  • arrayBuffers refers to memory allocated for ArrayBuffers and SharedArrayBuffers, including all Node.js Buffers. This is also included in the external value. When Node.js is used as an embedded library, this value may be 0 because allocations for ArrayBuffers may not be tracked in that case.

When using Worker threads, rss will be a value that is valid for the entire process, while the other fields will only refer to the current thread.

process.nextTick(callback[, ...args])#

History
VersionChanges
v1.8.1

Additional arguments after callback are now supported.

v0.1.26

Added in: v0.1.26

  • callback <Function>
  • ...args <any> Additional arguments to pass when invoking the callback

process.nextTick() adds callback to the "next tick queue". This queue is fully drained after the current operation on the JavaScript stack runs to completion and before the event loop is allowed to continue. It's possible to create an infinite loop if one were to recursively call process.nextTick(). See the Event Loop guide for more background.

console.log('start');
process.nextTick(() => {
  console.log('nextTick callback');
});
console.log('scheduled');
// Output:
// start
// scheduled
// nextTick callback

This is important when developing APIs in order to give users the opportunity to assign event handlers after an object has been constructed but before any I/O has occurred:

function MyThing(options) {
  this.setupOptions(options);

  process.nextTick(() => {
    this.startDoingStuff();
  });
}

const thing = new MyThing();
thing.getReadyForStuff();

// thing.startDoingStuff() gets called now, not before.

It is very important for APIs to be either 100% synchronous or 100% asynchronous. Consider this example:

// WARNING!  DO NOT USE!  BAD UNSAFE HAZARD!
function maybeSync(arg, cb) {
  if (arg) {
    cb();
    return;
  }

  fs.stat('file', cb);
}

This API is hazardous because in the following case:

const maybeTrue = Math.random() > 0.5;

maybeSync(maybeTrue, () => {
  foo();
});

bar();

It is not clear whether foo() or bar() will be called first.

The following approach is much better:

function definitelyAsync(arg, cb) {
  if (arg) {
    process.nextTick(cb);
    return;
  }

  fs.stat('file', cb);
}

process.noDeprecation#

Added in: v0.8.0

The process.noDeprecation property indicates whether the --no-deprecation flag is set on the current Node.js process. See the documentation for the 'warning' event and the emitWarning() method for more information about this flag's behavior.

process.pid#

Added in: v0.1.15

The process.pid property returns the PID of the process.

console.log(`This process is pid ${process.pid}`);

process.platform#

Added in: v0.1.16

The process.platform property returns a string identifying the operating system platform on which the Node.js process is running.

Currently possible values are:

  • 'aix'
  • 'darwin'
  • 'freebsd'
  • 'linux'
  • 'openbsd'
  • 'sunos'
  • 'win32'
console.log(`This platform is ${process.platform}`);

The value 'android' may also be returned if the Node.js is built on the Android operating system. However, Android support in Node.js is experimental.

process.ppid#

Added in: v9.2.0, v8.10.0, v6.13.0

The process.ppid property returns the PID of the current parent process.

console.log(`The parent process is pid ${process.ppid}`);

process.release#

History
VersionChanges
v4.2.0

The lts property is now supported.

v3.0.0

Added in: v3.0.0

The process.release property returns an Object containing metadata related to the current release, including URLs for the source tarball and headers-only tarball.

process.release contains the following properties:

  • name <string> A value that will always be 'node' for Node.js. For legacy io.js releases, this will be 'io.js'.
  • sourceUrl <string> an absolute URL pointing to a .tar.gz file containing the source code of the current release.
  • headersUrl<string> an absolute URL pointing to a .tar.gz file containing only the source header files for the current release. This file is significantly smaller than the full source file and can be used for compiling Node.js native add-ons.
  • libUrl <string> an absolute URL pointing to a node.lib file matching the architecture and version of the current release. This file is used for compiling Node.js native add-ons. This property is only present on Windows builds of Node.js and will be missing on all other platforms.

  • lts <string> a string label identifying the LTS label for this release. This property only exists for LTS releases and is undefined for all other release types, including Current releases. Currently the valid values are:

    • 'Argon' for the 4.x LTS line beginning with 4.2.0.
    • 'Boron' for the 6.x LTS line beginning with 6.9.0.
    • 'Carbon' for the 8.x LTS line beginning with 8.9.1.
{
  name: 'node',
  lts: 'Argon',
  sourceUrl: 'https://nodejs.org/download/release/v4.4.5/node-v4.4.5.tar.gz',
  headersUrl: 'https://nodejs.org/download/release/v4.4.5/node-v4.4.5-headers.tar.gz',
  libUrl: 'https://nodejs.org/download/release/v4.4.5/win-x64/node.lib'
}

In custom builds from non-release versions of the source tree, only the name property may be present. The additional properties should not be relied upon to exist.

process.report#

History
VersionChanges
v13.12.0

This API is no longer considered experimental.

v11.8.0

Added in: v11.8.0

process.report is an object whose methods are used to generate diagnostic reports for the current process. Additional documentation is available in the report documentation.

process.report.compact#

Added in: v13.12.0

Write reports in a compact format, single-line JSON, more easily consumable by log processing systems than the default multi-line format designed for human consumption.

console.log(`Reports are compact? ${process.report.compact}`);

process.report.directory#

History
VersionChanges
v13.12.0

This API is no longer considered experimental.

v11.12.0

Added in: v11.12.0

Directory where the report is written. The default value is the empty string, indicating that reports are written to the current working directory of the Node.js process.

console.log(`Report directory is ${process.report.directory}`);

process.report.filename#

History
VersionChanges
v13.12.0

This API is no longer considered experimental.

v11.12.0

Added in: v11.12.0

Filename where the report is written. If set to the empty string, the output filename will be comprised of a timestamp, PID, and sequence number. The default value is the empty string.

console.log(`Report filename is ${process.report.filename}`);

process.report.getReport([err])#

History
VersionChanges
v13.12.0

This API is no longer considered experimental.

v11.8.0

Added in: v11.8.0

  • err <Error> A custom error used for reporting the JavaScript stack.
  • Returns: <Object>

Returns a JavaScript Object representation of a diagnostic report for the running process. The report's JavaScript stack trace is taken from err, if present.

const data = process.report.getReport();
console.log(data.header.nodeJsVersion);

// Similar to process.report.writeReport()
const fs = require('fs');
fs.writeFileSync(util.inspect(data), 'my-report.log', 'utf8');

Additional documentation is available in the report documentation.

process.report.reportOnFatalError#

Added in: v11.12.0

Stability: 1 - Experimental

If true, a diagnostic report is generated on fatal errors, such as out of memory errors or failed C++ assertions.

console.log(`Report on fatal error: ${process.report.reportOnFatalError}`);

process.report.reportOnSignal#

History
VersionChanges
v13.12.0

This API is no longer considered experimental.

v11.12.0

Added in: v11.12.0

If true, a diagnostic report is generated when the process receives the signal specified by process.report.signal.

console.log(`Report on signal: ${process.report.reportOnSignal}`);

process.report.reportOnUncaughtException#

History
VersionChanges
v13.12.0

This API is no longer considered experimental.

v11.12.0

Added in: v11.12.0

If true, a diagnostic report is generated on uncaught exception.

console.log(`Report on exception: ${process.report.reportOnUncaughtException}`);

process.report.signal#

History
VersionChanges
v13.12.0

This API is no longer considered experimental.

v11.12.0

Added in: v11.12.0

The signal used to trigger the creation of a diagnostic report. Defaults to 'SIGUSR2'.

console.log(`Report signal: ${process.report.signal}`);

process.report.writeReport([filename][, err])#

History
VersionChanges
v13.12.0

This API is no longer considered experimental.

v11.8.0

Added in: v11.8.0

  • filename <string> Name of the file where the report is written. This should be a relative path, that will be appended to the directory specified in process.report.directory, or the current working directory of the Node.js process, if unspecified.

  • err <Error> A custom error used for reporting the JavaScript stack.

  • Returns: <string> Returns the filename of the generated report.

Writes a diagnostic report to a file. If filename is not provided, the default filename includes the date, time, PID, and a sequence number. The report's JavaScript stack trace is taken from err, if present.

process.report.writeReport();

Additional documentation is available in the report documentation.

process.resourceUsage()#

Added in: v12.6.0

  • Returns: <Object> the resource usage for the current process. All of these values come from the uv_getrusage call which returns a uv_rusage_t struct.

    • userCPUTime <integer> maps to ru_utime computed in microseconds. It is the same value as process.cpuUsage().user.
    • systemCPUTime <integer> maps to ru_stime computed in microseconds. It is the same value as process.cpuUsage().system.
    • maxRSS <integer> maps to ru_maxrss which is the maximum resident set size used in kilobytes.
    • sharedMemorySize <integer> maps to ru_ixrss but is not supported by any platform.
    • unsharedDataSize <integer> maps to ru_idrss but is not supported by any platform.
    • unsharedStackSize <integer> maps to ru_isrss but is not supported by any platform.
    • minorPageFault <integer> maps to ru_minflt which is the number of minor page faults for the process, see this article for more details.
    • majorPageFault <integer> maps to ru_majflt which is the number of major page faults for the process, see this article for more details. This field is not supported on Windows.
    • swappedOut <integer> maps to ru_nswap but is not supported by any platform.
    • fsRead <integer> maps to ru_inblock which is the number of times the file system had to perform input.
    • fsWrite <integer> maps to ru_oublock which is the number of times the file system had to perform output.
    • ipcSent <integer> maps to ru_msgsnd but is not supported by any platform.
    • ipcReceived <integer> maps to ru_msgrcv but is not supported by any platform.
    • signalsCount <integer> maps to ru_nsignals but is not supported by any platform.
    • voluntaryContextSwitches <integer> maps to ru_nvcsw which is the number of times a CPU context switch resulted due to a process voluntarily giving up the processor before its time slice was completed (usually to await availability of a resource). This field is not supported on Windows.
    • involuntaryContextSwitches <integer> maps to ru_nivcsw which is the number of times a CPU context switch resulted due to a higher priority process becoming runnable or because the current process exceeded its time slice. This field is not supported on Windows.
console.log(process.resourceUsage());
/*
  Will output:
  {
    userCPUTime: 82872,
    systemCPUTime: 4143,
    maxRSS: 33164,
    sharedMemorySize: 0,
    unsharedDataSize: 0,
    unsharedStackSize: 0,
    minorPageFault: 2469,
    majorPageFault: 0,
    swappedOut: 0,
    fsRead: 0,
    fsWrite: 8,
    ipcSent: 0,
    ipcReceived: 0,
    signalsCount: 0,
    voluntaryContextSwitches: 79,
    involuntaryContextSwitches: 1
  }
*/

process.send(message[, sendHandle[, options]][, callback])#

Added in: v0.5.9
  • message <Object>
  • sendHandle <net.Server> | <net.Socket>

  • options <Object> used to parameterize the sending of certain types of handles.options supports the following properties:

    • keepOpen <boolean> A value that can be used when passing instances of net.Socket. When true, the socket is kept open in the sending process. Default: false.
  • callback <Function>
  • Returns: <boolean>

If Node.js is spawned with an IPC channel, the process.send() method can be used to send messages to the parent process. Messages will be received as a 'message' event on the parent's ChildProcess object.

If Node.js was not spawned with an IPC channel, process.send will be undefined.

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

process.setegid(id)#

Added in: v2.0.0

The process.setegid() method sets the effective group identity of the process. (See setegid(2).) The id can be passed as either a numeric ID or a group name string. If a group name is specified, this method blocks while resolving the associated a numeric ID.

if (process.getegid && process.setegid) {
  console.log(`Current gid: ${process.getegid()}`);
  try {
    process.setegid(501);
    console.log(`New gid: ${process.getegid()}`);
  } catch (err) {
    console.log(`Failed to set gid: ${err}`);
  }
}

This function is only available on POSIX platforms (i.e. not Windows or Android). This feature is not available in Worker threads.

process.seteuid(id)#

Added in: v2.0.0

The process.seteuid() method sets the effective user identity of the process. (See seteuid(2).) The id can be passed as either a numeric ID or a username string. If a username is specified, the method blocks while resolving the associated numeric ID.

if (process.geteuid && process.seteuid) {
  console.log(`Current uid: ${process.geteuid()}`);
  try {
    process.seteuid(501);
    console.log(`New uid: ${process.geteuid()}`);
  } catch (err) {
    console.log(`Failed to set uid: ${err}`);
  }
}

This function is only available on POSIX platforms (i.e. not Windows or Android). This feature is not available in Worker threads.

process.setgid(id)#

Added in: v0.1.31

The process.setgid() method sets the group identity of the process. (See setgid(2).) The id can be passed as either a numeric ID or a group name string. If a group name is specified, this method blocks while resolving the associated numeric ID.

if (process.getgid && process.setgid) {
  console.log(`Current gid: ${process.getgid()}`);
  try {
    process.setgid(501);
    console.log(`New gid: ${process.getgid()}`);
  } catch (err) {
    console.log(`Failed to set gid: ${err}`);
  }
}

This function is only available on POSIX platforms (i.e. not Windows or Android). This feature is not available in Worker threads.

process.setgroups(groups)#

Added in: v0.9.4

The process.setgroups() method sets the supplementary group IDs for the Node.js process. This is a privileged operation that requires the Node.js process to have root or the CAP_SETGID capability.

The groups array can contain numeric group IDs, group names or both.

This function is only available on POSIX platforms (i.e. not Windows or Android). This feature is not available in Worker threads.

process.setuid(id)#

Added in: v0.1.28

The process.setuid(id) method sets the user identity of the process. (See setuid(2).) The id can be passed as either a numeric ID or a username string. If a username is specified, the method blocks while resolving the associated numeric ID.

if (process.getuid && process.setuid) {
  console.log(`Current uid: ${process.getuid()}`);
  try {
    process.setuid(501);
    console.log(`New uid: ${process.getuid()}`);
  } catch (err) {
    console.log(`Failed to set uid: ${err}`);
  }
}

This function is only available on POSIX platforms (i.e. not Windows or Android). This feature is not available in Worker threads.

process.setUncaughtExceptionCaptureCallback(fn)#

Added in: v9.3.0

The process.setUncaughtExceptionCaptureCallback() function sets a function that will be invoked when an uncaught exception occurs, which will receive the exception value itself as its first argument.

If such a function is set, the 'uncaughtException' event will not be emitted. If --abort-on-uncaught-exception was passed from the command line or set through v8.setFlagsFromString(), the process will not abort.

To unset the capture function, process.setUncaughtExceptionCaptureCallback(null) may be used. Calling this method with a non-null argument while another capture function is set will throw an error.

Using this function is mutually exclusive with using the deprecated domain built-in module.

process.stderr#

The process.stderr property returns a stream connected to stderr (fd 2). It is a net.Socket (which is a Duplex stream) unless fd 2 refers to a file, in which case it is a Writable stream.

process.stderr differs from other Node.js streams in important ways. See note on process I/O for more information.

process.stderr.fd#

This property refers to the value of underlying file descriptor of process.stderr. The value is fixed at 2. In Worker threads, this field does not exist.

process.stdin#

The process.stdin property returns a stream connected to stdin (fd 0). It is a net.Socket (which is a Duplex stream) unless fd 0 refers to a file, in which case it is a Readable stream.

process.stdin.setEncoding('utf8');

process.stdin.on('readable', () => {
  let chunk;
  // Use a loop to make sure we read all available data.
  while ((chunk = process.stdin.read()) !== null) {
    process.stdout.write(`data: ${chunk}`);
  }
});

process.stdin.on('end', () => {
  process.stdout.write('end');
});

As a Duplex stream, process.stdin can also be used in "old" mode that is compatible with scripts written for Node.js prior to v0.10. For more information see Stream compatibility.

In "old" streams mode the stdin stream is paused by default, so one must call process.stdin.resume() to read from it. Note also that calling process.stdin.resume() itself would switch stream to "old" mode.

process.stdin.fd#

This property refers to the value of underlying file descriptor of process.stdin. The value is fixed at 0. In Worker threads, this field does not exist.

process.stdout#

The process.stdout property returns a stream connected to stdout (fd 1). It is a net.Socket (which is a Duplex stream) unless fd 1 refers to a file, in which case it is a Writable stream.

For example, to copy process.stdin to process.stdout:

process.stdin.pipe(process.stdout);

process.stdout differs from other Node.js streams in important ways. See note on process I/O for more information.

process.stdout.fd#

This property refers to the value of underlying file descriptor of process.stdout. The value is fixed at 1. In Worker threads, this field does not exist.

A note on process I/O#

process.stdout and process.stderr differ from other Node.js streams in important ways:

  1. They are used internally by console.log() and console.error(), respectively.

  2. Writes may be synchronous depending on what the stream is connected to and whether the system is Windows or POSIX:

    • Files: synchronous on Windows and POSIX
    • TTYs (Terminals): asynchronous on Windows, synchronous on POSIX
    • Pipes (and sockets): synchronous on Windows, asynchronous on POSIX

These behaviors are partly for historical reasons, as changing them would create backwards incompatibility, but they are also expected by some users.

Synchronous writes avoid problems such as output written with console.log() or console.error() being unexpectedly interleaved, or not written at all if process.exit() is called before an asynchronous write completes. See process.exit() for more information.

Warning: Synchronous writes block the event loop until the write has completed. This can be near instantaneous in the case of output to a file, but under high system load, pipes that are not being read at the receiving end, or with slow terminals or file systems, its possible for the event loop to be blocked often enough and long enough to have severe negative performance impacts. This may not be a problem when writing to an interactive terminal session, but consider this particularly careful when doing production logging to the process output streams.

To check if a stream is connected to a TTY context, check the isTTY property.

For instance:

$ node -p "Boolean(process.stdin.isTTY)"
true
$ echo "foo" | node -p "Boolean(process.stdin.isTTY)"
false
$ node -p "Boolean(process.stdout.isTTY)"
true
$ node -p "Boolean(process.stdout.isTTY)" | cat
false

See the TTY documentation for more information.

process.throwDeprecation#

Added in: v0.9.12

The initial value of process.throwDeprecation indicates whether the --throw-deprecation flag is set on the current Node.js process. process.throwDeprecation is mutable, so whether or not deprecation warnings result in errors may be altered at runtime. See the documentation for the 'warning' event and the emitWarning() method for more information.

$ node --throw-deprecation -p "process.throwDeprecation"
true
$ node -p "process.throwDeprecation"
undefined
$ node
> process.emitWarning('test', 'DeprecationWarning');
undefined
> (node:26598) DeprecationWarning: test
> process.throwDeprecation = true;
true
> process.emitWarning('test', 'DeprecationWarning');
Thrown:
[DeprecationWarning: test] { name: 'DeprecationWarning' }

process.title#

Added in: v0.1.104

The process.title property returns the current process title (i.e. returns the current value of ps). Assigning a new value to process.title modifies the current value of ps.

When a new value is assigned, different platforms will impose different maximum length restrictions on the title. Usually such restrictions are quite limited. For instance, on Linux and macOS, process.title is limited to the size of the binary name plus the length of the command line arguments because setting the process.title overwrites the argv memory of the process. Node.js v0.8 allowed for longer process title strings by also overwriting the environ memory but that was potentially insecure and confusing in some (rather obscure) cases.

process.traceDeprecation#

Added in: v0.8.0

The process.traceDeprecation property indicates whether the --trace-deprecation flag is set on the current Node.js process. See the documentation for the 'warning' event and the emitWarning() method for more information about this flag's behavior.

process.umask()#

History
VersionChanges
v14.0.0

Calling process.umask() with no arguments is deprecated.

v0.1.19

Added in: v0.1.19

Stability: 0 - Deprecated. Calling process.umask() with no argument causes the process-wide umask to be written twice. This introduces a race condition between threads, and is a potential security vulnerability. There is no safe, cross-platform alternative API.

process.umask() returns the Node.js process's file mode creation mask. Child processes inherit the mask from the parent process.

process.umask(mask)#

Added in: v0.1.19

process.umask(mask) sets the Node.js process's file mode creation mask. Child processes inherit the mask from the parent process. Returns the previous mask.

const newmask = 0o022;
const oldmask = process.umask(newmask);
console.log(
  `Changed umask from ${oldmask.toString(8)} to ${newmask.toString(8)}`
);

In Worker threads, process.umask(mask) will throw an exception.

process.uptime()#

Added in: v0.5.0

The process.uptime() method returns the number of seconds the current Node.js process has been running.

The return value includes fractions of a second. Use Math.floor() to get whole seconds.

process.version#

Added in: v0.1.3

The process.version property returns the Node.js version string.

console.log(`Version: ${process.version}`);

process.versions#

History
VersionChanges
v9.0.0

The v8 property now includes a Node.js specific suffix.

v4.2.0

The icu property is now supported.

v0.2.0

Added in: v0.2.0

The process.versions property returns an object listing the version strings of Node.js and its dependencies. process.versions.modules indicates the current ABI version, which is increased whenever a C++ API changes. Node.js will refuse to load modules that were compiled against a different module ABI version.

console.log(process.versions);

Will generate an object similar to:

{ node: '11.13.0',
  v8: '7.0.276.38-node.18',
  uv: '1.27.0',
  zlib: '1.2.11',
  brotli: '1.0.7',
  ares: '1.15.0',
  modules: '67',
  nghttp2: '1.34.0',
  napi: '4',
  llhttp: '1.1.1',
  openssl: '1.1.1b',
  cldr: '34.0',
  icu: '63.1',
  tz: '2018e',
  unicode: '11.0' }

Exit Codes#

Node.js will normally exit with a 0 status code when no more async operations are pending. The following status codes are used in other cases:

  • 1 Uncaught Fatal Exception: There was an uncaught exception, and it was not handled by a domain or an 'uncaughtException' event handler.
  • 2: Unused (reserved by Bash for builtin misuse)
  • 3 Internal JavaScript Parse Error: The JavaScript source code internal in the Node.js bootstrapping process caused a parse error. This is extremely rare, and generally can only happen during development of Node.js itself.
  • 4 Internal JavaScript Evaluation Failure: The JavaScript source code internal in the Node.js bootstrapping process failed to return a function value when evaluated. This is extremely rare, and generally can only happen during development of Node.js itself.
  • 5 Fatal Error: There was a fatal unrecoverable error in V8. Typically a message will be printed to stderr with the prefix FATAL ERROR.
  • 6 Non-function Internal Exception Handler: There was an uncaught exception, but the internal fatal exception handler function was somehow set to a non-function, and could not be called.
  • 7 Internal Exception Handler Run-Time Failure: There was an uncaught exception, and the internal fatal exception handler function itself threw an error while attempting to handle it. This can happen, for example, if an 'uncaughtException' or domain.on('error') handler throws an error.
  • 8: Unused. In previous versions of Node.js, exit code 8 sometimes indicated an uncaught exception.
  • 9 Invalid Argument: Either an unknown option was specified, or an option requiring a value was provided without a value.
  • 10 Internal JavaScript Run-Time Failure: The JavaScript source code internal in the Node.js bootstrapping process threw an error when the bootstrapping function was called. This is extremely rare, and generally can only happen during development of Node.js itself.
  • 12 Invalid Debug Argument: The --inspect and/or --inspect-brk options were set, but the port number chosen was invalid or unavailable.
  • >128 Signal Exits: If Node.js receives a fatal signal such as SIGKILL or SIGHUP, then its exit code will be 128 plus the value of the signal code. This is a standard POSIX practice, since exit codes are defined to be 7-bit integers, and signal exits set the high-order bit, and then contain the value of the signal code. For example, signal SIGABRT has value 6, so the expected exit code will be 128 + 6, or 134.

Punycode#

History
VersionChanges
v7.0.0

Accessing this module will now emit a deprecation warning.

Stability: 0 - Deprecated

The version of the punycode module bundled in Node.js is being deprecated. In a future major version of Node.js this module will be removed. Users currently depending on the punycode module should switch to using the userland-provided Punycode.js module instead.

The punycode module is a bundled version of the Punycode.js module. It can be accessed using:

const punycode = require('punycode');

Punycode is a character encoding scheme defined by RFC 3492 that is primarily intended for use in Internationalized Domain Names. Because host names in URLs are limited to ASCII characters only, Domain Names that contain non-ASCII characters must be converted into ASCII using the Punycode scheme. For instance, the Japanese character that translates into the English word, 'example' is '例'. The Internationalized Domain Name, '例.com' (equivalent to 'example.com') is represented by Punycode as the ASCII string 'xn--fsq.com'.

The punycode module provides a simple implementation of the Punycode standard.

The punycode module is a third-party dependency used by Node.js and made available to developers as a convenience. Fixes or other modifications to the module must be directed to the Punycode.js project.

punycode.decode(string)#

Added in: v0.5.1

The punycode.decode() method converts a Punycode string of ASCII-only characters to the equivalent string of Unicode codepoints.

punycode.decode('maana-pta'); // 'mañana'
punycode.decode('--dqo34k'); // '☃-⌘'

punycode.encode(string)#

Added in: v0.5.1

The punycode.encode() method converts a string of Unicode codepoints to a Punycode string of ASCII-only characters.

punycode.encode('mañana'); // 'maana-pta'
punycode.encode('☃-⌘'); // '--dqo34k'

punycode.toASCII(domain)#

Added in: v0.6.1

The punycode.toASCII() method converts a Unicode string representing an Internationalized Domain Name to Punycode. Only the non-ASCII parts of the domain name will be converted. Calling punycode.toASCII() on a string that already only contains ASCII characters will have no effect.

// encode domain names
punycode.toASCII('mañana.com');  // 'xn--maana-pta.com'
punycode.toASCII('☃-⌘.com');   // 'xn----dqo34k.com'
punycode.toASCII('example.com'); // 'example.com'

punycode.toUnicode(domain)#

Added in: v0.6.1

The punycode.toUnicode() method converts a string representing a domain name containing Punycode encoded characters into Unicode. Only the Punycode encoded parts of the domain name are be converted.

// decode domain names
punycode.toUnicode('xn--maana-pta.com'); // 'mañana.com'
punycode.toUnicode('xn----dqo34k.com');  // '☃-⌘.com'
punycode.toUnicode('example.com');       // 'example.com'

punycode.ucs2#

Added in: v0.7.0

punycode.ucs2.decode(string)#

Added in: v0.7.0

The punycode.ucs2.decode() method returns an array containing the numeric codepoint values of each Unicode symbol in the string.

punycode.ucs2.decode('abc'); // [0x61, 0x62, 0x63]
// surrogate pair for U+1D306 tetragram for centre:
punycode.ucs2.decode('\uD834\uDF06'); // [0x1D306]

punycode.ucs2.encode(codePoints)#

Added in: v0.7.0

The punycode.ucs2.encode() method returns a string based on an array of numeric code point values.

punycode.ucs2.encode([0x61, 0x62, 0x63]); // 'abc'
punycode.ucs2.encode([0x1D306]); // '\uD834\uDF06'

punycode.version#

Added in: v0.6.1

Returns a string identifying the current Punycode.js version number.

Query String#

Stability: 2 - Stable

The querystring module provides utilities for parsing and formatting URL query strings. It can be accessed using:

const querystring = require('querystring');

querystring.decode()#

Added in: v0.1.99

The querystring.decode() function is an alias for querystring.parse().

querystring.encode()#

Added in: v0.1.99

The querystring.encode() function is an alias for querystring.stringify().

querystring.escape(str)#

Added in: v0.1.25

The querystring.escape() method performs URL percent-encoding on the given str in a manner that is optimized for the specific requirements of URL query strings.

The querystring.escape() method is used by querystring.stringify() and is generally not expected to be used directly. It is exported primarily to allow application code to provide a replacement percent-encoding implementation if necessary by assigning querystring.escape to an alternative function.

querystring.parse(str[, sep[, eq[, options]]])#

History
VersionChanges
v8.0.0

Multiple empty entries are now parsed correctly (e.g. &=&=).

v6.0.0

The returned object no longer inherits from Object.prototype.

v6.0.0, v4.2.4

The eq parameter may now have a length of more than 1.

v0.1.25

Added in: v0.1.25

  • str <string> The URL query string to parse
  • sep <string> The substring used to delimit key and value pairs in the query string. Default: '&'.
  • eq <string>. The substring used to delimit keys and values in the query string. Default: '='.

  • options <Object>

    • decodeURIComponent <Function> The function to use when decoding percent-encoded characters in the query string. Default: querystring.unescape().
    • maxKeys <number> Specifies the maximum number of keys to parse. Specify 0 to remove key counting limitations. Default: 1000.

The querystring.parse() method parses a URL query string (str) into a collection of key and value pairs.

For example, the query string 'foo=bar&abc=xyz&abc=123' is parsed into:

{
  foo: 'bar',
  abc: ['xyz', '123']
}

The object returned by the querystring.parse() method does not prototypically inherit from the JavaScript Object. This means that typical Object methods such as obj.toString(), obj.hasOwnProperty(), and others are not defined and will not work.

By default, percent-encoded characters within the query string will be assumed to use UTF-8 encoding. If an alternative character encoding is used, then an alternative decodeURIComponent option will need to be specified:

// Assuming gbkDecodeURIComponent function already exists...

querystring.parse('w=%D6%D0%CE%C4&foo=bar', null, null,
                  { decodeURIComponent: gbkDecodeURIComponent });

querystring.stringify(obj[, sep[, eq[, options]]])#

Added in: v0.1.25
  • obj <Object> The object to serialize into a URL query string
  • sep <string> The substring used to delimit key and value pairs in the query string. Default: '&'.
  • eq <string>. The substring used to delimit keys and values in the query string. Default: '='.

  • options

    • encodeURIComponent <Function> The function to use when converting URL-unsafe characters to percent-encoding in the query string. Default: querystring.escape().

The querystring.stringify() method produces a URL query string from a given obj by iterating through the object's "own properties".

It serializes the following types of values passed in obj: <string> | <number> | <boolean> | <string[]> | <number[]> | <boolean[]> Any other input values will be coerced to empty strings.

querystring.stringify({ foo: 'bar', baz: ['qux', 'quux'], corge: '' });
// Returns 'foo=bar&baz=qux&baz=quux&corge='

querystring.stringify({ foo: 'bar', baz: 'qux' }, ';', ':');
// Returns 'foo:bar;baz:qux'

By default, characters requiring percent-encoding within the query string will be encoded as UTF-8. If an alternative encoding is required, then an alternative encodeURIComponent option will need to be specified:

// Assuming gbkEncodeURIComponent function already exists,

querystring.stringify({ w: '中文', foo: 'bar' }, null, null,
                      { encodeURIComponent: gbkEncodeURIComponent });

querystring.unescape(str)#

Added in: v0.1.25

The querystring.unescape() method performs decoding of URL percent-encoded characters on the given str.

The querystring.unescape() method is used by querystring.parse() and is generally not expected to be used directly. It is exported primarily to allow application code to provide a replacement decoding implementation if necessary by assigning querystring.unescape to an alternative function.

By default, the querystring.unescape() method will attempt to use the JavaScript built-in decodeURIComponent() method to decode. If that fails, a safer equivalent that does not throw on malformed URLs will be used.

Readline#

Stability: 2 - Stable

The readline module provides an interface for reading data from a Readable stream (such as process.stdin) one line at a time. It can be accessed using:

const readline = require('readline');

The following simple example illustrates the basic use of the readline module.

const readline = require('readline');

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

rl.question('What do you think of Node.js? ', (answer) => {
  // TODO: Log the answer in a database
  console.log(`Thank you for your valuable feedback: ${answer}`);

  rl.close();
});

Once this code is invoked, the Node.js application will not terminate until the readline.Interface is closed because the interface waits for data to be received on the input stream.

Class: Interface#

Added in: v0.1.104

Instances of the readline.Interface class are constructed using the readline.createInterface() method. Every instance is associated with a single input Readable stream and a single output Writable stream. The output stream is used to print prompts for user input that arrives on, and is read from, the input stream.

Event: 'close'#

Added in: v0.1.98

The 'close' event is emitted when one of the following occur:

  • The rl.close() method is called and the readline.Interface instance has relinquished control over the input and output streams;
  • The input stream receives its 'end' event;
  • The input stream receives <ctrl>-D to signal end-of-transmission (EOT);
  • The input stream receives <ctrl>-C to signal SIGINT and there is no 'SIGINT' event listener registered on the readline.Interface instance.

The listener function is called without passing any arguments.

The readline.Interface instance is finished once the 'close' event is emitted.

Event: 'line'#

Added in: v0.1.98

The 'line' event is emitted whenever the input stream receives an end-of-line input (\n, \r, or \r\n). This usually occurs when the user presses the <Enter>, or <Return> keys.

The listener function is called with a string containing the single line of received input.

rl.on('line', (input) => {
  console.log(`Received: ${input}`);
});

Event: 'pause'#

Added in: v0.7.5

The 'pause' event is emitted when one of the following occur:

  • The input stream is paused.
  • The input stream is not paused and receives the 'SIGCONT' event. (See events 'SIGTSTP' and 'SIGCONT'.)

The listener function is called without passing any arguments.

rl.on('pause', () => {
  console.log('Readline paused.');
});

Event: 'resume'#

Added in: v0.7.5

The 'resume' event is emitted whenever the input stream is resumed.

The listener function is called without passing any arguments.

rl.on('resume', () => {
  console.log('Readline resumed.');
});

Event: 'SIGCONT'#

Added in: v0.7.5

The 'SIGCONT' event is emitted when a Node.js process previously moved into the background using <ctrl>-Z (i.e. SIGTSTP) is then brought back to the foreground using fg(1p).

If the input stream was paused before the SIGTSTP request, this event will not be emitted.

The listener function is invoked without passing any arguments.

rl.on('SIGCONT', () => {
  // `prompt` will automatically resume the stream
  rl.prompt();
});

The 'SIGCONT' event is not supported on Windows.

Event: 'SIGINT'#

Added in: v0.3.0

The 'SIGINT' event is emitted whenever the input stream receives a <ctrl>-C input, known typically as SIGINT. If there are no 'SIGINT' event listeners registered when the input stream receives a SIGINT, the 'pause' event will be emitted.

The listener function is invoked without passing any arguments.

rl.on('SIGINT', () => {
  rl.question('Are you sure you want to exit? ', (answer) => {
    if (answer.match(/^y(es)?$/i)) rl.pause();
  });
});

Event: 'SIGTSTP'#

Added in: v0.7.5

The 'SIGTSTP' event is emitted when the input stream receives a <ctrl>-Z input, typically known as SIGTSTP. If there are no 'SIGTSTP' event listeners registered when the input stream receives a SIGTSTP, the Node.js process will be sent to the background.

When the program is resumed using fg(1p), the 'pause' and 'SIGCONT' events will be emitted. These can be used to resume the input stream.

The 'pause' and 'SIGCONT' events will not be emitted if the input was paused before the process was sent to the background.

The listener function is invoked without passing any arguments.

rl.on('SIGTSTP', () => {
  // This will override SIGTSTP and prevent the program from going to the
  // background.
  console.log('Caught SIGTSTP.');
});

The 'SIGTSTP' event is not supported on Windows.

rl.close()#

Added in: v0.1.98

The rl.close() method closes the readline.Interface instance and relinquishes control over the input and output streams. When called, the 'close' event will be emitted.

Calling rl.close() does not immediately stop other events (including 'line') from being emitted by the readline.Interface instance.

rl.pause()#

Added in: v0.3.4

The rl.pause() method pauses the input stream, allowing it to be resumed later if necessary.

Calling rl.pause() does not immediately pause other events (including 'line') from being emitted by the readline.Interface instance.

rl.prompt([preserveCursor])#

Added in: v0.1.98
  • preserveCursor <boolean> If true, prevents the cursor placement from being reset to 0.

The rl.prompt() method writes the readline.Interface instances configured prompt to a new line in output in order to provide a user with a new location at which to provide input.

When called, rl.prompt() will resume the input stream if it has been paused.

If the readline.Interface was created with output set to null or undefined the prompt is not written.

rl.question(query, callback)#

Added in: v0.3.3
  • query <string> A statement or query to write to output, prepended to the prompt.
  • callback <Function> A callback function that is invoked with the user's input in response to the query.

The rl.question() method displays the query by writing it to the output, waits for user input to be provided on input, then invokes the callback function passing the provided input as the first argument.

When called, rl.question() will resume the input stream if it has been paused.

If the readline.Interface was created with output set to null or undefined the query is not written.

Example usage:

rl.question('What is your favorite food? ', (answer) => {
  console.log(`Oh, so your favorite food is ${answer}`);
});

The callback function passed to rl.question() does not follow the typical pattern of accepting an Error object or null as the first argument. The callback is called with the provided answer as the only argument.

rl.resume()#

Added in: v0.3.4

The rl.resume() method resumes the input stream if it has been paused.

rl.setPrompt(prompt)#

Added in: v0.1.98

The rl.setPrompt() method sets the prompt that will be written to output whenever rl.prompt() is called.

rl.write(data[, key])#

Added in: v0.1.98

The rl.write() method will write either data or a key sequence identified by key to the output. The key argument is supported only if output is a TTY text terminal. See TTY keybindings for a list of key combinations.

If key is specified, data is ignored.

When called, rl.write() will resume the input stream if it has been paused.

If the readline.Interface was created with output set to null or undefined the data and key are not written.

rl.write('Delete this!');
// Simulate Ctrl+u to delete the line written previously
rl.write(null, { ctrl: true, name: 'u' });

The rl.write() method will write the data to the readline Interface's input as if it were provided by the user.

rl[Symbol.asyncIterator]()#

History
VersionChanges
v11.4.0, v10.16.0

Added in: v11.4.0, v10.16.0

v11.14.0, v10.17.0

Symbol.asyncIterator support is no longer experimental.

Create an AsyncIterator object that iterates through each line in the input stream as a string. This method allows asynchronous iteration of readline.Interface objects through for await...of loops.

Errors in the input stream are not forwarded.

If the loop is terminated with break, throw, or return, rl.close() will be called. In other words, iterating over a readline.Interface will always consume the input stream fully.

Performance is not on par with the traditional 'line' event API. Use 'line' instead for performance-sensitive applications.

async function processLineByLine() {
  const rl = readline.createInterface({
    // ...
  });

  for await (const line of rl) {
    // Each line in the readline input will be successively available here as
    // `line`.
  }
}

rl.line#

Added in: v0.1.98

The current input data being processed by node.

This can be used when collecting input from a TTY stream to retrieve the current value that has been processed thus far, prior to the line event being emitted. Once the line event has been emitted, this property will be an empty string.

Be aware that modifying the value during the instance runtime may have unintended consequences if rl.cursor is not also controlled.

If not using a TTY stream for input, use the 'line' event.

One possible use case would be as follows:

const values = ['lorem ipsum', 'dolor sit amet'];
const rl = readline.createInterface(process.stdin);
const showResults = debounce(() => {
  console.log(
    '\n',
    values.filter((val) => val.startsWith(rl.line)).join(' ')
  );
}, 300);
process.stdin.on('keypress', (c, k) => {
  showResults();
});

rl.cursor#

Added in: v0.1.98

The cursor position relative to rl.line.

This will track where the current cursor lands in the input string, when reading input from a TTY stream. The position of cursor determines the portion of the input string that will be modified as input is processed, as well as the column where the terminal caret will be rendered.

rl.getCursorPos()#

Added in: v13.5.0, v12.16.0

  • Returns: <Object>

    • rows <number> the row of the prompt the cursor currently lands on
    • cols <number> the screen column the cursor currently lands on

Returns the real position of the cursor in relation to the input prompt + string. Long input (wrapping) strings, as well as multiple line prompts are included in the calculations.

readline.clearLine(stream, dir[, callback])#

History
VersionChanges
v12.7.0

The stream's write() callback and return value are exposed.

v0.7.7

Added in: v0.7.7

  • stream <stream.Writable>

  • dir <number>

    • -1: to the left from cursor
    • 1: to the right from cursor
    • 0: the entire line
  • callback <Function> Invoked once the operation completes.
  • Returns: <boolean> false if stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

The readline.clearLine() method clears current line of given TTY stream in a specified direction identified by dir.

readline.clearScreenDown(stream[, callback])#

History
VersionChanges
v12.7.0

The stream's write() callback and return value are exposed.

v0.7.7

Added in: v0.7.7

  • stream <stream.Writable>
  • callback <Function> Invoked once the operation completes.
  • Returns: <boolean> false if stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

The readline.clearScreenDown() method clears the given TTY stream from the current position of the cursor down.

readline.createInterface(options)#

History
VersionChanges
v13.9.0

The tabSize option is supported now.

v8.3.0, 6.11.4

Remove max limit of crlfDelay option.

v6.6.0

The crlfDelay option is supported now.

v6.3.0

The prompt option is supported now.

v6.0.0

The historySize option can be 0 now.

v0.1.98

Added in: v0.1.98

  • options <Object>

    • input <stream.Readable> The Readable stream to listen to. This option is required.
    • output <stream.Writable> The Writable stream to write readline data to.
    • completer <Function> An optional function used for Tab autocompletion.
    • terminal <boolean> true if the input and output streams should be treated like a TTY, and have ANSI/VT100 escape codes written to it. Default: checking isTTY on the output stream upon instantiation.
    • historySize <number> Maximum number of history lines retained. To disable the history set this value to 0. This option makes sense only if terminal is set to true by the user or by an internal output check, otherwise the history caching mechanism is not initialized at all. Default: 30.
    • prompt <string> The prompt string to use. Default: '> '.
    • crlfDelay <number> If the delay between \r and \n exceeds crlfDelay milliseconds, both \r and \n will be treated as separate end-of-line input. crlfDelay will be coerced to a number no less than 100. It can be set to Infinity, in which case \r followed by \n will always be considered a single newline (which may be reasonable for reading files with \r\n line delimiter). Default: 100.
    • removeHistoryDuplicates <boolean> If true, when a new input line added to the history list duplicates an older one, this removes the older line from the list. Default: false.
    • escapeCodeTimeout <number> The duration readline will wait for a character (when reading an ambiguous key sequence in milliseconds one that can both form a complete key sequence using the input read so far and can take additional input to complete a longer key sequence). Default: 500.
    • tabSize <integer> The number of spaces a tab is equal to (minimum 1). Default: 8.

The readline.createInterface() method creates a new readline.Interface instance.

const readline = require('readline');
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

Once the readline.Interface instance is created, the most common case is to listen for the 'line' event:

rl.on('line', (line) => {
  console.log(`Received: ${line}`);
});

If terminal is true for this instance then the output stream will get the best compatibility if it defines an output.columns property and emits a 'resize' event on the output if or when the columns ever change (process.stdout does this automatically when it is a TTY).

Use of the completer Function#

The completer function takes the current line entered by the user as an argument, and returns an Array with 2 entries:

  • An Array with matching entries for the completion.
  • The substring that was used for the matching.

For instance: [[substr1, substr2, ...], originalsubstring].

function completer(line) {
  const completions = '.help .error .exit .quit .q'.split(' ');
  const hits = completions.filter((c) => c.startsWith(line));
  // Show all completions if none found
  return [hits.length ? hits : completions, line];
}

The completer function can be called asynchronously if it accepts two arguments:

function completer(linePartial, callback) {
  callback(null, [['123'], linePartial]);
}

readline.cursorTo(stream, x[, y][, callback])#

History
VersionChanges
v12.7.0

The stream's write() callback and return value are exposed.

v0.7.7

Added in: v0.7.7

  • stream <stream.Writable>
  • x <number>
  • y <number>
  • callback <Function> Invoked once the operation completes.
  • Returns: <boolean> false if stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

The readline.cursorTo() method moves cursor to the specified position in a given TTY stream.

readline.emitKeypressEvents(stream[, interface])#

Added in: v0.7.7

The readline.emitKeypressEvents() method causes the given Readable stream to begin emitting 'keypress' events corresponding to received input.

Optionally, interface specifies a readline.Interface instance for which autocompletion is disabled when copy-pasted input is detected.

If the stream is a TTY, then it must be in raw mode.

This is automatically called by any readline instance on its input if the input is a terminal. Closing the readline instance does not stop the input from emitting 'keypress' events.

readline.emitKeypressEvents(process.stdin);
if (process.stdin.isTTY)
  process.stdin.setRawMode(true);

readline.moveCursor(stream, dx, dy[, callback])#

History
VersionChanges
v12.7.0

The stream's write() callback and return value are exposed.

v0.7.7

Added in: v0.7.7

  • stream <stream.Writable>
  • dx <number>
  • dy <number>
  • callback <Function> Invoked once the operation completes.
  • Returns: <boolean> false if stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

The readline.moveCursor() method moves the cursor relative to its current position in a given TTY stream.

Example: Tiny CLI#

The following example illustrates the use of readline.Interface class to implement a small command-line interface:

const readline = require('readline');
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
  prompt: 'OHAI> '
});

rl.prompt();

rl.on('line', (line) => {
  switch (line.trim()) {
    case 'hello':
      console.log('world!');
      break;
    default:
      console.log(`Say what? I might have heard '${line.trim()}'`);
      break;
  }
  rl.prompt();
}).on('close', () => {
  console.log('Have a great day!');
  process.exit(0);
});

Example: Read File Stream Line-by-Line#

A common use case for readline is to consume an input file one line at a time. The easiest way to do so is leveraging the fs.ReadStream API as well as a for await...of loop:

const fs = require('fs');
const readline = require('readline');

async function processLineByLine() {
  const fileStream = fs.createReadStream('input.txt');

  const rl = readline.createInterface({
    input: fileStream,
    crlfDelay: Infinity
  });
  // Note: we use the crlfDelay option to recognize all instances of CR LF
  // ('\r\n') in input.txt as a single line break.

  for await (const line of rl) {
    // Each line in input.txt will be successively available here as `line`.
    console.log(`Line from file: ${line}`);
  }
}

processLineByLine();

Alternatively, one could use the 'line' event:

const fs = require('fs');
const readline = require('readline');

const rl = readline.createInterface({
  input: fs.createReadStream('sample.txt'),
  crlfDelay: Infinity
});

rl.on('line', (line) => {
  console.log(`Line from file: ${line}`);
});

Currently, for await...of loop can be a bit slower. If async / await flow and speed are both essential, a mixed approach can be applied:

const { once } = require('events');
const { createReadStream } = require('fs');
const { createInterface } = require('readline');

(async function processLineByLine() {
  try {
    const rl = createInterface({
      input: createReadStream('big-file.txt'),
      crlfDelay: Infinity
    });

    rl.on('line', (line) => {
      // Process the line.
    });

    await once(rl, 'close');

    console.log('File processed.');
  } catch (err) {
    console.error(err);
  }
})();

TTY keybindings#

Keybindings Description Notes
ctrl + shift + backspace Delete line left Doesn't work on Linux, Mac and Windows
ctrl + shift + delete Delete line right Doesn't work on Linux and Mac
ctrl + c Emit SIGINT or close the readline instance
ctrl + h Delete left
ctrl + d Delete right or close the readline instance in case the current line is empty / EOF Doesn't work on Windows
ctrl + u Delete from the current position to the line start
ctrl + k Delete from the current position to the end of line
ctrl + a Go to start of line
ctrl + e Go to to end of line
ctrl + b Back one character
ctrl + f Forward one character
ctrl + l Clear screen
ctrl + n Next history item
ctrl + p Previous history item
ctrl + z Moves running process into background. Type fg and press enter to return. Doesn't work on Windows
ctrl + w or ctrl + backspace Delete backwards to a word boundary ctrl + backspace Doesn't work as expected on Windows
ctrl + delete Delete forward to a word boundary Doesn't work on Mac
ctrl + left or meta + b Word left ctrl + left Doesn't work on Mac
ctrl + right or meta + f Word right ctrl + right Doesn't work on Mac
meta + d or meta + delete Delete word right meta + delete Doesn't work on windows
meta + backspace Delete word left Doesn't work on Mac

REPL#

Stability: 2 - Stable

The repl module provides a Read-Eval-Print-Loop (REPL) implementation that is available both as a standalone program or includible in other applications. It can be accessed using:

const repl = require('repl');

Design and Features#

The repl module exports the repl.REPLServer class. While running, instances of repl.REPLServer will accept individual lines of user input, evaluate those according to a user-defined evaluation function, then output the result. Input and output may be from stdin and stdout, respectively, or may be connected to any Node.js stream.

Instances of repl.REPLServer support automatic completion of inputs, completion preview, simplistic Emacs-style line editing, multi-line inputs, ZSH-like reverse-i-search, ZSH-like substring-based history search, ANSI-styled output, saving and restoring current REPL session state, error recovery, and customizable evaluation functions. Terminals that do not support ANSI styles and Emacs-style line editing automatically fall back to a limited feature set.

Commands and Special Keys#

The following special commands are supported by all REPL instances:

  • .break: When in the process of inputting a multi-line expression, entering the .break command (or pressing the <ctrl>-C key combination) will abort further input or processing of that expression.
  • .clear: Resets the REPL context to an empty object and clears any multi-line expression currently being input.
  • .exit: Close the I/O stream, causing the REPL to exit.
  • .help: Show this list of special commands.
  • .save: Save the current REPL session to a file: > .save ./file/to/save.js
  • .load: Load a file into the current REPL session. > .load ./file/to/load.js
  • .editor: Enter editor mode (<ctrl>-D to finish, <ctrl>-C to cancel).
> .editor
// Entering editor mode (^D to finish, ^C to cancel)
function welcome(name) {
  return `Hello ${name}!`;
}

welcome('Node.js User');

// ^D
'Hello Node.js User!'
>

The following key combinations in the REPL have these special effects:

  • <ctrl>-C: When pressed once, has the same effect as the .break command. When pressed twice on a blank line, has the same effect as the .exit command.
  • <ctrl>-D: Has the same effect as the .exit command.
  • <tab>: When pressed on a blank line, displays global and local (scope) variables. When pressed while entering other input, displays relevant autocompletion options.

For key bindings related to the reverse-i-search, see reverse-i-search. For all other key bindings, see TTY keybindings.

Default Evaluation#

By default, all instances of repl.REPLServer use an evaluation function that evaluates JavaScript expressions and provides access to Node.js built-in modules. This default behavior can be overridden by passing in an alternative evaluation function when the repl.REPLServer instance is created.

JavaScript Expressions#

The default evaluator supports direct evaluation of JavaScript expressions:

> 1 + 1
2
> const m = 2
undefined
> m + 1
3

Unless otherwise scoped within blocks or functions, variables declared either implicitly or using the const, let, or var keywords are declared at the global scope.

Global and Local Scope#

The default evaluator provides access to any variables that exist in the global scope. It is possible to expose a variable to the REPL explicitly by assigning it to the context object associated with each REPLServer:

const repl = require('repl');
const msg = 'message';

repl.start('> ').context.m = msg;

Properties in the context object appear as local within the REPL:

$ node repl_test.js
> m
'message'

Context properties are not read-only by default. To specify read-only globals, context properties must be defined using Object.defineProperty():

const repl = require('repl');
const msg = 'message';

const r = repl.start('> ');
Object.defineProperty(r.context, 'm', {
  configurable: false,
  enumerable: true,
  value: msg
});

Accessing Core Node.js Modules#

The default evaluator will automatically load Node.js core modules into the REPL environment when used. For instance, unless otherwise declared as a global or scoped variable, the input fs will be evaluated on-demand as global.fs = require('fs').

> fs.createReadStream('./some/file');

Global Uncaught Exceptions#

History
VersionChanges
v12.3.0

The 'uncaughtException' event is from now on triggered if the repl is used as standalone program.

The REPL uses the domain module to catch all uncaught exceptions for that REPL session.

This use of the domain module in the REPL has these side effects:

As standalone program:

process.on('uncaughtException', () => console.log('Uncaught'));

throw new Error('foobar');
// Uncaught

When used in another application:

process.on('uncaughtException', () => console.log('Uncaught'));
// TypeError [ERR_INVALID_REPL_INPUT]: Listeners for `uncaughtException`
// cannot be used in the REPL

throw new Error('foobar');
// Thrown:
// Error: foobar

Assignment of the _ (underscore) variable#

History
VersionChanges
v9.8.0

Added _error support.

The default evaluator will, by default, assign the result of the most recently evaluated expression to the special variable _ (underscore). Explicitly setting _ to a value will disable this behavior.

> [ 'a', 'b', 'c' ]
[ 'a', 'b', 'c' ]
> _.length
3
> _ += 1
Expression assignment to _ now disabled.
4
> 1 + 1
2
> _
4

Similarly, _error will refer to the last seen error, if there was any. Explicitly setting _error to a value will disable this behavior.

> throw new Error('foo');
Error: foo
> _error.message
'foo'

await keyword#

With the --experimental-repl-await command line option specified, experimental support for the await keyword is enabled.

> await Promise.resolve(123)
123
> await Promise.reject(new Error('REPL await'))
Error: REPL await
    at repl:1:45
> const timeout = util.promisify(setTimeout);
undefined
> const old = Date.now(); await timeout(1000); console.log(Date.now() - old);
1002
undefined

Reverse-i-search#

Added in: v13.6.0

The REPL supports bi-directional reverse-i-search similar to ZSH. It is triggered with <ctrl> + R to search backwards and <ctrl> + S to search forwards.

Duplicated history entires will be skipped.

Entries are accepted as soon as any button is pressed that doesn't correspond with the reverse search. Cancelling is possible by pressing escape or <ctrl> + C.

Changing the direction immediately searches for the next entry in the expected direction from the current position on.

Custom Evaluation Functions#

When a new repl.REPLServer is created, a custom evaluation function may be provided. This can be used, for instance, to implement fully customized REPL applications.

The following illustrates a hypothetical example of a REPL that performs translation of text from one language to another:

const repl = require('repl');
const { Translator } = require('translator');

const myTranslator = new Translator('en', 'fr');

function myEval(cmd, context, filename, callback) {
  callback(null, myTranslator.translate(cmd));
}

repl.start({ prompt: '> ', eval: myEval });

Recoverable Errors#

As a user is typing input into the REPL prompt, pressing the <enter> key will send the current line of input to the eval function. In order to support multi-line input, the eval function can return an instance of repl.Recoverable to the provided callback function:

function myEval(cmd, context, filename, callback) {
  let result;
  try {
    result = vm.runInThisContext(cmd);
  } catch (e) {
    if (isRecoverableError(e)) {
      return callback(new repl.Recoverable(e));
    }
  }
  callback(null, result);
}

function isRecoverableError(error) {
  if (error.name === 'SyntaxError') {
    return /^(Unexpected end of input|Unexpected token)/.test(error.message);
  }
  return false;
}

Customizing REPL Output#

By default, repl.REPLServer instances format output using the util.inspect() method before writing the output to the provided Writable stream (process.stdout by default). The showProxy inspection option is set to true by default and the colors option is set to true depending on the REPL's useColors option.

The useColors boolean option can be specified at construction to instruct the default writer to use ANSI style codes to colorize the output from the util.inspect() method.

If the REPL is run as standalone program, it is also possible to change the REPL's inspection defaults from inside the REPL by using the inspect.replDefaults property which mirrors the defaultOptions from util.inspect().

> util.inspect.replDefaults.compact = false;
false
> [1]
[
  1
]
>

To fully customize the output of a repl.REPLServer instance pass in a new function for the writer option on construction. The following example, for instance, simply converts any input text to upper case:

const repl = require('repl');

const r = repl.start({ prompt: '> ', eval: myEval, writer: myWriter });

function myEval(cmd, context, filename, callback) {
  callback(null, cmd);
}

function myWriter(output) {
  return output.toUpperCase();
}

Class: REPLServer#

Added in: v0.1.91

Instances of repl.REPLServer are created using the repl.start() method or directly using the JavaScript new keyword.

const repl = require('repl');

const options = { useColors: true };

const firstInstance = repl.start(options);
const secondInstance = new repl.REPLServer(options);

Event: 'exit'#

Added in: v0.7.7

The 'exit' event is emitted when the REPL is exited either by receiving the .exit command as input, the user pressing <ctrl>-C twice to signal SIGINT, or by pressing <ctrl>-D to signal 'end' on the input stream. The listener callback is invoked without any arguments.

replServer.on('exit', () => {
  console.log('Received "exit" event from repl!');
  process.exit();
});

Event: 'reset'#

Added in: v0.11.0

The 'reset' event is emitted when the REPL's context is reset. This occurs whenever the .clear command is received as input unless the REPL is using the default evaluator and the repl.REPLServer instance was created with the useGlobal option set to true. The listener callback will be called with a reference to the context object as the only argument.

This can be used primarily to re-initialize REPL context to some pre-defined state:

const repl = require('repl');

function initializeContext(context) {
  context.m = 'test';
}

const r = repl.start({ prompt: '> ' });
initializeContext(r.context);

r.on('reset', initializeContext);

When this code is executed, the global 'm' variable can be modified but then reset to its initial value using the .clear command:

$ ./node example.js
> m
'test'
> m = 1
1
> m
1
> .clear
Clearing context...
> m
'test'
>

replServer.defineCommand(keyword, cmd)#

Added in: v0.3.0
  • keyword <string> The command keyword (without a leading . character).
  • cmd <Object> | <Function> The function to invoke when the command is processed.

The replServer.defineCommand() method is used to add new .-prefixed commands to the REPL instance. Such commands are invoked by typing a . followed by the keyword. The cmd is either a Function or an Object with the following properties:

  • help <string> Help text to be displayed when .help is entered (Optional).
  • action <Function> The function to execute, optionally accepting a single string argument.

The following example shows two new commands added to the REPL instance:

const repl = require('repl');

const replServer = repl.start({ prompt: '> ' });
replServer.defineCommand('sayhello', {
  help: 'Say hello',
  action(name) {
    this.clearBufferedCommand();
    console.log(`Hello, ${name}!`);
    this.displayPrompt();
  }
});
replServer.defineCommand('saybye', function saybye() {
  console.log('Goodbye!');
  this.close();
});

The new commands can then be used from within the REPL instance:

> .sayhello Node.js User
Hello, Node.js User!
> .saybye
Goodbye!

replServer.displayPrompt([preserveCursor])#

Added in: v0.1.91

The replServer.displayPrompt() method readies the REPL instance for input from the user, printing the configured prompt to a new line in the output and resuming the input to accept new input.

When multi-line input is being entered, an ellipsis is printed rather than the 'prompt'.

When preserveCursor is true, the cursor placement will not be reset to 0.

The replServer.displayPrompt method is primarily intended to be called from within the action function for commands registered using the replServer.defineCommand() method.

replServer.clearBufferedCommand()#

Added in: v9.0.0

The replServer.clearBufferedCommand() method clears any command that has been buffered but not yet executed. This method is primarily intended to be called from within the action function for commands registered using the replServer.defineCommand() method.

replServer.parseREPLKeyword(keyword[, rest])#

Added in: v0.8.9Deprecated since: v9.0.0

Stability: 0 - Deprecated.

  • keyword <string> the potential keyword to parse and execute
  • rest <any> any parameters to the keyword command
  • Returns: <boolean>

An internal method used to parse and execute REPLServer keywords. Returns true if keyword is a valid keyword, otherwise false.

replServer.setupHistory(historyPath, callback)#

Added in: v11.10.0

Initializes a history log file for the REPL instance. When executing the Node.js binary and using the command line REPL, a history file is initialized by default. However, this is not the case when creating a REPL programmatically. Use this method to initialize a history log file when working with REPL instances programmatically.

repl.start([options])#

History
VersionChanges
v13.4.0

The preview option is now available.

v12.0.0

The terminal option now follows the default description in all cases and useColors checks hasColors() if available.

v10.0.0

The REPL_MAGIC_MODE replMode was removed.

v5.8.0

The options parameter is optional now.

v0.1.91

Added in: v0.1.91

  • options <Object> | <string>

    • prompt <string> The input prompt to display. Default: '> ' (with a trailing space).
    • input <stream.Readable> The Readable stream from which REPL input will be read. Default: process.stdin.
    • output <stream.Writable> The Writable stream to which REPL output will be written. Default: process.stdout.
    • terminal <boolean> If true, specifies that the output should be treated as a TTY terminal. Default: checking the value of the isTTY property on the output stream upon instantiation.
    • eval <Function> The function to be used when evaluating each given line of input. Default: an async wrapper for the JavaScript eval() function. An eval function can error with repl.Recoverable to indicate the input was incomplete and prompt for additional lines.
    • useColors <boolean> If true, specifies that the default writer function should include ANSI color styling to REPL output. If a custom writer function is provided then this has no effect. Default: checking color support on the output stream if the REPL instance's terminal value is true.
    • useGlobal <boolean> If true, specifies that the default evaluation function will use the JavaScript global as the context as opposed to creating a new separate context for the REPL instance. The node CLI REPL sets this value to true. Default: false.
    • ignoreUndefined <boolean> If true, specifies that the default writer will not output the return value of a command if it evaluates to undefined. Default: false.
    • writer <Function> The function to invoke to format the output of each command before writing to output. Default: util.inspect().
    • completer <Function> An optional function used for custom Tab auto completion. See readline.InterfaceCompleter for an example.

    • replMode <symbol> A flag that specifies whether the default evaluator executes all JavaScript commands in strict mode or default (sloppy) mode. Acceptable values are:

      • repl.REPL_MODE_SLOPPY to evaluate expressions in sloppy mode.
      • repl.REPL_MODE_STRICT to evaluate expressions in strict mode. This is equivalent to prefacing every repl statement with 'use strict'.
    • breakEvalOnSigint <boolean> Stop evaluating the current piece of code when SIGINT is received, such as when Ctrl+C is pressed. This cannot be used together with a custom eval function. Default: false.
    • preview <boolean> Defines if the repl prints autocomplete and output previews or not. Default: true with the default eval function and false in case a custom eval function is used. If terminal is falsy, then there are no previews and the value of preview has no effect.
  • Returns: <repl.REPLServer>

The repl.start() method creates and starts a repl.REPLServer instance.

If options is a string, then it specifies the input prompt:

const repl = require('repl');

// a Unix style prompt
repl.start('$ ');

The Node.js REPL#

Node.js itself uses the repl module to provide its own interactive interface for executing JavaScript. This can be used by executing the Node.js binary without passing any arguments (or by passing the -i argument):

$ node
> const a = [1, 2, 3];
undefined
> a
[ 1, 2, 3 ]
> a.forEach((v) => {
...   console.log(v);
...   });
1
2
3

Environment Variable Options#

Various behaviors of the Node.js REPL can be customized using the following environment variables:

  • NODE_REPL_HISTORY: When a valid path is given, persistent REPL history will be saved to the specified file rather than .node_repl_history in the user's home directory. Setting this value to '' (an empty string) will disable persistent REPL history. Whitespace will be trimmed from the value. On Windows platforms environment variables with empty values are invalid so set this variable to one or more spaces to disable persistent REPL history.
  • NODE_REPL_HISTORY_SIZE: Controls how many lines of history will be persisted if history is available. Must be a positive number. Default: 1000.
  • NODE_REPL_MODE: May be either 'sloppy' or 'strict'. Default: 'sloppy', which will allow non-strict mode code to be run.

Persistent History#

By default, the Node.js REPL will persist history between node REPL sessions by saving inputs to a .node_repl_history file located in the user's home directory. This can be disabled by setting the environment variable NODE_REPL_HISTORY=''.

Using the Node.js REPL with advanced line-editors#

For advanced line-editors, start Node.js with the environment variable NODE_NO_READLINE=1. This will start the main and debugger REPL in canonical terminal settings, which will allow use with rlwrap.

For example, the following can be added to a .bashrc file:

alias node="env NODE_NO_READLINE=1 rlwrap node"

Starting multiple REPL instances against a single running instance#

It is possible to create and run multiple REPL instances against a single running instance of Node.js that share a single global object but have separate I/O interfaces.

The following example, for instance, provides separate REPLs on stdin, a Unix socket, and a TCP socket:

const net = require('net');
const repl = require('repl');
let connections = 0;

repl.start({
  prompt: 'Node.js via stdin> ',
  input: process.stdin,
  output: process.stdout
});

net.createServer((socket) => {
  connections += 1;
  repl.start({
    prompt: 'Node.js via Unix socket> ',
    input: socket,
    output: socket
  }).on('exit', () => {
    socket.end();
  });
}).listen('/tmp/node-repl-sock');

net.createServer((socket) => {
  connections += 1;
  repl.start({
    prompt: 'Node.js via TCP socket> ',
    input: socket,
    output: socket
  }).on('exit', () => {
    socket.end();
  });
}).listen(5001);

Running this application from the command line will start a REPL on stdin. Other REPL clients may connect through the Unix socket or TCP socket. telnet, for instance, is useful for connecting to TCP sockets, while socat can be used to connect to both Unix and TCP sockets.

By starting a REPL from a Unix socket-based server instead of stdin, it is possible to connect to a long-running Node.js process without restarting it.

For an example of running a "full-featured" (terminal) REPL over a net.Server and net.Socket instance, see: https://gist.github.com/TooTallNate/2209310.

For an example of running a REPL instance over curl(1), see: https://gist.github.com/TooTallNate/2053342.

Diagnostic Report#

Stability: 2 - Stable

Delivers a JSON-formatted diagnostic summary, written to a file.

The report is intended for development, test and production use, to capture and preserve information for problem determination. It includes JavaScript and native stack traces, heap statistics, platform information, resource usage etc. With the report option enabled, diagnostic reports can be triggered on unhandled exceptions, fatal errors and user signals, in addition to triggering programmatically through API calls.

A complete example report that was generated on an uncaught exception is provided below for reference.

{
  "header": {
    "reportVersion": 1,
    "event": "exception",
    "trigger": "Exception",
    "filename": "report.20181221.005011.8974.0.001.json",
    "dumpEventTime": "2018-12-21T00:50:11Z",
    "dumpEventTimeStamp": "1545371411331",
    "processId": 8974,
    "cwd": "/home/nodeuser/project/node",
    "commandLine": [
      "/home/nodeuser/project/node/out/Release/node",
      "--report-uncaught-exception",
      "/home/nodeuser/project/node/test/report/test-exception.js",
      "child"
    ],
    "nodejsVersion": "v12.0.0-pre",
    "glibcVersionRuntime": "2.17",
    "glibcVersionCompiler": "2.17",
    "wordSize": "64 bit",
    "arch": "x64",
    "platform": "linux",
    "componentVersions": {
      "node": "12.0.0-pre",
      "v8": "7.1.302.28-node.5",
      "uv": "1.24.1",
      "zlib": "1.2.11",
      "ares": "1.15.0",
      "modules": "68",
      "nghttp2": "1.34.0",
      "napi": "3",
      "llhttp": "1.0.1",
      "openssl": "1.1.0j"
    },
    "release": {
      "name": "node"
    },
    "osName": "Linux",
    "osRelease": "3.10.0-862.el7.x86_64",
    "osVersion": "#1 SMP Wed Mar 21 18:14:51 EDT 2018",
    "osMachine": "x86_64",
    "cpus": [
      {
        "model": "Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz",
        "speed": 2700,
        "user": 88902660,
        "nice": 0,
        "sys": 50902570,
        "idle": 241732220,
        "irq": 0
      },
      {
        "model": "Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz",
        "speed": 2700,
        "user": 88902660,
        "nice": 0,
        "sys": 50902570,
        "idle": 241732220,
        "irq": 0
      }
    ],
    "networkInterfaces": [
      {
        "name": "en0",
        "internal": false,
        "mac": "13:10:de:ad:be:ef",
        "address": "10.0.0.37",
        "netmask": "255.255.255.0",
        "family": "IPv4"
      }
    ],
    "host": "test_machine"
  },
  "javascriptStack": {
    "message": "Error: *** test-exception.js: throwing uncaught Error",
    "stack": [
      "at myException (/home/nodeuser/project/node/test/report/test-exception.js:9:11)",
      "at Object.<anonymous> (/home/nodeuser/project/node/test/report/test-exception.js:12:3)",
      "at Module._compile (internal/modules/cjs/loader.js:718:30)",
      "at Object.Module._extensions..js (internal/modules/cjs/loader.js:729:10)",
      "at Module.load (internal/modules/cjs/loader.js:617:32)",
      "at tryModuleLoad (internal/modules/cjs/loader.js:560:12)",
      "at Function.Module._load (internal/modules/cjs/loader.js:552:3)",
      "at Function.Module.runMain (internal/modules/cjs/loader.js:771:12)",
      "at executeUserCode (internal/bootstrap/node.js:332:15)"
    ]
  },
  "nativeStack": [
    {
      "pc": "0x000055b57f07a9ef",
      "symbol": "report::GetNodeReport(v8::Isolate*, node::Environment*, char const*, char const*, v8::Local<v8::String>, std::ostream&) [./node]"
    },
    {
      "pc": "0x000055b57f07cf03",
      "symbol": "report::GetReport(v8::FunctionCallbackInfo<v8::Value> const&) [./node]"
    },
    {
      "pc": "0x000055b57f1bccfd",
      "symbol": " [./node]"
    },
    {
      "pc": "0x000055b57f1be048",
      "symbol": "v8::internal::Builtin_HandleApiCall(int, v8::internal::Object**, v8::internal::Isolate*) [./node]"
    },
    {
      "pc": "0x000055b57feeda0e",
      "symbol": " [./node]"
    }
  ],
  "javascriptHeap": {
    "totalMemory": 6127616,
    "totalCommittedMemory": 4357352,
    "usedMemory": 3221136,
    "availableMemory": 1521370240,
    "memoryLimit": 1526909922,
    "heapSpaces": {
      "read_only_space": {
        "memorySize": 524288,
        "committedMemory": 39208,
        "capacity": 515584,
        "used": 30504,
        "available": 485080
      },
      "new_space": {
        "memorySize": 2097152,
        "committedMemory": 2019312,
        "capacity": 1031168,
        "used": 985496,
        "available": 45672
      },
      "old_space": {
        "memorySize": 2273280,
        "committedMemory": 1769008,
        "capacity": 1974640,
        "used": 1725488,
        "available": 249152
      },
      "code_space": {
        "memorySize": 696320,
        "committedMemory": 184896,
        "capacity": 152128,
        "used": 152128,
        "available": 0
      },
      "map_space": {
        "memorySize": 536576,
        "committedMemory": 344928,
        "capacity": 327520,
        "used": 327520,
        "available": 0
      },
      "large_object_space": {
        "memorySize": 0,
        "committedMemory": 0,
        "capacity": 1520590336,
        "used": 0,
        "available": 1520590336
      },
      "new_large_object_space": {
        "memorySize": 0,
        "committedMemory": 0,
        "capacity": 0,
        "used": 0,
        "available": 0
      }
    }
  },
  "resourceUsage": {
    "userCpuSeconds": 0.069595,
    "kernelCpuSeconds": 0.019163,
    "cpuConsumptionPercent": 0.000000,
    "maxRss": 18079744,
    "pageFaults": {
      "IORequired": 0,
      "IONotRequired": 4610
    },
    "fsActivity": {
      "reads": 0,
      "writes": 0
    }
  },
  "uvthreadResourceUsage": {
    "userCpuSeconds": 0.068457,
    "kernelCpuSeconds": 0.019127,
    "cpuConsumptionPercent": 0.000000,
    "fsActivity": {
      "reads": 0,
      "writes": 0
    }
  },
  "libuv": [
    {
      "type": "async",
      "is_active": true,
      "is_referenced": false,
      "address": "0x0000000102910900",
      "details": ""
    },
    {
      "type": "timer",
      "is_active": false,
      "is_referenced": false,
      "address": "0x00007fff5fbfeab0",
      "repeat": 0,
      "firesInMsFromNow": 94403548320796,
      "expired": true
    },
    {
      "type": "check",
      "is_active": true,
      "is_referenced": false,
      "address": "0x00007fff5fbfeb48"
    },
    {
      "type": "idle",
      "is_active": false,
      "is_referenced": true,
      "address": "0x00007fff5fbfebc0"
    },
    {
      "type": "prepare",
      "is_active": false,
      "is_referenced": false,
      "address": "0x00007fff5fbfec38"
    },
    {
      "type": "check",
      "is_active": false,
      "is_referenced": false,
      "address": "0x00007fff5fbfecb0"
    },
    {
      "type": "async",
      "is_active": true,
      "is_referenced": false,
      "address": "0x000000010188f2e0"
    },
    {
      "type": "tty",
      "is_active": false,
      "is_referenced": true,
      "address": "0x000055b581db0e18",
      "width": 204,
      "height": 55,
      "fd": 17,
      "writeQueueSize": 0,
      "readable": true,
      "writable": true
    },
    {
      "type": "signal",
      "is_active": true,
      "is_referenced": false,
      "address": "0x000055b581d80010",
      "signum": 28,
      "signal": "SIGWINCH"
    },
    {
      "type": "tty",
      "is_active": true,
      "is_referenced": true,
      "address": "0x000055b581df59f8",
      "width": 204,
      "height": 55,
      "fd": 19,
      "writeQueueSize": 0,
      "readable": true,
      "writable": true
    },
    {
      "type": "loop",
      "is_active": true,
      "address": "0x000055fc7b2cb180"
    }
  ],
  "workers": [],
  "environmentVariables": {
    "REMOTEHOST": "REMOVED",
    "MANPATH": "/opt/rh/devtoolset-3/root/usr/share/man:",
    "XDG_SESSION_ID": "66126",
    "HOSTNAME": "test_machine",
    "HOST": "test_machine",
    "TERM": "xterm-256color",
    "SHELL": "/bin/csh",
    "SSH_CLIENT": "REMOVED",
    "PERL5LIB": "/opt/rh/devtoolset-3/root//usr/lib64/perl5/vendor_perl:/opt/rh/devtoolset-3/root/usr/lib/perl5:/opt/rh/devtoolset-3/root//usr/share/perl5/vendor_perl",
    "OLDPWD": "/home/nodeuser/project/node/src",
    "JAVACONFDIRS": "/opt/rh/devtoolset-3/root/etc/java:/etc/java",
    "SSH_TTY": "/dev/pts/0",
    "PCP_DIR": "/opt/rh/devtoolset-3/root",
    "GROUP": "normaluser",
    "USER": "nodeuser",
    "LD_LIBRARY_PATH": "/opt/rh/devtoolset-3/root/usr/lib64:/opt/rh/devtoolset-3/root/usr/lib",
    "HOSTTYPE": "x86_64-linux",
    "XDG_CONFIG_DIRS": "/opt/rh/devtoolset-3/root/etc/xdg:/etc/xdg",
    "MAIL": "/var/spool/mail/nodeuser",
    "PATH": "/home/nodeuser/project/node:/opt/rh/devtoolset-3/root/usr/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin",
    "PWD": "/home/nodeuser/project/node",
    "LANG": "en_US.UTF-8",
    "PS1": "\\u@\\h : \\[\\e[31m\\]\\w\\[\\e[m\\] >  ",
    "SHLVL": "2",
    "HOME": "/home/nodeuser",
    "OSTYPE": "linux",
    "VENDOR": "unknown",
    "PYTHONPATH": "/opt/rh/devtoolset-3/root/usr/lib64/python2.7/site-packages:/opt/rh/devtoolset-3/root/usr/lib/python2.7/site-packages",
    "MACHTYPE": "x86_64",
    "LOGNAME": "nodeuser",
    "XDG_DATA_DIRS": "/opt/rh/devtoolset-3/root/usr/share:/usr/local/share:/usr/share",
    "LESSOPEN": "||/usr/bin/lesspipe.sh %s",
    "INFOPATH": "/opt/rh/devtoolset-3/root/usr/share/info",
    "XDG_RUNTIME_DIR": "/run/user/50141",
    "_": "./node"
  },
  "userLimits": {
    "core_file_size_blocks": {
      "soft": "",
      "hard": "unlimited"
    },
    "data_seg_size_kbytes": {
      "soft": "unlimited",
      "hard": "unlimited"
    },
    "file_size_blocks": {
      "soft": "unlimited",
      "hard": "unlimited"
    },
    "max_locked_memory_bytes": {
      "soft": "unlimited",
      "hard": 65536
    },
    "max_memory_size_kbytes": {
      "soft": "unlimited",
      "hard": "unlimited"
    },
    "open_files": {
      "soft": "unlimited",
      "hard": 4096
    },
    "stack_size_bytes": {
      "soft": "unlimited",
      "hard": "unlimited"
    },
    "cpu_time_seconds": {
      "soft": "unlimited",
      "hard": "unlimited"
    },
    "max_user_processes": {
      "soft": "unlimited",
      "hard": 4127290
    },
    "virtual_memory_kbytes": {
      "soft": "unlimited",
      "hard": "unlimited"
    }
  },
  "sharedObjects": [
    "/lib64/libdl.so.2",
    "/lib64/librt.so.1",
    "/lib64/libstdc++.so.6",
    "/lib64/libm.so.6",
    "/lib64/libgcc_s.so.1",
    "/lib64/libpthread.so.0",
    "/lib64/libc.so.6",
    "/lib64/ld-linux-x86-64.so.2"
  ]
}

Usage#

node --report-uncaught-exception --report-on-signal \
--report-on-fatalerror app.js

  • --report-uncaught-exception Enables report to be generated on un-caught exceptions. Useful when inspecting JavaScript stack in conjunction with native stack and other runtime environment data.

  • --report-on-signal Enables report to be generated upon receiving the specified (or predefined) signal to the running Node.js process. (See below on how to modify the signal that triggers the report.) Default signal is SIGUSR2. Useful when a report needs to be triggered from another program. Application monitors may leverage this feature to collect report at regular intervals and plot rich set of internal runtime data to their views.

Signal based report generation is not supported in Windows.

Under normal circumstances, there is no need to modify the report triggering signal. However, if SIGUSR2 is already used for other purposes, then this flag helps to change the signal for report generation and preserve the original meaning of SIGUSR2 for the said purposes.

  • --report-on-fatalerror Enables the report to be triggered on fatal errors (internal errors within the Node.js runtime, such as out of memory) that leads to termination of the application. Useful to inspect various diagnostic data elements such as heap, stack, event loop state, resource consumption etc. to reason about the fatal error.

  • --report-compact Write reports in a compact format, single-line JSON, more easily consumable by log processing systems than the default multi-line format designed for human consumption.

  • --report-directory Location at which the report will be generated.

  • --report-filename Name of the file to which the report will be written.

  • --report-signal Sets or resets the signal for report generation (not supported on Windows). Default signal is SIGUSR2.

A report can also be triggered via an API call from a JavaScript application:

process.report.writeReport();

This function takes an optional additional argument filename, which is the name of a file into which the report is written.

process.report.writeReport('./foo.json');

This function takes an optional additional argument err which is an Error object that will be used as the context for the JavaScript stack printed in the report. When using report to handle errors in a callback or an exception handler, this allows the report to include the location of the original error as well as where it was handled.

try {
  process.chdir('/non-existent-path');
} catch (err) {
  process.report.writeReport(err);
}
// Any other code

If both filename and error object are passed to writeReport() the error object must be the second parameter.

try {
  process.chdir('/non-existent-path');
} catch (err) {
  process.report.writeReport(filename, err);
}
// Any other code

The content of the diagnostic report can be returned as a JavaScript Object via an API call from a JavaScript application:

const report = process.report.getReport();
console.log(typeof report === 'object'); // true

// Similar to process.report.writeReport() output
console.log(JSON.stringify(report, null, 2));

This function takes an optional additional argument err, which is an Error object that will be used as the context for the JavaScript stack printed in the report.

const report = process.report.getReport(new Error('custom error'));
console.log(typeof report === 'object'); // true

The API versions are useful when inspecting the runtime state from within the application, in expectation of self-adjusting the resource consumption, load balancing, monitoring etc.

The content of the report consists of a header section containing the event type, date, time, PID and Node.js version, sections containing JavaScript and native stack traces, a section containing V8 heap information, a section containing libuv handle information and an OS platform information section showing CPU and memory usage and system limits. An example report can be triggered using the Node.js REPL:

$ node
> process.report.writeReport();
Writing Node.js report to file: report.20181126.091102.8480.0.001.json
Node.js report completed
>

When a report is written, start and end messages are issued to stderr and the filename of the report is returned to the caller. The default filename includes the date, time, PID and a sequence number. The sequence number helps in associating the report dump with the runtime state if generated multiple times for the same Node.js process.

Configuration#

Additional runtime configuration of report generation is available via the following properties of process.report:

reportOnFatalError triggers diagnostic reporting on fatal errors when true. Defaults to false.

reportOnSignal triggers diagnostic reporting on signal when true. This is not supported on Windows. Defaults to false.

reportOnUncaughtException triggers diagnostic reporting on uncaught exception when true. Defaults to false.

signal specifies the POSIX signal identifier that will be used to intercept external triggers for report generation. Defaults to 'SIGUSR2'.

filename specifies the name of the output file in the file system. Special meaning is attached to stdout and stderr. Usage of these will result in report being written to the associated standard streams. In cases where standard streams are used, the value in directory is ignored. URLs are not supported. Defaults to a composite filename that contains timestamp, PID and sequence number.

directory specifies the filesystem directory where the report will be written. URLs are not supported. Defaults to the current working directory of the Node.js process.

// Trigger report only on uncaught exceptions.
process.report.reportOnFatalError = false;
process.report.reportOnSignal = false;
process.report.reportOnUncaughtException = true;

// Trigger report for both internal errors as well as external signal.
process.report.reportOnFatalError = true;
process.report.reportOnSignal = true;
process.report.reportOnUncaughtException = false;

// Change the default signal to 'SIGQUIT' and enable it.
process.report.reportOnFatalError = false;
process.report.reportOnUncaughtException = false;
process.report.reportOnSignal = true;
process.report.signal = 'SIGQUIT';

Configuration on module initialization is also available via environment variables:

NODE_OPTIONS="--report-uncaught-exception \
  --report-on-fatalerror --report-on-signal \
  --report-signal=SIGUSR2  --report-filename=./report.json \
  --report-directory=/home/nodeuser"

Specific API documentation can be found under process API documentation section.

Interaction with Workers#

History
VersionChanges
v13.9.0, v12.16.2

Workers are now included in the report.

Worker threads can create reports in the same way that the main thread does.

Reports will include information on any Workers that are children of the current thread as part of the workers section, with each Worker generating a report in the standard report format.

The thread which is generating the report will wait for the reports from Worker threads to finish. However, the latency for this will usually be low, as both running JavaScript and the event loop are interrupted to generate the report.

Stream[src]#

Stability: 2 - Stable

A stream is an abstract interface for working with streaming data in Node.js. The stream module provides an API for implementing the stream interface.

There are many stream objects provided by Node.js. For instance, a request to an HTTP server and process.stdout are both stream instances.

Streams can be readable, writable, or both. All streams are instances of EventEmitter.

To access the stream module:

const stream = require('stream');

The stream module is useful for creating new types of stream instances. It is usually not necessary to use the stream module to consume streams.

Organization of this Document#

This document contains two primary sections and a third section for notes. The first section explains how to use existing streams within an application. The second section explains how to create new types of streams.

Types of Streams#

There are four fundamental stream types within Node.js:

Additionally, this module includes the utility functions stream.pipeline(), stream.finished() and stream.Readable.from().

Object Mode#

All streams created by Node.js APIs operate exclusively on strings and Buffer (or Uint8Array) objects. It is possible, however, for stream implementations to work with other types of JavaScript values (with the exception of null, which serves a special purpose within streams). Such streams are considered to operate in "object mode".

Stream instances are switched into object mode using the objectMode option when the stream is created. Attempting to switch an existing stream into object mode is not safe.

Buffering#

Both Writable and Readable streams will store data in an internal buffer that can be retrieved using writable.writableBuffer or readable.readableBuffer, respectively.

The amount of data potentially buffered depends on the highWaterMark option passed into the stream's constructor. For normal streams, the highWaterMark option specifies a total number of bytes. For streams operating in object mode, the highWaterMark specifies a total number of objects.

Data is buffered in Readable streams when the implementation calls stream.push(chunk). If the consumer of the Stream does not call stream.read(), the data will sit in the internal queue until it is consumed.

Once the total size of the internal read buffer reaches the threshold specified by highWaterMark, the stream will temporarily stop reading data from the underlying resource until the data currently buffered can be consumed (that is, the stream will stop calling the internal readable._read() method that is used to fill the read buffer).

Data is buffered in Writable streams when the writable.write(chunk) method is called repeatedly. While the total size of the internal write buffer is below the threshold set by highWaterMark, calls to writable.write() will return true. Once the size of the internal buffer reaches or exceeds the highWaterMark, false will be returned.

A key goal of the stream API, particularly the stream.pipe() method, is to limit the buffering of data to acceptable levels such that sources and destinations of differing speeds will not overwhelm the available memory.

Because Duplex and Transform streams are both Readable and Writable, each maintains two separate internal buffers used for reading and writing, allowing each side to operate independently of the other while maintaining an appropriate and efficient flow of data. For example, net.Socket instances are Duplex streams whose Readable side allows consumption of data received from the socket and whose Writable side allows writing data to the socket. Because data may be written to the socket at a faster or slower rate than data is received, each side should operate (and buffer) independently of the other.

API for Stream Consumers#

Almost all Node.js applications, no matter how simple, use streams in some manner. The following is an example of using streams in a Node.js application that implements an HTTP server:

const http = require('http');

const server = http.createServer((req, res) => {
  // `req` is an http.IncomingMessage, which is a Readable Stream.
  // `res` is an http.ServerResponse, which is a Writable Stream.

  let body = '';
  // Get the data as utf8 strings.
  // If an encoding is not set, Buffer objects will be received.
  req.setEncoding('utf8');

  // Readable streams emit 'data' events once a listener is added.
  req.on('data', (chunk) => {
    body += chunk;
  });

  // The 'end' event indicates that the entire body has been received.
  req.on('end', () => {
    try {
      const data = JSON.parse(body);
      // Write back something interesting to the user:
      res.write(typeof data);
      res.end();
    } catch (er) {
      // uh oh! bad json!
      res.statusCode = 400;
      return res.end(`error: ${er.message}`);
    }
  });
});

server.listen(1337);

// $ curl localhost:1337 -d "{}"
// object
// $ curl localhost:1337 -d "\"foo\""
// string
// $ curl localhost:1337 -d "not json"
// error: Unexpected token o in JSON at position 1

Writable streams (such as res in the example) expose methods such as write() and end() that are used to write data onto the stream.

Readable streams use the EventEmitter API for notifying application code when data is available to be read off the stream. That available data can be read from the stream in multiple ways.

Both Writable and Readable streams use the EventEmitter API in various ways to communicate the current state of the stream.

Duplex and Transform streams are both Writable and Readable.

Applications that are either writing data to or consuming data from a stream are not required to implement the stream interfaces directly and will generally have no reason to call require('stream').

Developers wishing to implement new types of streams should refer to the section API for Stream Implementers.

Writable Streams#

Writable streams are an abstraction for a destination to which data is written.

Examples of Writable streams include:

Some of these examples are actually Duplex streams that implement the Writable interface.

All Writable streams implement the interface defined by the stream.Writable class.

While specific instances of Writable streams may differ in various ways, all Writable streams follow the same fundamental usage pattern as illustrated in the example below:

const myStream = getWritableStreamSomehow();
myStream.write('some data');
myStream.write('some more data');
myStream.end('done writing data');

Class: stream.Writable#

Added in: v0.9.4
Event: 'close'#
History
VersionChanges
v10.0.0

Add emitClose option to specify if 'close' is emitted on destroy.

v0.9.4

Added in: v0.9.4

The 'close' event is emitted when the stream and any of its underlying resources (a file descriptor, for example) have been closed. The event indicates that no more events will be emitted, and no further computation will occur.

A Writable stream will always emit the 'close' event if it is created with the emitClose option.

Event: 'drain'#
Added in: v0.9.4

If a call to stream.write(chunk) returns false, the 'drain' event will be emitted when it is appropriate to resume writing data to the stream.

// Write the data to the supplied writable stream one million times.
// Be attentive to back-pressure.
function writeOneMillionTimes(writer, data, encoding, callback) {
  let i = 1000000;
  write();
  function write() {
    let ok = true;
    do {
      i--;
      if (i === 0) {
        // Last time!
        writer.write(data, encoding, callback);
      } else {
        // See if we should continue, or wait.
        // Don't pass the callback, because we're not done yet.
        ok = writer.write(data, encoding);
      }
    } while (i > 0 && ok);
    if (i > 0) {
      // Had to stop early!
      // Write some more once it drains.
      writer.once('drain', write);
    }
  }
}
Event: 'error'#
Added in: v0.9.4

The 'error' event is emitted if an error occurred while writing or piping data. The listener callback is passed a single Error argument when called.

The stream is closed when the 'error' event is emitted unless the autoDestroy option was set to false when creating the stream.

After 'error', no further events other than 'close' should be emitted (including 'error' events).

Event: 'finish'#
Added in: v0.9.4

The 'finish' event is emitted after the stream.end() method has been called, and all data has been flushed to the underlying system.

const writer = getWritableStreamSomehow();
for (let i = 0; i < 100; i++) {
  writer.write(`hello, #${i}!\n`);
}
writer.on('finish', () => {
  console.log('All writes are now complete.');
});
writer.end('This is the end\n');
Event: 'pipe'#
Added in: v0.9.4

The 'pipe' event is emitted when the stream.pipe() method is called on a readable stream, adding this writable to its set of destinations.

const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('pipe', (src) => {
  console.log('Something is piping into the writer.');
  assert.equal(src, reader);
});
reader.pipe(writer);
Event: 'unpipe'#
Added in: v0.9.4

The 'unpipe' event is emitted when the stream.unpipe() method is called on a Readable stream, removing this Writable from its set of destinations.

This is also emitted in case this Writable stream emits an error when a Readable stream pipes into it.

const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('unpipe', (src) => {
  console.log('Something has stopped piping into the writer.');
  assert.equal(src, reader);
});
reader.pipe(writer);
reader.unpipe(writer);
writable.cork()#
Added in: v0.11.2

The writable.cork() method forces all written data to be buffered in memory. The buffered data will be flushed when either the stream.uncork() or stream.end() methods are called.

The primary intent of writable.cork() is to accommodate a situation in which several small chunks are written to the stream in rapid succession. Instead of immediately forwarding them to the underlying destination, writable.cork() buffers all the chunks until writable.uncork() is called, which will pass them all to writable._writev(), if present. This prevents a head-of-line blocking situation where data is being buffered while waiting for the first small chunk to be processed. However, use of writable.cork() without implementing writable._writev() may have an adverse effect on throughput.

See also: writable.uncork(), writable._writev().

writable.destroy([error])#
Added in: v8.0.0
  • error <Error> Optional, an error to emit with 'error' event.
  • Returns: <this>

Destroy the stream. Optionally emit an 'error' event, and emit a 'close' event (unless emitClose is set to false). After this call, the writable stream has ended and subsequent calls to write() or end() will result in an ERR_STREAM_DESTROYED error. This is a destructive and immediate way to destroy a stream. Previous calls to write() may not have drained, and may trigger an ERR_STREAM_DESTROYED error. Use end() instead of destroy if data should flush before close, or wait for the 'drain' event before destroying the stream.

Once destroy() has been called any further calls will be a noop and no further errors except from _destroy may be emitted as 'error'.

Implementors should not override this method, but instead implement writable._destroy().

writable.destroyed#
Added in: v8.0.0

Is true after writable.destroy() has been called.

writable.end([chunk[, encoding]][, callback])#
History
VersionChanges
v14.0.0

The callback is invoked if 'finish' or 'error' is emitted.

v10.0.0

This method now returns a reference to writable.

v8.0.0

The chunk argument can now be a Uint8Array instance.

v0.9.4

Added in: v0.9.4

  • chunk <string> | <Buffer> | <Uint8Array> | <any> Optional data to write. For streams not operating in object mode, chunk must be a string, Buffer or Uint8Array. For object mode streams, chunk may be any JavaScript value other than null.
  • encoding <string> The encoding if chunk is a string
  • callback <Function> Optional callback for when the stream finishes or errors
  • Returns: <this>

Calling the writable.end() method signals that no more data will be written to the Writable. The optional chunk and encoding arguments allow one final additional chunk of data to be written immediately before closing the stream. If provided, the optional callback function is attached as a listener for the 'finish' and the 'error' event.

Calling the stream.write() method after calling stream.end() will raise an error.

// Write 'hello, ' and then end with 'world!'.
const fs = require('fs');
const file = fs.createWriteStream('example.txt');
file.write('hello, ');
file.end('world!');
// Writing more now is not allowed!
writable.setDefaultEncoding(encoding)#
History
VersionChanges
v6.1.0

This method now returns a reference to writable.

v0.11.15

Added in: v0.11.15

The writable.setDefaultEncoding() method sets the default encoding for a Writable stream.

writable.uncork()#
Added in: v0.11.2

The writable.uncork() method flushes all data buffered since stream.cork() was called.

When using writable.cork() and writable.uncork() to manage the buffering of writes to a stream, it is recommended that calls to writable.uncork() be deferred using process.nextTick(). Doing so allows batching of all writable.write() calls that occur within a given Node.js event loop phase.

stream.cork();
stream.write('some ');
stream.write('data ');
process.nextTick(() => stream.uncork());

If the writable.cork() method is called multiple times on a stream, the same number of calls to writable.uncork() must be called to flush the buffered data.

stream.cork();
stream.write('some ');
stream.cork();
stream.write('data ');
process.nextTick(() => {
  stream.uncork();
  // The data will not be flushed until uncork() is called a second time.
  stream.uncork();
});

See also: writable.cork().

writable.writable#
Added in: v11.4.0

Is true if it is safe to call writable.write(), which means the stream has not been destroyed, errored or ended.

writable.writableEnded#
Added in: v12.9.0

Is true after writable.end() has been called. This property does not indicate whether the data has been flushed, for this use writable.writableFinished instead.

writable.writableCorked#
Added in: v13.2.0, v12.16.0

Number of times writable.uncork() needs to be called in order to fully uncork the stream.

writable.writableFinished#
Added in: v12.6.0

Is set to true immediately before the 'finish' event is emitted.

writable.writableHighWaterMark#
Added in: v9.3.0

Return the value of highWaterMark passed when constructing this Writable.

writable.writableLength#
Added in: v9.4.0

This property contains the number of bytes (or objects) in the queue ready to be written. The value provides introspection data regarding the status of the highWaterMark.

writable.writableObjectMode#
Added in: v12.3.0

Getter for the property objectMode of a given Writable stream.

writable.write(chunk[, encoding][, callback])#
History
VersionChanges
v8.0.0

The chunk argument can now be a Uint8Array instance.

v6.0.0

Passing null as the chunk parameter will always be considered invalid now, even in object mode.

v0.9.4

Added in: v0.9.4

  • chunk <string> | <Buffer> | <Uint8Array> | <any> Optional data to write. For streams not operating in object mode, chunk must be a string, Buffer or Uint8Array. For object mode streams, chunk may be any JavaScript value other than null.
  • encoding <string> The encoding, if chunk is a string
  • callback <Function> Callback for when this chunk of data is flushed
  • Returns: <boolean> false if the stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

The writable.write() method writes some data to the stream, and calls the supplied callback once the data has been fully handled. If an error occurs, the callback may or may not be called with the error as its first argument. To reliably detect write errors, add a listener for the 'error' event. The callback is called asynchronously and before 'error' is emitted.

The return value is true if the internal buffer is less than the highWaterMark configured when the stream was created after admitting chunk. If false is returned, further attempts to write data to the stream should stop until the 'drain' event is emitted.

While a stream is not draining, calls to write() will buffer chunk, and return false. Once all currently buffered chunks are drained (accepted for delivery by the operating system), the 'drain' event will be emitted. It is recommended that once write() returns false, no more chunks be written until the 'drain' event is emitted. While calling write() on a stream that is not draining is allowed, Node.js will buffer all written chunks until maximum memory usage occurs, at which point it will abort unconditionally. Even before it aborts, high memory usage will cause poor garbage collector performance and high RSS (which is not typically released back to the system, even after the memory is no longer required). Since TCP sockets may never drain if the remote peer does not read the data, writing a socket that is not draining may lead to a remotely exploitable vulnerability.

Writing data while the stream is not draining is particularly problematic for a Transform, because the Transform streams are paused by default until they are piped or a 'data' or 'readable' event handler is added.

If the data to be written can be generated or fetched on demand, it is recommended to encapsulate the logic into a Readable and use stream.pipe(). However, if calling write() is preferred, it is possible to respect backpressure and avoid memory issues using the 'drain' event:

function write(data, cb) {
  if (!stream.write(data)) {
    stream.once('drain', cb);
  } else {
    process.nextTick(cb);
  }
}

// Wait for cb to be called before doing any other write.
write('hello', () => {
  console.log('Write completed, do more writes now.');
});

A Writable stream in object mode will always ignore the encoding argument.

Readable Streams#

Readable streams are an abstraction for a source from which data is consumed.

Examples of Readable streams include:

All Readable streams implement the interface defined by the stream.Readable class.

Two Reading Modes#

Readable streams effectively operate in one of two modes: flowing and paused. These modes are separate from object mode. A Readable stream can be in object mode or not, regardless of whether it is in flowing mode or paused mode.

  • In flowing mode, data is read from the underlying system automatically and provided to an application as quickly as possible using events via the EventEmitter interface.

  • In paused mode, the stream.read() method must be called explicitly to read chunks of data from the stream.

All Readable streams begin in paused mode but can be switched to flowing mode in one of the following ways:

The Readable can switch back to paused mode using one of the following:

  • If there are no pipe destinations, by calling the stream.pause() method.
  • If there are pipe destinations, by removing all pipe destinations. Multiple pipe destinations may be removed by calling the stream.unpipe() method.

The important concept to remember is that a Readable will not generate data until a mechanism for either consuming or ignoring that data is provided. If the consuming mechanism is disabled or taken away, the Readable will attempt to stop generating the data.

For backward compatibility reasons, removing 'data' event handlers will not automatically pause the stream. Also, if there are piped destinations, then calling stream.pause() will not guarantee that the stream will remain paused once those destinations drain and ask for more data.

If a Readable is switched into flowing mode and there are no consumers available to handle the data, that data will be lost. This can occur, for instance, when the readable.resume() method is called without a listener attached to the 'data' event, or when a 'data' event handler is removed from the stream.

Adding a 'readable' event handler automatically make the stream to stop flowing, and the data to be consumed via readable.read(). If the 'readable' event handler is removed, then the stream will start flowing again if there is a 'data' event handler.

Three States#

The "two modes" of operation for a Readable stream are a simplified abstraction for the more complicated internal state management that is happening within the Readable stream implementation.

Specifically, at any given point in time, every Readable is in one of three possible states:

  • readable.readableFlowing === null
  • readable.readableFlowing === false
  • readable.readableFlowing === true

When readable.readableFlowing is null, no mechanism for consuming the stream's data is provided. Therefore, the stream will not generate data. While in this state, attaching a listener for the 'data' event, calling the readable.pipe() method, or calling the readable.resume() method will switch readable.readableFlowing to true, causing the Readable to begin actively emitting events as data is generated.

Calling readable.pause(), readable.unpipe(), or receiving backpressure will cause the readable.readableFlowing to be set as false, temporarily halting the flowing of events but not halting the generation of data. While in this state, attaching a listener for the 'data' event will not switch readable.readableFlowing to true.

const { PassThrough, Writable } = require('stream');
const pass = new PassThrough();
const writable = new Writable();

pass.pipe(writable);
pass.unpipe(writable);
// readableFlowing is now false.

pass.on('data', (chunk) => { console.log(chunk.toString()); });
pass.write('ok');  // Will not emit 'data'.
pass.resume();     // Must be called to make stream emit 'data'.

While readable.readableFlowing is false, data may be accumulating within the stream's internal buffer.

Choose One API Style#

The Readable stream API evolved across multiple Node.js versions and provides multiple methods of consuming stream data. In general, developers should choose one of the methods of consuming data and should never use multiple methods to consume data from a single stream. Specifically, using a combination of on('data'), on('readable'), pipe(), or async iterators could lead to unintuitive behavior.

Use of the readable.pipe() method is recommended for most users as it has been implemented to provide the easiest way of consuming stream data. Developers that require more fine-grained control over the transfer and generation of data can use the EventEmitter and readable.on('readable')/readable.read() or the readable.pause()/readable.resume() APIs.

Class: stream.Readable#

Added in: v0.9.4
Event: 'close'#
History
VersionChanges
v10.0.0

Add emitClose option to specify if 'close' is emitted on destroy.

v0.9.4

Added in: v0.9.4

The 'close' event is emitted when the stream and any of its underlying resources (a file descriptor, for example) have been closed. The event indicates that no more events will be emitted, and no further computation will occur.

A Readable stream will always emit the 'close' event if it is created with the emitClose option.

Event: 'data'#
Added in: v0.9.4
  • chunk <Buffer> | <string> | <any> The chunk of data. For streams that are not operating in object mode, the chunk will be either a string or Buffer. For streams that are in object mode, the chunk can be any JavaScript value other than null.

The 'data' event is emitted whenever the stream is relinquishing ownership of a chunk of data to a consumer. This may occur whenever the stream is switched in flowing mode by calling readable.pipe(), readable.resume(), or by attaching a listener callback to the 'data' event. The 'data' event will also be emitted whenever the readable.read() method is called and a chunk of data is available to be returned.

Attaching a 'data' event listener to a stream that has not been explicitly paused will switch the stream into flowing mode. Data will then be passed as soon as it is available.

The listener callback will be passed the chunk of data as a string if a default encoding has been specified for the stream using the readable.setEncoding() method; otherwise the data will be passed as a Buffer.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
});
Event: 'end'#
Added in: v0.9.4

The 'end' event is emitted when there is no more data to be consumed from the stream.

The 'end' event will not be emitted unless the data is completely consumed. This can be accomplished by switching the stream into flowing mode, or by calling stream.read() repeatedly until all data has been consumed.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
});
readable.on('end', () => {
  console.log('There will be no more data.');
});
Event: 'error'#
Added in: v0.9.4

The 'error' event may be emitted by a Readable implementation at any time. Typically, this may occur if the underlying stream is unable to generate data due to an underlying internal failure, or when a stream implementation attempts to push an invalid chunk of data.

The listener callback will be passed a single Error object.

Event: 'pause'#
Added in: v0.9.4

The 'pause' event is emitted when stream.pause() is called and readableFlowing is not false.

Event: 'readable'#
History
VersionChanges
v10.0.0

The 'readable' is always emitted in the next tick after .push() is called.

v10.0.0

Using 'readable' requires calling .read().

v0.9.4

Added in: v0.9.4

The 'readable' event is emitted when there is data available to be read from the stream. In some cases, attaching a listener for the 'readable' event will cause some amount of data to be read into an internal buffer.

const readable = getReadableStreamSomehow();
readable.on('readable', function() {
  // There is some data to read now.
  let data;

  while (data = this.read()) {
    console.log(data);
  }
});

The 'readable' event will also be emitted once the end of the stream data has been reached but before the 'end' event is emitted.

Effectively, the 'readable' event indicates that the stream has new information: either new data is available or the end of the stream has been reached. In the former case, stream.read() will return the available data. In the latter case, stream.read() will return null. For instance, in the following example, foo.txt is an empty file:

const fs = require('fs');
const rr = fs.createReadStream('foo.txt');
rr.on('readable', () => {
  console.log(`readable: ${rr.read()}`);
});
rr.on('end', () => {
  console.log('end');
});

The output of running this script is:

$ node test.js
readable: null
end

In general, the readable.pipe() and 'data' event mechanisms are easier to understand than the 'readable' event. However, handling 'readable' might result in increased throughput.

If both 'readable' and 'data' are used at the same time, 'readable' takes precedence in controlling the flow, i.e. 'data' will be emitted only when stream.read() is called. The readableFlowing property would become false. If there are 'data' listeners when 'readable' is removed, the stream will start flowing, i.e. 'data' events will be emitted without calling .resume().

Event: 'resume'#
Added in: v0.9.4

The 'resume' event is emitted when stream.resume() is called and readableFlowing is not true.

readable.destroy([error])#
Added in: v8.0.0
  • error <Error> Error which will be passed as payload in 'error' event
  • Returns: <this>

Destroy the stream. Optionally emit an 'error' event, and emit a 'close' event (unless emitClose is set to false). After this call, the readable stream will release any internal resources and subsequent calls to push() will be ignored.

Once destroy() has been called any further calls will be a noop and no further errors except from _destroy may be emitted as 'error'.

Implementors should not override this method, but instead implement readable._destroy().

readable.destroyed#
Added in: v8.0.0

Is true after readable.destroy() has been called.

readable.isPaused()#
Added in: v0.11.14

The readable.isPaused() method returns the current operating state of the Readable. This is used primarily by the mechanism that underlies the readable.pipe() method. In most typical cases, there will be no reason to use this method directly.

const readable = new stream.Readable();

readable.isPaused(); // === false
readable.pause();
readable.isPaused(); // === true
readable.resume();
readable.isPaused(); // === false
readable.pause()#
Added in: v0.9.4

The readable.pause() method will cause a stream in flowing mode to stop emitting 'data' events, switching out of flowing mode. Any data that becomes available will remain in the internal buffer.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
  readable.pause();
  console.log('There will be no additional data for 1 second.');
  setTimeout(() => {
    console.log('Now data will start flowing again.');
    readable.resume();
  }, 1000);
});

The readable.pause() method has no effect if there is a 'readable' event listener.

readable.pipe(destination[, options])#
Added in: v0.9.4

The readable.pipe() method attaches a Writable stream to the readable, causing it to switch automatically into flowing mode and push all of its data to the attached Writable. The flow of data will be automatically managed so that the destination Writable stream is not overwhelmed by a faster Readable stream.

The following example pipes all of the data from the readable into a file named file.txt:

const fs = require('fs');
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt'.
readable.pipe(writable);

It is possible to attach multiple Writable streams to a single Readable stream.

The readable.pipe() method returns a reference to the destination stream making it possible to set up chains of piped streams:

const fs = require('fs');
const r = fs.createReadStream('file.txt');
const z = zlib.createGzip();
const w = fs.createWriteStream('file.txt.gz');
r.pipe(z).pipe(w);

By default, stream.end() is called on the destination Writable stream when the source Readable stream emits 'end', so that the destination is no longer writable. To disable this default behavior, the end option can be passed as false, causing the destination stream to remain open:

reader.pipe(writer, { end: false });
reader.on('end', () => {
  writer.end('Goodbye\n');
});

One important caveat is that if the Readable stream emits an error during processing, the Writable destination is not closed automatically. If an error occurs, it will be necessary to manually close each stream in order to prevent memory leaks.

The process.stderr and process.stdout Writable streams are never closed until the Node.js process exits, regardless of the specified options.

readable.read([size])#
Added in: v0.9.4

The readable.read() method pulls some data out of the internal buffer and returns it. If no data available to be read, null is returned. By default, the data will be returned as a Buffer object unless an encoding has been specified using the readable.setEncoding() method or the stream is operating in object mode.

The optional size argument specifies a specific number of bytes to read. If size bytes are not available to be read, null will be returned unless the stream has ended, in which case all of the data remaining in the internal buffer will be returned.

If the size argument is not specified, all of the data contained in the internal buffer will be returned.

The size argument must be less than or equal to 1 GB.

The readable.read() method should only be called on Readable streams operating in paused mode. In flowing mode, readable.read() is called automatically until the internal buffer is fully drained.

const readable = getReadableStreamSomehow();
readable.on('readable', () => {
  let chunk;
  while (null !== (chunk = readable.read())) {
    console.log(`Received ${chunk.length} bytes of data.`);
  }
});

The while loop is necessary when processing data with readable.read(). Only after readable.read() returns null, 'readable' will be emitted.

A Readable stream in object mode will always return a single item from a call to readable.read(size), regardless of the value of the size argument.

If the readable.read() method returns a chunk of data, a 'data' event will also be emitted.

Calling stream.read([size]) after the 'end' event has been emitted will return null. No runtime error will be raised.

readable.readable#
Added in: v11.4.0

Is true if it is safe to call readable.read(), which means the stream has not been destroyed or emitted 'error' or 'end'.

readable.readableEncoding#
Added in: v12.7.0

Getter for the property encoding of a given Readable stream. The encoding property can be set using the readable.setEncoding() method.

readable.readableEnded#
Added in: v12.9.0

Becomes true when 'end' event is emitted.

readable.readableFlowing#
Added in: v9.4.0

This property reflects the current state of a Readable stream as described in the Stream Three States section.

readable.readableHighWaterMark#
Added in: v9.3.0

Returns the value of highWaterMark passed when constructing this Readable.

readable.readableLength#
Added in: v9.4.0

This property contains the number of bytes (or objects) in the queue ready to be read. The value provides introspection data regarding the status of the highWaterMark.

readable.readableObjectMode#
Added in: v12.3.0

Getter for the property objectMode of a given Readable stream.

readable.resume()#
History
VersionChanges
v10.0.0

The resume() has no effect if there is a 'readable' event listening.

v0.9.4

Added in: v0.9.4

The readable.resume() method causes an explicitly paused Readable stream to resume emitting 'data' events, switching the stream into flowing mode.

The readable.resume() method can be used to fully consume the data from a stream without actually processing any of that data:

getReadableStreamSomehow()
  .resume()
  .on('end', () => {
    console.log('Reached the end, but did not read anything.');
  });

The readable.resume() method has no effect if there is a 'readable' event listener.

readable.setEncoding(encoding)#
Added in: v0.9.4

The readable.setEncoding() method sets the character encoding for data read from the Readable stream.

By default, no encoding is assigned and stream data will be returned as Buffer objects. Setting an encoding causes the stream data to be returned as strings of the specified encoding rather than as Buffer objects. For instance, calling readable.setEncoding('utf8') will cause the output data to be interpreted as UTF-8 data, and passed as strings. Calling readable.setEncoding('hex') will cause the data to be encoded in hexadecimal string format.

The Readable stream will properly handle multi-byte characters delivered through the stream that would otherwise become improperly decoded if simply pulled from the stream as Buffer objects.

const readable = getReadableStreamSomehow();
readable.setEncoding('utf8');
readable.on('data', (chunk) => {
  assert.equal(typeof chunk, 'string');
  console.log('Got %d characters of string data:', chunk.length);
});
readable.unpipe([destination])#
Added in: v0.9.4

The readable.unpipe() method detaches a Writable stream previously attached using the stream.pipe() method.

If the destination is not specified, then all pipes are detached.

If the destination is specified, but no pipe is set up for it, then the method does nothing.

const fs = require('fs');
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt',
// but only for the first second.
readable.pipe(writable);
setTimeout(() => {
  console.log('Stop writing to file.txt.');
  readable.unpipe(writable);
  console.log('Manually close the file stream.');
  writable.end();
}, 1000);
readable.unshift(chunk[, encoding])#
History
VersionChanges
v8.0.0

The chunk argument can now be a Uint8Array instance.

v0.9.11

Added in: v0.9.11

  • chunk <Buffer> | <Uint8Array> | <string> | <null> | <any> Chunk of data to unshift onto the read queue. For streams not operating in object mode, chunk must be a string, Buffer, Uint8Array or null. For object mode streams, chunk may be any JavaScript value.
  • encoding <string> Encoding of string chunks. Must be a valid Buffer encoding, such as 'utf8' or 'ascii'.

Passing chunk as null signals the end of the stream (EOF) and behaves the same as readable.push(null), after which no more data can be written. The EOF signal is put at the end of the buffer and any buffered data will still be flushed.

The readable.unshift() method pushes a chunk of data back into the internal buffer. This is useful in certain situations where a stream is being consumed by code that needs to "un-consume" some amount of data that it has optimistically pulled out of the source, so that the data can be passed on to some other party.

The stream.unshift(chunk) method cannot be called after the 'end' event has been emitted or a runtime error will be thrown.

Developers using stream.unshift() often should consider switching to use of a Transform stream instead. See the API for Stream Implementers section for more information.

// Pull off a header delimited by \n\n.
// Use unshift() if we get too much.
// Call the callback with (error, header, stream).
const { StringDecoder } = require('string_decoder');
function parseHeader(stream, callback) {
  stream.on('error', callback);
  stream.on('readable', onReadable);
  const decoder = new StringDecoder('utf8');
  let header = '';
  function onReadable() {
    let chunk;
    while (null !== (chunk = stream.read())) {
      const str = decoder.write(chunk);
      if (str.match(/\n\n/)) {
        // Found the header boundary.
        const split = str.split(/\n\n/);
        header += split.shift();
        const remaining = split.join('\n\n');
        const buf = Buffer.from(remaining, 'utf8');
        stream.removeListener('error', callback);
        // Remove the 'readable' listener before unshifting.
        stream.removeListener('readable', onReadable);
        if (buf.length)
          stream.unshift(buf);
        // Now the body of the message can be read from the stream.
        callback(null, header, stream);
      } else {
        // Still reading the header.
        header += str;
      }
    }
  }
}

Unlike stream.push(chunk), stream.unshift(chunk) will not end the reading process by resetting the internal reading state of the stream. This can cause unexpected results if readable.unshift() is called during a read (i.e. from within a stream._read() implementation on a custom stream). Following the call to readable.unshift() with an immediate stream.push('') will reset the reading state appropriately, however it is best to simply avoid calling readable.unshift() while in the process of performing a read.

readable.wrap(stream)#
Added in: v0.9.4

Prior to Node.js 0.10, streams did not implement the entire stream module API as it is currently defined. (See Compatibility for more information.)

When using an older Node.js library that emits 'data' events and has a stream.pause() method that is advisory only, the readable.wrap() method can be used to create a Readable stream that uses the old stream as its data source.

It will rarely be necessary to use readable.wrap() but the method has been provided as a convenience for interacting with older Node.js applications and libraries.

const { OldReader } = require('./old-api-module.js');
const { Readable } = require('stream');
const oreader = new OldReader();
const myReader = new Readable().wrap(oreader);

myReader.on('readable', () => {
  myReader.read(); // etc.
});
readable[Symbol.asyncIterator]()#
History
VersionChanges
v11.14.0

Symbol.asyncIterator support is no longer experimental.

v10.0.0

Added in: v10.0.0

const fs = require('fs');

async function print(readable) {
  readable.setEncoding('utf8');
  let data = '';
  for await (const chunk of readable) {
    data += chunk;
  }
  console.log(data);
}

print(fs.createReadStream('file')).catch(console.error);

If the loop terminates with a break or a throw, the stream will be destroyed. In other terms, iterating over a stream will consume the stream fully. The stream will be read in chunks of size equal to the highWaterMark option. In the code example above, data will be in a single chunk if the file has less then 64KB of data because no highWaterMark option is provided to fs.createReadStream().

Duplex and Transform Streams#

Class: stream.Duplex#

History
VersionChanges
v6.8.0

Instances of Duplex now return true when checking instanceof stream.Writable.

v0.9.4

Added in: v0.9.4

Duplex streams are streams that implement both the Readable and Writable interfaces.

Examples of Duplex streams include:

Class: stream.Transform#

Added in: v0.9.4

Transform streams are Duplex streams where the output is in some way related to the input. Like all Duplex streams, Transform streams implement both the Readable and Writable interfaces.

Examples of Transform streams include:

transform.destroy([error])#
Added in: v8.0.0

Destroy the stream, and optionally emit an 'error' event. After this call, the transform stream would release any internal resources. Implementors should not override this method, but instead implement readable._destroy(). The default implementation of _destroy() for Transform also emit 'close' unless emitClose is set in false.

Once destroy() has been called any further calls will be a noop and no further errors except from _destroy may be emitted as 'error'.

stream.finished(stream[, options], callback)#

History
VersionChanges
v14.0.0

The finished(stream, cb) will wait for the 'close' event before invoking the callback. The implementation tries to detect legacy streams and only apply this behavior to streams which are expected to emit 'close'.

v14.0.0

Emitting 'close' before 'end' on a Readable stream will cause an ERR_STREAM_PREMATURE_CLOSE error.

v14.0.0

Callback will be invoked on streams which have already finished before the call to finished(stream, cb).

v10.0.0

Added in: v10.0.0

  • stream <Stream> A readable and/or writable stream.

  • options <Object>

    • error <boolean> If set to false, then a call to emit('error', err) is not treated as finished. Default: true.
    • readable <boolean> When set to false, the callback will be called when the stream ends even though the stream might still be readable. Default: true.
    • writable <boolean> When set to false, the callback will be called when the stream ends even though the stream might still be writable. Default: true.
  • callback <Function> A callback function that takes an optional error argument.
  • Returns: <Function> A cleanup function which removes all registered listeners.

A function to get notified when a stream is no longer readable, writable or has experienced an error or a premature close event.

const { finished } = require('stream');

const rs = fs.createReadStream('archive.tar');

finished(rs, (err) => {
  if (err) {
    console.error('Stream failed.', err);
  } else {
    console.log('Stream is done reading.');
  }
});

rs.resume(); // Drain the stream.

Especially useful in error handling scenarios where a stream is destroyed prematurely (like an aborted HTTP request), and will not emit 'end' or 'finish'.

The finished API is promisify-able as well;

const finished = util.promisify(stream.finished);

const rs = fs.createReadStream('archive.tar');

async function run() {
  await finished(rs);
  console.log('Stream is done reading.');
}

run().catch(console.error);
rs.resume(); // Drain the stream.

stream.finished() leaves dangling event listeners (in particular 'error', 'end', 'finish' and 'close') after callback has been invoked. The reason for this is so that unexpected 'error' events (due to incorrect stream implementations) do not cause unexpected crashes. If this is unwanted behavior then the returned cleanup function needs to be invoked in the callback:

const cleanup = finished(rs, (err) => {
  cleanup();
  // ...
});

stream.pipeline(source[, ...transforms], destination, callback)#

History
VersionChanges
v14.0.0

The pipeline(..., cb) will wait for the 'close' event before invoking the callback. The implementation tries to detect legacy streams and only apply this behavior to streams which are expected to emit 'close'.

v13.10.0

Add support for async generators.

v10.0.0

Added in: v10.0.0

A module method to pipe between streams and generators forwarding errors and properly cleaning up and provide a callback when the pipeline is complete.

const { pipeline } = require('stream');
const fs = require('fs');
const zlib = require('zlib');

// Use the pipeline API to easily pipe a series of streams
// together and get notified when the pipeline is fully done.

// A pipeline to gzip a potentially huge tar file efficiently:

pipeline(
  fs.createReadStream('archive.tar'),
  zlib.createGzip(),
  fs.createWriteStream('archive.tar.gz'),
  (err) => {
    if (err) {
      console.error('Pipeline failed.', err);
    } else {
      console.log('Pipeline succeeded.');
    }
  }
);

The pipeline API is promisify-able as well:

const pipeline = util.promisify(stream.pipeline);

async function run() {
  await pipeline(
    fs.createReadStream('archive.tar'),
    zlib.createGzip(),
    fs.createWriteStream('archive.tar.gz')
  );
  console.log('Pipeline succeeded.');
}

run().catch(console.error);

The pipeline API also supports async generators:

const pipeline = util.promisify(stream.pipeline);
const fs = require('fs');

async function run() {
  await pipeline(
    fs.createReadStream('lowercase.txt'),
    async function* (source) {
      source.setEncoding('utf8');  // Work with strings rather than `Buffer`s.
      for await (const chunk of source) {
        yield chunk.toUpperCase();
      }
    },
    fs.createWriteStream('uppercase.txt')
  );
  console.log('Pipeline succeeded.');
}

run().catch(console.error);

stream.pipeline() will call stream.destroy(err) on all streams except:

  • Readable streams which have emitted 'end' or 'close'.
  • Writable streams which have emitted 'finish' or 'close'.

stream.pipeline() leaves dangling event listeners on the streams after the callback has been invoked. In the case of reuse of streams after failure, this can cause event listener leaks and swallowed errors.

stream.Readable.from(iterable, [options])#

Added in: v12.3.0, v10.17.0
  • iterable <Iterable> Object implementing the Symbol.asyncIterator or Symbol.iterator iterable protocol. Emits an 'error' event if a null value is passed.
  • options <Object> Options provided to new stream.Readable([options]). By default, Readable.from() will set options.objectMode to true, unless this is explicitly opted out by setting options.objectMode to false.
  • Returns: <stream.Readable>

A utility method for creating Readable Streams out of iterators.

const { Readable } = require('stream');

async function * generate() {
  yield 'hello';
  yield 'streams';
}

const readable = Readable.from(generate());

readable.on('data', (chunk) => {
  console.log(chunk);
});

Calling Readable.from(string) or Readable.from(buffer) will not have the strings or buffers be iterated to match the other streams semantics for performance reasons.

API for Stream Implementers#

The stream module API has been designed to make it possible to easily implement streams using JavaScript's prototypal inheritance model.

First, a stream developer would declare a new JavaScript class that extends one of the four basic stream classes (stream.Writable, stream.Readable, stream.Duplex, or stream.Transform), making sure they call the appropriate parent class constructor:

const { Writable } = require('stream');

class MyWritable extends Writable {
  constructor({ highWaterMark, ...options }) {
    super({ highWaterMark });
    // ...
  }
}

When extending streams, keep in mind what options the user can and should provide before forwarding these to the base constructor. For example, if the implementation makes assumptions in regard to the autoDestroy and emitClose options, do not allow the user to override these. Be explicit about what options are forwarded instead of implicitly forwarding all options.

The new stream class must then implement one or more specific methods, depending on the type of stream being created, as detailed in the chart below:

Use-caseClassMethod(s) to implement
Reading onlyReadable_read()
Writing onlyWritable_write(), _writev(), _final()
Reading and writingDuplex_read(), _write(), _writev(), _final()
Operate on written data, then read the resultTransform_transform(), _flush(), _final()

The implementation code for a stream should never call the "public" methods of a stream that are intended for use by consumers (as described in the API for Stream Consumers section). Doing so may lead to adverse side effects in application code consuming the stream.

Avoid overriding public methods such as write(), end(), cork(), uncork(), read() and destroy(), or emitting internal events such as 'error', 'data', 'end', 'finish' and 'close' through .emit(). Doing so can break current and future stream invariants leading to behavior and/or compatibility issues with other streams, stream utilities, and user expectations.

Simplified Construction#

Added in: v1.2.0

For many simple cases, it is possible to construct a stream without relying on inheritance. This can be accomplished by directly creating instances of the stream.Writable, stream.Readable, stream.Duplex or stream.Transform objects and passing appropriate methods as constructor options.

const { Writable } = require('stream');

const myWritable = new Writable({
  write(chunk, encoding, callback) {
    // ...
  }
});

Implementing a Writable Stream#

The stream.Writable class is extended to implement a Writable stream.

Custom Writable streams must call the new stream.Writable([options]) constructor and implement the writable._write() and/or writable._writev() method.

Constructor: new stream.Writable([options])#

History
VersionChanges
v14.0.0

Change autoDestroy option default to true.

v11.2.0, v10.16.0

Add autoDestroy option to automatically destroy() the stream when it emits 'finish' or errors.

v10.0.0

Add emitClose option to specify if 'close' is emitted on destroy.

  • options <Object>

    • highWaterMark <number> Buffer level when stream.write() starts returning false. Default: 16384 (16KB), or 16 for objectMode streams.
    • decodeStrings <boolean> Whether to encode strings passed to stream.write() to Buffers (with the encoding specified in the stream.write() call) before passing them to stream._write(). Other types of data are not converted (i.e. Buffers are not decoded into strings). Setting to false will prevent strings from being converted. Default: true.
    • defaultEncoding <string> The default encoding that is used when no encoding is specified as an argument to stream.write(). Default: 'utf8'.
    • objectMode <boolean> Whether or not the stream.write(anyObj) is a valid operation. When set, it becomes possible to write JavaScript values other than string, Buffer or Uint8Array if supported by the stream implementation. Default: false.
    • emitClose <boolean> Whether or not the stream should emit 'close' after it has been destroyed. Default: true.
    • write <Function> Implementation for the stream._write() method.
    • writev <Function> Implementation for the stream._writev() method.
    • destroy <Function> Implementation for the stream._destroy() method.
    • final <Function> Implementation for the stream._final() method.
    • autoDestroy <boolean> Whether this stream should automatically call .destroy() on itself after ending. Default: true.
const { Writable } = require('stream');

class MyWritable extends Writable {
  constructor(options) {
    // Calls the stream.Writable() constructor.
    super(options);
    // ...
  }
}

Or, when using pre-ES6 style constructors:

const { Writable } = require('stream');
const util = require('util');

function MyWritable(options) {
  if (!(this instanceof MyWritable))
    return new MyWritable(options);
  Writable.call(this, options);
}
util.inherits(MyWritable, Writable);

Or, using the Simplified Constructor approach:

const { Writable } = require('stream');

const myWritable = new Writable({
  write(chunk, encoding, callback) {
    // ...
  },
  writev(chunks, callback) {
    // ...
  }
});

writable._write(chunk, encoding, callback)#

History
VersionChanges
v12.11.0

_write() is optional when providing _writev().

  • chunk <Buffer> | <string> | <any> The Buffer to be written, converted from the string passed to stream.write(). If the stream's decodeStrings option is false or the stream is operating in object mode, the chunk will not be converted & will be whatever was passed to stream.write().
  • encoding <string> If the chunk is a string, then encoding is the character encoding of that string. If chunk is a Buffer, or if the stream is operating in object mode, encoding may be ignored.
  • callback <Function> Call this function (optionally with an error argument) when processing is complete for the supplied chunk.

All Writable stream implementations must provide a writable._write() and/or writable._writev() method to send data to the underlying resource.

Transform streams provide their own implementation of the writable._write().

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal Writable class methods only.

The callback function must be called synchronously inside of writable._write() or asynchronously (i.e. different tick) to signal either that the write completed successfully or failed with an error. The first argument passed to the callback must be the Error object if the call failed or null if the write succeeded.

All calls to writable.write() that occur between the time writable._write() is called and the callback is called will cause the written data to be buffered. When the callback is invoked, the stream might emit a 'drain' event. If a stream implementation is capable of processing multiple chunks of data at once, the writable._writev() method should be implemented.

If the decodeStrings property is explicitly set to false in the constructor options, then chunk will remain the same object that is passed to .write(), and may be a string rather than a Buffer. This is to support implementations that have an optimized handling for certain string data encodings. In that case, the encoding argument will indicate the character encoding of the string. Otherwise, the encoding argument can be safely ignored.

The writable._write() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

writable._writev(chunks, callback)#

  • chunks <Object[]> The chunks to be written. Each chunk has following format: { chunk: ..., encoding: ... }.
  • callback <Function> A callback function (optionally with an error argument) to be invoked when processing is complete for the supplied chunks.

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal Writable class methods only.

The writable._writev() method may be implemented in addition or alternatively to writable._write() in stream implementations that are capable of processing multiple chunks of data at once. If implemented and if there is buffered data from previous writes, _writev() will be called instead of _write().

The writable._writev() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

writable._destroy(err, callback)#

Added in: v8.0.0
  • err <Error> A possible error.
  • callback <Function> A callback function that takes an optional error argument.

The _destroy() method is called by writable.destroy(). It can be overridden by child classes but it must not be called directly.

writable._final(callback)#

Added in: v8.0.0
  • callback <Function> Call this function (optionally with an error argument) when finished writing any remaining data.

The _final() method must not be called directly. It may be implemented by child classes, and if so, will be called by the internal Writable class methods only.

This optional function will be called before the stream closes, delaying the 'finish' event until callback is called. This is useful to close resources or write buffered data before a stream ends.

Errors While Writing#

Errors occurring during the processing of the writable._write(), writable._writev() and writable._final() methods must be propagated by invoking the callback and passing the error as the first argument. Throwing an Error from within these methods or manually emitting an 'error' event results in undefined behavior.

If a Readable stream pipes into a Writable stream when Writable emits an error, the Readable stream will be unpiped.

const { Writable } = require('stream');

const myWritable = new Writable({
  write(chunk, encoding, callback) {
    if (chunk.toString().indexOf('a') >= 0) {
      callback(new Error('chunk is invalid'));
    } else {
      callback();
    }
  }
});

An Example Writable Stream#

The following illustrates a rather simplistic (and somewhat pointless) custom Writable stream implementation. While this specific Writable stream instance is not of any real particular usefulness, the example illustrates each of the required elements of a custom Writable stream instance:

const { Writable } = require('stream');

class MyWritable extends Writable {
  _write(chunk, encoding, callback) {
    if (chunk.toString().indexOf('a') >= 0) {
      callback(new Error('chunk is invalid'));
    } else {
      callback();
    }
  }
}

Decoding buffers in a Writable Stream#

Decoding buffers is a common task, for instance, when using transformers whose input is a string. This is not a trivial process when using multi-byte characters encoding, such as UTF-8. The following example shows how to decode multi-byte strings using StringDecoder and Writable.

const { Writable } = require('stream');
const { StringDecoder } = require('string_decoder');

class StringWritable extends Writable {
  constructor(options) {
    super(options);
    this._decoder = new StringDecoder(options && options.defaultEncoding);
    this.data = '';
  }
  _write(chunk, encoding, callback) {
    if (encoding === 'buffer') {
      chunk = this._decoder.write(chunk);
    }
    this.data += chunk;
    callback();
  }
  _final(callback) {
    this.data += this._decoder.end();
    callback();
  }
}

const euro = [[0xE2, 0x82], [0xAC]].map(Buffer.from);
const w = new StringWritable();

w.write('currency: ');
w.write(euro[0]);
w.end(euro[1]);

console.log(w.data); // currency: €

Implementing a Readable Stream#

The stream.Readable class is extended to implement a Readable stream.

Custom Readable streams must call the new stream.Readable([options]) constructor and implement the readable._read() method.

new stream.Readable([options])#

History
VersionChanges
v14.0.0

Change autoDestroy option default to true.

v11.2.0, v10.16.0

Add autoDestroy option to automatically destroy() the stream when it emits 'end' or errors.

  • options <Object>

    • highWaterMark <number> The maximum number of bytes to store in the internal buffer before ceasing to read from the underlying resource. Default: 16384 (16KB), or 16 for objectMode streams.
    • encoding <string> If specified, then buffers will be decoded to strings using the specified encoding. Default: null.
    • objectMode <boolean> Whether this stream should behave as a stream of objects. Meaning that stream.read(n) returns a single value instead of a Buffer of size n. Default: false.
    • emitClose <boolean> Whether or not the stream should emit 'close' after it has been destroyed. Default: true.
    • read <Function> Implementation for the stream._read() method.
    • destroy <Function> Implementation for the stream._destroy() method.
    • autoDestroy <boolean> Whether this stream should automatically call .destroy() on itself after ending. Default: true.
const { Readable } = require('stream');

class MyReadable extends Readable {
  constructor(options) {
    // Calls the stream.Readable(options) constructor.
    super(options);
    // ...
  }
}

Or, when using pre-ES6 style constructors:

const { Readable } = require('stream');
const util = require('util');

function MyReadable(options) {
  if (!(this instanceof MyReadable))
    return new MyReadable(options);
  Readable.call(this, options);
}
util.inherits(MyReadable, Readable);

Or, using the Simplified Constructor approach:

const { Readable } = require('stream');

const myReadable = new Readable({
  read(size) {
    // ...
  }
});

readable._read(size)#

Added in: v0.9.4
  • size <number> Number of bytes to read asynchronously

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal Readable class methods only.

All Readable stream implementations must provide an implementation of the readable._read() method to fetch data from the underlying resource.

When readable._read() is called, if data is available from the resource, the implementation should begin pushing that data into the read queue using the this.push(dataChunk) method. _read() should continue reading from the resource and pushing data until readable.push() returns false. Only when _read() is called again after it has stopped should it resume pushing additional data onto the queue.

Once the readable._read() method has been called, it will not be called again until more data is pushed through the readable.push() method. Empty data such as empty buffers and strings will not cause readable._read() to be called.

The size argument is advisory. For implementations where a "read" is a single operation that returns data can use the size argument to determine how much data to fetch. Other implementations may ignore this argument and simply provide data whenever it becomes available. There is no need to "wait" until size bytes are available before calling stream.push(chunk).

The readable._read() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

readable._destroy(err, callback)#

Added in: v8.0.0
  • err <Error> A possible error.
  • callback <Function> A callback function that takes an optional error argument.

The _destroy() method is called by readable.destroy(). It can be overridden by child classes but it must not be called directly.

readable.push(chunk[, encoding])#

History
VersionChanges
v8.0.0

The chunk argument can now be a Uint8Array instance.

  • chunk <Buffer> | <Uint8Array> | <string> | <null> | <any> Chunk of data to push into the read queue. For streams not operating in object mode, chunk must be a string, Buffer or Uint8Array. For object mode streams, chunk may be any JavaScript value.
  • encoding <string> Encoding of string chunks. Must be a valid Buffer encoding, such as 'utf8' or 'ascii'.
  • Returns: <boolean> true if additional chunks of data may continue to be pushed; false otherwise.

When chunk is a Buffer, Uint8Array or string, the chunk of data will be added to the internal queue for users of the stream to consume. Passing chunk as null signals the end of the stream (EOF), after which no more data can be written.

When the Readable is operating in paused mode, the data added with readable.push() can be read out by calling the readable.read() method when the 'readable' event is emitted.

When the Readable is operating in flowing mode, the data added with readable.push() will be delivered by emitting a 'data' event.

The readable.push() method is designed to be as flexible as possible. For example, when wrapping a lower-level source that provides some form of pause/resume mechanism, and a data callback, the low-level source can be wrapped by the custom Readable instance:

// `_source` is an object with readStop() and readStart() methods,
// and an `ondata` member that gets called when it has data, and
// an `onend` member that gets called when the data is over.

class SourceWrapper extends Readable {
  constructor(options) {
    super(options);

    this._source = getLowLevelSourceObject();

    // Every time there's data, push it into the internal buffer.
    this._source.ondata = (chunk) => {
      // If push() returns false, then stop reading from source.
      if (!this.push(chunk))
        this._source.readStop();
    };

    // When the source ends, push the EOF-signaling `null` chunk.
    this._source.onend = () => {
      this.push(null);
    };
  }
  // _read() will be called when the stream wants to pull more data in.
  // The advisory size argument is ignored in this case.
  _read(size) {
    this._source.readStart();
  }
}

The readable.push() method is used to push the content into the internal buffer. It can be driven by the readable._read() method.

For streams not operating in object mode, if the chunk parameter of readable.push() is undefined, it will be treated as empty string or buffer. See readable.push('') for more information.

Errors While Reading#

Errors occurring during processing of the readable._read() must be propagated through the readable.destroy(err) method. Throwing an Error from within readable._read() or manually emitting an 'error' event results in undefined behavior.

const { Readable } = require('stream');

const myReadable = new Readable({
  read(size) {
    const err = checkSomeErrorCondition();
    if (err) {
      this.destroy(err);
    } else {
      // Do some work.
    }
  }
});

An Example Counting Stream#

The following is a basic example of a Readable stream that emits the numerals from 1 to 1,000,000 in ascending order, and then ends.

const { Readable } = require('stream');

class Counter extends Readable {
  constructor(opt) {
    super(opt);
    this._max = 1000000;
    this._index = 1;
  }

  _read() {
    const i = this._index++;
    if (i > this._max)
      this.push(null);
    else {
      const str = String(i);
      const buf = Buffer.from(str, 'ascii');
      this.push(buf);
    }
  }
}

Implementing a Duplex Stream#

A Duplex stream is one that implements both Readable and Writable, such as a TCP socket connection.

Because JavaScript does not have support for multiple inheritance, the stream.Duplex class is extended to implement a Duplex stream (as opposed to extending the stream.Readable and stream.Writable classes).

The stream.Duplex class prototypically inherits from stream.Readable and parasitically from stream.Writable, but instanceof will work properly for both base classes due to overriding Symbol.hasInstance on stream.Writable.

Custom Duplex streams must call the new stream.Duplex([options]) constructor and implement both the readable._read() and writable._write() methods.

new stream.Duplex(options)#

History
VersionChanges
v8.4.0

The readableHighWaterMark and writableHighWaterMark options are supported now.

  • options <Object> Passed to both Writable and Readable constructors. Also has the following fields:

    • allowHalfOpen <boolean> If set to false, then the stream will automatically end the writable side when the readable side ends. Default: true.
    • readableObjectMode <boolean> Sets objectMode for readable side of the stream. Has no effect if objectMode is true. Default: false.
    • writableObjectMode <boolean> Sets objectMode for writable side of the stream. Has no effect if objectMode is true. Default: false.
    • readableHighWaterMark <number> Sets highWaterMark for the readable side of the stream. Has no effect if highWaterMark is provided.
    • writableHighWaterMark <number> Sets highWaterMark for the writable side of the stream. Has no effect if highWaterMark is provided.
const { Duplex } = require('stream');

class MyDuplex extends Duplex {
  constructor(options) {
    super(options);
    // ...
  }
}

Or, when using pre-ES6 style constructors:

const { Duplex } = require('stream');
const util = require('util');

function MyDuplex(options) {
  if (!(this instanceof MyDuplex))
    return new MyDuplex(options);
  Duplex.call(this, options);
}
util.inherits(MyDuplex, Duplex);

Or, using the Simplified Constructor approach:

const { Duplex } = require('stream');

const myDuplex = new Duplex({
  read(size) {
    // ...
  },
  write(chunk, encoding, callback) {
    // ...
  }
});

An Example Duplex Stream#

The following illustrates a simple example of a Duplex stream that wraps a hypothetical lower-level source object to which data can be written, and from which data can be read, albeit using an API that is not compatible with Node.js streams. The following illustrates a simple example of a Duplex stream that buffers incoming written data via the Writable interface that is read back out via the Readable interface.

const { Duplex } = require('stream');
const kSource = Symbol('source');

class MyDuplex extends Duplex {
  constructor(source, options) {
    super(options);
    this[kSource] = source;
  }

  _write(chunk, encoding, callback) {
    // The underlying source only deals with strings.
    if (Buffer.isBuffer(chunk))
      chunk = chunk.toString();
    this[kSource].writeSomeData(chunk);
    callback();
  }

  _read(size) {
    this[kSource].fetchSomeData(size, (data, encoding) => {
      this.push(Buffer.from(data, encoding));
    });
  }
}

The most important aspect of a Duplex stream is that the Readable and Writable sides operate independently of one another despite co-existing within a single object instance.

Object Mode Duplex Streams#

For Duplex streams, objectMode can be set exclusively for either the Readable or Writable side using the readableObjectMode and writableObjectMode options respectively.

In the following example, for instance, a new Transform stream (which is a type of Duplex stream) is created that has an object mode Writable side that accepts JavaScript numbers that are converted to hexadecimal strings on the Readable side.

const { Transform } = require('stream');

// All Transform streams are also Duplex Streams.
const myTransform = new Transform({
  writableObjectMode: true,

  transform(chunk, encoding, callback) {
    // Coerce the chunk to a number if necessary.
    chunk |= 0;

    // Transform the chunk into something else.
    const data = chunk.toString(16);

    // Push the data onto the readable queue.
    callback(null, '0'.repeat(data.length % 2) + data);
  }
});

myTransform.setEncoding('ascii');
myTransform.on('data', (chunk) => console.log(chunk));

myTransform.write(1);
// Prints: 01
myTransform.write(10);
// Prints: 0a
myTransform.write(100);
// Prints: 64

Implementing a Transform Stream#

A Transform stream is a Duplex stream where the output is computed in some way from the input. Examples include zlib streams or crypto streams that compress, encrypt, or decrypt data.

There is no requirement that the output be the same size as the input, the same number of chunks, or arrive at the same time. For example, a Hash stream will only ever have a single chunk of output which is provided when the input is ended. A zlib stream will produce output that is either much smaller or much larger than its input.

The stream.Transform class is extended to implement a Transform stream.

The stream.Transform class prototypically inherits from stream.Duplex and implements its own versions of the writable._write() and readable._read() methods. Custom Transform implementations must implement the transform._transform() method and may also implement the transform._flush() method.

Care must be taken when using Transform streams in that data written to the stream can cause the Writable side of the stream to become paused if the output on the Readable side is not consumed.

new stream.Transform([options])#

const { Transform } = require('stream');

class MyTransform extends Transform {
  constructor(options) {
    super(options);
    // ...
  }
}

Or, when using pre-ES6 style constructors:

const { Transform } = require('stream');
const util = require('util');

function MyTransform(options) {
  if (!(this instanceof MyTransform))
    return new MyTransform(options);
  Transform.call(this, options);
}
util.inherits(MyTransform, Transform);

Or, using the Simplified Constructor approach:

const { Transform } = require('stream');

const myTransform = new Transform({
  transform(chunk, encoding, callback) {
    // ...
  }
});

Events: 'finish' and 'end'#

The 'finish' and 'end' events are from the stream.Writable and stream.Readable classes, respectively. The 'finish' event is emitted after stream.end() is called and all chunks have been processed by stream._transform(). The 'end' event is emitted after all data has been output, which occurs after the callback in transform._flush() has been called. In the case of an error, neither 'finish' nor 'end' should be emitted.

transform._flush(callback)#

  • callback <Function> A callback function (optionally with an error argument and data) to be called when remaining data has been flushed.

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal Readable class methods only.

In some cases, a transform operation may need to emit an additional bit of data at the end of the stream. For example, a zlib compression stream will store an amount of internal state used to optimally compress the output. When the stream ends, however, that additional data needs to be flushed so that the compressed data will be complete.

Custom Transform implementations may implement the transform._flush() method. This will be called when there is no more written data to be consumed, but before the 'end' event is emitted signaling the end of the Readable stream.

Within the transform._flush() implementation, the readable.push() method may be called zero or more times, as appropriate. The callback function must be called when the flush operation is complete.

The transform._flush() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

transform._transform(chunk, encoding, callback)#

  • chunk <Buffer> | <string> | <any> The Buffer to be transformed, converted from the string passed to stream.write(). If the stream's decodeStrings option is false or the stream is operating in object mode, the chunk will not be converted & will be whatever was passed to stream.write().
  • encoding <string> If the chunk is a string, then this is the encoding type. If chunk is a buffer, then this is the special value 'buffer'. Ignore it in that case.
  • callback <Function> A callback function (optionally with an error argument and data) to be called after the supplied chunk has been processed.

This function MUST NOT be called by application code directly. It should be implemented by child classes, and called by the internal Readable class methods only.

All Transform stream implementations must provide a _transform() method to accept input and produce output. The transform._transform() implementation handles the bytes being written, computes an output, then passes that output off to the readable portion using the readable.push() method.

The transform.push() method may be called zero or more times to generate output from a single input chunk, depending on how much is to be output as a result of the chunk.

It is possible that no output is generated from any given chunk of input data.

The callback function must be called only when the current chunk is completely consumed. The first argument passed to the callback must be an Error object if an error occurred while processing the input or null otherwise. If a second argument is passed to the callback, it will be forwarded on to the readable.push() method. In other words, the following are equivalent:

transform.prototype._transform = function(data, encoding, callback) {
  this.push(data);
  callback();
};

transform.prototype._transform = function(data, encoding, callback) {
  callback(null, data);
};

The transform._transform() method is prefixed with an underscore because it is internal to the class that defines it, and should never be called directly by user programs.

transform._transform() is never called in parallel; streams implement a queue mechanism, and to receive the next chunk, callback must be called, either synchronously or asynchronously.

Class: stream.PassThrough#

The stream.PassThrough class is a trivial implementation of a Transform stream that simply passes the input bytes across to the output. Its purpose is primarily for examples and testing, but there are some use cases where stream.PassThrough is useful as a building block for novel sorts of streams.

Additional Notes#

Streams Compatibility with Async Generators and Async Iterators#

With the support of async generators and iterators in JavaScript, async generators are effectively a first-class language-level stream construct at this point.

Some common interop cases of using Node.js streams with async generators and async iterators are provided below.

Consuming Readable Streams with Async Iterators#

(async function() {
  for await (const chunk of readable) {
    console.log(chunk);
  }
})();

Async iterators register a permanent error handler on the stream to prevent any unhandled post-destroy errors.

Creating Readable Streams with Async Generators#

We can construct a Node.js Readable Stream from an asynchronous generator using the Readable.from() utility method:

const { Readable } = require('stream');

async function * generate() {
  yield 'a';
  yield 'b';
  yield 'c';
}

const readable = Readable.from(generate());

readable.on('data', (chunk) => {
  console.log(chunk);
});

Piping to Writable Streams from Async Iterators#

In the scenario of writing to a writable stream from an async iterator, ensure the correct handling of backpressure and errors.

const { once } = require('events');
const finished = util.promisify(stream.finished);

const writable = fs.createWriteStream('./file');

function drain(writable) {
  if (writable.destroyed) {
    return Promise.reject(new Error('premature close'));
  }
  return Promise.race([
    once(writable, 'drain'),
    once(writable, 'close')
      .then(() => Promise.reject(new Error('premature close')))
  ]);
}

async function pump(iterable, writable) {
  for await (const chunk of iterable) {
    // Handle backpressure on write().
    if (!writable.write(chunk)) {
      await drain(writable);
    }
  }
  writable.end();
}

(async function() {
  // Ensure completion without errors.
  await Promise.all([
    pump(iterable, writable),
    finished(writable)
  ]);
})();

In the above, errors on write() would be caught and thrown by the once() listener for the 'drain' event, since once() will also handle the 'error' event. To ensure completion of the write stream without errors, it is safer to use the finished() method as above, instead of using the once() listener for the 'finish' event. Under certain cases, an 'error' event could be emitted by the writable stream after 'finish' and as once() will release the 'error' handler on handling the 'finish' event, it could result in an unhandled error.

Alternatively, the readable stream could be wrapped with Readable.from() and then piped via .pipe():

const finished = util.promisify(stream.finished);

const writable = fs.createWriteStream('./file');

(async function() {
  const readable = Readable.from(iterable);
  readable.pipe(writable);
  // Ensure completion without errors.
  await finished(writable);
})();

Or, using stream.pipeline() to pipe streams:

const pipeline = util.promisify(stream.pipeline);

const writable = fs.createWriteStream('./file');

(async function() {
  await pipeline(iterable, writable);
})();

Compatibility with Older Node.js Versions#

Prior to Node.js 0.10, the Readable stream interface was simpler, but also less powerful and less useful.

  • Rather than waiting for calls to the stream.read() method, 'data' events would begin emitting immediately. Applications that would need to perform some amount of work to decide how to handle data were required to store read data into buffers so the data would not be lost.
  • The stream.pause() method was advisory, rather than guaranteed. This meant that it was still necessary to be prepared to receive 'data' events even when the stream was in a paused state.

In Node.js 0.10, the Readable class was added. For backward compatibility with older Node.js programs, Readable streams switch into "flowing mode" when a 'data' event handler is added, or when the stream.resume() method is called. The effect is that, even when not using the new stream.read() method and 'readable' event, it is no longer necessary to worry about losing 'data' chunks.

While most applications will continue to function normally, this introduces an edge case in the following conditions:

  • No 'data' event listener is added.
  • The stream.resume() method is never called.
  • The stream is not piped to any writable destination.

For example, consider the following code:

// WARNING!  BROKEN!
net.createServer((socket) => {

  // We add an 'end' listener, but never consume the data.
  socket.on('end', () => {
    // It will never get here.
    socket.end('The message was received but was not processed.\n');
  });

}).listen(1337);

Prior to Node.js 0.10, the incoming message data would be simply discarded. However, in Node.js 0.10 and beyond, the socket remains paused forever.

The workaround in this situation is to call the stream.resume() method to begin the flow of data:

// Workaround.
net.createServer((socket) => {
  socket.on('end', () => {
    socket.end('The message was received but was not processed.\n');
  });

  // Start the flow of data, discarding it.
  socket.resume();
}).listen(1337);

In addition to new Readable streams switching into flowing mode, pre-0.10 style streams can be wrapped in a Readable class using the readable.wrap() method.

readable.read(0)#

There are some cases where it is necessary to trigger a refresh of the underlying readable stream mechanisms, without actually consuming any data. In such cases, it is possible to call readable.read(0), which will always return null.

If the internal read buffer is below the highWaterMark, and the stream is not currently reading, then calling stream.read(0) will trigger a low-level stream._read() call.

While most applications will almost never need to do this, there are situations within Node.js where this is done, particularly in the Readable stream class internals.

readable.push('')#

Use of readable.push('') is not recommended.

Pushing a zero-byte string, Buffer or Uint8Array to a stream that is not in object mode has an interesting side effect. Because it is a call to readable.push(), the call will end the reading process. However, because the argument is an empty string, no data is added to the readable buffer so there is nothing for a user to consume.

highWaterMark discrepancy after calling readable.setEncoding()#

The use of readable.setEncoding() will change the behavior of how the highWaterMark operates in non-object mode.

Typically, the size of the current buffer is measured against the highWaterMark in bytes. However, after setEncoding() is called, the comparison function will begin to measure the buffer's size in characters.

This is not a problem in common cases with latin1 or ascii. But it is advised to be mindful about this behavior when working with strings that could contain multi-byte characters.

String Decoder#

Stability: 2 - Stable

The string_decoder module provides an API for decoding Buffer objects into strings in a manner that preserves encoded multi-byte UTF-8 and UTF-16 characters. It can be accessed using:

const { StringDecoder } = require('string_decoder');

The following example shows the basic use of the StringDecoder class.

const { StringDecoder } = require('string_decoder');
const decoder = new StringDecoder('utf8');

const cent = Buffer.from([0xC2, 0xA2]);
console.log(decoder.write(cent));

const euro = Buffer.from([0xE2, 0x82, 0xAC]);
console.log(decoder.write(euro));

When a Buffer instance is written to the StringDecoder instance, an internal buffer is used to ensure that the decoded string does not contain any incomplete multibyte characters. These are held in the buffer until the next call to stringDecoder.write() or until stringDecoder.end() is called.

In the following example, the three UTF-8 encoded bytes of the European Euro symbol (€) are written over three separate operations:

const { StringDecoder } = require('string_decoder');
const decoder = new StringDecoder('utf8');

decoder.write(Buffer.from([0xE2]));
decoder.write(Buffer.from([0x82]));
console.log(decoder.end(Buffer.from([0xAC])));

Class: StringDecoder#

new StringDecoder([encoding])#

Added in: v0.1.99
  • encoding <string> The character encoding the StringDecoder will use. Default: 'utf8'.

Creates a new StringDecoder instance.

stringDecoder.end([buffer])#

Added in: v0.9.3

Returns any remaining input stored in the internal buffer as a string. Bytes representing incomplete UTF-8 and UTF-16 characters will be replaced with substitution characters appropriate for the character encoding.

If the buffer argument is provided, one final call to stringDecoder.write() is performed before returning the remaining input.

stringDecoder.write(buffer)#

History
VersionChanges
v8.0.0

Each invalid character is now replaced by a single replacement character instead of one for each individual byte.

v0.1.99

Added in: v0.1.99

Returns a decoded string, ensuring that any incomplete multibyte characters at the end of the Buffer, or TypedArray, or DataView are omitted from the returned string and stored in an internal buffer for the next call to stringDecoder.write() or stringDecoder.end().

Timers#

Stability: 2 - Stable

The timer module exposes a global API for scheduling functions to be called at some future period of time. Because the timer functions are globals, there is no need to call require('timers') to use the API.

The timer functions within Node.js implement a similar API as the timers API provided by Web Browsers but use a different internal implementation that is built around the Node.js Event Loop.

Class: Immediate#

This object is created internally and is returned from setImmediate(). It can be passed to clearImmediate() in order to cancel the scheduled actions.

By default, when an immediate is scheduled, the Node.js event loop will continue running as long as the immediate is active. The Immediate object returned by setImmediate() exports both immediate.ref() and immediate.unref() functions that can be used to control this default behavior.

immediate.hasRef()#

Added in: v11.0.0

If true, the Immediate object will keep the Node.js event loop active.

immediate.ref()#

Added in: v9.7.0

When called, requests that the Node.js event loop not exit so long as the Immediate is active. Calling immediate.ref() multiple times will have no effect.

By default, all Immediate objects are "ref'ed", making it normally unnecessary to call immediate.ref() unless immediate.unref() had been called previously.

immediate.unref()#

Added in: v9.7.0

When called, the active Immediate object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the Immediate object's callback is invoked. Calling immediate.unref() multiple times will have no effect.

Class: Timeout#

This object is created internally and is returned from setTimeout() and setInterval(). It can be passed to either clearTimeout() or clearInterval() in order to cancel the scheduled actions.

By default, when a timer is scheduled using either setTimeout() or setInterval(), the Node.js event loop will continue running as long as the timer is active. Each of the Timeout objects returned by these functions export both timeout.ref() and timeout.unref() functions that can be used to control this default behavior.

timeout.hasRef()#

Added in: v11.0.0

If true, the Timeout object will keep the Node.js event loop active.

timeout.ref()#

Added in: v0.9.1

When called, requests that the Node.js event loop not exit so long as the Timeout is active. Calling timeout.ref() multiple times will have no effect.

By default, all Timeout objects are "ref'ed", making it normally unnecessary to call timeout.ref() unless timeout.unref() had been called previously.

timeout.refresh()#

Added in: v10.2.0

Sets the timer's start time to the current time, and reschedules the timer to call its callback at the previously specified duration adjusted to the current time. This is useful for refreshing a timer without allocating a new JavaScript object.

Using this on a timer that has already called its callback will reactivate the timer.

timeout.unref()#

Added in: v0.9.1

When called, the active Timeout object will not require the Node.js event loop to remain active. If there is no other activity keeping the event loop running, the process may exit before the Timeout object's callback is invoked. Calling timeout.unref() multiple times will have no effect.

Calling timeout.unref() creates an internal timer that will wake the Node.js event loop. Creating too many of these can adversely impact performance of the Node.js application.

Scheduling Timers#

A timer in Node.js is an internal construct that calls a given function after a certain period of time. When a timer's function is called varies depending on which method was used to create the timer and what other work the Node.js event loop is doing.

setImmediate(callback[, ...args])#

Added in: v0.9.1

Schedules the "immediate" execution of the callback after I/O events' callbacks.

When multiple calls to setImmediate() are made, the callback functions are queued for execution in the order in which they are created. The entire callback queue is processed every event loop iteration. If an immediate timer is queued from inside an executing callback, that timer will not be triggered until the next event loop iteration.

If callback is not a function, a TypeError will be thrown.

This method has a custom variant for promises that is available using util.promisify():

const util = require('util');
const setImmediatePromise = util.promisify(setImmediate);

setImmediatePromise('foobar').then((value) => {
  // value === 'foobar' (passing values is optional)
  // This is executed after all I/O callbacks.
});

// Or with async function
async function timerExample() {
  console.log('Before I/O callbacks');
  await setImmediatePromise();
  console.log('After I/O callbacks');
}
timerExample();

setInterval(callback, delay[, ...args])#

Added in: v0.0.1
  • callback <Function> The function to call when the timer elapses.
  • delay <number> The number of milliseconds to wait before calling the callback.
  • ...args <any> Optional arguments to pass when the callback is called.
  • Returns: <Timeout> for use with clearInterval()

Schedules repeated execution of callback every delay milliseconds.

When delay is larger than 2147483647 or less than 1, the delay will be set to 1. Non-integer delays are truncated to an integer.

If callback is not a function, a TypeError will be thrown.

setTimeout(callback, delay[, ...args])#

Added in: v0.0.1
  • callback <Function> The function to call when the timer elapses.
  • delay <number> The number of milliseconds to wait before calling the callback.
  • ...args <any> Optional arguments to pass when the callback is called.
  • Returns: <Timeout> for use with clearTimeout()

Schedules execution of a one-time callback after delay milliseconds.

The callback will likely not be invoked in precisely delay milliseconds. Node.js makes no guarantees about the exact timing of when callbacks will fire, nor of their ordering. The callback will be called as close as possible to the time specified.

When delay is larger than 2147483647 or less than 1, the delay will be set to 1. Non-integer delays are truncated to an integer.

If callback is not a function, a TypeError will be thrown.

This method has a custom variant for promises that is available using util.promisify():

const util = require('util');
const setTimeoutPromise = util.promisify(setTimeout);

setTimeoutPromise(40, 'foobar').then((value) => {
  // value === 'foobar' (passing values is optional)
  // This is executed after about 40 milliseconds.
});

Cancelling Timers#

The setImmediate(), setInterval(), and setTimeout() methods each return objects that represent the scheduled timers. These can be used to cancel the timer and prevent it from triggering.

It is not possible to cancel timers that were created using the promisified variants of setImmediate(), setTimeout().

clearImmediate(immediate)#

Added in: v0.9.1

Cancels an Immediate object created by setImmediate().

clearInterval(timeout)#

Added in: v0.0.1

Cancels a Timeout object created by setInterval().

clearTimeout(timeout)#

Added in: v0.0.1

Cancels a Timeout object created by setTimeout().

TLS (SSL)#

Stability: 2 - Stable

The tls module provides an implementation of the Transport Layer Security (TLS) and Secure Socket Layer (SSL) protocols that is built on top of OpenSSL. The module can be accessed using:

const tls = require('tls');

TLS/SSL Concepts#

The TLS/SSL is a public/private key infrastructure (PKI). For most common cases, each client and server must have a private key.

Private keys can be generated in multiple ways. The example below illustrates use of the OpenSSL command-line interface to generate a 2048-bit RSA private key:

openssl genrsa -out ryans-key.pem 2048

With TLS/SSL, all servers (and some clients) must have a certificate. Certificates are public keys that correspond to a private key, and that are digitally signed either by a Certificate Authority or by the owner of the private key (such certificates are referred to as "self-signed"). The first step to obtaining a certificate is to create a Certificate Signing Request (CSR) file.

The OpenSSL command-line interface can be used to generate a CSR for a private key:

openssl req -new -sha256 -key ryans-key.pem -out ryans-csr.pem

Once the CSR file is generated, it can either be sent to a Certificate Authority for signing or used to generate a self-signed certificate.

Creating a self-signed certificate using the OpenSSL command-line interface is illustrated in the example below:

openssl x509 -req -in ryans-csr.pem -signkey ryans-key.pem -out ryans-cert.pem

Once the certificate is generated, it can be used to generate a .pfx or .p12 file:

openssl pkcs12 -export -in ryans-cert.pem -inkey ryans-key.pem \
      -certfile ca-cert.pem -out ryans.pfx

Where:

  • in: is the signed certificate
  • inkey: is the associated private key
  • certfile: is a concatenation of all Certificate Authority (CA) certs into a single file, e.g. cat ca1-cert.pem ca2-cert.pem > ca-cert.pem

Perfect Forward Secrecy#

The term "Forward Secrecy" or "Perfect Forward Secrecy" describes a feature of key-agreement (i.e., key-exchange) methods. That is, the server and client keys are used to negotiate new temporary keys that are used specifically and only for the current communication session. Practically, this means that even if the server's private key is compromised, communication can only be decrypted by eavesdroppers if the attacker manages to obtain the key-pair specifically generated for the session.

Perfect Forward Secrecy is achieved by randomly generating a key pair for key-agreement on every TLS/SSL handshake (in contrast to using the same key for all sessions). Methods implementing this technique are called "ephemeral".

Currently two methods are commonly used to achieve Perfect Forward Secrecy (note the character "E" appended to the traditional abbreviations):

  • DHE: An ephemeral version of the Diffie Hellman key-agreement protocol.
  • ECDHE: An ephemeral version of the Elliptic Curve Diffie Hellman key-agreement protocol.

Ephemeral methods may have some performance drawbacks, because key generation is expensive.

To use Perfect Forward Secrecy using DHE with the tls module, it is required to generate Diffie-Hellman parameters and specify them with the dhparam option to tls.createSecureContext(). The following illustrates the use of the OpenSSL command-line interface to generate such parameters:

openssl dhparam -outform PEM -out dhparam.pem 2048

If using Perfect Forward Secrecy using ECDHE, Diffie-Hellman parameters are not required and a default ECDHE curve will be used. The ecdhCurve property can be used when creating a TLS Server to specify the list of names of supported curves to use, see tls.createServer() for more info.

Perfect Forward Secrecy was optional up to TLSv1.2, but it is not optional for TLSv1.3, because all TLSv1.3 cipher suites use ECDHE.

ALPN and SNI#

ALPN (Application-Layer Protocol Negotiation Extension) and SNI (Server Name Indication) are TLS handshake extensions:

  • ALPN: Allows the use of one TLS server for multiple protocols (HTTP, HTTP/2)
  • SNI: Allows the use of one TLS server for multiple hostnames with different SSL certificates.

Pre-shared keys#

TLS-PSK support is available as an alternative to normal certificate-based authentication. It uses a pre-shared key instead of certificates to authenticate a TLS connection, providing mutual authentication. TLS-PSK and public key infrastructure are not mutually exclusive. Clients and servers can accommodate both, choosing either of them during the normal cipher negotiation step.

TLS-PSK is only a good choice where means exist to securely share a key with every connecting machine, so it does not replace PKI (Public Key Infrastructure) for the majority of TLS uses. The TLS-PSK implementation in OpenSSL has seen many security flaws in recent years, mostly because it is used only by a minority of applications. Please consider all alternative solutions before switching to PSK ciphers. Upon generating PSK it is of critical importance to use sufficient entropy as discussed in RFC 4086. Deriving a shared secret from a password or other low-entropy sources is not secure.

PSK ciphers are disabled by default, and using TLS-PSK thus requires explicitly specifying a cipher suite with the ciphers option. The list of available ciphers can be retrieved via openssl ciphers -v 'PSK'. All TLS 1.3 ciphers are eligible for PSK but currently only those that use SHA256 digest are supported they can be retrieved via openssl ciphers -v -s -tls1_3 -psk.

According to the RFC 4279, PSK identities up to 128 bytes in length and PSKs up to 64 bytes in length must be supported. As of OpenSSL 1.1.0 maximum identity size is 128 bytes, and maximum PSK length is 256 bytes.

The current implementation doesn't support asynchronous PSK callbacks due to the limitations of the underlying OpenSSL API.

Client-initiated renegotiation attack mitigation#

The TLS protocol allows clients to renegotiate certain aspects of the TLS session. Unfortunately, session renegotiation requires a disproportionate amount of server-side resources, making it a potential vector for denial-of-service attacks.

To mitigate the risk, renegotiation is limited to three times every ten minutes. An 'error' event is emitted on the tls.TLSSocket instance when this threshold is exceeded. The limits are configurable:

  • tls.CLIENT_RENEG_LIMIT <number> Specifies the number of renegotiation requests. Default: 3.
  • tls.CLIENT_RENEG_WINDOW <number> Specifies the time renegotiation window in seconds. Default: 600 (10 minutes).

The default renegotiation limits should not be modified without a full understanding of the implications and risks.

TLSv1.3 does not support renegotiation.

Session Resumption#

Establishing a TLS session can be relatively slow. The process can be sped up by saving and later reusing the session state. There are several mechanisms to do so, discussed here from oldest to newest (and preferred).

Session Identifiers Servers generate a unique ID for new connections and send it to the client. Clients and servers save the session state. When reconnecting, clients send the ID of their saved session state and if the server also has the state for that ID, it can agree to use it. Otherwise, the server will create a new session. See RFC 2246 for more information, page 23 and 30.

Resumption using session identifiers is supported by most web browsers when making HTTPS requests.

For Node.js, clients wait for the 'session' event to get the session data, and provide the data to the session option of a subsequent tls.connect() to reuse the session. Servers must implement handlers for the 'newSession' and 'resumeSession' events to save and restore the session data using the session ID as the lookup key to reuse sessions. To reuse sessions across load balancers or cluster workers, servers must use a shared session cache (such as Redis) in their session handlers.

Session Tickets The servers encrypt the entire session state and send it to the client as a "ticket". When reconnecting, the state is sent to the server in the initial connection. This mechanism avoids the need for server-side session cache. If the server doesn't use the ticket, for any reason (failure to decrypt it, it's too old, etc.), it will create a new session and send a new ticket. See RFC 5077 for more information.

Resumption using session tickets is becoming commonly supported by many web browsers when making HTTPS requests.

For Node.js, clients use the same APIs for resumption with session identifiers as for resumption with session tickets. For debugging, if tls.TLSSocket.getTLSTicket() returns a value, the session data contains a ticket, otherwise it contains client-side session state.

With TLSv1.3, be aware that multiple tickets may be sent by the server, resulting in multiple 'session' events, see 'session' for more information.

Single process servers need no specific implementation to use session tickets. To use session tickets across server restarts or load balancers, servers must all have the same ticket keys. There are three 16-byte keys internally, but the tls API exposes them as a single 48-byte buffer for convenience.

Its possible to get the ticket keys by calling server.getTicketKeys() on one server instance and then distribute them, but it is more reasonable to securely generate 48 bytes of secure random data and set them with the ticketKeys option of tls.createServer(). The keys should be regularly regenerated and server's keys can be reset with server.setTicketKeys().

Session ticket keys are cryptographic keys, and they must be stored securely. With TLS 1.2 and below, if they are compromised all sessions that used tickets encrypted with them can be decrypted. They should not be stored on disk, and they should be regenerated regularly.

If clients advertise support for tickets, the server will send them. The server can disable tickets by supplying require('constants').SSL_OP_NO_TICKET in secureOptions.

Both session identifiers and session tickets timeout, causing the server to create new sessions. The timeout can be configured with the sessionTimeout option of tls.createServer().

For all the mechanisms, when resumption fails, servers will create new sessions. Since failing to resume the session does not cause TLS/HTTPS connection failures, it is easy to not notice unnecessarily poor TLS performance. The OpenSSL CLI can be used to verify that servers are resuming sessions. Use the -reconnect option to openssl s_client, for example:

$ openssl s_client -connect localhost:443 -reconnect

Read through the debug output. The first connection should say "New", for example:

New, TLSv1.2, Cipher is ECDHE-RSA-AES128-GCM-SHA256

Subsequent connections should say "Reused", for example:

Reused, TLSv1.2, Cipher is ECDHE-RSA-AES128-GCM-SHA256

Modifying the Default TLS Cipher suite#

Node.js is built with a default suite of enabled and disabled TLS ciphers. Currently, the default cipher suite is:

TLS_AES_256_GCM_SHA384:
TLS_CHACHA20_POLY1305_SHA256:
TLS_AES_128_GCM_SHA256:
ECDHE-RSA-AES128-GCM-SHA256:
ECDHE-ECDSA-AES128-GCM-SHA256:
ECDHE-RSA-AES256-GCM-SHA384:
ECDHE-ECDSA-AES256-GCM-SHA384:
DHE-RSA-AES128-GCM-SHA256:
ECDHE-RSA-AES128-SHA256:
DHE-RSA-AES128-SHA256:
ECDHE-RSA-AES256-SHA384:
DHE-RSA-AES256-SHA384:
ECDHE-RSA-AES256-SHA256:
DHE-RSA-AES256-SHA256:
HIGH:
!aNULL:
!eNULL:
!EXPORT:
!DES:
!RC4:
!MD5:
!PSK:
!SRP:
!CAMELLIA

This default can be replaced entirely using the --tls-cipher-list command line switch (directly, or via the NODE_OPTIONS environment variable). For instance, the following makes ECDHE-RSA-AES128-GCM-SHA256:!RC4 the default TLS cipher suite:

node --tls-cipher-list="ECDHE-RSA-AES128-GCM-SHA256:!RC4" server.js

export NODE_OPTIONS=--tls-cipher-list="ECDHE-RSA-AES128-GCM-SHA256:!RC4"
node server.js

The default can also be replaced on a per client or server basis using the ciphers option from tls.createSecureContext(), which is also available in tls.createServer(), tls.connect(), and when creating new tls.TLSSockets.

The ciphers list can contain a mixture of TLSv1.3 cipher suite names, the ones that start with 'TLS_', and specifications for TLSv1.2 and below cipher suites. The TLSv1.2 ciphers support a legacy specification format, consult the OpenSSL cipher list format documentation for details, but those specifications do not apply to TLSv1.3 ciphers. The TLSv1.3 suites can only be enabled by including their full name in the cipher list. They cannot, for example, be enabled or disabled by using the legacy TLSv1.2 'EECDH' or '!EECDH' specification.

Despite the relative order of TLSv1.3 and TLSv1.2 cipher suites, the TLSv1.3 protocol is significantly more secure than TLSv1.2, and will always be chosen over TLSv1.2 if the handshake indicates it is supported, and if any TLSv1.3 cipher suites are enabled.

The default cipher suite included within Node.js has been carefully selected to reflect current security best practices and risk mitigation. Changing the default cipher suite can have a significant impact on the security of an application. The --tls-cipher-list switch and ciphers option should by used only if absolutely necessary.

The default cipher suite prefers GCM ciphers for Chrome's 'modern cryptography' setting and also prefers ECDHE and DHE ciphers for Perfect Forward Secrecy, while offering some backward compatibility.

128 bit AES is preferred over 192 and 256 bit AES in light of specific attacks affecting larger AES key sizes.

Old clients that rely on insecure and deprecated RC4 or DES-based ciphers (like Internet Explorer 6) cannot complete the handshaking process with the default configuration. If these clients must be supported, the TLS recommendations may offer a compatible cipher suite. For more details on the format, see the OpenSSL cipher list format documentation.

There are only 5 TLSv1.3 cipher suites:

  • 'TLS_AES_256_GCM_SHA384'
  • 'TLS_CHACHA20_POLY1305_SHA256'
  • 'TLS_AES_128_GCM_SHA256'
  • 'TLS_AES_128_CCM_SHA256'
  • 'TLS_AES_128_CCM_8_SHA256'

The first 3 are enabled by default. The last 2 CCM-based suites are supported by TLSv1.3 because they may be more performant on constrained systems, but they are not enabled by default since they offer less security.

Class: tls.Server#

Added in: v0.3.2

Accepts encrypted connections using TLS or SSL.

Event: 'keylog'#

Added in: v12.3.0, v10.20.0
  • line <Buffer> Line of ASCII text, in NSS SSLKEYLOGFILE format.
  • tlsSocket <tls.TLSSocket> The tls.TLSSocket instance on which it was generated.

The keylog event is emitted when key material is generated or received by a connection to this server (typically before handshake has completed, but not necessarily). This keying material can be stored for debugging, as it allows captured TLS traffic to be decrypted. It may be emitted multiple times for each socket.

A typical use case is to append received lines to a common text file, which is later used by software (such as Wireshark) to decrypt the traffic:

const logFile = fs.createWriteStream('/tmp/ssl-keys.log', { flags: 'a' });
// ...
server.on('keylog', (line, tlsSocket) => {
  if (tlsSocket.remoteAddress !== '...')
    return; // Only log keys for a particular IP
  logFile.write(line);
});

Event: 'newSession'#

Added in: v0.9.2

The 'newSession' event is emitted upon creation of a new TLS session. This may be used to store sessions in external storage. The data should be provided to the 'resumeSession' callback.

The listener callback is passed three arguments when called:

  • sessionId <Buffer> The TLS session identifier
  • sessionData <Buffer> The TLS session data
  • callback <Function> A callback function taking no arguments that must be invoked in order for data to be sent or received over the secure connection.

Listening for this event will have an effect only on connections established after the addition of the event listener.

Event: 'OCSPRequest'#

Added in: v0.11.13

The 'OCSPRequest' event is emitted when the client sends a certificate status request. The listener callback is passed three arguments when called:

  • certificate <Buffer> The server certificate
  • issuer <Buffer> The issuer's certificate
  • callback <Function> A callback function that must be invoked to provide the results of the OCSP request.

The server's current certificate can be parsed to obtain the OCSP URL and certificate ID; after obtaining an OCSP response, callback(null, resp) is then invoked, where resp is a Buffer instance containing the OCSP response. Both certificate and issuer are Buffer DER-representations of the primary and issuer's certificates. These can be used to obtain the OCSP certificate ID and OCSP endpoint URL.

Alternatively, callback(null, null) may be called, indicating that there was no OCSP response.

Calling callback(err) will result in a socket.destroy(err) call.

The typical flow of an OCSP Request is as follows:

  1. Client connects to the server and sends an 'OCSPRequest' (via the status info extension in ClientHello).
  2. Server receives the request and emits the 'OCSPRequest' event, calling the listener if registered.
  3. Server extracts the OCSP URL from either the certificate or issuer and performs an OCSP request to the CA.
  4. Server receives 'OCSPResponse' from the CA and sends it back to the client via the callback argument
  5. Client validates the response and either destroys the socket or performs a handshake.

The issuer can be null if the certificate is either self-signed or the issuer is not in the root certificates list. (An issuer may be provided via the ca option when establishing the TLS connection.)

Listening for this event will have an effect only on connections established after the addition of the event listener.

An npm module like asn1.js may be used to parse the certificates.

Event: 'resumeSession'#

Added in: v0.9.2

The 'resumeSession' event is emitted when the client requests to resume a previous TLS session. The listener callback is passed two arguments when called:

  • sessionId <Buffer> The TLS session identifier

  • callback <Function> A callback function to be called when the prior session has been recovered: callback([err[, sessionData]])

The event listener should perform a lookup in external storage for the sessionData saved by the 'newSession' event handler using the given sessionId. If found, call callback(null, sessionData) to resume the session. If not found, the session cannot be resumed. callback() must be called without sessionData so that the handshake can continue and a new session can be created. It is possible to call callback(err) to terminate the incoming connection and destroy the socket.

Listening for this event will have an effect only on connections established after the addition of the event listener.

The following illustrates resuming a TLS session:

const tlsSessionStore = {};
server.on('newSession', (id, data, cb) => {
  tlsSessionStore[id.toString('hex')] = data;
  cb();
});
server.on('resumeSession', (id, cb) => {
  cb(null, tlsSessionStore[id.toString('hex')] || null);
});

Event: 'secureConnection'#

Added in: v0.3.2

The 'secureConnection' event is emitted after the handshaking process for a new connection has successfully completed. The listener callback is passed a single argument when called:

The tlsSocket.authorized property is a boolean indicating whether the client has been verified by one of the supplied Certificate Authorities for the server. If tlsSocket.authorized is false, then socket.authorizationError is set to describe how authorization failed. Depending on the settings of the TLS server, unauthorized connections may still be accepted.

The tlsSocket.alpnProtocol property is a string that contains the selected ALPN protocol. When ALPN has no selected protocol, tlsSocket.alpnProtocol equals false.

The tlsSocket.servername property is a string containing the server name requested via SNI.

Event: 'tlsClientError'#

Added in: v6.0.0

The 'tlsClientError' event is emitted when an error occurs before a secure connection is established. The listener callback is passed two arguments when called:

  • exception <Error> The Error object describing the error
  • tlsSocket <tls.TLSSocket> The tls.TLSSocket instance from which the error originated.

server.addContext(hostname, context)#

Added in: v0.5.3
  • hostname <string> A SNI host name or wildcard (e.g. '*')
  • context <Object> An object containing any of the possible properties from the tls.createSecureContext() options arguments (e.g. key, cert, ca, etc).

The server.addContext() method adds a secure context that will be used if the client request's SNI name matches the supplied hostname (or wildcard).

server.address()#

Added in: v0.6.0

Returns the bound address, the address family name, and port of the server as reported by the operating system. See net.Server.address() for more information.

server.close([callback])#

Added in: v0.3.2
  • callback <Function> A listener callback that will be registered to listen for the server instance's 'close' event.
  • Returns: <tls.Server>

The server.close() method stops the server from accepting new connections.

This function operates asynchronously. The 'close' event will be emitted when the server has no more open connections.

server.connections#

Added in: v0.3.2Deprecated since: v0.9.7

Stability: 0 - Deprecated: Use server.getConnections() instead.

Returns the current number of concurrent connections on the server.

server.getTicketKeys()#

Added in: v3.0.0
  • Returns: <Buffer> A 48-byte buffer containing the session ticket keys.

Returns the session ticket keys.

See Session Resumption for more information.

server.listen()#

Starts the server listening for encrypted connections. This method is identical to server.listen() from net.Server.

server.setSecureContext(options)#

Added in: v11.0.0

The server.setSecureContext() method replaces the secure context of an existing server. Existing connections to the server are not interrupted.

server.setTicketKeys(keys)#

Added in: v3.0.0
  • keys <Buffer> A 48-byte buffer containing the session ticket keys.

Sets the session ticket keys.

Changes to the ticket keys are effective only for future server connections. Existing or currently pending server connections will use the previous keys.

See Session Resumption for more information.

Class: tls.TLSSocket#

Added in: v0.11.4

Performs transparent encryption of written data and all required TLS negotiation.

Instances of tls.TLSSocket implement the duplex Stream interface.

Methods that return TLS connection metadata (e.g. tls.TLSSocket.getPeerCertificate() will only return data while the connection is open.

new tls.TLSSocket(socket[, options])#

History
VersionChanges
v12.2.0

The enableTrace option is now supported.

v5.0.0

ALPN options are supported now.

v0.11.4

Added in: v0.11.4

  • socket <net.Socket> | <stream.Duplex> On the server side, any Duplex stream. On the client side, any instance of net.Socket (for generic Duplex stream support on the client side, tls.connect() must be used).

  • options <Object>

    • enableTrace: See tls.createServer()
    • isServer: The SSL/TLS protocol is asymmetrical, TLSSockets must know if they are to behave as a server or a client. If true the TLS socket will be instantiated as a server. Default: false.
    • server <net.Server> A net.Server instance.
    • requestCert: Whether to authenticate the remote peer by requesting a certificate. Clients always request a server certificate. Servers (isServer is true) may set requestCert to true to request a client certificate.
    • rejectUnauthorized: See tls.createServer()
    • ALPNProtocols: See tls.createServer()
    • SNICallback: See tls.createServer()
    • session <Buffer> A Buffer instance containing a TLS session.
    • requestOCSP <boolean> If true, specifies that the OCSP status request extension will be added to the client hello and an 'OCSPResponse' event will be emitted on the socket before establishing a secure communication
    • secureContext: TLS context object created with tls.createSecureContext(). If a secureContext is not provided, one will be created by passing the entire options object to tls.createSecureContext().
    • ...: tls.createSecureContext() options that are used if the secureContext option is missing. Otherwise, they are ignored.

Construct a new tls.TLSSocket object from an existing TCP socket.

Event: 'keylog'#

Added in: v12.3.0, v10.20.0
  • line <Buffer> Line of ASCII text, in NSS SSLKEYLOGFILE format.

The keylog event is emitted on a tls.TLSSocket when key material is generated or received by the socket. This keying material can be stored for debugging, as it allows captured TLS traffic to be decrypted. It may be emitted multiple times, before or after the handshake completes.

A typical use case is to append received lines to a common text file, which is later used by software (such as Wireshark) to decrypt the traffic:

const logFile = fs.createWriteStream('/tmp/ssl-keys.log', { flags: 'a' });
// ...
tlsSocket.on('keylog', (line) => logFile.write(line));

Event: 'OCSPResponse'#

Added in: v0.11.13

The 'OCSPResponse' event is emitted if the requestOCSP option was set when the tls.TLSSocket was created and an OCSP response has been received. The listener callback is passed a single argument when called:

  • response <Buffer> The server's OCSP response

Typically, the response is a digitally signed object from the server's CA that contains information about server's certificate revocation status.

Event: 'secureConnect'#

Added in: v0.11.4

The 'secureConnect' event is emitted after the handshaking process for a new connection has successfully completed. The listener callback will be called regardless of whether or not the server's certificate has been authorized. It is the client's responsibility to check the tlsSocket.authorized property to determine if the server certificate was signed by one of the specified CAs. If tlsSocket.authorized === false, then the error can be found by examining the tlsSocket.authorizationError property. If ALPN was used, the tlsSocket.alpnProtocol property can be checked to determine the negotiated protocol.

Event: 'session'#

Added in: v11.10.0

The 'session' event is emitted on a client tls.TLSSocket when a new session or TLS ticket is available. This may or may not be before the handshake is complete, depending on the TLS protocol version that was negotiated. The event is not emitted on the server, or if a new session was not created, for example, when the connection was resumed. For some TLS protocol versions the event may be emitted multiple times, in which case all the sessions can be used for resumption.

On the client, the session can be provided to the session option of tls.connect() to resume the connection.

See Session Resumption for more information.

For TLSv1.2 and below, tls.TLSSocket.getSession() can be called once the handshake is complete. For TLSv1.3, only ticket-based resumption is allowed by the protocol, multiple tickets are sent, and the tickets aren't sent until after the handshake completes. So it is necessary to wait for the 'session' event to get a resumable session. Applications should use the 'session' event instead of getSession() to ensure they will work for all TLS versions. Applications that only expect to get or use one session should listen for this event only once:

tlsSocket.once('session', (session) => {
  // The session can be used immediately or later.
  tls.connect({
    session: session,
    // Other connect options...
  });
});

tlsSocket.address()#

Added in: v0.11.4

Returns the bound address, the address family name, and port of the underlying socket as reported by the operating system: { port: 12346, family: 'IPv4', address: '127.0.0.1' }.

tlsSocket.authorizationError#

Added in: v0.11.4

Returns the reason why the peer's certificate was not been verified. This property is set only when tlsSocket.authorized === false.

tlsSocket.authorized#

Added in: v0.11.4

Returns true if the peer certificate was signed by one of the CAs specified when creating the tls.TLSSocket instance, otherwise false.

tlsSocket.disableRenegotiation()#

Added in: v8.4.0

Disables TLS renegotiation for this TLSSocket instance. Once called, attempts to renegotiate will trigger an 'error' event on the TLSSocket.

tlsSocket.enableTrace()#

Added in: v12.2.0

When enabled, TLS packet trace information is written to stderr. This can be used to debug TLS connection problems.

Note: The format of the output is identical to the output of openssl s_client -trace or openssl s_server -trace. While it is produced by OpenSSL's SSL_trace() function, the format is undocumented, can change without notice, and should not be relied on.

tlsSocket.encrypted#

Added in: v0.11.4

Always returns true. This may be used to distinguish TLS sockets from regular net.Socket instances.

tlsSocket.getCertificate()#

Added in: v11.2.0

Returns an object representing the local certificate. The returned object has some properties corresponding to the fields of the certificate.

See tls.TLSSocket.getPeerCertificate() for an example of the certificate structure.

If there is no local certificate, an empty object will be returned. If the socket has been destroyed, null will be returned.

tlsSocket.getCipher()#

History
VersionChanges
v13.4.0, v12.16.0

Return the IETF cipher name as standardName.

v12.0.0

Return the minimum cipher version, instead of a fixed string ('TLSv1/SSLv3').

v0.11.4

Added in: v0.11.4

  • Returns: <Object>

    • name <string> OpenSSL name for the cipher suite.
    • standardName <string> IETF name for the cipher suite.
    • version <string> The minimum TLS protocol version supported by this cipher suite.

Returns an object containing information on the negotiated cipher suite.

For example:

{
    "name": "AES128-SHA256",
    "standardName": "TLS_RSA_WITH_AES_128_CBC_SHA256",
    "version": "TLSv1.2"
}

See SSL_CIPHER_get_name for more information.

tlsSocket.getEphemeralKeyInfo()#

Added in: v5.0.0

Returns an object representing the type, name, and size of parameter of an ephemeral key exchange in Perfect Forward Secrecy on a client connection. It returns an empty object when the key exchange is not ephemeral. As this is only supported on a client socket; null is returned if called on a server socket. The supported types are 'DH' and 'ECDH'. The name property is available only when type is 'ECDH'.

For example: { type: 'ECDH', name: 'prime256v1', size: 256 }.

tlsSocket.getFinished()#

Added in: v9.9.0
  • Returns: <Buffer> | <undefined> The latest Finished message that has been sent to the socket as part of a SSL/TLS handshake, or undefined if no Finished message has been sent yet.

As the Finished messages are message digests of the complete handshake (with a total of 192 bits for TLS 1.0 and more for SSL 3.0), they can be used for external authentication procedures when the authentication provided by SSL/TLS is not desired or is not enough.

Corresponds to the SSL_get_finished routine in OpenSSL and may be used to implement the tls-unique channel binding from RFC 5929.

tlsSocket.getPeerCertificate([detailed])#

Added in: v0.11.4
  • detailed <boolean> Include the full certificate chain if true, otherwise include just the peer's certificate.
  • Returns: <Object> A certificate object.

Returns an object representing the peer's certificate. If the peer does not provide a certificate, an empty object will be returned. If the socket has been destroyed, null will be returned.

If the full certificate chain was requested, each certificate will include an issuerCertificate property containing an object representing its issuer's certificate.

Certificate Object#

History
VersionChanges
v11.4.0

Support Elliptic Curve public key info.

A certificate object has properties corresponding to the fields of the certificate.

  • raw <Buffer> The DER encoded X.509 certificate data.
  • subject <Object> The certificate subject, described in terms of Country (C:), StateOrProvince (ST), Locality (L), Organization (O), OrganizationalUnit (OU), and CommonName (CN). The CommonName is typically a DNS name with TLS certificates. Example: {C: 'UK', ST: 'BC', L: 'Metro', O: 'Node Fans', OU: 'Docs', CN: 'example.com'}.
  • issuer <Object> The certificate issuer, described in the same terms as the subject.
  • valid_from <string> The date-time the certificate is valid from.
  • valid_to <string> The date-time the certificate is valid to.
  • serialNumber <string> The certificate serial number, as a hex string. Example: 'B9B0D332A1AA5635'.
  • fingerprint <string> The SHA-1 digest of the DER encoded certificate. It is returned as a : separated hexadecimal string. Example: '2A:7A:C2:DD:...'.
  • fingerprint256 <string> The SHA-256 digest of the DER encoded certificate. It is returned as a : separated hexadecimal string. Example: '2A:7A:C2:DD:...'.
  • ext_key_usage <Array> (Optional) The extended key usage, a set of OIDs.
  • subjectaltname <string> (Optional) A string containing concatenated names for the subject, an alternative to the subject names.
  • infoAccess <Array> (Optional) An array describing the AuthorityInfoAccess, used with OCSP.
  • issuerCertificate <Object> (Optional) The issuer certificate object. For self-signed certificates, this may be a circular reference.

The certificate may contain information about the public key, depending on the key type.

For RSA keys, the following properties may be defined:

  • bits <number> The RSA bit size. Example: 1024.
  • exponent <string> The RSA exponent, as a string in hexadecimal number notation. Example: '0x010001'.
  • modulus <string> The RSA modulus, as a hexadecimal string. Example: 'B56CE45CB7...'.
  • pubkey <Buffer> The public key.

For EC keys, the following properties may be defined:

  • pubkey <Buffer> The public key.
  • bits <number> The key size in bits. Example: 256.
  • asn1Curve <string> (Optional) The ASN.1 name of the OID of the elliptic curve. Well-known curves are identified by an OID. While it is unusual, it is possible that the curve is identified by its mathematical properties, in which case it will not have an OID. Example: 'prime256v1'.
  • nistCurve <string> (Optional) The NIST name for the elliptic curve, if it has one (not all well-known curves have been assigned names by NIST). Example: 'P-256'.

Example certificate:

{ subject:
   { OU: [ 'Domain Control Validated', 'PositiveSSL Wildcard' ],
     CN: '*.nodejs.org' },
  issuer:
   { C: 'GB',
     ST: 'Greater Manchester',
     L: 'Salford',
     O: 'COMODO CA Limited',
     CN: 'COMODO RSA Domain Validation Secure Server CA' },
  subjectaltname: 'DNS:*.nodejs.org, DNS:nodejs.org',
  infoAccess:
   { 'CA Issuers - URI':
      [ 'http://crt.comodoca.com/COMODORSADomainValidationSecureServerCA.crt' ],
     'OCSP - URI': [ 'http://ocsp.comodoca.com' ] },
  modulus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
  exponent: '0x10001',
  pubkey: <Buffer ... >,
  valid_from: 'Aug 14 00:00:00 2017 GMT',
  valid_to: 'Nov 20 23:59:59 2019 GMT',
  fingerprint: '01:02:59:D9:C3:D2:0D:08:F7:82:4E:44:A4:B4:53:C5:E2:3A:87:4D',
  fingerprint256: '69:AE:1A:6A:D4:3D:C6:C1:1B:EA:C6:23:DE:BA:2A:14:62:62:93:5C:7A:EA:06:41:9B:0B:BC:87:CE:48:4E:02',
  ext_key_usage: [ '1.3.6.1.5.5.7.3.1', '1.3.6.1.5.5.7.3.2' ],
  serialNumber: '66593D57F20CBC573E433381B5FEC280',
  raw: <Buffer ... > }

tlsSocket.getPeerFinished()#

Added in: v9.9.0
  • Returns: <Buffer> | <undefined> The latest Finished message that is expected or has actually been received from the socket as part of a SSL/TLS handshake, or undefined if there is no Finished message so far.

As the Finished messages are message digests of the complete handshake (with a total of 192 bits for TLS 1.0 and more for SSL 3.0), they can be used for external authentication procedures when the authentication provided by SSL/TLS is not desired or is not enough.

Corresponds to the SSL_get_peer_finished routine in OpenSSL and may be used to implement the tls-unique channel binding from RFC 5929.

tlsSocket.getProtocol()#

Added in: v5.7.0

Returns a string containing the negotiated SSL/TLS protocol version of the current connection. The value 'unknown' will be returned for connected sockets that have not completed the handshaking process. The value null will be returned for server sockets or disconnected client sockets.

Protocol versions are:

  • 'SSLv3'
  • 'TLSv1'
  • 'TLSv1.1'
  • 'TLSv1.2'
  • 'TLSv1.3'

See the OpenSSL SSL_get_version documentation for more information.

tlsSocket.getSession()#

Added in: v0.11.4

Returns the TLS session data or undefined if no session was negotiated. On the client, the data can be provided to the session option of tls.connect() to resume the connection. On the server, it may be useful for debugging.

See Session Resumption for more information.

Note: getSession() works only for TLSv1.2 and below. For TLSv1.3, applications must use the 'session' event (it also works for TLSv1.2 and below).

tlsSocket.getSharedSigalgs()#

Added in: v12.11.0
  • Returns: <Array> List of signature algorithms shared between the server and the client in the order of decreasing preference.

See SSL_get_shared_sigalgs for more information.

tlsSocket.exportKeyingMaterial(length, label[, context])#

Added in: v13.10.0

Keying material is used for validations to prevent different kind of attacks in network protocols, for example in the specifications of IEEE 802.1X.

Example

const keyingMaterial = tlsSocket.exportKeyingMaterial(
  128,
  'client finished');

/**
 Example return value of keyingMaterial:
 <Buffer 76 26 af 99 c5 56 8e 42 09 91 ef 9f 93 cb ad 6c 7b 65 f8 53 f1 d8 d9
    12 5a 33 b8 b5 25 df 7b 37 9f e0 e2 4f b8 67 83 a3 2f cd 5d 41 42 4c 91
    74 ef 2c ... 78 more bytes>
*/

See the OpenSSL SSL_export_keying_material documentation for more information.

tlsSocket.getTLSTicket()#

Added in: v0.11.4

For a client, returns the TLS session ticket if one is available, or undefined. For a server, always returns undefined.

It may be useful for debugging.

See Session Resumption for more information.

tlsSocket.isSessionReused()#

Added in: v0.5.6
  • Returns: <boolean> true if the session was reused, false otherwise.

See Session Resumption for more information.

tlsSocket.localAddress#

Added in: v0.11.4

Returns the string representation of the local IP address.

tlsSocket.localPort#

Added in: v0.11.4

Returns the numeric representation of the local port.

tlsSocket.remoteAddress#

Added in: v0.11.4

Returns the string representation of the remote IP address. For example, '74.125.127.100' or '2001:4860:a005::68'.

tlsSocket.remoteFamily#

Added in: v0.11.4

Returns the string representation of the remote IP family. 'IPv4' or 'IPv6'.

tlsSocket.remotePort#

Added in: v0.11.4

Returns the numeric representation of the remote port. For example, 443.

tlsSocket.renegotiate(options, callback)#

Added in: v0.11.8

  • options <Object>

    • rejectUnauthorized <boolean> If not false, the server certificate is verified against the list of supplied CAs. An 'error' event is emitted if verification fails; err.code contains the OpenSSL error code. Default: true.
    • requestCert

  • callback <Function> If renegotiate() returned true, callback is attached once to the 'secure' event. If renegotiate() returned false, callback will be called in the next tick with an error, unless the tlsSocket has been destroyed, in which case callback will not be called at all.

  • Returns: <boolean> true if renegotiation was initiated, false otherwise.

The tlsSocket.renegotiate() method initiates a TLS renegotiation process. Upon completion, the callback function will be passed a single argument that is either an Error (if the request failed) or null.

This method can be used to request a peer's certificate after the secure connection has been established.

When running as the server, the socket will be destroyed with an error after handshakeTimeout timeout.

For TLSv1.3, renegotiation cannot be initiated, it is not supported by the protocol.

tlsSocket.setMaxSendFragment(size)#

Added in: v0.11.11
  • size <number> The maximum TLS fragment size. The maximum value is 16384. Default: 16384.
  • Returns: <boolean>

The tlsSocket.setMaxSendFragment() method sets the maximum TLS fragment size. Returns true if setting the limit succeeded; false otherwise.

Smaller fragment sizes decrease the buffering latency on the client: larger fragments are buffered by the TLS layer until the entire fragment is received and its integrity is verified; large fragments can span multiple roundtrips and their processing can be delayed due to packet loss or reordering. However, smaller fragments add extra TLS framing bytes and CPU overhead, which may decrease overall server throughput.

tls.checkServerIdentity(hostname, cert)#

Added in: v0.8.4

Verifies the certificate cert is issued to hostname.

Returns <Error> object, populating it with reason, host, and cert on failure. On success, returns <undefined>.

This function can be overwritten by providing alternative function as part of the options.checkServerIdentity option passed to tls.connect(). The overwriting function can call tls.checkServerIdentity() of course, to augment the checks done with additional verification.

This function is only called if the certificate passed all other checks, such as being issued by trusted CA (options.ca).

tls.connect(options[, callback])#

History
VersionChanges
v14.1.0

The highWaterMark option is accepted now.

v13.6.0, v12.16.0

The pskCallback option is now supported.

v12.9.0

Support the allowHalfOpen option.

v12.4.0

The hints option is now supported.

v12.2.0

The enableTrace option is now supported.

v11.8.0, v10.16.0

The timeout option is supported now.

v8.0.0

The lookup option is supported now.

v8.0.0

The ALPNProtocols option can be a TypedArray or DataView now.

v5.3.0, v4.7.0

The secureContext option is supported now.

v5.0.0

ALPN options are supported now.

v0.11.3

Added in: v0.11.3

  • options <Object>

    • enableTrace: See tls.createServer()
    • host <string> Host the client should connect to. Default: 'localhost'.
    • port <number> Port the client should connect to.
    • path <string> Creates Unix socket connection to path. If this option is specified, host and port are ignored.
    • socket <stream.Duplex> Establish secure connection on a given socket rather than creating a new socket. Typically, this is an instance of net.Socket, but any Duplex stream is allowed. If this option is specified, path, host and port are ignored, except for certificate validation. Usually, a socket is already connected when passed to tls.connect(), but it can be connected later. Connection/disconnection/destruction of socket is the user's responsibility; calling tls.connect() will not cause net.connect() to be called.
    • allowHalfOpen <boolean> If the socket option is missing, indicates whether or not to allow the internally created socket to be half-open, otherwise the option is ignored. See the allowHalfOpen option of net.Socket for details. Default: false.
    • rejectUnauthorized <boolean> If not false, the server certificate is verified against the list of supplied CAs. An 'error' event is emitted if verification fails; err.code contains the OpenSSL error code. Default: true.

    • pskCallback <Function>

      • hint: <string> optional message sent from the server to help client decide which identity to use during negotiation. Always null if TLS 1.3 is used.
      • Returns: <Object> in the form { psk: <Buffer|TypedArray|DataView>, identity: <string> } or null to stop the negotiation process. psk must be compatible with the selected cipher's digest. identity must use UTF-8 encoding. When negotiating TLS-PSK (pre-shared keys), this function is called with optional identity hint provided by the server or null in case of TLS 1.3 where hint was removed. It will be necessary to provide a custom tls.checkServerIdentity() for the connection as the default one will try to check host name/IP of the server against the certificate but that's not applicable for PSK because there won't be a certificate present. More information can be found in the RFC 4279.
    • ALPNProtocols: <string[]> | <Buffer[]> | <TypedArray[]> | <DataView[]> | <Buffer> | <TypedArray> | <DataView> An array of strings, Buffers or TypedArrays or DataViews, or a single Buffer or TypedArray or DataView containing the supported ALPN protocols. Buffers should have the format [len][name][len][name]... e.g. '\x08http/1.1\x08http/1.0', where the len byte is the length of the next protocol name. Passing an array is usually much simpler, e.g. ['http/1.1', 'http/1.0']. Protocols earlier in the list have higher preference than those later.
    • servername: <string> Server name for the SNI (Server Name Indication) TLS extension. It is the name of the host being connected to, and must be a host name, and not an IP address. It can be used by a multi-homed server to choose the correct certificate to present to the client, see the SNICallback option to tls.createServer().
    • checkServerIdentity(servername, cert) <Function> A callback function to be used (instead of the builtin tls.checkServerIdentity() function) when checking the server's host name (or the provided servername when explicitly set) against the certificate. This should return an <Error> if verification fails. The method should return undefined if the servername and cert are verified.
    • session <Buffer> A Buffer instance, containing TLS session.
    • minDHSize <number> Minimum size of the DH parameter in bits to accept a TLS connection. When a server offers a DH parameter with a size less than minDHSize, the TLS connection is destroyed and an error is thrown. Default: 1024.
    • highWaterMark: <number> Consistent with the readable stream highWaterMark parameter. Default: 16 * 1024.
    • secureContext: TLS context object created with tls.createSecureContext(). If a secureContext is not provided, one will be created by passing the entire options object to tls.createSecureContext().
    • ...: tls.createSecureContext() options that are used if the secureContext option is missing, otherwise they are ignored.
    • ...: Any socket.connect() option not already listed.
  • callback <Function>
  • Returns: <tls.TLSSocket>

The callback function, if specified, will be added as a listener for the 'secureConnect' event.

tls.connect() returns a tls.TLSSocket object.

The following illustrates a client for the echo server example from tls.createServer():

// Assumes an echo server that is listening on port 8000.
const tls = require('tls');
const fs = require('fs');

const options = {
  // Necessary only if the server requires client certificate authentication.
  key: fs.readFileSync('client-key.pem'),
  cert: fs.readFileSync('client-cert.pem'),

  // Necessary only if the server uses a self-signed certificate.
  ca: [ fs.readFileSync('server-cert.pem') ],

  // Necessary only if the server's cert isn't for "localhost".
  checkServerIdentity: () => { return null; },
};

const socket = tls.connect(8000, options, () => {
  console.log('client connected',
              socket.authorized ? 'authorized' : 'unauthorized');
  process.stdin.pipe(socket);
  process.stdin.resume();
});
socket.setEncoding('utf8');
socket.on('data', (data) => {
  console.log(data);
});
socket.on('end', () => {
  console.log('server ends connection');
});

tls.connect(path[, options][, callback])#

Added in: v0.11.3

Same as tls.connect() except that path can be provided as an argument instead of an option.

A path option, if specified, will take precedence over the path argument.

tls.connect(port[, host][, options][, callback])#

Added in: v0.11.3

Same as tls.connect() except that port and host can be provided as arguments instead of options.

A port or host option, if specified, will take precedence over any port or host argument.

tls.createSecureContext([options])#

History
VersionChanges
v12.12.0

Added privateKeyIdentifier and privateKeyEngine options to get private key from an OpenSSL engine.

v12.11.0

Added sigalgs option to override supported signature algorithms.

v12.0.0

TLSv1.3 support added.

v11.5.0

The ca: option now supports BEGIN TRUSTED CERTIFICATE.

v11.4.0, v10.16.0

The minVersion and maxVersion can be used to restrict the allowed TLS protocol versions.

v10.0.0

The ecdhCurve cannot be set to false anymore due to a change in OpenSSL.

v9.3.0

The options parameter can now include clientCertEngine.

v9.0.0

The ecdhCurve option can now be multiple ':' separated curve names or 'auto'.

v7.3.0

If the key option is an array, individual entries do not need a passphrase property anymore. Array entries can also just be strings or Buffers now.

v5.2.0

The ca option can now be a single string containing multiple CA certificates.

v0.11.13

Added in: v0.11.13

  • options <Object>

    • ca <string> | <string[]> | <Buffer> | <Buffer[]> Optionally override the trusted CA certificates. Default is to trust the well-known CAs curated by Mozilla. Mozilla's CAs are completely replaced when CAs are explicitly specified using this option. The value can be a string or Buffer, or an Array of strings and/or Buffers. Any string or Buffer can contain multiple PEM CAs concatenated together. The peer's certificate must be chainable to a CA trusted by the server for the connection to be authenticated. When using certificates that are not chainable to a well-known CA, the certificate's CA must be explicitly specified as a trusted or the connection will fail to authenticate. If the peer uses a certificate that doesn't match or chain to one of the default CAs, use the ca option to provide a CA certificate that the peer's certificate can match or chain to. For self-signed certificates, the certificate is its own CA, and must be provided. For PEM encoded certificates, supported types are "TRUSTED CERTIFICATE", "X509 CERTIFICATE", and "CERTIFICATE". See also tls.rootCertificates.
    • cert <string> | <string[]> | <Buffer> | <Buffer[]> Cert chains in PEM format. One cert chain should be provided per private key. Each cert chain should consist of the PEM formatted certificate for a provided private key, followed by the PEM formatted intermediate certificates (if any), in order, and not including the root CA (the root CA must be pre-known to the peer, see ca). When providing multiple cert chains, they do not have to be in the same order as their private keys in key. If the intermediate certificates are not provided, the peer will not be able to validate the certificate, and the handshake will fail.
    • sigalgs <string> Colon-separated list of supported signature algorithms. The list can contain digest algorithms (SHA256, MD5 etc.), public key algorithms (RSA-PSS, ECDSA etc.), combination of both (e.g 'RSA+SHA384') or TLS v1.3 scheme names (e.g. rsa_pss_pss_sha512). See OpenSSL man pages for more info.
    • ciphers <string> Cipher suite specification, replacing the default. For more information, see modifying the default cipher suite. Permitted ciphers can be obtained via tls.getCiphers(). Cipher names must be uppercased in order for OpenSSL to accept them.
    • clientCertEngine <string> Name of an OpenSSL engine which can provide the client certificate.
    • crl <string> | <string[]> | <Buffer> | <Buffer[]> PEM formatted CRLs (Certificate Revocation Lists).
    • dhparam <string> | <Buffer> Diffie Hellman parameters, required for Perfect Forward Secrecy. Use openssl dhparam to create the parameters. The key length must be greater than or equal to 1024 bits or else an error will be thrown. Although 1024 bits is permissible, use 2048 bits or larger for stronger security. If omitted or invalid, the parameters are silently discarded and DHE ciphers will not be available.
    • ecdhCurve <string> A string describing a named curve or a colon separated list of curve NIDs or names, for example P-521:P-384:P-256, to use for ECDH key agreement. Set to auto to select the curve automatically. Use crypto.getCurves() to obtain a list of available curve names. On recent releases, openssl ecparam -list_curves will also display the name and description of each available elliptic curve. Default: tls.DEFAULT_ECDH_CURVE.
    • honorCipherOrder <boolean> Attempt to use the server's cipher suite preferences instead of the client's. When true, causes SSL_OP_CIPHER_SERVER_PREFERENCE to be set in secureOptions, see OpenSSL Options for more information.
    • key <string> | <string[]> | <Buffer> | <Buffer[]> | <Object[]> Private keys in PEM format. PEM allows the option of private keys being encrypted. Encrypted keys will be decrypted with options.passphrase. Multiple keys using different algorithms can be provided either as an array of unencrypted key strings or buffers, or an array of objects in the form {pem: <string|buffer>[, passphrase: <string>]}. The object form can only occur in an array. object.passphrase is optional. Encrypted keys will be decrypted with object.passphrase if provided, or options.passphrase if it is not.
    • privateKeyEngine <string> Name of an OpenSSL engine to get private key from. Should be used together with privateKeyIdentifier.
    • privateKeyIdentifier <string> Identifier of a private key managed by an OpenSSL engine. Should be used together with privateKeyEngine. Should not be set together with key, because both options define a private key in different ways.
    • maxVersion <string> Optionally set the maximum TLS version to allow. One of 'TLSv1.3', 'TLSv1.2', 'TLSv1.1', or 'TLSv1'. Cannot be specified along with the secureProtocol option, use one or the other. Default: tls.DEFAULT_MAX_VERSION.
    • minVersion <string> Optionally set the minimum TLS version to allow. One of 'TLSv1.3', 'TLSv1.2', 'TLSv1.1', or 'TLSv1'. Cannot be specified along with the secureProtocol option, use one or the other. It is not recommended to use less than TLSv1.2, but it may be required for interoperability. Default: tls.DEFAULT_MIN_VERSION.
    • passphrase <string> Shared passphrase used for a single private key and/or a PFX.
    • pfx <string> | <string[]> | <Buffer> | <Buffer[]> | <Object[]> PFX or PKCS12 encoded private key and certificate chain. pfx is an alternative to providing key and cert individually. PFX is usually encrypted, if it is, passphrase will be used to decrypt it. Multiple PFX can be provided either as an array of unencrypted PFX buffers, or an array of objects in the form {buf: <string|buffer>[, passphrase: <string>]}. The object form can only occur in an array. object.passphrase is optional. Encrypted PFX will be decrypted with object.passphrase if provided, or options.passphrase if it is not.
    • secureOptions <number> Optionally affect the OpenSSL protocol behavior, which is not usually necessary. This should be used carefully if at all! Value is a numeric bitmask of the SSL_OP_* options from OpenSSL Options.
    • secureProtocol <string> Legacy mechanism to select the TLS protocol version to use, it does not support independent control of the minimum and maximum version, and does not support limiting the protocol to TLSv1.3. Use minVersion and maxVersion instead. The possible values are listed as SSL_METHODS, use the function names as strings. For example, use 'TLSv1_1_method' to force TLS version 1.1, or 'TLS_method' to allow any TLS protocol version up to TLSv1.3. It is not recommended to use TLS versions less than 1.2, but it may be required for interoperability. Default: none, see minVersion.
    • sessionIdContext <string> Opaque identifier used by servers to ensure session state is not shared between applications. Unused by clients.

tls.createServer() sets the default value of the honorCipherOrder option to true, other APIs that create secure contexts leave it unset.

tls.createServer() uses a 128 bit truncated SHA1 hash value generated from process.argv as the default value of the sessionIdContext option, other APIs that create secure contexts have no default value.

The tls.createSecureContext() method creates a SecureContext object. It is usable as an argument to several tls APIs, such as tls.createServer() and server.addContext(), but has no public methods.

A key is required for ciphers that make use of certificates. Either key or pfx can be used to provide it.

If the ca option is not given, then Node.js will default to using Mozilla's publicly trusted list of CAs.

tls.createServer([options][, secureConnectionListener])#

History
VersionChanges
v12.3.0

The options parameter now supports net.createServer() options.

v9.3.0

The options parameter can now include clientCertEngine.

v8.0.0

The ALPNProtocols option can be a TypedArray or DataView now.

v5.0.0

ALPN options are supported now.

v0.3.2

Added in: v0.3.2

  • options <Object>

    • ALPNProtocols: <string[]> | <Buffer[]> | <TypedArray[]> | <DataView[]> | <Buffer> | <TypedArray> | <DataView> An array of strings, Buffers or TypedArrays or DataViews, or a single Buffer or TypedArray or DataView containing the supported ALPN protocols. Buffers should have the format [len][name][len][name]... e.g. 0x05hello0x05world, where the first byte is the length of the next protocol name. Passing an array is usually much simpler, e.g. ['hello', 'world']. (Protocols should be ordered by their priority.)
    • clientCertEngine <string> Name of an OpenSSL engine which can provide the client certificate.
    • enableTrace <boolean> If true, tls.TLSSocket.enableTrace() will be called on new connections. Tracing can be enabled after the secure connection is established, but this option must be used to trace the secure connection setup. Default: false.
    • handshakeTimeout <number> Abort the connection if the SSL/TLS handshake does not finish in the specified number of milliseconds. A 'tlsClientError' is emitted on the tls.Server object whenever a handshake times out. Default: 120000 (120 seconds).
    • rejectUnauthorized <boolean> If not false the server will reject any connection which is not authorized with the list of supplied CAs. This option only has an effect if requestCert is true. Default: true.
    • requestCert <boolean> If true the server will request a certificate from clients that connect and attempt to verify that certificate. Default: false.
    • sessionTimeout <number> The number of seconds after which a TLS session created by the server will no longer be resumable. See Session Resumption for more information. Default: 300.
    • SNICallback(servername, cb) <Function> A function that will be called if the client supports SNI TLS extension. Two arguments will be passed when called: servername and cb. SNICallback should invoke cb(null, ctx), where ctx is a SecureContext instance. (tls.createSecureContext(...) can be used to get a proper SecureContext.) If SNICallback wasn't provided the default callback with high-level API will be used (see below).
    • ticketKeys: <Buffer> 48-bytes of cryptographically strong pseudo-random data. See Session Resumption for more information.

    • pskCallback <Function>

      • socket: <tls.TLSSocket> the server tls.TLSSocket instance for this connection.
      • identity: <string> identity parameter sent from the client.
      • Returns: <Buffer> | <TypedArray> | <DataView> pre-shared key that must either be a buffer or null to stop the negotiation process. Returned PSK must be compatible with the selected cipher's digest. When negotiating TLS-PSK (pre-shared keys), this function is called with the identity provided by the client. If the return value is null the negotiation process will stop and an "unknown_psk_identity" alert message will be sent to the other party. If the server wishes to hide the fact that the PSK identity was not known, the callback must provide some random data as psk to make the connection fail with "decrypt_error" before negotiation is finished. PSK ciphers are disabled by default, and using TLS-PSK thus requires explicitly specifying a cipher suite with the ciphers option. More information can be found in the RFC 4279.
    • pskIdentityHint <string> optional hint to send to a client to help with selecting the identity during TLS-PSK negotiation. Will be ignored in TLS 1.3. Upon failing to set pskIdentityHint 'tlsClientError' will be emitted with 'ERR_TLS_PSK_SET_IDENTIY_HINT_FAILED' code.
    • ...: Any tls.createSecureContext() option can be provided. For servers, the identity options (pfx, key/cert or pskCallback) are usually required.
    • ...: Any net.createServer() option can be provided.
  • secureConnectionListener <Function>
  • Returns: <tls.Server>

Creates a new tls.Server. The secureConnectionListener, if provided, is automatically set as a listener for the 'secureConnection' event.

The ticketKeys options is automatically shared between cluster module workers.

The following illustrates a simple echo server:

const tls = require('tls');
const fs = require('fs');

const options = {
  key: fs.readFileSync('server-key.pem'),
  cert: fs.readFileSync('server-cert.pem'),

  // This is necessary only if using client certificate authentication.
  requestCert: true,

  // This is necessary only if the client uses a self-signed certificate.
  ca: [ fs.readFileSync('client-cert.pem') ]
};

const server = tls.createServer(options, (socket) => {
  console.log('server connected',
              socket.authorized ? 'authorized' : 'unauthorized');
  socket.write('welcome!\n');
  socket.setEncoding('utf8');
  socket.pipe(socket);
});
server.listen(8000, () => {
  console.log('server bound');
});

The server can be tested by connecting to it using the example client from tls.connect().

tls.getCiphers()#

Added in: v0.10.2

Returns an array with the names of the supported TLS ciphers. The names are lower-case for historical reasons, but must be uppercased to be used in the ciphers option of tls.createSecureContext().

Cipher names that start with 'tls_' are for TLSv1.3, all the others are for TLSv1.2 and below.

console.log(tls.getCiphers()); // ['aes128-gcm-sha256', 'aes128-sha', ...]

tls.rootCertificates#

Added in: v12.3.0

An immutable array of strings representing the root certificates (in PEM format) used for verifying peer certificates. This is the default value of the ca option to tls.createSecureContext().

tls.DEFAULT_ECDH_CURVE#

History
VersionChanges
v10.0.0

Default value changed to 'auto'.

v0.11.13

Added in: v0.11.13

The default curve name to use for ECDH key agreement in a tls server. The default value is 'auto'. See tls.createSecureContext() for further information.

tls.DEFAULT_MAX_VERSION#

Added in: v11.4.0
  • <string> The default value of the maxVersion option of tls.createSecureContext(). It can be assigned any of the supported TLS protocol versions, 'TLSv1.3', 'TLSv1.2', 'TLSv1.1', or 'TLSv1'. Default: 'TLSv1.3', unless changed using CLI options. Using --tls-max-v1.2 sets the default to 'TLSv1.2'. Using --tls-max-v1.3 sets the default to 'TLSv1.3'. If multiple of the options are provided, the highest maximum is used.

tls.DEFAULT_MIN_VERSION#

Added in: v11.4.0
  • <string> The default value of the minVersion option of tls.createSecureContext(). It can be assigned any of the supported TLS protocol versions, 'TLSv1.3', 'TLSv1.2', 'TLSv1.1', or 'TLSv1'. Default: 'TLSv1.2', unless changed using CLI options. Using --tls-min-v1.0 sets the default to 'TLSv1'. Using --tls-min-v1.1 sets the default to 'TLSv1.1'. Using --tls-min-v1.3 sets the default to 'TLSv1.3'. If multiple of the options are provided, the lowest minimum is used.

Deprecated APIs#

Class: CryptoStream#

Added in: v0.3.4Deprecated since: v0.11.3

Stability: 0 - Deprecated: Use tls.TLSSocket instead.

The tls.CryptoStream class represents a stream of encrypted data. This class is deprecated and should no longer be used.

cryptoStream.bytesWritten#

Added in: v0.3.4Deprecated since: v0.11.3

The cryptoStream.bytesWritten property returns the total number of bytes written to the underlying socket including the bytes required for the implementation of the TLS protocol.

Class: SecurePair#

Added in: v0.3.2Deprecated since: v0.11.3

Stability: 0 - Deprecated: Use tls.TLSSocket instead.

Returned by tls.createSecurePair().

Event: 'secure'#

Added in: v0.3.2Deprecated since: v0.11.3

The 'secure' event is emitted by the SecurePair object once a secure connection has been established.

As with checking for the server 'secureConnection' event, pair.cleartext.authorized should be inspected to confirm whether the certificate used is properly authorized.

tls.createSecurePair([context][, isServer][, requestCert][, rejectUnauthorized][, options])#

History
VersionChanges
v5.0.0

ALPN options are supported now.

v0.11.3

Deprecated since: v0.11.3

v0.3.2

Added in: v0.3.2

Stability: 0 - Deprecated: Use tls.TLSSocket instead.

  • context <Object> A secure context object as returned by tls.createSecureContext()
  • isServer <boolean> true to specify that this TLS connection should be opened as a server.
  • requestCert <boolean> true to specify whether a server should request a certificate from a connecting client. Only applies when isServer is true.
  • rejectUnauthorized <boolean> If not false a server automatically reject clients with invalid certificates. Only applies when isServer is true.

  • options

Creates a new secure pair object with two streams, one of which reads and writes the encrypted data and the other of which reads and writes the cleartext data. Generally, the encrypted stream is piped to/from an incoming encrypted data stream and the cleartext one is used as a replacement for the initial encrypted stream.

tls.createSecurePair() returns a tls.SecurePair object with cleartext and encrypted stream properties.

Using cleartext has the same API as tls.TLSSocket.

The tls.createSecurePair() method is now deprecated in favor of tls.TLSSocket(). For example, the code:

pair = tls.createSecurePair(/* ... */);
pair.encrypted.pipe(socket);
socket.pipe(pair.encrypted);

can be replaced by:

secureSocket = tls.TLSSocket(socket, options);

where secureSocket has the same API as pair.cleartext.

Trace Events#

Stability: 1 - Experimental

Trace Event provides a mechanism to centralize tracing information generated by V8, Node.js core, and userspace code.

Tracing can be enabled with the --trace-event-categories command-line flag or by using the trace_events module. The --trace-event-categories flag accepts a list of comma-separated category names.

The available categories are:

  • node: An empty placeholder.
  • node.async_hooks: Enables capture of detailed async_hooks trace data. The async_hooks events have a unique asyncId and a special triggerId triggerAsyncId property.
  • node.bootstrap: Enables capture of Node.js bootstrap milestones.
  • node.console: Enables capture of console.time() and console.count() output.
  • node.dns.native: Enables capture of trace data for DNS queries.
  • node.environment: Enables capture of Node.js Environment milestones.
  • node.fs.sync: Enables capture of trace data for file system sync methods.

  • node.perf: Enables capture of Performance API measurements.

    • node.perf.usertiming: Enables capture of only Performance API User Timing measures and marks.
    • node.perf.timerify: Enables capture of only Performance API timerify measurements.
  • node.promises.rejections: Enables capture of trace data tracking the number of unhandled Promise rejections and handled-after-rejections.
  • node.vm.script: Enables capture of trace data for the vm module's runInNewContext(), runInContext(), and runInThisContext() methods.
  • v8: The V8 events are GC, compiling, and execution related.

By default the node, node.async_hooks, and v8 categories are enabled.

node --trace-event-categories v8,node,node.async_hooks server.js

Prior versions of Node.js required the use of the --trace-events-enabled flag to enable trace events. This requirement has been removed. However, the --trace-events-enabled flag may still be used and will enable the node, node.async_hooks, and v8 trace event categories by default.

node --trace-events-enabled

// is equivalent to

node --trace-event-categories v8,node,node.async_hooks

Alternatively, trace events may be enabled using the trace_events module:

const trace_events = require('trace_events');
const tracing = trace_events.createTracing({ categories: ['node.perf'] });
tracing.enable();  // Enable trace event capture for the 'node.perf' category

// do work

tracing.disable();  // Disable trace event capture for the 'node.perf' category

Running Node.js with tracing enabled will produce log files that can be opened in the chrome://tracing tab of Chrome.

The logging file is by default called node_trace.${rotation}.log, where ${rotation} is an incrementing log-rotation id. The filepath pattern can be specified with --trace-event-file-pattern that accepts a template string that supports ${rotation} and ${pid}:

node --trace-event-categories v8 --trace-event-file-pattern '${pid}-${rotation}.log' server.js

Starting with Node.js 10.0.0, the tracing system uses the same time source as the one used by process.hrtime() however the trace-event timestamps are expressed in microseconds, unlike process.hrtime() which returns nanoseconds.

The features from this module are not available in Worker threads.

The trace_events module#

Added in: v10.0.0

Tracing object#

Added in: v10.0.0

The Tracing object is used to enable or disable tracing for sets of categories. Instances are created using the trace_events.createTracing() method.

When created, the Tracing object is disabled. Calling the tracing.enable() method adds the categories to the set of enabled trace event categories. Calling tracing.disable() will remove the categories from the set of enabled trace event categories.

tracing.categories#

Added in: v10.0.0

A comma-separated list of the trace event categories covered by this Tracing object.

tracing.disable()#

Added in: v10.0.0

Disables this Tracing object.

Only trace event categories not covered by other enabled Tracing objects and not specified by the --trace-event-categories flag will be disabled.

const trace_events = require('trace_events');
const t1 = trace_events.createTracing({ categories: ['node', 'v8'] });
const t2 = trace_events.createTracing({ categories: ['node.perf', 'node'] });
t1.enable();
t2.enable();

// Prints 'node,node.perf,v8'
console.log(trace_events.getEnabledCategories());

t2.disable(); // Will only disable emission of the 'node.perf' category

// Prints 'node,v8'
console.log(trace_events.getEnabledCategories());

tracing.enable()#

Added in: v10.0.0

Enables this Tracing object for the set of categories covered by the Tracing object.

tracing.enabled#

Added in: v10.0.0
  • <boolean> true only if the Tracing object has been enabled.

trace_events.createTracing(options)#

Added in: v10.0.0

  • options <Object>

    • categories <string[]> An array of trace category names. Values included in the array are coerced to a string when possible. An error will be thrown if the value cannot be coerced.
  • Returns: <Tracing>.

Creates and returns a Tracing object for the given set of categories.

const trace_events = require('trace_events');
const categories = ['node.perf', 'node.async_hooks'];
const tracing = trace_events.createTracing({ categories });
tracing.enable();
// do stuff
tracing.disable();

trace_events.getEnabledCategories()#

Added in: v10.0.0

Returns a comma-separated list of all currently-enabled trace event categories. The current set of enabled trace event categories is determined by the union of all currently-enabled Tracing objects and any categories enabled using the --trace-event-categories flag.

Given the file test.js below, the command node --trace-event-categories node.perf test.js will print 'node.async_hooks,node.perf' to the console.

const trace_events = require('trace_events');
const t1 = trace_events.createTracing({ categories: ['node.async_hooks'] });
const t2 = trace_events.createTracing({ categories: ['node.perf'] });
const t3 = trace_events.createTracing({ categories: ['v8'] });

t1.enable();
t2.enable();

console.log(trace_events.getEnabledCategories());

TTY#

Stability: 2 - Stable

The tty module provides the tty.ReadStream and tty.WriteStream classes. In most cases, it will not be necessary or possible to use this module directly. However, it can be accessed using:

const tty = require('tty');

When Node.js detects that it is being run with a text terminal ("TTY") attached, process.stdin will, by default, be initialized as an instance of tty.ReadStream and both process.stdout and process.stderr will, by default be instances of tty.WriteStream. The preferred method of determining whether Node.js is being run within a TTY context is to check that the value of the process.stdout.isTTY property is true:

$ node -p -e "Boolean(process.stdout.isTTY)"
true
$ node -p -e "Boolean(process.stdout.isTTY)" | cat
false

In most cases, there should be little to no reason for an application to manually create instances of the tty.ReadStream and tty.WriteStream classes.

Class: tty.ReadStream#

Added in: v0.5.8

Represents the readable side of a TTY. In normal circumstances process.stdin will be the only tty.ReadStream instance in a Node.js process and there should be no reason to create additional instances.

readStream.isRaw#

Added in: v0.7.7

A boolean that is true if the TTY is currently configured to operate as a raw device. Defaults to false.

readStream.isTTY#

Added in: v0.5.8

A boolean that is always true for tty.ReadStream instances.

readStream.setRawMode(mode)#

Added in: v0.7.7
  • mode <boolean> If true, configures the tty.ReadStream to operate as a raw device. If false, configures the tty.ReadStream to operate in its default mode. The readStream.isRaw property will be set to the resulting mode.
  • Returns: <this> The read stream instance.

Allows configuration of tty.ReadStream so that it operates as a raw device.

When in raw mode, input is always available character-by-character, not including modifiers. Additionally, all special processing of characters by the terminal is disabled, including echoing input characters. CTRL+C will no longer cause a SIGINT when in this mode.

Class: tty.WriteStream#

Added in: v0.5.8

Represents the writable side of a TTY. In normal circumstances, process.stdout and process.stderr will be the only tty.WriteStream instances created for a Node.js process and there should be no reason to create additional instances.

Event: 'resize'#

Added in: v0.7.7

The 'resize' event is emitted whenever either of the writeStream.columns or writeStream.rows properties have changed. No arguments are passed to the listener callback when called.

process.stdout.on('resize', () => {
  console.log('screen size has changed!');
  console.log(`${process.stdout.columns}x${process.stdout.rows}`);
});

writeStream.clearLine(dir[, callback])#

History
VersionChanges
v12.7.0

The stream's write() callback and return value are exposed.

v0.7.7

Added in: v0.7.7

  • dir <number>

    • -1: to the left from cursor
    • 1: to the right from cursor
    • 0: the entire line
  • callback <Function> Invoked once the operation completes.
  • Returns: <boolean> false if the stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

writeStream.clearLine() clears the current line of this WriteStream in a direction identified by dir.

writeStream.clearScreenDown([callback])#

History
VersionChanges
v12.7.0

The stream's write() callback and return value are exposed.

v0.7.7

Added in: v0.7.7

  • callback <Function> Invoked once the operation completes.
  • Returns: <boolean> false if the stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

writeStream.clearScreenDown() clears this WriteStream from the current cursor down.

writeStream.columns#

Added in: v0.7.7

A number specifying the number of columns the TTY currently has. This property is updated whenever the 'resize' event is emitted.

writeStream.cursorTo(x[, y][, callback])#

History
VersionChanges
v12.7.0

The stream's write() callback and return value are exposed.

v0.7.7

Added in: v0.7.7

  • x <number>
  • y <number>
  • callback <Function> Invoked once the operation completes.
  • Returns: <boolean> false if the stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

writeStream.cursorTo() moves this WriteStream's cursor to the specified position.

writeStream.getColorDepth([env])#

Added in: v9.9.0
  • env <Object> An object containing the environment variables to check. This enables simulating the usage of a specific terminal. Default: process.env.
  • Returns: <number>

Returns:

  • 1 for 2,
  • 4 for 16,
  • 8 for 256,
  • 24 for 16,777,216 colors supported.

Use this to determine what colors the terminal supports. Due to the nature of colors in terminals it is possible to either have false positives or false negatives. It depends on process information and the environment variables that may lie about what terminal is used. It is possible to pass in an env object to simulate the usage of a specific terminal. This can be useful to check how specific environment settings behave.

To enforce a specific color support, use one of the below environment settings.

  • 2 colors: FORCE_COLOR = 0 (Disables colors)
  • 16 colors: FORCE_COLOR = 1
  • 256 colors: FORCE_COLOR = 2
  • 16,777,216 colors: FORCE_COLOR = 3

Disabling color support is also possible by using the NO_COLOR and NODE_DISABLE_COLORS environment variables.

writeStream.getWindowSize()#

Added in: v0.7.7

writeStream.getWindowSize() returns the size of the TTY corresponding to this WriteStream. The array is of the type [numColumns, numRows] where numColumns and numRows represent the number of columns and rows in the corresponding TTY.

writeStream.hasColors([count][, env])#

Added in: v11.13.0, v10.16.0
  • count <integer> The number of colors that are requested (minimum 2). Default: 16.
  • env <Object> An object containing the environment variables to check. This enables simulating the usage of a specific terminal. Default: process.env.
  • Returns: <boolean>

Returns true if the writeStream supports at least as many colors as provided in count. Minimum support is 2 (black and white).

This has the same false positives and negatives as described in writeStream.getColorDepth().

process.stdout.hasColors();
// Returns true or false depending on if `stdout` supports at least 16 colors.
process.stdout.hasColors(256);
// Returns true or false depending on if `stdout` supports at least 256 colors.
process.stdout.hasColors({ TMUX: '1' });
// Returns true.
process.stdout.hasColors(2 ** 24, { TMUX: '1' });
// Returns false (the environment setting pretends to support 2 ** 8 colors).

writeStream.isTTY#

Added in: v0.5.8

A boolean that is always true.

writeStream.moveCursor(dx, dy[, callback])#

History
VersionChanges
v12.7.0

The stream's write() callback and return value are exposed.

v0.7.7

Added in: v0.7.7

  • dx <number>
  • dy <number>
  • callback <Function> Invoked once the operation completes.
  • Returns: <boolean> false if the stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

writeStream.moveCursor() moves this WriteStream's cursor relative to its current position.

writeStream.rows#

Added in: v0.7.7

A number specifying the number of rows the TTY currently has. This property is updated whenever the 'resize' event is emitted.

tty.isatty(fd)#

Added in: v0.5.8

The tty.isatty() method returns true if the given fd is associated with a TTY and false if it is not, including whenever fd is not a non-negative integer.

UDP/Datagram Sockets#

Stability: 2 - Stable

The dgram module provides an implementation of UDP Datagram sockets.

const dgram = require('dgram');
const server = dgram.createSocket('udp4');

server.on('error', (err) => {
  console.log(`server error:\n${err.stack}`);
  server.close();
});

server.on('message', (msg, rinfo) => {
  console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});

server.on('listening', () => {
  const address = server.address();
  console.log(`server listening ${address.address}:${address.port}`);
});

server.bind(41234);
// Prints: server listening 0.0.0.0:41234

Class: dgram.Socket#

Added in: v0.1.99

Encapsulates the datagram functionality.

New instances of dgram.Socket are created using dgram.createSocket(). The new keyword is not to be used to create dgram.Socket instances.

Event: 'close'#

Added in: v0.1.99

The 'close' event is emitted after a socket is closed with close(). Once triggered, no new 'message' events will be emitted on this socket.

Event: 'connect'#

Added in: v12.0.0

The 'connect' event is emitted after a socket is associated to a remote address as a result of a successful connect() call.

Event: 'error'#

Added in: v0.1.99

The 'error' event is emitted whenever any error occurs. The event handler function is passed a single Error object.

Event: 'listening'#

Added in: v0.1.99

The 'listening' event is emitted once the dgram.Socket is addressable and can receive data. This happens either explicitly with socket.bind() or implicitly the first time data is sent using socket.send(). Until the dgram.Socket is listening, the underlying system resources do not exist and calls such as socket.address() and socket.setTTL() will fail.

Event: 'message'#

Added in: v0.1.99

The 'message' event is emitted when a new datagram is available on a socket. The event handler function is passed two arguments: msg and rinfo.

socket.addMembership(multicastAddress[, multicastInterface])#

Added in: v0.6.9

Tells the kernel to join a multicast group at the given multicastAddress and multicastInterface using the IP_ADD_MEMBERSHIP socket option. If the multicastInterface argument is not specified, the operating system will choose one interface and will add membership to it. To add membership to every available interface, call addMembership multiple times, once per interface.

When sharing a UDP socket across multiple cluster workers, the socket.addMembership() function must be called only once or an EADDRINUSE error will occur:

const cluster = require('cluster');
const dgram = require('dgram');
if (cluster.isMaster) {
  cluster.fork(); // Works ok.
  cluster.fork(); // Fails with EADDRINUSE.
} else {
  const s = dgram.createSocket('udp4');
  s.bind(1234, () => {
    s.addMembership('224.0.0.114');
  });
}

socket.addSourceSpecificMembership(sourceAddress, groupAddress[, multicastInterface])#

Added in: v13.1.0, v12.16.0

Tells the kernel to join a source-specific multicast channel at the given sourceAddress and groupAddress, using the multicastInterface with the IP_ADD_SOURCE_MEMBERSHIP socket option. If the multicastInterface argument is not specified, the operating system will choose one interface and will add membership to it. To add membership to every available interface, call socket.addSourceSpecificMembership() multiple times, once per interface.

socket.address()#

Added in: v0.1.99

Returns an object containing the address information for a socket. For UDP sockets, this object will contain address, family and port properties.

socket.bind([port][, address][, callback])#

History
VersionChanges
v0.10

The method was changed to an asynchronous execution model. Legacy code would need to be changed to pass a callback function to the method call.

v0.1.99

Added in: v0.1.99

For UDP sockets, causes the dgram.Socket to listen for datagram messages on a named port and optional address. If port is not specified or is 0, the operating system will attempt to bind to a random port. If address is not specified, the operating system will attempt to listen on all addresses. Once binding is complete, a 'listening' event is emitted and the optional callback function is called.

Specifying both a 'listening' event listener and passing a callback to the socket.bind() method is not harmful but not very useful.

A bound datagram socket keeps the Node.js process running to receive datagram messages.

If binding fails, an 'error' event is generated. In rare case (e.g. attempting to bind with a closed socket), an Error may be thrown.

Example of a UDP server listening on port 41234:

const dgram = require('dgram');
const server = dgram.createSocket('udp4');

server.on('error', (err) => {
  console.log(`server error:\n${err.stack}`);
  server.close();
});

server.on('message', (msg, rinfo) => {
  console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});

server.on('listening', () => {
  const address = server.address();
  console.log(`server listening ${address.address}:${address.port}`);
});

server.bind(41234);
// Prints: server listening 0.0.0.0:41234

socket.bind(options[, callback])#

Added in: v0.11.14

For UDP sockets, causes the dgram.Socket to listen for datagram messages on a named port and optional address that are passed as properties of an options object passed as the first argument. If port is not specified or is 0, the operating system will attempt to bind to a random port. If address is not specified, the operating system will attempt to listen on all addresses. Once binding is complete, a 'listening' event is emitted and the optional callback function is called.

The options object may contain a fd property. When a fd greater than 0 is set, it will wrap around an existing socket with the given file descriptor. In this case, the properties of port and address will be ignored.

Specifying both a 'listening' event listener and passing a callback to the socket.bind() method is not harmful but not very useful.

The options object may contain an additional exclusive property that is used when using dgram.Socket objects with the cluster module. When exclusive is set to false (the default), cluster workers will use the same underlying socket handle allowing connection handling duties to be shared. When exclusive is true, however, the handle is not shared and attempted port sharing results in an error.

A bound datagram socket keeps the Node.js process running to receive datagram messages.

If binding fails, an 'error' event is generated. In rare case (e.g. attempting to bind with a closed socket), an Error may be thrown.

An example socket listening on an exclusive port is shown below.

socket.bind({
  address: 'localhost',
  port: 8000,
  exclusive: true
});

socket.close([callback])#

Added in: v0.1.99
  • callback <Function> Called when the socket has been closed.

Close the underlying socket and stop listening for data on it. If a callback is provided, it is added as a listener for the 'close' event.

socket.connect(port[, address][, callback])#

Added in: v12.0.0

Associates the dgram.Socket to a remote address and port. Every message sent by this handle is automatically sent to that destination. Also, the socket will only receive messages from that remote peer. Trying to call connect() on an already connected socket will result in an ERR_SOCKET_DGRAM_IS_CONNECTED exception. If address is not provided, '127.0.0.1' (for udp4 sockets) or '::1' (for udp6 sockets) will be used by default. Once the connection is complete, a 'connect' event is emitted and the optional callback function is called. In case of failure, the callback is called or, failing this, an 'error' event is emitted.

socket.disconnect()#

Added in: v12.0.0

A synchronous function that disassociates a connected dgram.Socket from its remote address. Trying to call disconnect() on an already disconnected socket will result in an ERR_SOCKET_DGRAM_NOT_CONNECTED exception.

socket.dropMembership(multicastAddress[, multicastInterface])#

Added in: v0.6.9

Instructs the kernel to leave a multicast group at multicastAddress using the IP_DROP_MEMBERSHIP socket option. This method is automatically called by the kernel when the socket is closed or the process terminates, so most apps will never have reason to call this.

If multicastInterface is not specified, the operating system will attempt to drop membership on all valid interfaces.

socket.dropSourceSpecificMembership(sourceAddress, groupAddress[, multicastInterface])#

Added in: v13.1.0, v12.16.0

Instructs the kernel to leave a source-specific multicast channel at the given sourceAddress and groupAddress using the IP_DROP_SOURCE_MEMBERSHIP socket option. This method is automatically called by the kernel when the socket is closed or the process terminates, so most apps will never have reason to call this.

If multicastInterface is not specified, the operating system will attempt to drop membership on all valid interfaces.

socket.getRecvBufferSize()#

Added in: v8.7.0
  • Returns: <number> the SO_RCVBUF socket receive buffer size in bytes.

socket.getSendBufferSize()#

Added in: v8.7.0
  • Returns: <number> the SO_SNDBUF socket send buffer size in bytes.

socket.ref()#

Added in: v0.9.1

By default, binding a socket will cause it to block the Node.js process from exiting as long as the socket is open. The socket.unref() method can be used to exclude the socket from the reference counting that keeps the Node.js process active. The socket.ref() method adds the socket back to the reference counting and restores the default behavior.

Calling socket.ref() multiples times will have no additional effect.

The socket.ref() method returns a reference to the socket so calls can be chained.

socket.remoteAddress()#

Added in: v12.0.0

Returns an object containing the address, family, and port of the remote endpoint. It throws an ERR_SOCKET_DGRAM_NOT_CONNECTED exception if the socket is not connected.

socket.send(msg[, offset, length][, port][, address][, callback])#

History
VersionChanges
v12.0.0

Added support for sending data on connected sockets.

v8.0.0

The msg parameter can be an Uint8Array now.

v8.0.0

The address parameter is always optional now.

v6.0.0

On success, callback will now be called with an error argument of null rather than 0.

v5.7.0

The msg parameter can be an array now. Also, the offset and length parameters are optional now.

v0.1.99

Added in: v0.1.99

Broadcasts a datagram on the socket. For connectionless sockets, the destination port and address must be specified. Connected sockets, on the other hand, will use their associated remote endpoint, so the port and address arguments must not be set.

The msg argument contains the message to be sent. Depending on its type, different behavior can apply. If msg is a Buffer or Uint8Array, the offset and length specify the offset within the Buffer where the message begins and the number of bytes in the message, respectively. If msg is a String, then it is automatically converted to a Buffer with 'utf8' encoding. With messages that contain multi-byte characters, offset and length will be calculated with respect to byte length and not the character position. If msg is an array, offset and length must not be specified.

The address argument is a string. If the value of address is a host name, DNS will be used to resolve the address of the host. If address is not provided or otherwise falsy, '127.0.0.1' (for udp4 sockets) or '::1' (for udp6 sockets) will be used by default.

If the socket has not been previously bound with a call to bind, the socket is assigned a random port number and is bound to the "all interfaces" address ('0.0.0.0' for udp4 sockets, '::0' for udp6 sockets.)

An optional callback function may be specified to as a way of reporting DNS errors or for determining when it is safe to reuse the buf object. DNS lookups delay the time to send for at least one tick of the Node.js event loop.

The only way to know for sure that the datagram has been sent is by using a callback. If an error occurs and a callback is given, the error will be passed as the first argument to the callback. If a callback is not given, the error is emitted as an 'error' event on the socket object.

Offset and length are optional but both must be set if either are used. They are supported only when the first argument is a Buffer or Uint8Array.

Example of sending a UDP packet to a port on localhost;

const dgram = require('dgram');
const message = Buffer.from('Some bytes');
const client = dgram.createSocket('udp4');
client.send(message, 41234, 'localhost', (err) => {
  client.close();
});

Example of sending a UDP packet composed of multiple buffers to a port on 127.0.0.1;

const dgram = require('dgram');
const buf1 = Buffer.from('Some ');
const buf2 = Buffer.from('bytes');
const client = dgram.createSocket('udp4');
client.send([buf1, buf2], 41234, (err) => {
  client.close();
});

Sending multiple buffers might be faster or slower depending on the application and operating system. Run benchmarks to determine the optimal strategy on a case-by-case basis. Generally speaking, however, sending multiple buffers is faster.

Example of sending a UDP packet using a socket connected to a port on localhost:

const dgram = require('dgram');
const message = Buffer.from('Some bytes');
const client = dgram.createSocket('udp4');
client.connect(41234, 'localhost', (err) => {
  client.send(message, (err) => {
    client.close();
  });
});

Note about UDP datagram size#

The maximum size of an IPv4/v6 datagram depends on the MTU (Maximum Transmission Unit) and on the Payload Length field size.

  • The Payload Length field is 16 bits wide, which means that a normal payload exceed 64K octets including the internet header and data (65,507 bytes = 65,535 − 8 bytes UDP header − 20 bytes IP header); this is generally true for loopback interfaces, but such long datagram messages are impractical for most hosts and networks.

  • The MTU is the largest size a given link layer technology can support for datagram messages. For any link, IPv4 mandates a minimum MTU of 68 octets, while the recommended MTU for IPv4 is 576 (typically recommended as the MTU for dial-up type applications), whether they arrive whole or in fragments.

    For IPv6, the minimum MTU is 1280 octets, however, the mandatory minimum fragment reassembly buffer size is 1500 octets. The value of 68 octets is very small, since most current link layer technologies, like Ethernet, have a minimum MTU of 1500.

It is impossible to know in advance the MTU of each link through which a packet might travel. Sending a datagram greater than the receiver MTU will not work because the packet will get silently dropped without informing the source that the data did not reach its intended recipient.

socket.setBroadcast(flag)#

Added in: v0.6.9

Sets or clears the SO_BROADCAST socket option. When set to true, UDP packets may be sent to a local interface's broadcast address.

socket.setMulticastInterface(multicastInterface)#

Added in: v8.6.0

All references to scope in this section are referring to IPv6 Zone Indices, which are defined by RFC 4007. In string form, an IP with a scope index is written as 'IP%scope' where scope is an interface name or interface number.

Sets the default outgoing multicast interface of the socket to a chosen interface or back to system interface selection. The multicastInterface must be a valid string representation of an IP from the socket's family.

For IPv4 sockets, this should be the IP configured for the desired physical interface. All packets sent to multicast on the socket will be sent on the interface determined by the most recent successful use of this call.

For IPv6 sockets, multicastInterface should include a scope to indicate the interface as in the examples that follow. In IPv6, individual send calls can also use explicit scope in addresses, so only packets sent to a multicast address without specifying an explicit scope are affected by the most recent successful use of this call.

Examples: IPv6 Outgoing Multicast Interface#

On most systems, where scope format uses the interface name:

const socket = dgram.createSocket('udp6');

socket.bind(1234, () => {
  socket.setMulticastInterface('::%eth1');
});

On Windows, where scope format uses an interface number:

const socket = dgram.createSocket('udp6');

socket.bind(1234, () => {
  socket.setMulticastInterface('::%2');
});

Example: IPv4 Outgoing Multicast Interface#

All systems use an IP of the host on the desired physical interface:

const socket = dgram.createSocket('udp4');

socket.bind(1234, () => {
  socket.setMulticastInterface('10.0.0.2');
});

Call Results#

A call on a socket that is not ready to send or no longer open may throw a Not running Error.

If multicastInterface can not be parsed into an IP then an EINVAL System Error is thrown.

On IPv4, if multicastInterface is a valid address but does not match any interface, or if the address does not match the family then a System Error such as EADDRNOTAVAIL or EPROTONOSUP is thrown.

On IPv6, most errors with specifying or omitting scope will result in the socket continuing to use (or returning to) the system's default interface selection.

A socket's address family's ANY address (IPv4 '0.0.0.0' or IPv6 '::') can be used to return control of the sockets default outgoing interface to the system for future multicast packets.

socket.setMulticastLoopback(flag)#

Added in: v0.3.8

Sets or clears the IP_MULTICAST_LOOP socket option. When set to true, multicast packets will also be received on the local interface.

socket.setMulticastTTL(ttl)#

Added in: v0.3.8

Sets the IP_MULTICAST_TTL socket option. While TTL generally stands for "Time to Live", in this context it specifies the number of IP hops that a packet is allowed to travel through, specifically for multicast traffic. Each router or gateway that forwards a packet decrements the TTL. If the TTL is decremented to 0 by a router, it will not be forwarded.

The ttl argument may be between 0 and 255. The default on most systems is 1.

socket.setRecvBufferSize(size)#

Added in: v8.7.0

Sets the SO_RCVBUF socket option. Sets the maximum socket receive buffer in bytes.

socket.setSendBufferSize(size)#

Added in: v8.7.0

Sets the SO_SNDBUF socket option. Sets the maximum socket send buffer in bytes.

socket.setTTL(ttl)#

Added in: v0.1.101

Sets the IP_TTL socket option. While TTL generally stands for "Time to Live", in this context it specifies the number of IP hops that a packet is allowed to travel through. Each router or gateway that forwards a packet decrements the TTL. If the TTL is decremented to 0 by a router, it will not be forwarded. Changing TTL values is typically done for network probes or when multicasting.

The ttl argument may be between between 1 and 255. The default on most systems is 64.

socket.unref()#

Added in: v0.9.1

By default, binding a socket will cause it to block the Node.js process from exiting as long as the socket is open. The socket.unref() method can be used to exclude the socket from the reference counting that keeps the Node.js process active, allowing the process to exit even if the socket is still listening.

Calling socket.unref() multiple times will have no addition effect.

The socket.unref() method returns a reference to the socket so calls can be chained.

dgram module functions#

dgram.createSocket(options[, callback])#

History
VersionChanges
v11.4.0

The ipv6Only option is supported.

v8.7.0

The recvBufferSize and sendBufferSize options are supported now.

v8.6.0

The lookup option is supported.

v0.11.13

Added in: v0.11.13

  • options <Object> Available options are:

    • type <string> The family of socket. Must be either 'udp4' or 'udp6'. Required.
    • reuseAddr <boolean> When true socket.bind() will reuse the address, even if another process has already bound a socket on it. Default: false.
    • ipv6Only <boolean> Setting ipv6Only to true will disable dual-stack support, i.e., binding to address :: won't make 0.0.0.0 be bound. Default: false.
    • recvBufferSize <number> Sets the SO_RCVBUF socket value.
    • sendBufferSize <number> Sets the SO_SNDBUF socket value.
    • lookup <Function> Custom lookup function. Default: dns.lookup().
  • callback <Function> Attached as a listener for 'message' events. Optional.
  • Returns: <dgram.Socket>

Creates a dgram.Socket object. Once the socket is created, calling socket.bind() will instruct the socket to begin listening for datagram messages. When address and port are not passed to socket.bind() the method will bind the socket to the "all interfaces" address on a random port (it does the right thing for both udp4 and udp6 sockets). The bound address and port can be retrieved using socket.address().address and socket.address().port.

dgram.createSocket(type[, callback])#

Added in: v0.1.99

Creates a dgram.Socket object of the specified type.

Once the socket is created, calling socket.bind() will instruct the socket to begin listening for datagram messages. When address and port are not passed to socket.bind() the method will bind the socket to the "all interfaces" address on a random port (it does the right thing for both udp4 and udp6 sockets). The bound address and port can be retrieved using socket.address().address and socket.address().port.

URL#

Stability: 2 - Stable

The url module provides utilities for URL resolution and parsing. It can be accessed using:

const url = require('url');

URL Strings and URL Objects#

A URL string is a structured string containing multiple meaningful components. When parsed, a URL object is returned containing properties for each of these components.

The url module provides two APIs for working with URLs: a legacy API that is Node.js specific, and a newer API that implements the same WHATWG URL Standard used by web browsers.

A comparison between the WHATWG and Legacy APIs is provided below. Above the URL 'http://user:pass@sub.example.com:8080/p/a/t/h?query=string#hash', properties of an object returned by the legacy url.parse() are shown. Below it are properties of a WHATWG URL object.

WHATWG URL's origin property includes protocol and host, but not username or password.

┌────────────────────────────────────────────────────────────────────────────────────────────────┐
│                                              href                                              │
├──────────┬──┬─────────────────────┬────────────────────────┬───────────────────────────┬───────┤
│ protocol │  │        auth         │          host          │           path            │ hash  │
│          │  │                     ├─────────────────┬──────┼──────────┬────────────────┤       │
│          │  │                     │    hostname     │ port │ pathname │     search     │       │
│          │  │                     │                 │      │          ├─┬──────────────┤       │
│          │  │                     │                 │      │          │ │    query     │       │
"  https:   //    user   :   pass   @ sub.example.com : 8080   /p/a/t/h  ?  query=string   #hash "
│          │  │          │          │    hostname     │ port │          │                │       │
│          │  │          │          ├─────────────────┴──────┤          │                │       │
│ protocol │  │ username │ password │          host          │          │                │       │
├──────────┴──┼──────────┴──────────┼────────────────────────┤          │                │       │
│   origin    │                     │         origin         │ pathname │     search     │ hash  │
├─────────────┴─────────────────────┴────────────────────────┴──────────┴────────────────┴───────┤
│                                              href                                              │
└────────────────────────────────────────────────────────────────────────────────────────────────┘
(All spaces in the "" line should be ignored. They are purely for formatting.)

Parsing the URL string using the WHATWG API:

const myURL =
  new URL('https://user:pass@sub.example.com:8080/p/a/t/h?query=string#hash');

Parsing the URL string using the Legacy API:

const url = require('url');
const myURL =
  url.parse('https://user:pass@sub.example.com:8080/p/a/t/h?query=string#hash');

The WHATWG URL API#

Class: URL#

History
VersionChanges
v10.0.0

The class is now available on the global object.

v7.0.0, v6.13.0

Added in: v7.0.0, v6.13.0

Browser-compatible URL class, implemented by following the WHATWG URL Standard. Examples of parsed URLs may be found in the Standard itself. The URL class is also available on the global object.

In accordance with browser conventions, all properties of URL objects are implemented as getters and setters on the class prototype, rather than as data properties on the object itself. Thus, unlike legacy urlObjects, using the delete keyword on any properties of URL objects (e.g. delete myURL.protocol, delete myURL.pathname, etc) has no effect but will still return true.

Constructor: new URL(input[, base])#

  • input <string> The absolute or relative input URL to parse. If input is relative, then base is required. If input is absolute, the base is ignored.
  • base <string> | <URL> The base URL to resolve against if the input is not absolute.

Creates a new URL object by parsing the input relative to the base. If base is passed as a string, it will be parsed equivalent to new URL(base).

const myURL = new URL('/foo', 'https://example.org/');
// https://example.org/foo

The URL constructor is accessible as a property on the global object. It can also be imported from the built-in url module:

console.log(URL === require('url').URL); // Prints 'true'.

A TypeError will be thrown if the input or base are not valid URLs. Note that an effort will be made to coerce the given values into strings. For instance:

const myURL = new URL({ toString: () => 'https://example.org/' });
// https://example.org/

Unicode characters appearing within the host name of input will be automatically converted to ASCII using the Punycode algorithm.

const myURL = new URL('https://測試');
// https://xn--g6w251d/

This feature is only available if the node executable was compiled with ICU enabled. If not, the domain names are passed through unchanged.

In cases where it is not known in advance if input is an absolute URL and a base is provided, it is advised to validate that the origin of the URL object is what is expected.

let myURL = new URL('http://Example.com/', 'https://example.org/');
// http://example.com/

myURL = new URL('https://Example.com/', 'https://example.org/');
// https://example.com/

myURL = new URL('foo://Example.com/', 'https://example.org/');
// foo://Example.com/

myURL = new URL('http:Example.com/', 'https://example.org/');
// http://example.com/

myURL = new URL('https:Example.com/', 'https://example.org/');
// https://example.org/Example.com/

myURL = new URL('foo:Example.com/', 'https://example.org/');
// foo:Example.com/

url.hash#

Gets and sets the fragment portion of the URL.

const myURL = new URL('https://example.org/foo#bar');
console.log(myURL.hash);
// Prints #bar

myURL.hash = 'baz';
console.log(myURL.href);
// Prints https://example.org/foo#baz

Invalid URL characters included in the value assigned to the hash property are percent-encoded. The selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.host#

Gets and sets the host portion of the URL.

const myURL = new URL('https://example.org:81/foo');
console.log(myURL.host);
// Prints example.org:81

myURL.host = 'example.com:82';
console.log(myURL.href);
// Prints https://example.com:82/foo

Invalid host values assigned to the host property are ignored.

url.hostname#

Gets and sets the host name portion of the URL. The key difference between url.host and url.hostname is that url.hostname does not include the port.

const myURL = new URL('https://example.org:81/foo');
console.log(myURL.hostname);
// Prints example.org

myURL.hostname = 'example.com:82';
console.log(myURL.href);
// Prints https://example.com:81/foo

Invalid host name values assigned to the hostname property are ignored.

url.href#

Gets and sets the serialized URL.

const myURL = new URL('https://example.org/foo');
console.log(myURL.href);
// Prints https://example.org/foo

myURL.href = 'https://example.com/bar';
console.log(myURL.href);
// Prints https://example.com/bar

Getting the value of the href property is equivalent to calling url.toString().

Setting the value of this property to a new value is equivalent to creating a new URL object using new URL(value). Each of the URL object's properties will be modified.

If the value assigned to the href property is not a valid URL, a TypeError will be thrown.

url.origin#

Gets the read-only serialization of the URL's origin.

const myURL = new URL('https://example.org/foo/bar?baz');
console.log(myURL.origin);
// Prints https://example.org

const idnURL = new URL('https://測試');
console.log(idnURL.origin);
// Prints https://xn--g6w251d

console.log(idnURL.hostname);
// Prints xn--g6w251d

url.password#

Gets and sets the password portion of the URL.

const myURL = new URL('https://abc:xyz@example.com');
console.log(myURL.password);
// Prints xyz

myURL.password = '123';
console.log(myURL.href);
// Prints https://abc:123@example.com

Invalid URL characters included in the value assigned to the password property are percent-encoded. The selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.pathname#

Gets and sets the path portion of the URL.

const myURL = new URL('https://example.org/abc/xyz?123');
console.log(myURL.pathname);
// Prints /abc/xyz

myURL.pathname = '/abcdef';
console.log(myURL.href);
// Prints https://example.org/abcdef?123

Invalid URL characters included in the value assigned to the pathname property are percent-encoded. The selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.port#

Gets and sets the port portion of the URL.

The port value may be a number or a string containing a number in the range 0 to 65535 (inclusive). Setting the value to the default port of the URL objects given protocol will result in the port value becoming the empty string ('').

The port value can be an empty string in which case the port depends on the protocol/scheme:

protocolport
"ftp"21
"file"
"gopher"70
"http"80
"https"443
"ws"80
"wss"443

Upon assigning a value to the port, the value will first be converted to a string using .toString().

If that string is invalid but it begins with a number, the leading number is assigned to port. If the number lies outside the range denoted above, it is ignored.

const myURL = new URL('https://example.org:8888');
console.log(myURL.port);
// Prints 8888

// Default ports are automatically transformed to the empty string
// (HTTPS protocol's default port is 443)
myURL.port = '443';
console.log(myURL.port);
// Prints the empty string
console.log(myURL.href);
// Prints https://example.org/

myURL.port = 1234;
console.log(myURL.port);
// Prints 1234
console.log(myURL.href);
// Prints https://example.org:1234/

// Completely invalid port strings are ignored
myURL.port = 'abcd';
console.log(myURL.port);
// Prints 1234

// Leading numbers are treated as a port number
myURL.port = '5678abcd';
console.log(myURL.port);
// Prints 5678

// Non-integers are truncated
myURL.port = 1234.5678;
console.log(myURL.port);
// Prints 1234

// Out-of-range numbers which are not represented in scientific notation
// will be ignored.
myURL.port = 1e10; // 10000000000, will be range-checked as described below
console.log(myURL.port);
// Prints 1234

Numbers which contain a decimal point, such as floating-point numbers or numbers in scientific notation, are not an exception to this rule. Leading numbers up to the decimal point will be set as the URL's port, assuming they are valid:

myURL.port = 4.567e21;
console.log(myURL.port);
// Prints 4 (because it is the leading number in the string '4.567e21')

url.protocol#

Gets and sets the protocol portion of the URL.

const myURL = new URL('https://example.org');
console.log(myURL.protocol);
// Prints https:

myURL.protocol = 'ftp';
console.log(myURL.href);
// Prints ftp://example.org/

Invalid URL protocol values assigned to the protocol property are ignored.

Special Schemes#

The WHATWG URL Standard considers a handful of URL protocol schemes to be special in terms of how they are parsed and serialized. When a URL is parsed using one of these special protocols, the url.protocol property may be changed to another special protocol but cannot be changed to a non-special protocol, and vice versa.

For instance, changing from http to https works:

const u = new URL('http://example.org');
u.protocol = 'https';
console.log(u.href);
// https://example.org

However, changing from http to a hypothetical fish protocol does not because the new protocol is not special.

const u = new URL('http://example.org');
u.protocol = 'fish';
console.log(u.href);
// http://example.org

Likewise, changing from a non-special protocol to a special protocol is also not permitted:

const u = new URL('fish://example.org');
u.protocol = 'http';
console.log(u.href);
// fish://example.org

According to the WHATWG URL Standard, special protocol schemes are ftp, file, gopher, http, https, ws, and wss.

url.search#

Gets and sets the serialized query portion of the URL.

const myURL = new URL('https://example.org/abc?123');
console.log(myURL.search);
// Prints ?123

myURL.search = 'abc=xyz';
console.log(myURL.href);
// Prints https://example.org/abc?abc=xyz

Any invalid URL characters appearing in the value assigned the search property will be percent-encoded. The selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.searchParams#

Gets the URLSearchParams object representing the query parameters of the URL. This property is read-only but the URLSearchParams object it provides can be used to mutate the URL instance; to replace the entirety of query parameters of the URL, use the url.search setter. See URLSearchParams documentation for details.

Use care when using .searchParams to modify the URL because, per the WHATWG specification, the URLSearchParams object uses different rules to determine which characters to percent-encode. For instance, the URL object will not percent encode the ASCII tilde (~) character, while URLSearchParams will always encode it:

const myUrl = new URL('https://example.org/abc?foo=~bar');

console.log(myUrl.search);  // prints ?foo=~bar

// Modify the URL via searchParams...
myUrl.searchParams.sort();

console.log(myUrl.search);  // prints ?foo=%7Ebar

url.username#

Gets and sets the username portion of the URL.

const myURL = new URL('https://abc:xyz@example.com');
console.log(myURL.username);
// Prints abc

myURL.username = '123';
console.log(myURL.href);
// Prints https://123:xyz@example.com/

Any invalid URL characters appearing in the value assigned the username property will be percent-encoded. The selection of which characters to percent-encode may vary somewhat from what the url.parse() and url.format() methods would produce.

url.toString()#

The toString() method on the URL object returns the serialized URL. The value returned is equivalent to that of url.href and url.toJSON().

Because of the need for standard compliance, this method does not allow users to customize the serialization process of the URL. For more flexibility, require('url').format() method might be of interest.

url.toJSON()#

The toJSON() method on the URL object returns the serialized URL. The value returned is equivalent to that of url.href and url.toString().

This method is automatically called when an URL object is serialized with JSON.stringify().

const myURLs = [
  new URL('https://www.example.com'),
  new URL('https://test.example.org')
];
console.log(JSON.stringify(myURLs));
// Prints ["https://www.example.com/","https://test.example.org/"]

Class: URLSearchParams#

History
VersionChanges
v10.0.0

The class is now available on the global object.

v7.5.0, v6.13.0

Added in: v7.5.0, v6.13.0

The URLSearchParams API provides read and write access to the query of a URL. The URLSearchParams class can also be used standalone with one of the four following constructors. The URLSearchParams class is also available on the global object.

The WHATWG URLSearchParams interface and the querystring module have similar purpose, but the purpose of the querystring module is more general, as it allows the customization of delimiter characters (& and =). On the other hand, this API is designed purely for URL query strings.

const myURL = new URL('https://example.org/?abc=123');
console.log(myURL.searchParams.get('abc'));
// Prints 123

myURL.searchParams.append('abc', 'xyz');
console.log(myURL.href);
// Prints https://example.org/?abc=123&abc=xyz

myURL.searchParams.delete('abc');
myURL.searchParams.set('a', 'b');
console.log(myURL.href);
// Prints https://example.org/?a=b

const newSearchParams = new URLSearchParams(myURL.searchParams);
// The above is equivalent to
// const newSearchParams = new URLSearchParams(myURL.search);

newSearchParams.append('a', 'c');
console.log(myURL.href);
// Prints https://example.org/?a=b
console.log(newSearchParams.toString());
// Prints a=b&a=c

// newSearchParams.toString() is implicitly called
myURL.search = newSearchParams;
console.log(myURL.href);
// Prints https://example.org/?a=b&a=c
newSearchParams.delete('a');
console.log(myURL.href);
// Prints https://example.org/?a=b&a=c

Constructor: new URLSearchParams()#

Instantiate a new empty URLSearchParams object.

Constructor: new URLSearchParams(string)#

Parse the string as a query string, and use it to instantiate a new URLSearchParams object. A leading '?', if present, is ignored.

let params;

params = new URLSearchParams('user=abc&query=xyz');
console.log(params.get('user'));
// Prints 'abc'
console.log(params.toString());
// Prints 'user=abc&query=xyz'

params = new URLSearchParams('?user=abc&query=xyz');
console.log(params.toString());
// Prints 'user=abc&query=xyz'

Constructor: new URLSearchParams(obj)#

Added in: v7.10.0, v6.13.0
  • obj <Object> An object representing a collection of key-value pairs

Instantiate a new URLSearchParams object with a query hash map. The key and value of each property of obj are always coerced to strings.

Unlike querystring module, duplicate keys in the form of array values are not allowed. Arrays are stringified using array.toString(), which simply joins all array elements with commas.

const params = new URLSearchParams({
  user: 'abc',
  query: ['first', 'second']
});
console.log(params.getAll('query'));
// Prints [ 'first,second' ]
console.log(params.toString());
// Prints 'user=abc&query=first%2Csecond'

Constructor: new URLSearchParams(iterable)#

Added in: v7.10.0, v6.13.0
  • iterable <Iterable> An iterable object whose elements are key-value pairs

Instantiate a new URLSearchParams object with an iterable map in a way that is similar to Map's constructor. iterable can be an Array or any iterable object. That means iterable can be another URLSearchParams, in which case the constructor will simply create a clone of the provided URLSearchParams. Elements of iterable are key-value pairs, and can themselves be any iterable object.

Duplicate keys are allowed.

let params;

// Using an array
params = new URLSearchParams([
  ['user', 'abc'],
  ['query', 'first'],
  ['query', 'second']
]);
console.log(params.toString());
// Prints 'user=abc&query=first&query=second'

// Using a Map object
const map = new Map();
map.set('user', 'abc');
map.set('query', 'xyz');
params = new URLSearchParams(map);
console.log(params.toString());
// Prints 'user=abc&query=xyz'

// Using a generator function
function* getQueryPairs() {
  yield ['user', 'abc'];
  yield ['query', 'first'];
  yield ['query', 'second'];
}
params = new URLSearchParams(getQueryPairs());
console.log(params.toString());
// Prints 'user=abc&query=first&query=second'

// Each key-value pair must have exactly two elements
new URLSearchParams([
  ['user', 'abc', 'error']
]);
// Throws TypeError [ERR_INVALID_TUPLE]:
//        Each query pair must be an iterable [name, value] tuple

urlSearchParams.append(name, value)#

Append a new name-value pair to the query string.

urlSearchParams.delete(name)#

Remove all name-value pairs whose name is name.

urlSearchParams.entries()#

Returns an ES6 Iterator over each of the name-value pairs in the query. Each item of the iterator is a JavaScript Array. The first item of the Array is the name, the second item of the Array is the value.

Alias for urlSearchParams[@@iterator]().

urlSearchParams.forEach(fn[, thisArg])#

  • fn <Function> Invoked for each name-value pair in the query
  • thisArg <Object> To be used as this value for when fn is called

Iterates over each name-value pair in the query and invokes the given function.

const myURL = new URL('https://example.org/?a=b&c=d');
myURL.searchParams.forEach((value, name, searchParams) => {
  console.log(name, value, myURL.searchParams === searchParams);
});
// Prints:
//   a b true
//   c d true

urlSearchParams.get(name)#

  • name <string>
  • Returns: <string> or null if there is no name-value pair with the given name.

Returns the value of the first name-value pair whose name is name. If there are no such pairs, null is returned.

urlSearchParams.getAll(name)#

Returns the values of all name-value pairs whose name is name. If there are no such pairs, an empty array is returned.

urlSearchParams.has(name)#

Returns true if there is at least one name-value pair whose name is name.

urlSearchParams.keys()#

Returns an ES6 Iterator over the names of each name-value pair.

const params = new URLSearchParams('foo=bar&foo=baz');
for (const name of params.keys()) {
  console.log(name);
}
// Prints:
//   foo
//   foo

urlSearchParams.set(name, value)#

Sets the value in the URLSearchParams object associated with name to value. If there are any pre-existing name-value pairs whose names are name, set the first such pair's value to value and remove all others. If not, append the name-value pair to the query string.

const params = new URLSearchParams();
params.append('foo', 'bar');
params.append('foo', 'baz');
params.append('abc', 'def');
console.log(params.toString());
// Prints foo=bar&foo=baz&abc=def

params.set('foo', 'def');
params.set('xyz', 'opq');
console.log(params.toString());
// Prints foo=def&abc=def&xyz=opq

urlSearchParams.sort()#

Added in: v7.7.0, v6.13.0

Sort all existing name-value pairs in-place by their names. Sorting is done with a stable sorting algorithm, so relative order between name-value pairs with the same name is preserved.

This method can be used, in particular, to increase cache hits.

const params = new URLSearchParams('query[]=abc&type=search&query[]=123');
params.sort();
console.log(params.toString());
// Prints query%5B%5D=abc&query%5B%5D=123&type=search

urlSearchParams.toString()#

Returns the search parameters serialized as a string, with characters percent-encoded where necessary.

urlSearchParams.values()#

Returns an ES6 Iterator over the values of each name-value pair.

urlSearchParams[Symbol.iterator]()#

Returns an ES6 Iterator over each of the name-value pairs in the query string. Each item of the iterator is a JavaScript Array. The first item of the Array is the name, the second item of the Array is the value.

Alias for urlSearchParams.entries().

const params = new URLSearchParams('foo=bar&xyz=baz');
for (const [name, value] of params) {
  console.log(name, value);
}
// Prints:
//   foo bar
//   xyz baz

url.domainToASCII(domain)#

Added in: v7.4.0, v6.13.0

Returns the Punycode ASCII serialization of the domain. If domain is an invalid domain, the empty string is returned.

It performs the inverse operation to url.domainToUnicode().

const url = require('url');
console.log(url.domainToASCII('español.com'));
// Prints xn--espaol-zwa.com
console.log(url.domainToASCII('中文.com'));
// Prints xn--fiq228c.com
console.log(url.domainToASCII('xn--iñvalid.com'));
// Prints an empty string

url.domainToUnicode(domain)#

Added in: v7.4.0, v6.13.0

Returns the Unicode serialization of the domain. If domain is an invalid domain, the empty string is returned.

It performs the inverse operation to url.domainToASCII().

const url = require('url');
console.log(url.domainToUnicode('xn--espaol-zwa.com'));
// Prints español.com
console.log(url.domainToUnicode('xn--fiq228c.com'));
// Prints 中文.com
console.log(url.domainToUnicode('xn--iñvalid.com'));
// Prints an empty string

url.fileURLToPath(url)#

Added in: v10.12.0
  • url <URL> | <string> The file URL string or URL object to convert to a path.
  • Returns: <string> The fully-resolved platform-specific Node.js file path.

This function ensures the correct decodings of percent-encoded characters as well as ensuring a cross-platform valid absolute path string.

new URL('file:///C:/path/').pathname;    // Incorrect: /C:/path/
fileURLToPath('file:///C:/path/');       // Correct:   C:\path\ (Windows)

new URL('file://nas/foo.txt').pathname;  // Incorrect: /foo.txt
fileURLToPath('file://nas/foo.txt');     // Correct:   \\nas\foo.txt (Windows)

new URL('file:///你好.txt').pathname;    // Incorrect: /%E4%BD%A0%E5%A5%BD.txt
fileURLToPath('file:///你好.txt');       // Correct:   /你好.txt (POSIX)

new URL('file:///hello world').pathname; // Incorrect: /hello%20world
fileURLToPath('file:///hello world');    // Correct:   /hello world (POSIX)

url.format(URL[, options])#

Added in: v7.6.0
  • URL <URL> A WHATWG URL object

  • options <Object>

    • auth <boolean> true if the serialized URL string should include the username and password, false otherwise. Default: true.
    • fragment <boolean> true if the serialized URL string should include the fragment, false otherwise. Default: true.
    • search <boolean> true if the serialized URL string should include the search query, false otherwise. Default: true.
    • unicode <boolean> true if Unicode characters appearing in the host component of the URL string should be encoded directly as opposed to being Punycode encoded. Default: false.
  • Returns: <string>

Returns a customizable serialization of a URL String representation of a WHATWG URL object.

The URL object has both a toString() method and href property that return string serializations of the URL. These are not, however, customizable in any way. The url.format(URL[, options]) method allows for basic customization of the output.

const myURL = new URL('https://a:b@測試?abc#foo');

console.log(myURL.href);
// Prints https://a:b@xn--g6w251d/?abc#foo

console.log(myURL.toString());
// Prints https://a:b@xn--g6w251d/?abc#foo

console.log(url.format(myURL, { fragment: false, unicode: true, auth: false }));
// Prints 'https://測試/?abc'

url.pathToFileURL(path)#

Added in: v10.12.0
  • path <string> The path to convert to a File URL.
  • Returns: <URL> The file URL object.

This function ensures that path is resolved absolutely, and that the URL control characters are correctly encoded when converting into a File URL.

new URL(__filename);                // Incorrect: throws (POSIX)
new URL(__filename);                // Incorrect: C:\... (Windows)
pathToFileURL(__filename);          // Correct:   file:///... (POSIX)
pathToFileURL(__filename);          // Correct:   file:///C:/... (Windows)

new URL('/foo#1', 'file:');         // Incorrect: file:///foo#1
pathToFileURL('/foo#1');            // Correct:   file:///foo%231 (POSIX)

new URL('/some/path%.js', 'file:'); // Incorrect: file:///some/path%
pathToFileURL('/some/path%.js');    // Correct:   file:///some/path%25 (POSIX)

Legacy URL API#

Stability: 0 - Deprecated: Use the WHATWG URL API instead.

Legacy urlObject#

History
VersionChanges
v11.0.0

The Legacy URL API is deprecated. Use the WHATWG URL API.

The legacy urlObject (require('url').Url) is created and returned by the url.parse() function.

urlObject.auth#

The auth property is the username and password portion of the URL, also referred to as userinfo. This string subset follows the protocol and double slashes (if present) and precedes the host component, delimited by @. The string is either the username, or it is the username and password separated by :.

For example: 'user:pass'.

urlObject.hash#

The hash property is the fragment identifier portion of the URL including the leading # character.

For example: '#hash'.

urlObject.host#

The host property is the full lower-cased host portion of the URL, including the port if specified.

For example: 'sub.example.com:8080'.

urlObject.hostname#

The hostname property is the lower-cased host name portion of the host component without the port included.

For example: 'sub.example.com'.

urlObject.href#

The href property is the full URL string that was parsed with both the protocol and host components converted to lower-case.

For example: 'http://user:pass@sub.example.com:8080/p/a/t/h?query=string#hash'.

urlObject.path#

The path property is a concatenation of the pathname and search components.

For example: '/p/a/t/h?query=string'.

No decoding of the path is performed.

urlObject.pathname#

The pathname property consists of the entire path section of the URL. This is everything following the host (including the port) and before the start of the query or hash components, delimited by either the ASCII question mark (?) or hash (#) characters.

For example: '/p/a/t/h'.

No decoding of the path string is performed.

urlObject.port#

The port property is the numeric port portion of the host component.

For example: '8080'.

urlObject.protocol#

The protocol property identifies the URL's lower-cased protocol scheme.

For example: 'http:'.

urlObject.query#

The query property is either the query string without the leading ASCII question mark (?), or an object returned by the querystring module's parse() method. Whether the query property is a string or object is determined by the parseQueryString argument passed to url.parse().

For example: 'query=string' or {'query': 'string'}.

If returned as a string, no decoding of the query string is performed. If returned as an object, both keys and values are decoded.

urlObject.search#

The search property consists of the entire "query string" portion of the URL, including the leading ASCII question mark (?) character.

For example: '?query=string'.

No decoding of the query string is performed.

urlObject.slashes#

The slashes property is a boolean with a value of true if two ASCII forward-slash characters (/) are required following the colon in the protocol.

url.format(urlObject)#

History
VersionChanges
v11.0.0

The Legacy URL API is deprecated. Use the WHATWG URL API.

v7.0.0

URLs with a file: scheme will now always use the correct number of slashes regardless of slashes option. A false-y slashes option with no protocol is now also respected at all times.

v0.1.25

Added in: v0.1.25

  • urlObject <Object> | <string> A URL object (as returned by url.parse() or constructed otherwise). If a string, it is converted to an object by passing it to url.parse().

The url.format() method returns a formatted URL string derived from urlObject.

url.format({
  protocol: 'https',
  hostname: 'example.com',
  pathname: '/some/path',
  query: {
    page: 1,
    format: 'json'
  }
});

// => 'https://example.com/some/path?page=1&format=json'

If urlObject is not an object or a string, url.format() will throw a TypeError.

The formatting process operates as follows:

  • A new empty string result is created.
  • If urlObject.protocol is a string, it is appended as-is to result.
  • Otherwise, if urlObject.protocol is not undefined and is not a string, an Error is thrown.
  • For all string values of urlObject.protocol that do not end with an ASCII colon (:) character, the literal string : will be appended to result.

  • If either of the following conditions is true, then the literal string // will be appended to result:

    • urlObject.slashes property is true;
    • urlObject.protocol begins with http, https, ftp, gopher, or file;
  • If the value of the urlObject.auth property is truthy, and either urlObject.host or urlObject.hostname are not undefined, the value of urlObject.auth will be coerced into a string and appended to result followed by the literal string @.

  • If the urlObject.host property is undefined then:

    • If the urlObject.hostname is a string, it is appended to result.
    • Otherwise, if urlObject.hostname is not undefined and is not a string, an Error is thrown.

    • If the urlObject.port property value is truthy, and urlObject.hostname is not undefined:

      • The literal string : is appended to result, and
      • The value of urlObject.port is coerced to a string and appended to result.
  • Otherwise, if the urlObject.host property value is truthy, the value of urlObject.host is coerced to a string and appended to result.

  • If the urlObject.pathname property is a string that is not an empty string:

    • If the urlObject.pathname does not start with an ASCII forward slash (/), then the literal string '/' is appended to result.
    • The value of urlObject.pathname is appended to result.
  • Otherwise, if urlObject.pathname is not undefined and is not a string, an Error is thrown.
  • If the urlObject.search property is undefined and if the urlObject.query property is an Object, the literal string ? is appended to result followed by the output of calling the querystring module's stringify() method passing the value of urlObject.query.

  • Otherwise, if urlObject.search is a string:

    • If the value of urlObject.search does not start with the ASCII question mark (?) character, the literal string ? is appended to result.
    • The value of urlObject.search is appended to result.
  • Otherwise, if urlObject.search is not undefined and is not a string, an Error is thrown.

  • If the urlObject.hash property is a string:

    • If the value of urlObject.hash does not start with the ASCII hash (#) character, the literal string # is appended to result.
    • The value of urlObject.hash is appended to result.
  • Otherwise, if the urlObject.hash property is not undefined and is not a string, an Error is thrown.
  • result is returned.

url.parse(urlString[, parseQueryString[, slashesDenoteHost]])#

History
VersionChanges
v11.14.0

The pathname property on the returned URL object is now / when there is no path and the protocol scheme is ws: or wss:.

v11.0.0

The Legacy URL API is deprecated. Use the WHATWG URL API.

v9.0.0

The search property on the returned URL object is now null when no query string is present.

v0.1.25

Added in: v0.1.25

  • urlString <string> The URL string to parse.
  • parseQueryString <boolean> If true, the query property will always be set to an object returned by the querystring module's parse() method. If false, the query property on the returned URL object will be an unparsed, undecoded string. Default: false.
  • slashesDenoteHost <boolean> If true, the first token after the literal string // and preceding the next / will be interpreted as the host. For instance, given //foo/bar, the result would be {host: 'foo', pathname: '/bar'} rather than {pathname: '//foo/bar'}. Default: false.

The url.parse() method takes a URL string, parses it, and returns a URL object.

A TypeError is thrown if urlString is not a string.

A URIError is thrown if the auth property is present but cannot be decoded.

url.resolve(from, to)#

History
VersionChanges
v11.0.0

The Legacy URL API is deprecated. Use the WHATWG URL API.

v6.6.0

The auth fields are now kept intact when from and to refer to the same host.

v6.5.0, v4.6.2

The port field is copied correctly now.

v6.0.0

The auth fields is cleared now the to parameter contains a hostname.

v0.1.25

Added in: v0.1.25

  • from <string> The Base URL being resolved against.
  • to <string> The HREF URL being resolved.

The url.resolve() method resolves a target URL relative to a base URL in a manner similar to that of a Web browser resolving an anchor tag HREF.

const url = require('url');
url.resolve('/one/two/three', 'four');         // '/one/two/four'
url.resolve('http://example.com/', '/one');    // 'http://example.com/one'
url.resolve('http://example.com/one', '/two'); // 'http://example.com/two'

Percent-Encoding in URLs#

URLs are permitted to only contain a certain range of characters. Any character falling outside of that range must be encoded. How such characters are encoded, and which characters to encode depends entirely on where the character is located within the structure of the URL.

Legacy API#

Within the Legacy API, spaces (' ') and the following characters will be automatically escaped in the properties of URL objects:

< > " ` \r \n \t { } | \ ^ '

For example, the ASCII space character (' ') is encoded as %20. The ASCII forward slash (/) character is encoded as %3C.

WHATWG API#

The WHATWG URL Standard uses a more selective and fine grained approach to selecting encoded characters than that used by the Legacy API.

The WHATWG algorithm defines four "percent-encode sets" that describe ranges of characters that must be percent-encoded:

  • The C0 control percent-encode set includes code points in range U+0000 to U+001F (inclusive) and all code points greater than U+007E.

  • The fragment percent-encode set includes the C0 control percent-encode set and code points U+0020, U+0022, U+003C, U+003E, and U+0060.

  • The path percent-encode set includes the C0 control percent-encode set and code points U+0020, U+0022, U+0023, U+003C, U+003E, U+003F, U+0060, U+007B, and U+007D.

  • The userinfo encode set includes the path percent-encode set and code points U+002F, U+003A, U+003B, U+003D, U+0040, U+005B, U+005C, U+005D, U+005E, and U+007C.

The userinfo percent-encode set is used exclusively for username and passwords encoded within the URL. The path percent-encode set is used for the path of most URLs. The fragment percent-encode set is used for URL fragments. The C0 control percent-encode set is used for host and path under certain specific conditions, in addition to all other cases.

When non-ASCII characters appear within a host name, the host name is encoded using the Punycode algorithm. Note, however, that a host name may contain both Punycode encoded and percent-encoded characters:

const myURL = new URL('https://%CF%80.example.com/foo');
console.log(myURL.href);
// Prints https://xn--1xa.example.com/foo
console.log(myURL.origin);
// Prints https://xn--1xa.example.com

Util#

Stability: 2 - Stable

The util module supports the needs of Node.js internal APIs. Many of the utilities are useful for application and module developers as well. To access it:

const util = require('util');

util.callbackify(original)#

Added in: v8.2.0

Takes an async function (or a function that returns a Promise) and returns a function following the error-first callback style, i.e. taking an (err, value) => ... callback as the last argument. In the callback, the first argument will be the rejection reason (or null if the Promise resolved), and the second argument will be the resolved value.

const util = require('util');

async function fn() {
  return 'hello world';
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
  if (err) throw err;
  console.log(ret);
});

Will print:

hello world

The callback is executed asynchronously, and will have a limited stack trace. If the callback throws, the process will emit an 'uncaughtException' event, and if not handled will exit.

Since null has a special meaning as the first argument to a callback, if a wrapped function rejects a Promise with a falsy value as a reason, the value is wrapped in an Error with the original value stored in a field named reason.

function fn() {
  return Promise.reject(null);
}
const callbackFunction = util.callbackify(fn);

callbackFunction((err, ret) => {
  // When the Promise was rejected with `null` it is wrapped with an Error and
  // the original value is stored in `reason`.
  err && err.hasOwnProperty('reason') && err.reason === null;  // true
});

util.debuglog(section)#

Added in: v0.11.3
  • section <string> A string identifying the portion of the application for which the debuglog function is being created.
  • Returns: <Function> The logging function

The util.debuglog() method is used to create a function that conditionally writes debug messages to stderr based on the existence of the NODE_DEBUG environment variable. If the section name appears within the value of that environment variable, then the returned function operates similar to console.error(). If not, then the returned function is a no-op.

const util = require('util');
const debuglog = util.debuglog('foo');

debuglog('hello from foo [%d]', 123);

If this program is run with NODE_DEBUG=foo in the environment, then it will output something like:

FOO 3245: hello from foo [123]

where 3245 is the process id. If it is not run with that environment variable set, then it will not print anything.

The section supports wildcard also:

const util = require('util');
const debuglog = util.debuglog('foo-bar');

debuglog('hi there, it\'s foo-bar [%d]', 2333);

if it is run with NODE_DEBUG=foo* in the environment, then it will output something like:

FOO-BAR 3257: hi there, it's foo-bar [2333]

Multiple comma-separated section names may be specified in the NODE_DEBUG environment variable: NODE_DEBUG=fs,net,tls.

util.deprecate(fn, msg[, code])#

History
VersionChanges
v10.0.0

Deprecation warnings are only emitted once for each code.

v0.8.0

Added in: v0.8.0

  • fn <Function> The function that is being deprecated.
  • msg <string> A warning message to display when the deprecated function is invoked.
  • code <string> A deprecation code. See the list of deprecated APIs for a list of codes.
  • Returns: <Function> The deprecated function wrapped to emit a warning.

The util.deprecate() method wraps fn (which may be a function or class) in such a way that it is marked as deprecated.

const util = require('util');

exports.obsoleteFunction = util.deprecate(() => {
  // Do something here.
}, 'obsoleteFunction() is deprecated. Use newShinyFunction() instead.');

When called, util.deprecate() will return a function that will emit a DeprecationWarning using the 'warning' event. The warning will be emitted and printed to stderr the first time the returned function is called. After the warning is emitted, the wrapped function is called without emitting a warning.

If the same optional code is supplied in multiple calls to util.deprecate(), the warning will be emitted only once for that code.

const util = require('util');

const fn1 = util.deprecate(someFunction, someMessage, 'DEP0001');
const fn2 = util.deprecate(someOtherFunction, someOtherMessage, 'DEP0001');
fn1(); // Emits a deprecation warning with code DEP0001
fn2(); // Does not emit a deprecation warning because it has the same code

If either the --no-deprecation or --no-warnings command line flags are used, or if the process.noDeprecation property is set to true prior to the first deprecation warning, the util.deprecate() method does nothing.

If the --trace-deprecation or --trace-warnings command line flags are set, or the process.traceDeprecation property is set to true, a warning and a stack trace are printed to stderr the first time the deprecated function is called.

If the --throw-deprecation command line flag is set, or the process.throwDeprecation property is set to true, then an exception will be thrown when the deprecated function is called.

The --throw-deprecation command line flag and process.throwDeprecation property take precedence over --trace-deprecation and process.traceDeprecation.

util.format(format[, ...args])#

History
VersionChanges
v12.11.0

The %c specifier is ignored now.

v12.0.0

The format argument is now only taken as such if it actually contains format specifiers.

v12.0.0

If the format argument is not a format string, the output string's formatting is no longer dependent on the type of the first argument. This change removes previously present quotes from strings that were being output when the first argument was not a string.

v11.4.0

The %d, %f and %i specifiers now support Symbols properly.

v11.4.0

The %o specifier's depth has default depth of 4 again.

v11.0.0

The %o specifier's depth option will now fall back to the default depth.

v10.12.0

The %d and %i specifiers now support BigInt.

v8.4.0

The %o and %O specifiers are supported now.

v0.5.3

Added in: v0.5.3

  • format <string> A printf-like format string.

The util.format() method returns a formatted string using the first argument as a printf-like format string which can contain zero or more format specifiers. Each specifier is replaced with the converted value from the corresponding argument. Supported specifiers are:

  • %s: String will be used to convert all values except BigInt, Object and -0. BigInt values will be represented with an n and Objects that have no user defined toString function are inspected using util.inspect() with options { depth: 0, colors: false, compact: 3 }.
  • %d: Number will be used to convert all values except BigInt and Symbol.
  • %i: parseInt(value, 10) is used for all values except BigInt and Symbol.
  • %f: parseFloat(value) is used for all values expect Symbol.
  • %j: JSON. Replaced with the string '[Circular]' if the argument contains circular references.
  • %o: Object. A string representation of an object with generic JavaScript object formatting. Similar to util.inspect() with options { showHidden: true, showProxy: true }. This will show the full object including non-enumerable properties and proxies.
  • %O: Object. A string representation of an object with generic JavaScript object formatting. Similar to util.inspect() without options. This will show the full object not including non-enumerable properties and proxies.
  • %c: CSS. This specifier is currently ignored, and will skip any CSS passed in.
  • %%: single percent sign ('%'). This does not consume an argument.
  • Returns: <string> The formatted string

If a specifier does not have a corresponding argument, it is not replaced:

util.format('%s:%s', 'foo');
// Returns: 'foo:%s'

Values that are not part of the format string are formatted using util.inspect() if their type is not string.

If there are more arguments passed to the util.format() method than the number of specifiers, the extra arguments are concatenated to the returned string, separated by spaces:

util.format('%s:%s', 'foo', 'bar', 'baz');
// Returns: 'foo:bar baz'

If the first argument does not contain a valid format specifier, util.format() returns a string that is the concatenation of all arguments separated by spaces:

util.format(1, 2, 3);
// Returns: '1 2 3'

If only one argument is passed to util.format(), it is returned as it is without any formatting:

util.format('%% %s');
// Returns: '%% %s'

util.format() is a synchronous method that is intended as a debugging tool. Some input values can have a significant performance overhead that can block the event loop. Use this function with care and never in a hot code path.

util.formatWithOptions(inspectOptions, format[, ...args])#

Added in: v10.0.0

This function is identical to util.format(), except in that it takes an inspectOptions argument which specifies options that are passed along to util.inspect().

util.formatWithOptions({ colors: true }, 'See object %O', { foo: 42 });
// Returns 'See object { foo: 42 }', where `42` is colored as a number
// when printed to a terminal.

util.getSystemErrorName(err)#

Added in: v9.7.0

Returns the string name for a numeric error code that comes from a Node.js API. The mapping between error codes and error names is platform-dependent. See Common System Errors for the names of common errors.

fs.access('file/that/does/not/exist', (err) => {
  const name = util.getSystemErrorName(err.errno);
  console.error(name);  // ENOENT
});

util.inherits(constructor, superConstructor)#

History
VersionChanges
v5.0.0

The constructor parameter can refer to an ES6 class now.

v0.3.0

Added in: v0.3.0

Usage of util.inherits() is discouraged. Please use the ES6 class and extends keywords to get language level inheritance support. Also note that the two styles are semantically incompatible.

Inherit the prototype methods from one constructor into another. The prototype of constructor will be set to a new object created from superConstructor.

This mainly adds some input validation on top of Object.setPrototypeOf(constructor.prototype, superConstructor.prototype). As an additional convenience, superConstructor will be accessible through the constructor.super_ property.

const util = require('util');
const EventEmitter = require('events');

function MyStream() {
  EventEmitter.call(this);
}

util.inherits(MyStream, EventEmitter);

MyStream.prototype.write = function(data) {
  this.emit('data', data);
};

const stream = new MyStream();

console.log(stream instanceof EventEmitter); // true
console.log(MyStream.super_ === EventEmitter); // true

stream.on('data', (data) => {
  console.log(`Received data: "${data}"`);
});
stream.write('It works!'); // Received data: "It works!"

ES6 example using class and extends:

const EventEmitter = require('events');

class MyStream extends EventEmitter {
  write(data) {
    this.emit('data', data);
  }
}

const stream = new MyStream();

stream.on('data', (data) => {
  console.log(`Received data: "${data}"`);
});
stream.write('With ES6');

util.inspect(object[, options])#

util.inspect(object[, showHidden[, depth[, colors]]])#

History
VersionChanges
v14.0.0

The maxStringLength option is supported now.

v13.0.0

Circular references now include a marker to the reference.

v13.5.0, v12.16.0

User defined prototype properties are inspected in case showHidden is true.

v12.0.0

The compact options default is changed to 3 and the breakLength options default is changed to 80.

v12.0.0

Internal properties no longer appear in the context argument of a custom inspection function.

v11.11.0

The compact option accepts numbers for a new output mode.

v11.7.0

ArrayBuffers now also show their binary contents.

v11.5.0

The getters option is supported now.

v11.4.0

The depth default changed back to 2.

v11.0.0

The depth default changed to 20.

v11.0.0

The inspection output is now limited to about 128 MB. Data above that size will not be fully inspected.

v10.12.0

The sorted option is supported now.

v10.6.0

Inspecting linked lists and similar objects is now possible up to the maximum call stack size.

v10.0.0

The WeakMap and WeakSet entries can now be inspected as well.

v9.9.0

The compact option is supported now.

v6.6.0

Custom inspection functions can now return this.

v6.3.0

The breakLength option is supported now.

v6.1.0

The maxArrayLength option is supported now; in particular, long arrays are truncated by default.

v6.1.0

The showProxy option is supported now.

v0.3.0

Added in: v0.3.0

  • object <any> Any JavaScript primitive or Object.

  • options <Object>

    • showHidden <boolean> If true, object's non-enumerable symbols and properties are included in the formatted result. WeakMap and WeakSet entries are also included as well as user defined prototype properties (excluding method properties). Default: false.
    • depth <number> Specifies the number of times to recurse while formatting object. This is useful for inspecting large objects. To recurse up to the maximum call stack size pass Infinity or null. Default: 2.
    • colors <boolean> If true, the output is styled with ANSI color codes. Colors are customizable. See Customizing util.inspect colors. Default: false.
    • customInspect <boolean> If false, [util.inspect.custom](depth, opts) functions are not invoked. Default: true.
    • showProxy <boolean> If true, Proxy inspection includes the target and handler objects. Default: false.
    • maxArrayLength <integer> Specifies the maximum number of Array, TypedArray, WeakMap and WeakSet elements to include when formatting. Set to null or Infinity to show all elements. Set to 0 or negative to show no elements. Default: 100.
    • maxStringLength <integer> Specifies the maximum number of characters to include when formatting. Set to null or Infinity to show all elements. Set to 0 or negative to show no characters. Default: Infinity.
    • breakLength <integer> The length at which input values are split across multiple lines. Set to Infinity to format the input as a single line (in combination with compact set to true or any number >= 1). Default: 80.
    • compact <boolean> | <integer> Setting this to false causes each object key to be displayed on a new line. It will also add new lines to text that is longer than breakLength. If set to a number, the most n inner elements are united on a single line as long as all properties fit into breakLength. Short array elements are also grouped together. No text will be reduced below 16 characters, no matter the breakLength size. For more information, see the example below. Default: 3.
    • sorted <boolean> | <Function> If set to true or a function, all properties of an object, and Set and Map entries are sorted in the resulting string. If set to true the default sort is used. If set to a function, it is used as a compare function.
    • getters <boolean> | <string> If set to true, getters are inspected. If set to 'get', only getters without a corresponding setter are inspected. If set to 'set', only getters with a corresponding setter are inspected. This might cause side effects depending on the getter function. Default: false.
  • Returns: <string> The representation of object.

The util.inspect() method returns a string representation of object that is intended for debugging. The output of util.inspect may change at any time and should not be depended upon programmatically. Additional options may be passed that alter the result. util.inspect() will use the constructor's name and/or @@toStringTag to make an identifiable tag for an inspected value.

class Foo {
  get [Symbol.toStringTag]() {
    return 'bar';
  }
}

class Bar {}

const baz = Object.create(null, { [Symbol.toStringTag]: { value: 'foo' } });

util.inspect(new Foo()); // 'Foo [bar] {}'
util.inspect(new Bar()); // 'Bar {}'
util.inspect(baz);       // '[foo] {}'

Circular references point to their anchor by using a reference index:

const { inspect } = require('util');

const obj = {};
obj.a = [obj];
obj.b = {};
obj.b.inner = obj.b;
obj.b.obj = obj;

console.log(inspect(obj));
// <ref *1> {
//   a: [ [Circular *1] ],
//   b: <ref *2> { inner: [Circular *2], obj: [Circular *1] }
// }

The following example inspects all properties of the util object:

const util = require('util');

console.log(util.inspect(util, { showHidden: true, depth: null }));

The following example highlights the effect of the compact option:

const util = require('util');

const o = {
  a: [1, 2, [[
    'Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do ' +
      'eiusmod tempor incididunt ut labore et dolore magna aliqua.',
    'test',
    'foo']], 4],
  b: new Map([['za', 1], ['zb', 'test']])
};
console.log(util.inspect(o, { compact: true, depth: 5, breakLength: 80 }));

// { a:
//   [ 1,
//     2,
//     [ [ 'Lorem ipsum dolor sit amet, consectetur [...]', // A long line
//           'test',
//           'foo' ] ],
//     4 ],
//   b: Map(2) { 'za' => 1, 'zb' => 'test' } }

// Setting `compact` to false changes the output to be more reader friendly.
console.log(util.inspect(o, { compact: false, depth: 5, breakLength: 80 }));

// {
//   a: [
//     1,
//     2,
//     [
//       [
//         'Lorem ipsum dolor sit amet, consectetur ' +
//           'adipiscing elit, sed do eiusmod tempor ' +
//           'incididunt ut labore et dolore magna ' +
//           'aliqua.,
//         'test',
//         'foo'
//       ]
//     ],
//     4
//   ],
//   b: Map(2) {
//     'za' => 1,
//     'zb' => 'test'
//   }
// }

// Setting `breakLength` to e.g. 150 will print the "Lorem ipsum" text in a
// single line.
// Reducing the `breakLength` will split the "Lorem ipsum" text in smaller
// chunks.

The showHidden option allows WeakMap and WeakSet entries to be inspected. If there are more entries than maxArrayLength, there is no guarantee which entries are displayed. That means retrieving the same WeakSet entries twice may result in different output. Furthermore, entries with no remaining strong references may be garbage collected at any time.

const { inspect } = require('util');

const obj = { a: 1 };
const obj2 = { b: 2 };
const weakSet = new WeakSet([obj, obj2]);

console.log(inspect(weakSet, { showHidden: true }));
// WeakSet { { a: 1 }, { b: 2 } }

The sorted option ensures that an object's property insertion order does not impact the result of util.inspect().

const { inspect } = require('util');
const assert = require('assert');

const o1 = {
  b: [2, 3, 1],
  a: '`a` comes before `b`',
  c: new Set([2, 3, 1])
};
console.log(inspect(o1, { sorted: true }));
// { a: '`a` comes before `b`', b: [ 2, 3, 1 ], c: Set(3) { 1, 2, 3 } }
console.log(inspect(o1, { sorted: (a, b) => b.localeCompare(a) }));
// { c: Set(3) { 3, 2, 1 }, b: [ 2, 3, 1 ], a: '`a` comes before `b`' }

const o2 = {
  c: new Set([2, 1, 3]),
  a: '`a` comes before `b`',
  b: [2, 3, 1]
};
assert.strict.equal(
  inspect(o1, { sorted: true }),
  inspect(o2, { sorted: true })
);

util.inspect() is a synchronous method intended for debugging. Its maximum output length is approximately 128 MB. Inputs that result in longer output will be truncated.

Customizing util.inspect colors#

Color output (if enabled) of util.inspect is customizable globally via the util.inspect.styles and util.inspect.colors properties.

util.inspect.styles is a map associating a style name to a color from util.inspect.colors.

The default styles and associated colors are:

  • bigint: yellow
  • boolean: yellow
  • date: magenta
  • module: underline
  • name: (no styling)
  • null: bold
  • number: yellow
  • regexp: red
  • special: cyan (e.g., Proxies)
  • string: green
  • symbol: green
  • undefined: grey

Color styling uses ANSI control codes that may not be supported on all terminals. To verify color support use tty.hasColors().

Predefined control codes are listed below (grouped as "Modifiers", "Foreground colors", and "Background colors").

Modifiers#

Modifier support varies throughout different terminals. They will mostly be ignored, if not supported.

  • reset - Resets all (color) modifiers to their defaults
  • bold - Make text bold
  • italic - Make text italic
  • underline - Make text underlined
  • strikethrough - Puts a horizontal line through the center of the text (Alias: strikeThrough, crossedout, crossedOut)
  • hidden - Prints the text, but makes it invisible (Alias: conceal)
  • dim - Decreased color intensity (Alias: faint)
  • overlined - Make text overlined
  • blink - Hides and shows the text in an interval
  • inverse - Swap foreground and background colors (Alias: swapcolors, swapColors)
  • doubleunderline - Make text double underlined (Alias: doubleUnderline)
  • framed - Draw a frame around the text

Foreground colors#

  • black
  • red
  • green
  • yellow
  • blue
  • magenta
  • cyan
  • white
  • gray (alias: grey, blackBright)
  • redBright
  • greenBright
  • yellowBright
  • blueBright
  • magentaBright
  • cyanBright
  • whiteBright

Background colors#

  • bgBlack
  • bgRed
  • bgGreen
  • bgYellow
  • bgBlue
  • bgMagenta
  • bgCyan
  • bgWhite
  • bgGray (alias: bgGrey, bgBlackBright)
  • bgRedBright
  • bgGreenBright
  • bgYellowBright
  • bgBlueBright
  • bgMagentaBright
  • bgCyanBright
  • bgWhiteBright

Custom inspection functions on Objects#

Objects may also define their own [util.inspect.custom](depth, opts) function, which util.inspect() will invoke and use the result of when inspecting the object:

const util = require('util');

class Box {
  constructor(value) {
    this.value = value;
  }

  [util.inspect.custom](depth, options) {
    if (depth < 0) {
      return options.stylize('[Box]', 'special');
    }

    const newOptions = Object.assign({}, options, {
      depth: options.depth === null ? null : options.depth - 1
    });

    // Five space padding because that's the size of "Box< ".
    const padding = ' '.repeat(5);
    const inner = util.inspect(this.value, newOptions)
                      .replace(/\n/g, `\n${padding}`);
    return `${options.stylize('Box', 'special')}< ${inner} >`;
  }
}

const box = new Box(true);

util.inspect(box);
// Returns: "Box< true >"

Custom [util.inspect.custom](depth, opts) functions typically return a string but may return a value of any type that will be formatted accordingly by util.inspect().

const util = require('util');

const obj = { foo: 'this will not show up in the inspect() output' };
obj[util.inspect.custom] = (depth) => {
  return { bar: 'baz' };
};

util.inspect(obj);
// Returns: "{ bar: 'baz' }"

util.inspect.custom#

History
VersionChanges
v10.12.0

This is now defined as a shared symbol.

v6.6.0

Added in: v6.6.0

  • <symbol> that can be used to declare custom inspect functions.

In addition to being accessible through util.inspect.custom, this symbol is registered globally and can be accessed in any environment as Symbol.for('nodejs.util.inspect.custom').

const inspect = Symbol.for('nodejs.util.inspect.custom');

class Password {
  constructor(value) {
    this.value = value;
  }

  toString() {
    return 'xxxxxxxx';
  }

  [inspect]() {
    return `Password <${this.toString()}>`;
  }
}

const password = new Password('r0sebud');
console.log(password);
// Prints Password <xxxxxxxx>

See Custom inspection functions on Objects for more details.

util.inspect.defaultOptions#

Added in: v6.4.0

The defaultOptions value allows customization of the default options used by util.inspect. This is useful for functions like console.log or util.format which implicitly call into util.inspect. It shall be set to an object containing one or more valid util.inspect() options. Setting option properties directly is also supported.

const util = require('util');
const arr = Array(101).fill(0);

console.log(arr); // Logs the truncated array
util.inspect.defaultOptions.maxArrayLength = null;
console.log(arr); // logs the full array

util.isDeepStrictEqual(val1, val2)#

Added in: v9.0.0

Returns true if there is deep strict equality between val1 and val2. Otherwise, returns false.

See assert.deepStrictEqual() for more information about deep strict equality.

util.promisify(original)#

Added in: v8.0.0

Takes a function following the common error-first callback style, i.e. taking an (err, value) => ... callback as the last argument, and returns a version that returns promises.

const util = require('util');
const fs = require('fs');

const stat = util.promisify(fs.stat);
stat('.').then((stats) => {
  // Do something with `stats`
}).catch((error) => {
  // Handle the error.
});

Or, equivalently using async functions:

const util = require('util');
const fs = require('fs');

const stat = util.promisify(fs.stat);

async function callStat() {
  const stats = await stat('.');
  console.log(`This directory is owned by ${stats.uid}`);
}

If there is an original[util.promisify.custom] property present, promisify will return its value, see Custom promisified functions.

promisify() assumes that original is a function taking a callback as its final argument in all cases. If original is not a function, promisify() will throw an error. If original is a function but its last argument is not an error-first callback, it will still be passed an error-first callback as its last argument.

Using promisify() on class methods or other methods that use this may not work as expected unless handled specially:

const util = require('util');

class Foo {
  constructor() {
    this.a = 42;
  }

  bar(callback) {
    callback(null, this.a);
  }
}

const foo = new Foo();

const naiveBar = util.promisify(foo.bar);
// TypeError: Cannot read property 'a' of undefined
// naiveBar().then(a => console.log(a));

naiveBar.call(foo).then((a) => console.log(a)); // '42'

const bindBar = naiveBar.bind(foo);
bindBar().then((a) => console.log(a)); // '42'

Custom promisified functions#

Using the util.promisify.custom symbol one can override the return value of util.promisify():

const util = require('util');

function doSomething(foo, callback) {
  // ...
}

doSomething[util.promisify.custom] = (foo) => {
  return getPromiseSomehow();
};

const promisified = util.promisify(doSomething);
console.log(promisified === doSomething[util.promisify.custom]);
// prints 'true'

This can be useful for cases where the original function does not follow the standard format of taking an error-first callback as the last argument.

For example, with a function that takes in (foo, onSuccessCallback, onErrorCallback):

doSomething[util.promisify.custom] = (foo) => {
  return new Promise((resolve, reject) => {
    doSomething(foo, resolve, reject);
  });
};

If promisify.custom is defined but is not a function, promisify() will throw an error.

util.promisify.custom#

History
VersionChanges
v13.12.0, v12.16.2

This is now defined as a shared symbol.

v8.0.0

Added in: v8.0.0

In addition to being accessible through util.promisify.custom, this symbol is registered globally and can be accessed in any environment as Symbol.for('nodejs.util.promisify.custom').

For example, with a function that takes in (foo, onSuccessCallback, onErrorCallback):

const kCustomPromisifiedSymbol = Symbol.for('nodejs.util.promisify.custom');

doSomething[kCustomPromisifiedSymbol] = (foo) => {
  return new Promise((resolve, reject) => {
    doSomething(foo, resolve, reject);
  });
};

Class: util.TextDecoder#

Added in: v8.3.0

An implementation of the WHATWG Encoding Standard TextDecoder API.

const decoder = new TextDecoder('shift_jis');
let string = '';
let buffer;
while (buffer = getNextChunkSomehow()) {
  string += decoder.decode(buffer, { stream: true });
}
string += decoder.decode(); // end-of-stream

WHATWG Supported Encodings#

Per the WHATWG Encoding Standard, the encodings supported by the TextDecoder API are outlined in the tables below. For each encoding, one or more aliases may be used.

Different Node.js build configurations support different sets of encodings. (see Internationalization)

Encodings Supported by Default (With Full ICU Data)#

EncodingAliases
'ibm866''866', 'cp866', 'csibm866'
'iso-8859-2''csisolatin2', 'iso-ir-101', 'iso8859-2', 'iso88592', 'iso_8859-2', 'iso_8859-2:1987', 'l2', 'latin2'
'iso-8859-3''csisolatin3', 'iso-ir-109', 'iso8859-3', 'iso88593', 'iso_8859-3', 'iso_8859-3:1988', 'l3', 'latin3'
'iso-8859-4''csisolatin4', 'iso-ir-110', 'iso8859-4', 'iso88594', 'iso_8859-4', 'iso_8859-4:1988', 'l4', 'latin4'
'iso-8859-5''csisolatincyrillic', 'cyrillic', 'iso-ir-144', 'iso8859-5', 'iso88595', 'iso_8859-5', 'iso_8859-5:1988'
'iso-8859-6''arabic', 'asmo-708', 'csiso88596e', 'csiso88596i', 'csisolatinarabic', 'ecma-114', 'iso-8859-6-e', 'iso-8859-6-i', 'iso-ir-127', 'iso8859-6', 'iso88596', 'iso_8859-6', 'iso_8859-6:1987'
'iso-8859-7''csisolatingreek', 'ecma-118', 'elot_928', 'greek', 'greek8', 'iso-ir-126', 'iso8859-7', 'iso88597', 'iso_8859-7', 'iso_8859-7:1987', 'sun_eu_greek'
'iso-8859-8''csiso88598e', 'csisolatinhebrew', 'hebrew', 'iso-8859-8-e', 'iso-ir-138', 'iso8859-8', 'iso88598', 'iso_8859-8', 'iso_8859-8:1988', 'visual'
'iso-8859-8-i''csiso88598i', 'logical'
'iso-8859-10''csisolatin6', 'iso-ir-157', 'iso8859-10', 'iso885910', 'l6', 'latin6'
'iso-8859-13''iso8859-13', 'iso885913'
'iso-8859-14''iso8859-14', 'iso885914'
'iso-8859-15''csisolatin9', 'iso8859-15', 'iso885915', 'iso_8859-15', 'l9'
'koi8-r''cskoi8r', 'koi', 'koi8', 'koi8_r'
'koi8-u''koi8-ru'
'macintosh''csmacintosh', 'mac', 'x-mac-roman'
'windows-874''dos-874', 'iso-8859-11', 'iso8859-11', 'iso885911', 'tis-620'
'windows-1250''cp1250', 'x-cp1250'
'windows-1251''cp1251', 'x-cp1251'
'windows-1252''ansi_x3.4-1968', 'ascii', 'cp1252', 'cp819', 'csisolatin1', 'ibm819', 'iso-8859-1', 'iso-ir-100', 'iso8859-1', 'iso88591', 'iso_8859-1', 'iso_8859-1:1987', 'l1', 'latin1', 'us-ascii', 'x-cp1252'
'windows-1253''cp1253', 'x-cp1253'
'windows-1254''cp1254', 'csisolatin5', 'iso-8859-9', 'iso-ir-148', 'iso8859-9', 'iso88599', 'iso_8859-9', 'iso_8859-9:1989', 'l5', 'latin5', 'x-cp1254'
'windows-1255''cp1255', 'x-cp1255'
'windows-1256''cp1256', 'x-cp1256'
'windows-1257''cp1257', 'x-cp1257'
'windows-1258''cp1258', 'x-cp1258'
'x-mac-cyrillic''x-mac-ukrainian'
'gbk''chinese', 'csgb2312', 'csiso58gb231280', 'gb2312', 'gb_2312', 'gb_2312-80', 'iso-ir-58', 'x-gbk'
'gb18030'
'big5''big5-hkscs', 'cn-big5', 'csbig5', 'x-x-big5'
'euc-jp''cseucpkdfmtjapanese', 'x-euc-jp'
'iso-2022-jp''csiso2022jp'
'shift_jis''csshiftjis', 'ms932', 'ms_kanji', 'shift-jis', 'sjis', 'windows-31j', 'x-sjis'
'euc-kr''cseuckr', 'csksc56011987', 'iso-ir-149', 'korean', 'ks_c_5601-1987', 'ks_c_5601-1989', 'ksc5601', 'ksc_5601', 'windows-949'

Encodings Supported when Node.js is built with the small-icu option#

EncodingAliases
'utf-8''unicode-1-1-utf-8', 'utf8'
'utf-16le''utf-16'
'utf-16be'

Encodings Supported when ICU is disabled#

EncodingAliases
'utf-8''unicode-1-1-utf-8', 'utf8'
'utf-16le''utf-16'

The 'iso-8859-16' encoding listed in the WHATWG Encoding Standard is not supported.

new TextDecoder([encoding[, options]])#

History
VersionChanges
v11.0.0

The class is now available on the global object.

v8.3.0

Added in: v8.3.0

  • encoding <string> Identifies the encoding that this TextDecoder instance supports. Default: 'utf-8'.

  • options <Object>

    • fatal <boolean> true if decoding failures are fatal. This option is not supported when ICU is disabled (see Internationalization). Default: false.
    • ignoreBOM <boolean> When true, the TextDecoder will include the byte order mark in the decoded result. When false, the byte order mark will be removed from the output. This option is only used when encoding is 'utf-8', 'utf-16be' or 'utf-16le'. Default: false.

Creates an new TextDecoder instance. The encoding may specify one of the supported encodings or an alias.

The TextDecoder class is also available on the global object.

textDecoder.decode([input[, options]])#

Decodes the input and returns a string. If options.stream is true, any incomplete byte sequences occurring at the end of the input are buffered internally and emitted after the next call to textDecoder.decode().

If textDecoder.fatal is true, decoding errors that occur will result in a TypeError being thrown.

textDecoder.encoding#

The encoding supported by the TextDecoder instance.

textDecoder.fatal#

The value will be true if decoding errors result in a TypeError being thrown.

textDecoder.ignoreBOM#

The value will be true if the decoding result will include the byte order mark.

Class: util.TextEncoder#

History
VersionChanges
v11.0.0

The class is now available on the global object.

v8.3.0

Added in: v8.3.0

An implementation of the WHATWG Encoding Standard TextEncoder API. All instances of TextEncoder only support UTF-8 encoding.

const encoder = new TextEncoder();
const uint8array = encoder.encode('this is some data');

The TextEncoder class is also available on the global object.

textEncoder.encode([input])#

UTF-8 encodes the input string and returns a Uint8Array containing the encoded bytes.

textEncoder.encodeInto(src, dest)#

UTF-8 encodes the src string to the dest Uint8Array and returns an object containing the read Unicode code units and written UTF-8 bytes.

const encoder = new TextEncoder();
const src = 'this is some data';
const dest = new Uint8Array(10);
const { read, written } = encoder.encodeInto(src, dest);

textEncoder.encoding#

The encoding supported by the TextEncoder instance. Always set to 'utf-8'.

util.types#

Added in: v10.0.0

util.types provides type checks for different kinds of built-in objects. Unlike instanceof or Object.prototype.toString.call(value), these checks do not inspect properties of the object that are accessible from JavaScript (like their prototype), and usually have the overhead of calling into C++.

The result generally does not make any guarantees about what kinds of properties or behavior a value exposes in JavaScript. They are primarily useful for addon developers who prefer to do type checking in JavaScript.

util.types.isAnyArrayBuffer(value)#

Added in: v10.0.0

Returns true if the value is a built-in ArrayBuffer or SharedArrayBuffer instance.

See also util.types.isArrayBuffer() and util.types.isSharedArrayBuffer().

util.types.isAnyArrayBuffer(new ArrayBuffer());  // Returns true
util.types.isAnyArrayBuffer(new SharedArrayBuffer());  // Returns true

util.types.isArrayBufferView(value)#

Added in: v10.0.0

Returns true if the value is an instance of one of the ArrayBuffer views, such as typed array objects or DataView. Equivalent to ArrayBuffer.isView().

util.types.isArrayBufferView(new Int8Array());  // true
util.types.isArrayBufferView(Buffer.from('hello world')); // true
util.types.isArrayBufferView(new DataView(new ArrayBuffer(16)));  // true
util.types.isArrayBufferView(new ArrayBuffer());  // false

util.types.isArgumentsObject(value)#

Added in: v10.0.0

Returns true if the value is an arguments object.

function foo() {
  util.types.isArgumentsObject(arguments);  // Returns true
}

util.types.isArrayBuffer(value)#

Added in: v10.0.0

Returns true if the value is a built-in ArrayBuffer instance. This does not include SharedArrayBuffer instances. Usually, it is desirable to test for both; See util.types.isAnyArrayBuffer() for that.

util.types.isArrayBuffer(new ArrayBuffer());  // Returns true
util.types.isArrayBuffer(new SharedArrayBuffer());  // Returns false

util.types.isAsyncFunction(value)#

Added in: v10.0.0

Returns true if the value is an async function. This only reports back what the JavaScript engine is seeing; in particular, the return value may not match the original source code if a transpilation tool was used.

util.types.isAsyncFunction(function foo() {});  // Returns false
util.types.isAsyncFunction(async function foo() {});  // Returns true

util.types.isBigInt64Array(value)#

Added in: v10.0.0

Returns true if the value is a BigInt64Array instance.

util.types.isBigInt64Array(new BigInt64Array());   // Returns true
util.types.isBigInt64Array(new BigUint64Array());  // Returns false

util.types.isBigUint64Array(value)#

Added in: v10.0.0

Returns true if the value is a BigUint64Array instance.

util.types.isBigUint64Array(new BigInt64Array());   // Returns false
util.types.isBigUint64Array(new BigUint64Array());  // Returns true

util.types.isBooleanObject(value)#

Added in: v10.0.0

Returns true if the value is a boolean object, e.g. created by new Boolean().

util.types.isBooleanObject(false);  // Returns false
util.types.isBooleanObject(true);   // Returns false
util.types.isBooleanObject(new Boolean(false)); // Returns true
util.types.isBooleanObject(new Boolean(true));  // Returns true
util.types.isBooleanObject(Boolean(false)); // Returns false
util.types.isBooleanObject(Boolean(true));  // Returns false

util.types.isBoxedPrimitive(value)#

Added in: v10.11.0

Returns true if the value is any boxed primitive object, e.g. created by new Boolean(), new String() or Object(Symbol()).

For example:

util.types.isBoxedPrimitive(false); // Returns false
util.types.isBoxedPrimitive(new Boolean(false)); // Returns true
util.types.isBoxedPrimitive(Symbol('foo')); // Returns false
util.types.isBoxedPrimitive(Object(Symbol('foo'))); // Returns true
util.types.isBoxedPrimitive(Object(BigInt(5))); // Returns true

util.types.isDataView(value)#

Added in: v10.0.0

Returns true if the value is a built-in DataView instance.

const ab = new ArrayBuffer(20);
util.types.isDataView(new DataView(ab));  // Returns true
util.types.isDataView(new Float64Array());  // Returns false

See also ArrayBuffer.isView().

util.types.isDate(value)#

Added in: v10.0.0

Returns true if the value is a built-in Date instance.

util.types.isDate(new Date());  // Returns true

util.types.isExternal(value)#

Added in: v10.0.0

Returns true if the value is a native External value.

A native External value is a special type of object that contains a raw C++ pointer (void*) for access from native code, and has no other properties. Such objects are created either by Node.js internals or native addons. In JavaScript, they are frozen objects with a null prototype.

#include <js_native_api.h>
#include <stdlib.h>
napi_value result;
static napi_value MyNapi(napi_env env, napi_callback_info info) {
  int* raw = (int*) malloc(1024);
  napi_status status = napi_create_external(env, (void*) raw, NULL, NULL, &result);
  if (status != napi_ok) {
    napi_throw_error(env, NULL, "napi_create_external failed");
    return NULL;
  }
  return result;
}
...
DECLARE_NAPI_PROPERTY("myNapi", MyNapi)
...

const native = require('napi_addon.node');
const data = native.myNapi();
util.types.isExternal(data); // returns true
util.types.isExternal(0); // returns false
util.types.isExternal(new String('foo')); // returns false

For further information on napi_create_external, refer to napi_create_external().

util.types.isFloat32Array(value)#

Added in: v10.0.0

Returns true if the value is a built-in Float32Array instance.

util.types.isFloat32Array(new ArrayBuffer());  // Returns false
util.types.isFloat32Array(new Float32Array());  // Returns true
util.types.isFloat32Array(new Float64Array());  // Returns false

util.types.isFloat64Array(value)#

Added in: v10.0.0

Returns true if the value is a built-in Float64Array instance.

util.types.isFloat64Array(new ArrayBuffer());  // Returns false
util.types.isFloat64Array(new Uint8Array());  // Returns false
util.types.isFloat64Array(new Float64Array());  // Returns true

util.types.isGeneratorFunction(value)#

Added in: v10.0.0

Returns true if the value is a generator function. This only reports back what the JavaScript engine is seeing; in particular, the return value may not match the original source code if a transpilation tool was used.

util.types.isGeneratorFunction(function foo() {});  // Returns false
util.types.isGeneratorFunction(function* foo() {});  // Returns true

util.types.isGeneratorObject(value)#

Added in: v10.0.0

Returns true if the value is a generator object as returned from a built-in generator function. This only reports back what the JavaScript engine is seeing; in particular, the return value may not match the original source code if a transpilation tool was used.

function* foo() {}
const generator = foo();
util.types.isGeneratorObject(generator);  // Returns true

util.types.isInt8Array(value)#

Added in: v10.0.0

Returns true if the value is a built-in Int8Array instance.

util.types.isInt8Array(new ArrayBuffer());  // Returns false
util.types.isInt8Array(new Int8Array());  // Returns true
util.types.isInt8Array(new Float64Array());  // Returns false

util.types.isInt16Array(value)#

Added in: v10.0.0

Returns true if the value is a built-in Int16Array instance.

util.types.isInt16Array(new ArrayBuffer());  // Returns false
util.types.isInt16Array(new Int16Array());  // Returns true
util.types.isInt16Array(new Float64Array());  // Returns false

util.types.isInt32Array(value)#

Added in: v10.0.0

Returns true if the value is a built-in Int32Array instance.

util.types.isInt32Array(new ArrayBuffer());  // Returns false
util.types.isInt32Array(new Int32Array());  // Returns true
util.types.isInt32Array(new Float64Array());  // Returns false

util.types.isMap(value)#

Added in: v10.0.0

Returns true if the value is a built-in Map instance.

util.types.isMap(new Map());  // Returns true

util.types.isMapIterator(value)#

Added in: v10.0.0

Returns true if the value is an iterator returned for a built-in Map instance.

const map = new Map();
util.types.isMapIterator(map.keys());  // Returns true
util.types.isMapIterator(map.values());  // Returns true
util.types.isMapIterator(map.entries());  // Returns true
util.types.isMapIterator(map[Symbol.iterator]());  // Returns true

util.types.isModuleNamespaceObject(value)#

Added in: v10.0.0

Returns true if the value is an instance of a Module Namespace Object.

import * as ns from './a.js';

util.types.isModuleNamespaceObject(ns);  // Returns true

util.types.isNativeError(value)#

Added in: v10.0.0

Returns true if the value is an instance of a built-in Error type.

util.types.isNativeError(new Error());  // Returns true
util.types.isNativeError(new TypeError());  // Returns true
util.types.isNativeError(new RangeError());  // Returns true

util.types.isNumberObject(value)#

Added in: v10.0.0

Returns true if the value is a number object, e.g. created by new Number().

util.types.isNumberObject(0);  // Returns false
util.types.isNumberObject(new Number(0));   // Returns true

util.types.isPromise(value)#

Added in: v10.0.0

Returns true if the value is a built-in Promise.

util.types.isPromise(Promise.resolve(42));  // Returns true

util.types.isProxy(value)#

Added in: v10.0.0

Returns true if the value is a Proxy instance.

const target = {};
const proxy = new Proxy(target, {});
util.types.isProxy(target);  // Returns false
util.types.isProxy(proxy);  // Returns true

util.types.isRegExp(value)#

Added in: v10.0.0

Returns true if the value is a regular expression object.

util.types.isRegExp(/abc/);  // Returns true
util.types.isRegExp(new RegExp('abc'));  // Returns true

util.types.isSet(value)#

Added in: v10.0.0

Returns true if the value is a built-in Set instance.

util.types.isSet(new Set());  // Returns true

util.types.isSetIterator(value)#

Added in: v10.0.0

Returns true if the value is an iterator returned for a built-in Set instance.

const set = new Set();
util.types.isSetIterator(set.keys());  // Returns true
util.types.isSetIterator(set.values());  // Returns true
util.types.isSetIterator(set.entries());  // Returns true
util.types.isSetIterator(set[Symbol.iterator]());  // Returns true

util.types.isSharedArrayBuffer(value)#

Added in: v10.0.0

Returns true if the value is a built-in SharedArrayBuffer instance. This does not include ArrayBuffer instances. Usually, it is desirable to test for both; See util.types.isAnyArrayBuffer() for that.

util.types.isSharedArrayBuffer(new ArrayBuffer());  // Returns false
util.types.isSharedArrayBuffer(new SharedArrayBuffer());  // Returns true

util.types.isStringObject(value)#

Added in: v10.0.0

Returns true if the value is a string object, e.g. created by new String().

util.types.isStringObject('foo');  // Returns false
util.types.isStringObject(new String('foo'));   // Returns true

util.types.isSymbolObject(value)#

Added in: v10.0.0

Returns true if the value is a symbol object, created by calling Object() on a Symbol primitive.

const symbol = Symbol('foo');
util.types.isSymbolObject(symbol);  // Returns false
util.types.isSymbolObject(Object(symbol));   // Returns true

util.types.isTypedArray(value)#

Added in: v10.0.0

Returns true if the value is a built-in TypedArray instance.

util.types.isTypedArray(new ArrayBuffer());  // Returns false
util.types.isTypedArray(new Uint8Array());  // Returns true
util.types.isTypedArray(new Float64Array());  // Returns true

See also ArrayBuffer.isView().

util.types.isUint8Array(value)#

Added in: v10.0.0

Returns true if the value is a built-in Uint8Array instance.

util.types.isUint8Array(new ArrayBuffer());  // Returns false
util.types.isUint8Array(new Uint8Array());  // Returns true
util.types.isUint8Array(new Float64Array());  // Returns false

util.types.isUint8ClampedArray(value)#

Added in: v10.0.0

Returns true if the value is a built-in Uint8ClampedArray instance.

util.types.isUint8ClampedArray(new ArrayBuffer());  // Returns false
util.types.isUint8ClampedArray(new Uint8ClampedArray());  // Returns true
util.types.isUint8ClampedArray(new Float64Array());  // Returns false

util.types.isUint16Array(value)#

Added in: v10.0.0

Returns true if the value is a built-in Uint16Array instance.

util.types.isUint16Array(new ArrayBuffer());  // Returns false
util.types.isUint16Array(new Uint16Array());  // Returns true
util.types.isUint16Array(new Float64Array());  // Returns false

util.types.isUint32Array(value)#

Added in: v10.0.0

Returns true if the value is a built-in Uint32Array instance.

util.types.isUint32Array(new ArrayBuffer());  // Returns false
util.types.isUint32Array(new Uint32Array());  // Returns true
util.types.isUint32Array(new Float64Array());  // Returns false

util.types.isWeakMap(value)#

Added in: v10.0.0

Returns true if the value is a built-in WeakMap instance.

util.types.isWeakMap(new WeakMap());  // Returns true

util.types.isWeakSet(value)#

Added in: v10.0.0

Returns true if the value is a built-in WeakSet instance.

util.types.isWeakSet(new WeakSet());  // Returns true

util.types.isWebAssemblyCompiledModule(value)#

Added in: v10.0.0Deprecated since: v14.0.0

Stability: 0 - Deprecated: Use value instanceof WebAssembly.Module instead.

Returns true if the value is a built-in WebAssembly.Module instance.

const module = new WebAssembly.Module(wasmBuffer);
util.types.isWebAssemblyCompiledModule(module);  // Returns true

Deprecated APIs#

The following APIs are deprecated and should no longer be used. Existing applications and modules should be updated to find alternative approaches.

util._extend(target, source)#

Added in: v0.7.5Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use Object.assign() instead.

The util._extend() method was never intended to be used outside of internal Node.js modules. The community found and used it anyway.

It is deprecated and should not be used in new code. JavaScript comes with very similar built-in functionality through Object.assign().

util.isArray(object)#

Added in: v0.6.0Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use Array.isArray() instead.

Alias for Array.isArray().

Returns true if the given object is an Array. Otherwise, returns false.

const util = require('util');

util.isArray([]);
// Returns: true
util.isArray(new Array());
// Returns: true
util.isArray({});
// Returns: false

util.isBoolean(object)#

Added in: v0.11.5Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use typeof value === 'boolean' instead.

Returns true if the given object is a Boolean. Otherwise, returns false.

const util = require('util');

util.isBoolean(1);
// Returns: false
util.isBoolean(0);
// Returns: false
util.isBoolean(false);
// Returns: true

util.isBuffer(object)#

Added in: v0.11.5Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use Buffer.isBuffer() instead.

Returns true if the given object is a Buffer. Otherwise, returns false.

const util = require('util');

util.isBuffer({ length: 0 });
// Returns: false
util.isBuffer([]);
// Returns: false
util.isBuffer(Buffer.from('hello world'));
// Returns: true

util.isDate(object)#

Added in: v0.6.0Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use util.types.isDate() instead.

Returns true if the given object is a Date. Otherwise, returns false.

const util = require('util');

util.isDate(new Date());
// Returns: true
util.isDate(Date());
// false (without 'new' returns a String)
util.isDate({});
// Returns: false

util.isError(object)#

Added in: v0.6.0Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use util.types.isNativeError() instead.

Returns true if the given object is an Error. Otherwise, returns false.

const util = require('util');

util.isError(new Error());
// Returns: true
util.isError(new TypeError());
// Returns: true
util.isError({ name: 'Error', message: 'an error occurred' });
// Returns: false

This method relies on Object.prototype.toString() behavior. It is possible to obtain an incorrect result when the object argument manipulates @@toStringTag.

const util = require('util');
const obj = { name: 'Error', message: 'an error occurred' };

util.isError(obj);
// Returns: false
obj[Symbol.toStringTag] = 'Error';
util.isError(obj);
// Returns: true

util.isFunction(object)#

Added in: v0.11.5Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use typeof value === 'function' instead.

Returns true if the given object is a Function. Otherwise, returns false.

const util = require('util');

function Foo() {}
const Bar = () => {};

util.isFunction({});
// Returns: false
util.isFunction(Foo);
// Returns: true
util.isFunction(Bar);
// Returns: true

util.isNull(object)#

Added in: v0.11.5Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use value === null instead.

Returns true if the given object is strictly null. Otherwise, returns false.

const util = require('util');

util.isNull(0);
// Returns: false
util.isNull(undefined);
// Returns: false
util.isNull(null);
// Returns: true

util.isNullOrUndefined(object)#

Added in: v0.11.5Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use value === undefined || value === null instead.

Returns true if the given object is null or undefined. Otherwise, returns false.

const util = require('util');

util.isNullOrUndefined(0);
// Returns: false
util.isNullOrUndefined(undefined);
// Returns: true
util.isNullOrUndefined(null);
// Returns: true

util.isNumber(object)#

Added in: v0.11.5Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use typeof value === 'number' instead.

Returns true if the given object is a Number. Otherwise, returns false.

const util = require('util');

util.isNumber(false);
// Returns: false
util.isNumber(Infinity);
// Returns: true
util.isNumber(0);
// Returns: true
util.isNumber(NaN);
// Returns: true

util.isObject(object)#

Added in: v0.11.5Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use value !== null && typeof value === 'object' instead.

Returns true if the given object is strictly an Object and not a Function (even though functions are objects in JavaScript). Otherwise, returns false.

const util = require('util');

util.isObject(5);
// Returns: false
util.isObject(null);
// Returns: false
util.isObject({});
// Returns: true
util.isObject(() => {});
// Returns: false

util.isPrimitive(object)#

Added in: v0.11.5Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use (typeof value !== 'object' && typeof value !== 'function') || value === null instead.

Returns true if the given object is a primitive type. Otherwise, returns false.

const util = require('util');

util.isPrimitive(5);
// Returns: true
util.isPrimitive('foo');
// Returns: true
util.isPrimitive(false);
// Returns: true
util.isPrimitive(null);
// Returns: true
util.isPrimitive(undefined);
// Returns: true
util.isPrimitive({});
// Returns: false
util.isPrimitive(() => {});
// Returns: false
util.isPrimitive(/^$/);
// Returns: false
util.isPrimitive(new Date());
// Returns: false

util.isRegExp(object)#

Added in: v0.6.0Deprecated since: v4.0.0

Stability: 0 - Deprecated

Returns true if the given object is a RegExp. Otherwise, returns false.

const util = require('util');

util.isRegExp(/some regexp/);
// Returns: true
util.isRegExp(new RegExp('another regexp'));
// Returns: true
util.isRegExp({});
// Returns: false

util.isString(object)#

Added in: v0.11.5Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use typeof value === 'string' instead.

Returns true if the given object is a string. Otherwise, returns false.

const util = require('util');

util.isString('');
// Returns: true
util.isString('foo');
// Returns: true
util.isString(String('foo'));
// Returns: true
util.isString(5);
// Returns: false

util.isSymbol(object)#

Added in: v0.11.5Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use typeof value === 'symbol' instead.

Returns true if the given object is a Symbol. Otherwise, returns false.

const util = require('util');

util.isSymbol(5);
// Returns: false
util.isSymbol('foo');
// Returns: false
util.isSymbol(Symbol('foo'));
// Returns: true

util.isUndefined(object)#

Added in: v0.11.5Deprecated since: v4.0.0

Stability: 0 - Deprecated: Use value === undefined instead.

Returns true if the given object is undefined. Otherwise, returns false.

const util = require('util');

const foo = undefined;
util.isUndefined(5);
// Returns: false
util.isUndefined(foo);
// Returns: true
util.isUndefined(null);
// Returns: false

util.log(string)#

Added in: v0.3.0Deprecated since: v6.0.0

Stability: 0 - Deprecated: Use a third party module instead.

The util.log() method prints the given string to stdout with an included timestamp.

const util = require('util');

util.log('Timestamped message.');

V8#

The v8 module exposes APIs that are specific to the version of V8 built into the Node.js binary. It can be accessed using:

const v8 = require('v8');

The APIs and implementation are subject to change at any time.

v8.cachedDataVersionTag()#

Added in: v8.0.0

Returns an integer representing a "version tag" derived from the V8 version, command line flags and detected CPU features. This is useful for determining whether a vm.Script cachedData buffer is compatible with this instance of V8.

v8.getHeapSpaceStatistics()#

History
VersionChanges
v7.5.0

Support values exceeding the 32-bit unsigned integer range.

v6.0.0

Added in: v6.0.0

Returns statistics about the V8 heap spaces, i.e. the segments which make up the V8 heap. Neither the ordering of heap spaces, nor the availability of a heap space can be guaranteed as the statistics are provided via the V8 GetHeapSpaceStatistics function and may change from one V8 version to the next.

The value returned is an array of objects containing the following properties:

[
  {
    "space_name": "new_space",
    "space_size": 2063872,
    "space_used_size": 951112,
    "space_available_size": 80824,
    "physical_space_size": 2063872
  },
  {
    "space_name": "old_space",
    "space_size": 3090560,
    "space_used_size": 2493792,
    "space_available_size": 0,
    "physical_space_size": 3090560
  },
  {
    "space_name": "code_space",
    "space_size": 1260160,
    "space_used_size": 644256,
    "space_available_size": 960,
    "physical_space_size": 1260160
  },
  {
    "space_name": "map_space",
    "space_size": 1094160,
    "space_used_size": 201608,
    "space_available_size": 0,
    "physical_space_size": 1094160
  },
  {
    "space_name": "large_object_space",
    "space_size": 0,
    "space_used_size": 0,
    "space_available_size": 1490980608,
    "physical_space_size": 0
  }
]

v8.getHeapSnapshot()#

Added in: v11.13.0

Generates a snapshot of the current V8 heap and returns a Readable Stream that may be used to read the JSON serialized representation. This JSON stream format is intended to be used with tools such as Chrome DevTools. The JSON schema is undocumented and specific to the V8 engine, and may change from one version of V8 to the next.

const stream = v8.getHeapSnapshot();
stream.pipe(process.stdout);

v8.getHeapStatistics()#

History
VersionChanges
v7.5.0

Support values exceeding the 32-bit unsigned integer range.

v7.2.0

Added malloced_memory, peak_malloced_memory, and does_zap_garbage.

v1.0.0

Added in: v1.0.0

Returns an object with the following properties:

does_zap_garbage is a 0/1 boolean, which signifies whether the --zap_code_space option is enabled or not. This makes V8 overwrite heap garbage with a bit pattern. The RSS footprint (resident memory set) gets bigger because it continuously touches all heap pages and that makes them less likely to get swapped out by the operating system.

number_of_native_contexts The value of native_context is the number of the top-level contexts currently active. Increase of this number over time indicates a memory leak.

number_of_detached_contexts The value of detached_context is the number of contexts that were detached and not yet garbage collected. This number being non-zero indicates a potential memory leak.

{
  total_heap_size: 7326976,
  total_heap_size_executable: 4194304,
  total_physical_size: 7326976,
  total_available_size: 1152656,
  used_heap_size: 3476208,
  heap_size_limit: 1535115264,
  malloced_memory: 16384,
  peak_malloced_memory: 1127496,
  does_zap_garbage: 0,
  number_of_native_contexts: 1,
  number_of_detached_contexts: 0
}

v8.getHeapCodeStatistics()#

Added in: v12.8.0

Returns an object with the following properties:

{
  code_and_metadata_size: 212208,
  bytecode_and_metadata_size: 161368,
  external_script_source_size: 1410794
}

v8.setFlagsFromString(flags)#

Added in: v1.0.0

The v8.setFlagsFromString() method can be used to programmatically set V8 command line flags. This method should be used with care. Changing settings after the VM has started may result in unpredictable behavior, including crashes and data loss; or it may simply do nothing.

The V8 options available for a version of Node.js may be determined by running node --v8-options.

Usage:

// Print GC events to stdout for one minute.
const v8 = require('v8');
v8.setFlagsFromString('--trace_gc');
setTimeout(() => { v8.setFlagsFromString('--notrace_gc'); }, 60e3);

v8.writeHeapSnapshot([filename])#

Added in: v11.13.0
  • filename <string> The file path where the V8 heap snapshot is to be saved. If not specified, a file name with the pattern 'Heap-${yyyymmdd}-${hhmmss}-${pid}-${thread_id}.heapsnapshot' will be generated, where {pid} will be the PID of the Node.js process, {thread_id} will be 0 when writeHeapSnapshot() is called from the main Node.js thread or the id of a worker thread.
  • Returns: <string> The filename where the snapshot was saved.

Generates a snapshot of the current V8 heap and writes it to a JSON file. This file is intended to be used with tools such as Chrome DevTools. The JSON schema is undocumented and specific to the V8 engine, and may change from one version of V8 to the next.

A heap snapshot is specific to a single V8 isolate. When using Worker Threads, a heap snapshot generated from the main thread will not contain any information about the workers, and vice versa.

const { writeHeapSnapshot } = require('v8');
const {
  Worker,
  isMainThread,
  parentPort
} = require('worker_threads');

if (isMainThread) {
  const worker = new Worker(__filename);

  worker.once('message', (filename) => {
    console.log(`worker heapdump: ${filename}`);
    // Now get a heapdump for the main thread.
    console.log(`main thread heapdump: ${writeHeapSnapshot()}`);
  });

  // Tell the worker to create a heapdump.
  worker.postMessage('heapdump');
} else {
  parentPort.once('message', (message) => {
    if (message === 'heapdump') {
      // Generate a heapdump for the worker
      // and return the filename to the parent.
      parentPort.postMessage(writeHeapSnapshot());
    }
  });
}

Serialization API#

The serialization API provides means of serializing JavaScript values in a way that is compatible with the HTML structured clone algorithm.

The format is backward-compatible (i.e. safe to store to disk). Equal JavaScript values may result in different serialized output.

v8.serialize(value)#

Added in: v8.0.0

Uses a DefaultSerializer to serialize value into a buffer.

v8.deserialize(buffer)#

Added in: v8.0.0

Uses a DefaultDeserializer with default options to read a JS value from a buffer.

Class: v8.Serializer#

Added in: v8.0.0

new Serializer()#

Creates a new Serializer object.

serializer.writeHeader()#

Writes out a header, which includes the serialization format version.

serializer.writeValue(value)#

Serializes a JavaScript value and adds the serialized representation to the internal buffer.

This throws an error if value cannot be serialized.

serializer.releaseBuffer()#

Returns the stored internal buffer. This serializer should not be used once the buffer is released. Calling this method results in undefined behavior if a previous write has failed.

serializer.transferArrayBuffer(id, arrayBuffer)#

Marks an ArrayBuffer as having its contents transferred out of band. Pass the corresponding ArrayBuffer in the deserializing context to deserializer.transferArrayBuffer().

serializer.writeUint32(value)#

Write a raw 32-bit unsigned integer. For use inside of a custom serializer._writeHostObject().

serializer.writeUint64(hi, lo)#

Write a raw 64-bit unsigned integer, split into high and low 32-bit parts. For use inside of a custom serializer._writeHostObject().

serializer.writeDouble(value)#

Write a JS number value. For use inside of a custom serializer._writeHostObject().

serializer.writeRawBytes(buffer)#

Write raw bytes into the serializer’s internal buffer. The deserializer will require a way to compute the length of the buffer. For use inside of a custom serializer._writeHostObject().

serializer._writeHostObject(object)#

This method is called to write some kind of host object, i.e. an object created by native C++ bindings. If it is not possible to serialize object, a suitable exception should be thrown.

This method is not present on the Serializer class itself but can be provided by subclasses.

serializer._getDataCloneError(message)#

This method is called to generate error objects that will be thrown when an object can not be cloned.

This method defaults to the Error constructor and can be overridden on subclasses.

serializer._getSharedArrayBufferId(sharedArrayBuffer)#

This method is called when the serializer is going to serialize a SharedArrayBuffer object. It must return an unsigned 32-bit integer ID for the object, using the same ID if this SharedArrayBuffer has already been serialized. When deserializing, this ID will be passed to deserializer.transferArrayBuffer().

If the object cannot be serialized, an exception should be thrown.

This method is not present on the Serializer class itself but can be provided by subclasses.

serializer._setTreatArrayBufferViewsAsHostObjects(flag)#

Indicate whether to treat TypedArray and DataView objects as host objects, i.e. pass them to serializer._writeHostObject().

Class: v8.Deserializer#

Added in: v8.0.0

new Deserializer(buffer)#

Creates a new Deserializer object.

deserializer.readHeader()#

Reads and validates a header (including the format version). May, for example, reject an invalid or unsupported wire format. In that case, an Error is thrown.

deserializer.readValue()#

Deserializes a JavaScript value from the buffer and returns it.

deserializer.transferArrayBuffer(id, arrayBuffer)#

Marks an ArrayBuffer as having its contents transferred out of band. Pass the corresponding ArrayBuffer in the serializing context to serializer.transferArrayBuffer() (or return the id from serializer._getSharedArrayBufferId() in the case of SharedArrayBuffers).

deserializer.getWireFormatVersion()#

Reads the underlying wire format version. Likely mostly to be useful to legacy code reading old wire format versions. May not be called before .readHeader().

deserializer.readUint32()#

Read a raw 32-bit unsigned integer and return it. For use inside of a custom deserializer._readHostObject().

deserializer.readUint64()#

Read a raw 64-bit unsigned integer and return it as an array [hi, lo] with two 32-bit unsigned integer entries. For use inside of a custom deserializer._readHostObject().

deserializer.readDouble()#

Read a JS number value. For use inside of a custom deserializer._readHostObject().

deserializer.readRawBytes(length)#

Read raw bytes from the deserializer’s internal buffer. The length parameter must correspond to the length of the buffer that was passed to serializer.writeRawBytes(). For use inside of a custom deserializer._readHostObject().

deserializer._readHostObject()#

This method is called to read some kind of host object, i.e. an object that is created by native C++ bindings. If it is not possible to deserialize the data, a suitable exception should be thrown.

This method is not present on the Deserializer class itself but can be provided by subclasses.

Class: v8.DefaultSerializer#

Added in: v8.0.0

A subclass of Serializer that serializes TypedArray (in particular Buffer) and DataView objects as host objects, and only stores the part of their underlying ArrayBuffers that they are referring to.

Class: v8.DefaultDeserializer#

Added in: v8.0.0

A subclass of Deserializer corresponding to the format written by DefaultSerializer.

VM (Executing JavaScript)#

Stability: 2 - Stable

The vm module enables compiling and running code within V8 Virtual Machine contexts. The vm module is not a security mechanism. Do not use it to run untrusted code.

JavaScript code can be compiled and run immediately or compiled, saved, and run later.

A common use case is to run the code in a different V8 Context. This means invoked code has a different global object than the invoking code.

One can provide the context by contextifying an object. The invoked code treats any property in the context like a global variable. Any changes to global variables caused by the invoked code are reflected in the context object.

const vm = require('vm');

const x = 1;

const context = { x: 2 };
vm.createContext(context); // Contextify the object.

const code = 'x += 40; var y = 17;';
// `x` and `y` are global variables in the context.
// Initially, x has the value 2 because that is the value of context.x.
vm.runInContext(code, context);

console.log(context.x); // 42
console.log(context.y); // 17

console.log(x); // 1; y is not defined.

Class: vm.Script#

Added in: v0.3.1

Instances of the vm.Script class contain precompiled scripts that can be executed in specific contexts.

Constructor: new vm.Script(code[, options])#

History
VersionChanges
v10.6.0

The produceCachedData is deprecated in favour of script.createCachedData()

v5.7.0

The cachedData and produceCachedData options are supported now.

v0.3.1

Added in: v0.3.1

  • code <string> The JavaScript code to compile.

  • options <Object> | <string>

    • filename <string> Specifies the filename used in stack traces produced by this script. Default: 'evalmachine.<anonymous>'.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script. Default: 0.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script. Default: 0.
    • cachedData <Buffer> | <TypedArray> | <DataView> Provides an optional Buffer or TypedArray, or DataView with V8's code cache data for the supplied source. When supplied, the cachedDataRejected value will be set to either true or false depending on acceptance of the data by V8.
    • produceCachedData <boolean> When true and no cachedData is present, V8 will attempt to produce code cache data for code. Upon success, a Buffer with V8's code cache data will be produced and stored in the cachedData property of the returned vm.Script instance. The cachedDataProduced value will be set to either true or false depending on whether code cache data is produced successfully. This option is deprecated in favor of script.createCachedData(). Default: false.

    • importModuleDynamically <Function> Called during evaluation of this module when import() is called. If this option is not specified, calls to import() will reject with ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING. This option is part of the experimental modules API, and should not be considered stable.

If options is a string, then it specifies the filename.

Creating a new vm.Script object compiles code but does not run it. The compiled vm.Script can be run later multiple times. The code is not bound to any global object; rather, it is bound before each run, just for that run.

script.createCachedData()#

Added in: v10.6.0

Creates a code cache that can be used with the Script constructor's cachedData option. Returns a Buffer. This method may be called at any time and any number of times.

const script = new vm.Script(`
function add(a, b) {
  return a + b;
}

const x = add(1, 2);
`);

const cacheWithoutX = script.createCachedData();

script.runInThisContext();

const cacheWithX = script.createCachedData();

script.runInContext(contextifiedObject[, options])#

History
VersionChanges
v6.3.0

The breakOnSigint option is supported now.

v0.3.1

Added in: v0.3.1

  • contextifiedObject <Object> A contextified object as returned by the vm.createContext() method.

  • options <Object>

    • displayErrors <boolean> When true, if an Error occurs while compiling the code, the line of code causing the error is attached to the stack trace. Default: true.
    • timeout <integer> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown. This value must be a strictly positive integer.
    • breakOnSigint <boolean> If true, the execution will be terminated when SIGINT (Ctrl+C) is received. Existing handlers for the event that have been attached via process.on('SIGINT') will be disabled during script execution, but will continue to work after that. If execution is terminated, an Error will be thrown. Default: false.
  • Returns: <any> the result of the very last statement executed in the script.

Runs the compiled code contained by the vm.Script object within the given contextifiedObject and returns the result. Running code does not have access to local scope.

The following example compiles code that increments a global variable, sets the value of another global variable, then execute the code multiple times. The globals are contained in the context object.

const vm = require('vm');

const context = {
  animal: 'cat',
  count: 2
};

const script = new vm.Script('count += 1; name = "kitty";');

vm.createContext(context);
for (let i = 0; i < 10; ++i) {
  script.runInContext(context);
}

console.log(context);
// Prints: { animal: 'cat', count: 12, name: 'kitty' }

Using the timeout or breakOnSigint options will result in new event loops and corresponding threads being started, which have a non-zero performance overhead.

script.runInNewContext([contextObject[, options]])#

History
VersionChanges
v10.0.0

The contextCodeGeneration option is supported now.

v6.3.0

The breakOnSigint option is supported now.

v0.3.1

Added in: v0.3.1

  • contextObject <Object> An object that will be contextified. If undefined, a new object will be created.

  • options <Object>

    • displayErrors <boolean> When true, if an Error occurs while compiling the code, the line of code causing the error is attached to the stack trace. Default: true.
    • timeout <integer> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown. This value must be a strictly positive integer.
    • breakOnSigint <boolean> If true, the execution will be terminated when SIGINT (Ctrl+C) is received. Existing handlers for the event that have been attached via process.on('SIGINT') will be disabled during script execution, but will continue to work after that. If execution is terminated, an Error will be thrown. Default: false.
    • contextName <string> Human-readable name of the newly created context. Default: 'VM Context i', where i is an ascending numerical index of the created context.
    • contextOrigin <string> Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of the url.origin property of a URL object. Most notably, this string should omit the trailing slash, as that denotes a path. Default: ''.

    • contextCodeGeneration <Object>

      • strings <boolean> If set to false any calls to eval or function constructors (Function, GeneratorFunction, etc) will throw an EvalError. Default: true.
      • wasm <boolean> If set to false any attempt to compile a WebAssembly module will throw a WebAssembly.CompileError. Default: true.
  • Returns: <any> the result of the very last statement executed in the script.

First contextifies the given contextObject, runs the compiled code contained by the vm.Script object within the created context, and returns the result. Running code does not have access to local scope.

The following example compiles code that sets a global variable, then executes the code multiple times in different contexts. The globals are set on and contained within each individual context.

const vm = require('vm');

const script = new vm.Script('globalVar = "set"');

const contexts = [{}, {}, {}];
contexts.forEach((context) => {
  script.runInNewContext(context);
});

console.log(contexts);
// Prints: [{ globalVar: 'set' }, { globalVar: 'set' }, { globalVar: 'set' }]

script.runInThisContext([options])#

History
VersionChanges
v6.3.0

The breakOnSigint option is supported now.

v0.3.1

Added in: v0.3.1

  • options <Object>

    • displayErrors <boolean> When true, if an Error occurs while compiling the code, the line of code causing the error is attached to the stack trace. Default: true.
    • timeout <integer> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown. This value must be a strictly positive integer.
    • breakOnSigint <boolean> If true, the execution will be terminated when SIGINT (Ctrl+C) is received. Existing handlers for the event that have been attached via process.on('SIGINT') will be disabled during script execution, but will continue to work after that. If execution is terminated, an Error will be thrown. Default: false.
  • Returns: <any> the result of the very last statement executed in the script.

Runs the compiled code contained by the vm.Script within the context of the current global object. Running code does not have access to local scope, but does have access to the current global object.

The following example compiles code that increments a global variable then executes that code multiple times:

const vm = require('vm');

global.globalVar = 0;

const script = new vm.Script('globalVar += 1', { filename: 'myfile.vm' });

for (let i = 0; i < 1000; ++i) {
  script.runInThisContext();
}

console.log(globalVar);

// 1000

vm.measureMemory([options])#

Added in: v13.10.0

Stability: 1 - Experimental

Measure the memory known to V8 and used by all contexts known to the current V8 isolate, or the main context.

  • options <Object> Optional.

    • mode <string> Either 'summary' or 'detailed'. In summary mode, only the memory measured for the main context will be returned. In detailed mode, the measure measured for all contexts known to the current V8 isolate will be returned. Default: 'summary'
    • execution <string> Either 'default' or 'eager'. With default execution, the promise will not resolve until after the next scheduled garbage collection starts, which may take a while (or never if the program exits before the next GC). With eager execution, the GC will be started right away to measure the memory. Default: 'default'
  • Returns: <Promise> If the memory is successfully measured the promise will resolve with an object containing information about the memory usage.

The format of the object that the returned Promise may resolve with is specific to the V8 engine and may change from one version of V8 to the next.

The returned result is different from the statistics returned by v8.getHeapSpaceStatistics() in that vm.measureMemory() measure the memory reachable by each V8 specific contexts in the current instance of the V8 engine, while the result of v8.getHeapSpaceStatistics() measure the memory occupied by each heap space in the current V8 instance.

const vm = require('vm');
// Measure the memory used by the main context.
vm.measureMemory({ mode: 'summary' })
  // This is the same as vm.measureMemory()
  .then((result) => {
    // The current format is:
    // {
    //   total: {
    //      jsMemoryEstimate: 2418479, jsMemoryRange: [ 2418479, 2745799 ]
    //    }
    // }
    console.log(result);
  });

const context = vm.createContext({ a: 1 });
vm.measureMemory({ mode: 'detailed', execution: 'eager' })
  .then((result) => {
    // Reference the context here so that it won't be GC'ed
    // until the measurement is complete.
    console.log(context.a);
    // {
    //   total: {
    //     jsMemoryEstimate: 2574732,
    //     jsMemoryRange: [ 2574732, 2904372 ]
    //   },
    //   current: {
    //     jsMemoryEstimate: 2438996,
    //     jsMemoryRange: [ 2438996, 2768636 ]
    //   },
    //   other: [
    //     {
    //       jsMemoryEstimate: 135736,
    //       jsMemoryRange: [ 135736, 465376 ]
    //     }
    //   ]
    // }
    console.log(result);
  });

Class: vm.Module#

Added in: v13.0.0, v12.16.0

Stability: 1 - Experimental

This feature is only available with the --experimental-vm-modules command flag enabled.

The vm.Module class provides a low-level interface for using ECMAScript modules in VM contexts. It is the counterpart of the vm.Script class that closely mirrors Module Records as defined in the ECMAScript specification.

Unlike vm.Script however, every vm.Module object is bound to a context from its creation. Operations on vm.Module objects are intrinsically asynchronous, in contrast with the synchronous nature of vm.Script objects. The use of 'async' functions can help with manipulating vm.Module objects.

Using a vm.Module object requires three distinct steps: creation/parsing, linking, and evaluation. These three steps are illustrated in the following example.

This implementation lies at a lower level than the ECMAScript Module loader. There is also currently no way to interact with the Loader, though support is planned.

const vm = require('vm');

const contextifiedObject = vm.createContext({
  secret: 42,
  print: console.log,
});

(async () => {
  // Step 1
  //
  // Create a Module by constructing a new `vm.SourceTextModule` object. This
  // parses the provided source text, throwing a `SyntaxError` if anything goes
  // wrong. By default, a Module is created in the top context. But here, we
  // specify `contextifiedObject` as the context this Module belongs to.
  //
  // Here, we attempt to obtain the default export from the module "foo", and
  // put it into local binding "secret".

  const bar = new vm.SourceTextModule(`
    import s from 'foo';
    s;
    print(s);
  `, { context: contextifiedObject });

  // Step 2
  //
  // "Link" the imported dependencies of this Module to it.
  //
  // The provided linking callback (the "linker") accepts two arguments: the
  // parent module (`bar` in this case) and the string that is the specifier of
  // the imported module. The callback is expected to return a Module that
  // corresponds to the provided specifier, with certain requirements documented
  // in `module.link()`.
  //
  // If linking has not started for the returned Module, the same linker
  // callback will be called on the returned Module.
  //
  // Even top-level Modules without dependencies must be explicitly linked. The
  // callback provided would never be called, however.
  //
  // The link() method returns a Promise that will be resolved when all the
  // Promises returned by the linker resolve.
  //
  // Note: This is a contrived example in that the linker function creates a new
  // "foo" module every time it is called. In a full-fledged module system, a
  // cache would probably be used to avoid duplicated modules.

  async function linker(specifier, referencingModule) {
    if (specifier === 'foo') {
      return new vm.SourceTextModule(`
        // The "secret" variable refers to the global variable we added to
        // "contextifiedObject" when creating the context.
        export default secret;
      `, { context: referencingModule.context });

      // Using `contextifiedObject` instead of `referencingModule.context`
      // here would work as well.
    }
    throw new Error(`Unable to resolve dependency: ${specifier}`);
  }
  await bar.link(linker);

  // Step 3
  //
  // Evaluate the Module. The evaluate() method returns a promise which will
  // resolve after the module has finished evaluating.

  // Prints 42.
  await bar.evaluate();
})();

module.dependencySpecifiers#

The specifiers of all dependencies of this module. The returned array is frozen to disallow any changes to it.

Corresponds to the [[RequestedModules]] field of Cyclic Module Records in the ECMAScript specification.

module.error#

If the module.status is 'errored', this property contains the exception thrown by the module during evaluation. If the status is anything else, accessing this property will result in a thrown exception.

The value undefined cannot be used for cases where there is not a thrown exception due to possible ambiguity with throw undefined;.

Corresponds to the [[EvaluationError]] field of Cyclic Module Records in the ECMAScript specification.

module.evaluate([options])#

  • options <Object>

    • timeout <integer> Specifies the number of milliseconds to evaluate before terminating execution. If execution is interrupted, an Error will be thrown. This value must be a strictly positive integer.
    • breakOnSigint <boolean> If true, the execution will be terminated when SIGINT (Ctrl+C) is received. Existing handlers for the event that have been attached via process.on('SIGINT') will be disabled during script execution, but will continue to work after that. If execution is interrupted, an Error will be thrown. Default: false.
  • Returns: <Promise>

Evaluate the module.

This must be called after the module has been linked; otherwise it will reject. It could be called also when the module has already been evaluated, in which case it will either do nothing if the initial evaluation ended in success (module.status is 'evaluated') or it will re-throw the exception that the initial evaluation resulted in (module.status is 'errored').

This method cannot be called while the module is being evaluated (module.status is 'evaluating').

Corresponds to the Evaluate() concrete method field of Cyclic Module Records in the ECMAScript specification.

module.link(linker)#

Link module dependencies. This method must be called before evaluation, and can only be called once per module.

The function is expected to return a Module object or a Promise that eventually resolves to a Module object. The returned Module must satisfy the following two invariants:

  • It must belong to the same context as the parent Module.
  • Its status must not be 'errored'.

If the returned Module's status is 'unlinked', this method will be recursively called on the returned Module with the same provided linker function.

link() returns a Promise that will either get resolved when all linking instances resolve to a valid Module, or rejected if the linker function either throws an exception or returns an invalid Module.

The linker function roughly corresponds to the implementation-defined HostResolveImportedModule abstract operation in the ECMAScript specification, with a few key differences:

The actual HostResolveImportedModule implementation used during module linking is one that returns the modules linked during linking. Since at that point all modules would have been fully linked already, the HostResolveImportedModule implementation is fully synchronous per specification.

Corresponds to the Link() concrete method field of Cyclic Module Records in the ECMAScript specification.

module.namespace#

The namespace object of the module. This is only available after linking (module.link()) has completed.

Corresponds to the GetModuleNamespace abstract operation in the ECMAScript specification.

module.status#

The current status of the module. Will be one of:

  • 'unlinked': module.link() has not yet been called.

  • 'linking': module.link() has been called, but not all Promises returned by the linker function have been resolved yet.

  • 'linked': The module has been linked successfully, and all of its dependencies are linked, but module.evaluate() has not yet been called.

  • 'evaluating': The module is being evaluated through a module.evaluate() on itself or a parent module.

  • 'evaluated': The module has been successfully evaluated.

  • 'errored': The module has been evaluated, but an exception was thrown.

Other than 'errored', this status string corresponds to the specification's Cyclic Module Record's [[Status]] field. 'errored' corresponds to 'evaluated' in the specification, but with [[EvaluationError]] set to a value that is not undefined.

module.identifier#

The identifier of the current module, as set in the constructor.

Class: vm.SourceTextModule#

Added in: v9.6.0

Stability: 1 - Experimental

This feature is only available with the --experimental-vm-modules command flag enabled.

The vm.SourceTextModule class provides the Source Text Module Record as defined in the ECMAScript specification.

Constructor: new vm.SourceTextModule(code[, options])#

  • code <string> JavaScript Module code to parse

  • options

    • identifier <string> String used in stack traces. Default: 'vm:module(i)' where i is a context-specific ascending index.
    • cachedData <Buffer> | <TypedArray> | <DataView> Provides an optional Buffer or TypedArray, or DataView with V8's code cache data for the supplied source. The code must be the same as the module from which this cachedData was created.
    • context <Object> The contextified object as returned by the vm.createContext() method, to compile and evaluate this Module in.
    • lineOffset <integer> Specifies the line number offset that is displayed in stack traces produced by this Module. Default: 0.
    • columnOffset <integer> Specifies the column number offset that is displayed in stack traces produced by this Module. Default: 0.

    • initializeImportMeta <Function> Called during evaluation of this Module to initialize the import.meta.

    • importModuleDynamically <Function> Called during evaluation of this module when import() is called. If this option is not specified, calls to import() will reject with ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING.

Creates a new SourceTextModule instance.

Properties assigned to the import.meta object that are objects may allow the module to access information outside the specified context. Use vm.runInContext() to create objects in a specific context.

const vm = require('vm');

const contextifiedObject = vm.createContext({ secret: 42 });

(async () => {
  const module = new vm.SourceTextModule(
    'Object.getPrototypeOf(import.meta.prop).secret = secret;',
    {
      initializeImportMeta(meta) {
        // Note: this object is created in the top context. As such,
        // Object.getPrototypeOf(import.meta.prop) points to the
        // Object.prototype in the top context rather than that in
        // the contextified object.
        meta.prop = {};
      }
    });
  // Since module has no dependencies, the linker function will never be called.
  await module.link(() => {});
  await module.evaluate();

  // Now, Object.prototype.secret will be equal to 42.
  //
  // To fix this problem, replace
  //     meta.prop = {};
  // above with
  //     meta.prop = vm.runInContext('{}', contextifiedObject);
})();

sourceTextModule.createCachedData()#

Added in: v13.7.0

Creates a code cache that can be used with the SourceTextModule constructor's cachedData option. Returns a Buffer. This method may be called any number of times before the module has been evaluated.

// Create an initial module
const module = new vm.SourceTextModule('const a = 1;');

// Create cached data from this module
const cachedData = module.createCachedData();

// Create a new module using the cached data. The code must be the same.
const module2 = new vm.SourceTextModule('const a = 1;', { cachedData });

Class: vm.SyntheticModule#

Added in: v13.0.0, v12.16.0

Stability: 1 - Experimental

This feature is only available with the --experimental-vm-modules command flag enabled.

The vm.SyntheticModule class provides the Synthetic Module Record as defined in the WebIDL specification. The purpose of synthetic modules is to provide a generic interface for exposing non-JavaScript sources to ECMAScript module graphs.

const vm = require('vm');

const source = '{ "a": 1 }';
const module = new vm.SyntheticModule(['default'], function() {
  const obj = JSON.parse(source);
  this.setExport('default', obj);
});

// Use `module` in linking...

Constructor: new vm.SyntheticModule(exportNames, evaluateCallback[, options])#

Added in: v13.0.0, v12.16.0
  • exportNames <string[]> Array of names that will be exported from the module.
  • evaluateCallback <Function> Called when the module is evaluated.

  • options

    • identifier <string> String used in stack traces. Default: 'vm:module(i)' where i is a context-specific ascending index.
    • context <Object> The contextified object as returned by the vm.createContext() method, to compile and evaluate this Module in.

Creates a new SyntheticModule instance.

Objects assigned to the exports of this instance may allow importers of the module to access information outside the specified context. Use vm.runInContext() to create objects in a specific context.

syntheticModule.setExport(name, value)#

Added in: v13.0.0, v12.16.0
  • name <string> Name of the export to set.
  • value <any> The value to set the export to.

This method is used after the module is linked to set the values of exports. If it is called before the module is linked, an ERR_VM_MODULE_STATUS error will be thrown.

const vm = require('vm');

(async () => {
  const m = new vm.SyntheticModule(['x'], () => {
    m.setExport('x', 1);
  });

  await m.link(() => {});
  await m.evaluate();

  assert.strictEqual(m.namespace.x, 1);
})();

vm.compileFunction(code[, params[, options]])#

History
VersionChanges
v14.3.0

Removal of importModuleDynamically due to compatibility issues

v14.1.0

The importModuleDynamically option is now supported.

v10.10.0

Added in: v10.10.0

  • code <string> The body of the function to compile.
  • params <string[]> An array of strings containing all parameters for the function.

  • options <Object>

    • filename <string> Specifies the filename used in stack traces produced by this script. Default: ''.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script. Default: 0.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script. Default: 0.
    • cachedData <Buffer> | <TypedArray> | <DataView> Provides an optional Buffer or TypedArray, or DataView with V8's code cache data for the supplied source.
    • produceCachedData <boolean> Specifies whether to produce new cache data. Default: false.
    • parsingContext <Object> The contextified object in which the said function should be compiled in.
    • contextExtensions <Object[]> An array containing a collection of context extensions (objects wrapping the current scope) to be applied while compiling. Default: [].
  • Returns: <Function>

Compiles the given code into the provided context (if no context is supplied, the current context is used), and returns it wrapped inside a function with the given params.

vm.createContext([contextObject[, options]])#

History
VersionChanges
v10.0.0

The first argument can no longer be a function.

v10.0.0

The codeGeneration option is supported now.

v0.3.1

Added in: v0.3.1

  • contextObject <Object>

  • options <Object>

    • name <string> Human-readable name of the newly created context. Default: 'VM Context i', where i is an ascending numerical index of the created context.
    • origin <string> Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of the url.origin property of a URL object. Most notably, this string should omit the trailing slash, as that denotes a path. Default: ''.

    • codeGeneration <Object>

      • strings <boolean> If set to false any calls to eval or function constructors (Function, GeneratorFunction, etc) will throw an EvalError. Default: true.
      • wasm <boolean> If set to false any attempt to compile a WebAssembly module will throw a WebAssembly.CompileError. Default: true.
  • Returns: <Object> contextified object.

If given a contextObject, the vm.createContext() method will prepare that object so that it can be used in calls to vm.runInContext() or script.runInContext(). Inside such scripts, the contextObject will be the global object, retaining all of its existing properties but also having the built-in objects and functions any standard global object has. Outside of scripts run by the vm module, global variables will remain unchanged.

const vm = require('vm');

global.globalVar = 3;

const context = { globalVar: 1 };
vm.createContext(context);

vm.runInContext('globalVar *= 2;', context);

console.log(context);
// Prints: { globalVar: 2 }

console.log(global.globalVar);
// Prints: 3

If contextObject is omitted (or passed explicitly as undefined), a new, empty contextified object will be returned.

The vm.createContext() method is primarily useful for creating a single context that can be used to run multiple scripts. For instance, if emulating a web browser, the method can be used to create a single context representing a window's global object, then run all <script> tags together within that context.

The provided name and origin of the context are made visible through the Inspector API.

vm.isContext(object)#

Added in: v0.11.7

Returns true if the given oject object has been contextified using vm.createContext().

vm.runInContext(code, contextifiedObject[, options])#

History
VersionChanges
v6.3.0

The breakOnSigint option is supported now.

v0.3.1

Added in: v0.3.1

  • code <string> The JavaScript code to compile and run.
  • contextifiedObject <Object> The contextified object that will be used as the global when the code is compiled and run.

  • options <Object> | <string>

    • filename <string> Specifies the filename used in stack traces produced by this script. Default: 'evalmachine.<anonymous>'.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script. Default: 0.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script. Default: 0.
    • displayErrors <boolean> When true, if an Error occurs while compiling the code, the line of code causing the error is attached to the stack trace. Default: true.
    • timeout <integer> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown. This value must be a strictly positive integer.
    • breakOnSigint <boolean> If true, the execution will be terminated when SIGINT (Ctrl+C) is received. Existing handlers for the event that have been attached via process.on('SIGINT') will be disabled during script execution, but will continue to work after that. If execution is terminated, an Error will be thrown. Default: false.
    • cachedData <Buffer> | <TypedArray> | <DataView> Provides an optional Buffer or TypedArray, or DataView with V8's code cache data for the supplied source. When supplied, the cachedDataRejected value will be set to either true or false depending on acceptance of the data by V8.
    • produceCachedData <boolean> When true and no cachedData is present, V8 will attempt to produce code cache data for code. Upon success, a Buffer with V8's code cache data will be produced and stored in the cachedData property of the returned vm.Script instance. The cachedDataProduced value will be set to either true or false depending on whether code cache data is produced successfully. This option is deprecated in favor of script.createCachedData(). Default: false.

    • importModuleDynamically <Function> Called during evaluation of this module when import() is called. If this option is not specified, calls to import() will reject with ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING. This option is part of the experimental modules API, and should not be considered stable.

  • Returns: <any> the result of the very last statement executed in the script.

The vm.runInContext() method compiles code, runs it within the context of the contextifiedObject, then returns the result. Running code does not have access to the local scope. The contextifiedObject object must have been previously contextified using the vm.createContext() method.

If options is a string, then it specifies the filename.

The following example compiles and executes different scripts using a single contextified object:

const vm = require('vm');

const contextObject = { globalVar: 1 };
vm.createContext(contextObject);

for (let i = 0; i < 10; ++i) {
  vm.runInContext('globalVar *= 2;', contextObject);
}
console.log(contextObject);
// Prints: { globalVar: 1024 }

vm.runInNewContext(code[, contextObject[, options]])#

History
VersionChanges
v10.0.0

The contextCodeGeneration option is supported now.

v6.3.0

The breakOnSigint option is supported now.

v0.3.1

Added in: v0.3.1

  • code <string> The JavaScript code to compile and run.
  • contextObject <Object> An object that will be contextified. If undefined, a new object will be created.

  • options <Object> | <string>

    • filename <string> Specifies the filename used in stack traces produced by this script. Default: 'evalmachine.<anonymous>'.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script. Default: 0.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script. Default: 0.
    • displayErrors <boolean> When true, if an Error occurs while compiling the code, the line of code causing the error is attached to the stack trace. Default: true.
    • timeout <integer> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown. This value must be a strictly positive integer.
    • breakOnSigint <boolean> If true, the execution will be terminated when SIGINT (Ctrl+C) is received. Existing handlers for the event that have been attached via process.on('SIGINT') will be disabled during script execution, but will continue to work after that. If execution is terminated, an Error will be thrown. Default: false.
    • contextName <string> Human-readable name of the newly created context. Default: 'VM Context i', where i is an ascending numerical index of the created context.
    • contextOrigin <string> Origin corresponding to the newly created context for display purposes. The origin should be formatted like a URL, but with only the scheme, host, and port (if necessary), like the value of the url.origin property of a URL object. Most notably, this string should omit the trailing slash, as that denotes a path. Default: ''.

    • contextCodeGeneration <Object>

      • strings <boolean> If set to false any calls to eval or function constructors (Function, GeneratorFunction, etc) will throw an EvalError. Default: true.
      • wasm <boolean> If set to false any attempt to compile a WebAssembly module will throw a WebAssembly.CompileError. Default: true.
    • cachedData <Buffer> | <TypedArray> | <DataView> Provides an optional Buffer or TypedArray, or DataView with V8's code cache data for the supplied source. When supplied, the cachedDataRejected value will be set to either true or false depending on acceptance of the data by V8.
    • produceCachedData <boolean> When true and no cachedData is present, V8 will attempt to produce code cache data for code. Upon success, a Buffer with V8's code cache data will be produced and stored in the cachedData property of the returned vm.Script instance. The cachedDataProduced value will be set to either true or false depending on whether code cache data is produced successfully. This option is deprecated in favor of script.createCachedData(). Default: false.

    • importModuleDynamically <Function> Called during evaluation of this module when import() is called. If this option is not specified, calls to import() will reject with ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING. This option is part of the experimental modules API, and should not be considered stable.

  • Returns: <any> the result of the very last statement executed in the script.

The vm.runInNewContext() first contextifies the given contextObject (or creates a new contextObject if passed as undefined), compiles the code, runs it within the created context, then returns the result. Running code does not have access to the local scope.

If options is a string, then it specifies the filename.

The following example compiles and executes code that increments a global variable and sets a new one. These globals are contained in the contextObject.

const vm = require('vm');

const contextObject = {
  animal: 'cat',
  count: 2
};

vm.runInNewContext('count += 1; name = "kitty"', contextObject);
console.log(contextObject);
// Prints: { animal: 'cat', count: 3, name: 'kitty' }

vm.runInThisContext(code[, options])#

History
VersionChanges
v6.3.0

The breakOnSigint option is supported now.

v0.3.1

Added in: v0.3.1

  • code <string> The JavaScript code to compile and run.

  • options <Object> | <string>

    • filename <string> Specifies the filename used in stack traces produced by this script. Default: 'evalmachine.<anonymous>'.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script. Default: 0.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script. Default: 0.
    • displayErrors <boolean> When true, if an Error occurs while compiling the code, the line of code causing the error is attached to the stack trace. Default: true.
    • timeout <integer> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown. This value must be a strictly positive integer.
    • breakOnSigint <boolean> If true, the execution will be terminated when SIGINT (Ctrl+C) is received. Existing handlers for the event that have been attached via process.on('SIGINT') will be disabled during script execution, but will continue to work after that. If execution is terminated, an Error will be thrown. Default: false.
    • cachedData <Buffer> | <TypedArray> | <DataView> Provides an optional Buffer or TypedArray, or DataView with V8's code cache data for the supplied source. When supplied, the cachedDataRejected value will be set to either true or false depending on acceptance of the data by V8.
    • produceCachedData <boolean> When true and no cachedData is present, V8 will attempt to produce code cache data for code. Upon success, a Buffer with V8's code cache data will be produced and stored in the cachedData property of the returned vm.Script instance. The cachedDataProduced value will be set to either true or false depending on whether code cache data is produced successfully. This option is deprecated in favor of script.createCachedData(). Default: false.

    • importModuleDynamically <Function> Called during evaluation of this module when import() is called. If this option is not specified, calls to import() will reject with ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING. This option is part of the experimental modules API, and should not be considered stable.

  • Returns: <any> the result of the very last statement executed in the script.

vm.runInThisContext() compiles code, runs it within the context of the current global and returns the result. Running code does not have access to local scope, but does have access to the current global object.

If options is a string, then it specifies the filename.

The following example illustrates using both vm.runInThisContext() and the JavaScript eval() function to run the same code:

const vm = require('vm');
let localVar = 'initial value';

const vmResult = vm.runInThisContext('localVar = "vm";');
console.log(`vmResult: '${vmResult}', localVar: '${localVar}'`);
// Prints: vmResult: 'vm', localVar: 'initial value'

const evalResult = eval('localVar = "eval";');
console.log(`evalResult: '${evalResult}', localVar: '${localVar}'`);
// Prints: evalResult: 'eval', localVar: 'eval'

Because vm.runInThisContext() does not have access to the local scope, localVar is unchanged. In contrast, eval() does have access to the local scope, so the value localVar is changed. In this way vm.runInThisContext() is much like an indirect eval() call, e.g. (0,eval)('code').

Example: Running an HTTP Server within a VM#

When using either script.runInThisContext() or vm.runInThisContext(), the code is executed within the current V8 global context. The code passed to this VM context will have its own isolated scope.

In order to run a simple web server using the http module the code passed to the context must either call require('http') on its own, or have a reference to the http module passed to it. For instance:

'use strict';
const vm = require('vm');

const code = `
((require) => {
  const http = require('http');

  http.createServer((request, response) => {
    response.writeHead(200, { 'Content-Type': 'text/plain' });
    response.end('Hello World\\n');
  }).listen(8124);

  console.log('Server running at http://127.0.0.1:8124/');
})`;

vm.runInThisContext(code)(require);

The require() in the above case shares the state with the context it is passed from. This may introduce risks when untrusted code is executed, e.g. altering objects in the context in unwanted ways.

What does it mean to "contextify" an object?#

All JavaScript executed within Node.js runs within the scope of a "context". According to the V8 Embedder's Guide:

In V8, a context is an execution environment that allows separate, unrelated, JavaScript applications to run in a single instance of V8. You must explicitly specify the context in which you want any JavaScript code to be run.

When the method vm.createContext() is called, the contextObject argument (or a newly-created object if contextObject is undefined) is associated internally with a new instance of a V8 Context. This V8 Context provides the code run using the vm module's methods with an isolated global environment within which it can operate. The process of creating the V8 Context and associating it with the contextObject is what this document refers to as "contextifying" the object.

Timeout limitations when using process.nextTick(), Promises, and queueMicrotask()#

Because of the internal mechanics of how the process.nextTick() queue and the microtask queue that underlies Promises are implemented within V8 and Node.js, it is possible for code running within a context to "escape" the timeout set using vm.runInContext(), vm.runInNewContext(), and vm.runInThisContext().

For example, the following code executed by vm.runInNewContext() with a timeout of 5 milliseconds schedules an infinite loop to run after a promise resolves. The scheduled loop is never interrupted by the timeout:

const vm = require('vm');

function loop() {
  while (1) console.log(Date.now());
}

vm.runInNewContext(
  'Promise.resolve().then(loop);',
  { loop, console },
  { timeout: 5 }
);

This issue also occurs when the loop() call is scheduled using the process.nextTick() and queueMicrotask() functions.

This issue occurs because all contexts share the same microtask and nextTick queues.

WebAssembly System Interface (WASI)#

Stability: 1 - Experimental

The WASI API provides an implementation of the WebAssembly System Interface specification. WASI gives sandboxed WebAssembly applications access to the underlying operating system via a collection of POSIX-like functions.

'use strict';
const fs = require('fs');
const { WASI } = require('wasi');
const wasi = new WASI({
  args: process.argv,
  env: process.env,
  preopens: {
    '/sandbox': '/some/real/path/that/wasm/can/access'
  }
});
const importObject = { wasi_snapshot_preview1: wasi.wasiImport };

(async () => {
  const wasm = await WebAssembly.compile(fs.readFileSync('./binary.wasm'));
  const instance = await WebAssembly.instantiate(wasm, importObject);

  wasi.start(instance);
})();

The --experimental-wasi-unstable-preview1 and --experimental-wasm-bigint CLI arguments are needed for the previous example to run.

Class: WASI#

Added in: v13.3.0, v12.16.0

The WASI class provides the WASI system call API and additional convenience methods for working with WASI-based applications. Each WASI instance represents a distinct sandbox environment. For security purposes, each WASI instance must have its command line arguments, environment variables, and sandbox directory structure configured explicitly.

new WASI([options])#

Added in: v13.3.0, v12.16.0

  • options <Object>

    • args <Array> An array of strings that the WebAssembly application will see as command line arguments. The first argument is the virtual path to the WASI command itself. Default: [].
    • env <Object> An object similar to process.env that the WebAssembly application will see as its environment. Default: {}.
    • preopens <Object> This object represents the WebAssembly application's sandbox directory structure. The string keys of preopens are treated as directories within the sandbox. The corresponding values in preopens are the real paths to those directories on the host machine.
    • returnOnExit <boolean> By default, WASI applications terminate the Node.js process via the __wasi_proc_exit() function. Setting this option to true causes wasi.start() to return the exit code rather than terminate the process. Default: false.

wasi.start(instance)#

Added in: v13.3.0, v12.16.0

Attempt to begin execution of instance as a WASI command by invoking its _start() export. If instance does not contain a _start() export, or if instance contains an _initialize() export, then an exception is thrown.

start() requires that instance exports a WebAssembly.Memory named memory. If instance does not have a memory export an exception is thrown.

If start() is called more than once, an exception is thrown.

wasi.wasiImport#

Added in: v13.3.0, v12.16.0

wasiImport is an object that implements the WASI system call API. This object should be passed as the wasi_snapshot_preview1 import during the instantiation of a WebAssembly.Instance.

Worker Threads#

Stability: 2 - Stable

The worker_threads module enables the use of threads that execute JavaScript in parallel. To access it:

const worker = require('worker_threads');

Workers (threads) are useful for performing CPU-intensive JavaScript operations. They will not help much with I/O-intensive work. Node.js’s built-in asynchronous I/O operations are more efficient than Workers can be.

Unlike child_process or cluster, worker_threads can share memory. They do so by transferring ArrayBuffer instances or sharing SharedArrayBuffer instances.

const {
  Worker, isMainThread, parentPort, workerData
} = require('worker_threads');

if (isMainThread) {
  module.exports = function parseJSAsync(script) {
    return new Promise((resolve, reject) => {
      const worker = new Worker(__filename, {
        workerData: script
      });
      worker.on('message', resolve);
      worker.on('error', reject);
      worker.on('exit', (code) => {
        if (code !== 0)
          reject(new Error(`Worker stopped with exit code ${code}`));
      });
    });
  };
} else {
  const { parse } = require('some-js-parsing-library');
  const script = workerData;
  parentPort.postMessage(parse(script));
}

The above example spawns a Worker thread for each parse() call. In actual practice, use a pool of Workers instead for these kinds of tasks. Otherwise, the overhead of creating Workers would likely exceed their benefit.

When implementing a worker pool, use the AsyncResource API to inform diagnostic tools (e.g. in order to provide asynchronous stack traces) about the correlation between tasks and their outcomes. See "Using AsyncResource for a Worker thread pool" in the async_hooks documentation for an example implementation.

Worker threads inherit non-process-specific options by default. Refer to Worker constructor options to know how to customize worker thread options, specifically argv and execArgv options.

worker.isMainThread#

Added in: v10.5.0

Is true if this code is not running inside of a Worker thread.

const { Worker, isMainThread } = require('worker_threads');

if (isMainThread) {
  // This re-loads the current file inside a Worker instance.
  new Worker(__filename);
} else {
  console.log('Inside Worker!');
  console.log(isMainThread);  // Prints 'false'.
}

worker.moveMessagePortToContext(port, contextifiedSandbox)#

Added in: v11.13.0

Transfer a MessagePort to a different vm Context. The original port object will be rendered unusable, and the returned MessagePort instance will take its place.

The returned MessagePort will be an object in the target context, and will inherit from its global Object class. Objects passed to the port.onmessage() listener will also be created in the target context and inherit from its global Object class.

However, the created MessagePort will no longer inherit from EventEmitter, and only port.onmessage() can be used to receive events using it.

worker.parentPort#

Added in: v10.5.0

If this thread was spawned as a Worker, this will be a MessagePort allowing communication with the parent thread. Messages sent using parentPort.postMessage() will be available in the parent thread using worker.on('message'), and messages sent from the parent thread using worker.postMessage() will be available in this thread using parentPort.on('message').

const { Worker, isMainThread, parentPort } = require('worker_threads');

if (isMainThread) {
  const worker = new Worker(__filename);
  worker.once('message', (message) => {
    console.log(message);  // Prints 'Hello, world!'.
  });
  worker.postMessage('Hello, world!');
} else {
  // When a message from the parent thread is received, send it back:
  parentPort.once('message', (message) => {
    parentPort.postMessage(message);
  });
}

worker.receiveMessageOnPort(port)#

Added in: v12.3.0

Receive a single message from a given MessagePort. If no message is available, undefined is returned, otherwise an object with a single message property that contains the message payload, corresponding to the oldest message in the MessagePort’s queue.

const { MessageChannel, receiveMessageOnPort } = require('worker_threads');
const { port1, port2 } = new MessageChannel();
port1.postMessage({ hello: 'world' });

console.log(receiveMessageOnPort(port2));
// Prints: { message: { hello: 'world' } }
console.log(receiveMessageOnPort(port2));
// Prints: undefined

When this function is used, no 'message' event will be emitted and the onmessage listener will not be invoked.

worker.resourceLimits#

Added in: v13.2.0, v12.16.0

Provides the set of JS engine resource constraints inside this Worker thread. If the resourceLimits option was passed to the Worker constructor, this matches its values.

If this is used in the main thread, its value is an empty object.

worker.SHARE_ENV#

Added in: v11.14.0

A special value that can be passed as the env option of the Worker constructor, to indicate that the current thread and the Worker thread should share read and write access to the same set of environment variables.

const { Worker, SHARE_ENV } = require('worker_threads');
new Worker('process.env.SET_IN_WORKER = "foo"', { eval: true, env: SHARE_ENV })
  .on('exit', () => {
    console.log(process.env.SET_IN_WORKER);  // Prints 'foo'.
  });

worker.threadId#

Added in: v10.5.0

An integer identifier for the current thread. On the corresponding worker object (if there is any), it is available as worker.threadId. This value is unique for each Worker instance inside a single process.

worker.workerData#

Added in: v10.5.0

An arbitrary JavaScript value that contains a clone of the data passed to this thread’s Worker constructor.

The data is cloned as if using postMessage(), according to the HTML structured clone algorithm.

const { Worker, isMainThread, workerData } = require('worker_threads');

if (isMainThread) {
  const worker = new Worker(__filename, { workerData: 'Hello, world!' });
} else {
  console.log(workerData);  // Prints 'Hello, world!'.
}

Class: MessageChannel#

Added in: v10.5.0

Instances of the worker.MessageChannel class represent an asynchronous, two-way communications channel. The MessageChannel has no methods of its own. new MessageChannel() yields an object with port1 and port2 properties, which refer to linked MessagePort instances.

const { MessageChannel } = require('worker_threads');

const { port1, port2 } = new MessageChannel();
port1.on('message', (message) => console.log('received', message));
port2.postMessage({ foo: 'bar' });
// Prints: received { foo: 'bar' } from the `port1.on('message')` listener

Class: MessagePort#

Added in: v10.5.0

Instances of the worker.MessagePort class represent one end of an asynchronous, two-way communications channel. It can be used to transfer structured data, memory regions and other MessagePorts between different Workers.

With the exception of MessagePorts being EventEmitters rather than EventTargets, this implementation matches browser MessagePorts.

Event: 'close'#

Added in: v10.5.0

The 'close' event is emitted once either side of the channel has been disconnected.

const { MessageChannel } = require('worker_threads');
const { port1, port2 } = new MessageChannel();

// Prints:
//   foobar
//   closed!
port2.on('message', (message) => console.log(message));
port2.on('close', () => console.log('closed!'));

port1.postMessage('foobar');
port1.close();

Event: 'message'#

Added in: v10.5.0
  • value <any> The transmitted value

The 'message' event is emitted for any incoming message, containing the cloned input of port.postMessage().

Listeners on this event will receive a clone of the value parameter as passed to postMessage() and no further arguments.

port.close()#

Added in: v10.5.0

Disables further sending of messages on either side of the connection. This method can be called when no further communication will happen over this MessagePort.

The 'close' event will be emitted on both MessagePort instances that are part of the channel.

port.postMessage(value[, transferList])#

Added in: v10.5.0

Sends a JavaScript value to the receiving side of this channel. value will be transferred in a way which is compatible with the HTML structured clone algorithm.

In particular, the significant differences to JSON are:

  • value may contain circular references.
  • value may contain instances of builtin JS types such as RegExps, BigInts, Maps, Sets, etc.
  • value may contain typed arrays, both using ArrayBuffers and SharedArrayBuffers.
  • value may contain WebAssembly.Module instances.
  • value may not contain native (C++-backed) objects other than MessagePorts.
const { MessageChannel } = require('worker_threads');
const { port1, port2 } = new MessageChannel();

port1.on('message', (message) => console.log(message));

const circularData = {};
circularData.foo = circularData;
// Prints: { foo: [Circular] }
port2.postMessage(circularData);

transferList may be a list of ArrayBuffer and MessagePort objects. After transferring, they will not be usable on the sending side of the channel anymore (even if they are not contained in value). Unlike with child processes, transferring handles such as network sockets is currently not supported.

If value contains SharedArrayBuffer instances, those will be accessible from either thread. They cannot be listed in transferList.

value may still contain ArrayBuffer instances that are not in transferList; in that case, the underlying memory is copied rather than moved.

const { MessageChannel } = require('worker_threads');
const { port1, port2 } = new MessageChannel();

port1.on('message', (message) => console.log(message));

const uint8Array = new Uint8Array([ 1, 2, 3, 4 ]);
// This posts a copy of `uint8Array`:
port2.postMessage(uint8Array);
// This does not copy data, but renders `uint8Array` unusable:
port2.postMessage(uint8Array, [ uint8Array.buffer ]);

// The memory for the `sharedUint8Array` will be accessible from both the
// original and the copy received by `.on('message')`:
const sharedUint8Array = new Uint8Array(new SharedArrayBuffer(4));
port2.postMessage(sharedUint8Array);

// This transfers a freshly created message port to the receiver.
// This can be used, for example, to create communication channels between
// multiple `Worker` threads that are children of the same parent thread.
const otherChannel = new MessageChannel();
port2.postMessage({ port: otherChannel.port1 }, [ otherChannel.port1 ]);

Because the object cloning uses the structured clone algorithm, non-enumerable properties, property accessors, and object prototypes are not preserved. In particular, Buffer objects will be read as plain Uint8Arrays on the receiving side.

The message object will be cloned immediately, and can be modified after posting without having side effects.

For more information on the serialization and deserialization mechanisms behind this API, see the serialization API of the v8 module.

Considerations when transferring TypedArrays and Buffers#

All TypedArray and Buffer instances are views over an underlying ArrayBuffer. That is, it is the ArrayBuffer that actually stores the raw data while the TypedArray and Buffer objects provide a way of viewing and manipulating the data. It is possible and common for multiple views to be created over the same ArrayBuffer instance. Great care must be taken when using a transfer list to transfer an ArrayBuffer as doing so will cause all TypedArray and Buffer instances that share that same ArrayBuffer to become unusable.

const ab = new ArrayBuffer(10);

const u1 = new Uint8Array(ab);
const u2 = new Uint16Array(ab);

console.log(u2.length);  // prints 5

port.postMessage(u1, [u1.buffer]);

console.log(u2.length);  // prints 0

For Buffer instances, specifically, whether the underlying ArrayBuffer can be transferred or cloned depends entirely on how instances were created, which often cannot be reliably determined.

Depending on how a Buffer instance was created, it may or may not own its underlying ArrayBuffer. An ArrayBuffer must not be transferred unless it is known that the Buffer instance owns it. In particular, for Buffers created from the internal Buffer pool (using, for instance Buffer.from() or Buffer.alloc()), transferring them is not possible and they will always be cloned, which sends a copy of the entire Buffer pool. This behavior may come with unintended higher memory usage and possible security concerns.

See Buffer.allocUnsafe() for more details on Buffer pooling.

The ArrayBuffers for Buffer instances created using Buffer.alloc() or Buffer.allocUnsafeSlow() can always be transferred but doing so will render all other existing views of those ArrayBuffers unusable.

port.ref()#

Added in: v10.5.0

Opposite of unref(). Calling ref() on a previously unref()ed port will not let the program exit if it's the only active handle left (the default behavior). If the port is ref()ed, calling ref() again will have no effect.

If listeners are attached or removed using .on('message'), the port will be ref()ed and unref()ed automatically depending on whether listeners for the event exist.

port.start()#

Added in: v10.5.0

Starts receiving messages on this MessagePort. When using this port as an event emitter, this will be called automatically once 'message' listeners are attached.

This method exists for parity with the Web MessagePort API. In Node.js, it is only useful for ignoring messages when no event listener is present. Node.js also diverges in its handling of .onmessage. Setting it will automatically call .start(), but unsetting it will let messages queue up until a new handler is set or the port is discarded.

port.unref()#

Added in: v10.5.0

Calling unref() on a port will allow the thread to exit if this is the only active handle in the event system. If the port is already unref()ed calling unref() again will have no effect.

If listeners are attached or removed using .on('message'), the port will be ref()ed and unref()ed automatically depending on whether listeners for the event exist.

Class: Worker#

Added in: v10.5.0

The Worker class represents an independent JavaScript execution thread. Most Node.js APIs are available inside of it.

Notable differences inside a Worker environment are:

Creating Worker instances inside of other Workers is possible.

Like Web Workers and the cluster module, two-way communication can be achieved through inter-thread message passing. Internally, a Worker has a built-in pair of MessagePorts that are already associated with each other when the Worker is created. While the MessagePort object on the parent side is not directly exposed, its functionalities are exposed through worker.postMessage() and the worker.on('message') event on the Worker object for the parent thread.

To create custom messaging channels (which is encouraged over using the default global channel because it facilitates separation of concerns), users can create a MessageChannel object on either thread and pass one of the MessagePorts on that MessageChannel to the other thread through a pre-existing channel, such as the global one.

See port.postMessage() for more information on how messages are passed, and what kind of JavaScript values can be successfully transported through the thread barrier.

const assert = require('assert');
const {
  Worker, MessageChannel, MessagePort, isMainThread, parentPort
} = require('worker_threads');
if (isMainThread) {
  const worker = new Worker(__filename);
  const subChannel = new MessageChannel();
  worker.postMessage({ hereIsYourPort: subChannel.port1 }, [subChannel.port1]);
  subChannel.port2.on('message', (value) => {
    console.log('received:', value);
  });
} else {
  parentPort.once('message', (value) => {
    assert(value.hereIsYourPort instanceof MessagePort);
    value.hereIsYourPort.postMessage('the worker is sending this');
    value.hereIsYourPort.close();
  });
}

new Worker(filename[, options])#

History
VersionChanges
v14.0.0

The transferList option was introduced.

v13.12.0

The filename parameter can be a WHATWG URL object using file: protocol.

v13.2.0, v12.16.0

The resourceLimits option was introduced.

v13.4.0, v12.16.0

The argv option was introduced.

v10.5.0

Added in: v10.5.0

  • filename <string> | <URL> The path to the Worker’s main script or module. Must be either an absolute path or a relative path (i.e. relative to the current working directory) starting with ./ or ../, or a WHATWG URL object using file: protocol. If options.eval is true, this is a string containing JavaScript code rather than a path.

  • options <Object>

    • argv <any[]> List of arguments which would be stringified and appended to process.argv in the worker. This is mostly similar to the workerData but the values will be available on the global process.argv as if they were passed as CLI options to the script.
    • env <Object> If set, specifies the initial value of process.env inside the Worker thread. As a special value, worker.SHARE_ENV may be used to specify that the parent thread and the child thread should share their environment variables; in that case, changes to one thread’s process.env object will affect the other thread as well. Default: process.env.
    • eval <boolean> If true and the first argument is a string, interpret the first argument to the constructor as a script that is executed once the worker is online.
    • execArgv <string[]> List of node CLI options passed to the worker. V8 options (such as --max-old-space-size) and options that affect the process (such as --title) are not supported. If set, this will be provided as process.execArgv inside the worker. By default, options will be inherited from the parent thread.
    • stdin <boolean> If this is set to true, then worker.stdin will provide a writable stream whose contents will appear as process.stdin inside the Worker. By default, no data is provided.
    • stdout <boolean> If this is set to true, then worker.stdout will not automatically be piped through to process.stdout in the parent.
    • stderr <boolean> If this is set to true, then worker.stderr will not automatically be piped through to process.stderr in the parent.
    • workerData <any> Any JavaScript value that will be cloned and made available as require('worker_threads').workerData. The cloning will occur as described in the HTML structured clone algorithm, and an error will be thrown if the object cannot be cloned (e.g. because it contains functions).
    • transferList <Object[]> If one or more MessagePort-like objects are passed in workerData, a transferList is required for those items or ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST will be thrown. See port.postMessage() for more information.

    • resourceLimits <Object> An optional set of resource limits for the new JS engine instance. Reaching these limits will lead to termination of the Worker instance. These limits only affect the JS engine, and no external data, including no ArrayBuffers. Even if these limits are set, the process may still abort if it encounters a global out-of-memory situation.

      • maxOldGenerationSizeMb <number> The maximum size of the main heap in MB.
      • maxYoungGenerationSizeMb <number> The maximum size of a heap space for recently created objects.
      • codeRangeSizeMb <number> The size of a pre-allocated memory range used for generated code.
      • stackSizeMb <number> The default maximum stack size for the thread. Small values may lead to unusable Worker instances. Default: 4.

Event: 'error'#

Added in: v10.5.0

The 'error' event is emitted if the worker thread throws an uncaught exception. In that case, the worker will be terminated.

Event: 'exit'#

Added in: v10.5.0

The 'exit' event is emitted once the worker has stopped. If the worker exited by calling process.exit(), the exitCode parameter will be the passed exit code. If the worker was terminated, the exitCode parameter will be 1.

This is the final event emitted by any Worker instance.

Event: 'message'#

Added in: v10.5.0
  • value <any> The transmitted value

The 'message' event is emitted when the worker thread has invoked require('worker_threads').parentPort.postMessage(). See the port.on('message') event for more details.

All messages sent from the worker thread will be emitted before the 'exit' event is emitted on the Worker object.

Event: 'online'#

Added in: v10.5.0

The 'online' event is emitted when the worker thread has started executing JavaScript code.

worker.getHeapSnapshot()#

Added in: v13.9.0
  • Returns: <Promise> A promise for a Readable Stream containing a V8 heap snapshot

Returns a readable stream for a V8 snapshot of the current state of the Worker. See v8.getHeapSnapshot() for more details.

If the Worker thread is no longer running, which may occur before the 'exit' event is emitted, the returned Promise will be rejected immediately with an ERR_WORKER_NOT_RUNNING error.

worker.postMessage(value[, transferList])#

Added in: v10.5.0

Send a message to the worker that will be received via require('worker_threads').parentPort.on('message'). See port.postMessage() for more details.

worker.ref()#

Added in: v10.5.0

Opposite of unref(), calling ref() on a previously unref()ed worker will not let the program exit if it's the only active handle left (the default behavior). If the worker is ref()ed, calling ref() again will have no effect.

worker.resourceLimits#

Added in: v13.2.0, v12.16.0

Provides the set of JS engine resource constraints for this Worker thread. If the resourceLimits option was passed to the Worker constructor, this matches its values.

If the worker has stopped, the return value is an empty object.

worker.stderr#

Added in: v10.5.0

This is a readable stream which contains data written to process.stderr inside the worker thread. If stderr: true was not passed to the Worker constructor, then data will be piped to the parent thread's process.stderr stream.

worker.stdin#

Added in: v10.5.0

If stdin: true was passed to the Worker constructor, this is a writable stream. The data written to this stream will be made available in the worker thread as process.stdin.

worker.stdout#

Added in: v10.5.0

This is a readable stream which contains data written to process.stdout inside the worker thread. If stdout: true was not passed to the Worker constructor, then data will be piped to the parent thread's process.stdout stream.

worker.terminate()#

History
VersionChanges
v12.5.0

This function now returns a Promise. Passing a callback is deprecated, and was useless up to this version, as the Worker was actually terminated synchronously. Terminating is now a fully asynchronous operation.

v10.5.0

Added in: v10.5.0

Stop all JavaScript execution in the worker thread as soon as possible. Returns a Promise for the exit code that is fulfilled when the 'exit' event is emitted.

worker.threadId#

Added in: v10.5.0

An integer identifier for the referenced thread. Inside the worker thread, it is available as require('worker_threads').threadId. This value is unique for each Worker instance inside a single process.

worker.unref()#

Added in: v10.5.0

Calling unref() on a worker will allow the thread to exit if this is the only active handle in the event system. If the worker is already unref()ed calling unref() again will have no effect.

Zlib#

Stability: 2 - Stable

The zlib module provides compression functionality implemented using Gzip, Deflate/Inflate, and Brotli.

To access it:

const zlib = require('zlib');

Compression and decompression are built around the Node.js Streams API.

Compressing or decompressing a stream (such as a file) can be accomplished by piping the source stream through a zlib Transform stream into a destination stream:

const { createGzip } = require('zlib');
const { pipeline } = require('stream');
const {
  createReadStream,
  createWriteStream
} = require('fs');

const gzip = createGzip();
const source = createReadStream('input.txt');
const destination = createWriteStream('input.txt.gz');

pipeline(source, gzip, destination, (err) => {
  if (err) {
    console.error('An error occurred:', err);
    process.exitCode = 1;
  }
});

// Or, Promisified

const { promisify } = require('util');
const pipe = promisify(pipeline);

async function do_gzip(input, output) {
  const gzip = createGzip();
  const source = createReadStream(input);
  const destination = createWriteStream(output);
  await pipe(source, gzip, destination);
}

do_gzip('input.txt', 'input.txt.gz')
  .catch((err) => {
    console.error('An error occurred:', err);
    process.exitCode = 1;
  });

It is also possible to compress or decompress data in a single step:

const { deflate, unzip } = require('zlib');

const input = '.................................';
deflate(input, (err, buffer) => {
  if (err) {
    console.error('An error occurred:', err);
    process.exitCode = 1;
  }
  console.log(buffer.toString('base64'));
});

const buffer = Buffer.from('eJzT0yMAAGTvBe8=', 'base64');
unzip(buffer, (err, buffer) => {
  if (err) {
    console.error('An error occurred:', err);
    process.exitCode = 1;
  }
  console.log(buffer.toString());
});

// Or, Promisified

const { promisify } = require('util');
const do_unzip = promisify(unzip);

do_unzip(buffer)
  .then((buf) => console.log(buf.toString()))
  .catch((err) => {
    console.error('An error occurred:', err);
    process.exitCode = 1;
  });

Threadpool Usage and Performance Considerations#

All zlib APIs, except those that are explicitly synchronous, use the Node.js internal threadpool. This can lead to surprising effects and performance limitations in some applications.

Creating and using a large number of zlib objects simultaneously can cause significant memory fragmentation.

const zlib = require('zlib');

const payload = Buffer.from('This is some data');

// WARNING: DO NOT DO THIS!
for (let i = 0; i < 30000; ++i) {
  zlib.deflate(payload, (err, buffer) => {});
}

In the preceding example, 30,000 deflate instances are created concurrently. Because of how some operating systems handle memory allocation and deallocation, this may lead to to significant memory fragmentation.

It is strongly recommended that the results of compression operations be cached to avoid duplication of effort.

Compressing HTTP requests and responses#

The zlib module can be used to implement support for the gzip, deflate and br content-encoding mechanisms defined by HTTP.

The HTTP Accept-Encoding header is used within an http request to identify the compression encodings accepted by the client. The Content-Encoding header is used to identify the compression encodings actually applied to a message.

The examples given below are drastically simplified to show the basic concept. Using zlib encoding can be expensive, and the results ought to be cached. See Memory Usage Tuning for more information on the speed/memory/compression tradeoffs involved in zlib usage.

// Client request example
const zlib = require('zlib');
const http = require('http');
const fs = require('fs');
const { pipeline } = require('stream');

const request = http.get({ host: 'example.com',
                           path: '/',
                           port: 80,
                           headers: { 'Accept-Encoding': 'br,gzip,deflate' } });
request.on('response', (response) => {
  const output = fs.createWriteStream('example.com_index.html');

  const onError = (err) => {
    if (err) {
      console.error('An error occurred:', err);
      process.exitCode = 1;
    }
  };

  switch (response.headers['content-encoding']) {
    case 'br':
      pipeline(response, zlib.createBrotliDecompress(), output, onError);
      break;
    // Or, just use zlib.createUnzip() to handle both of the following cases:
    case 'gzip':
      pipeline(response, zlib.createGunzip(), output, onError);
      break;
    case 'deflate':
      pipeline(response, zlib.createInflate(), output, onError);
      break;
    default:
      pipeline(response, output, onError);
      break;
  }
});

// server example
// Running a gzip operation on every request is quite expensive.
// It would be much more efficient to cache the compressed buffer.
const zlib = require('zlib');
const http = require('http');
const fs = require('fs');
const { pipeline } = require('stream');

http.createServer((request, response) => {
  const raw = fs.createReadStream('index.html');
  // Store both a compressed and an uncompressed version of the resource.
  response.setHeader('Vary', 'Accept-Encoding');
  let acceptEncoding = request.headers['accept-encoding'];
  if (!acceptEncoding) {
    acceptEncoding = '';
  }

  const onError = (err) => {
    if (err) {
      // If an error occurs, there's not much we can do because
      // the server has already sent the 200 response code and
      // some amount of data has already been sent to the client.
      // The best we can do is terminate the response immediately
      // and log the error.
      response.end();
      console.error('An error occurred:', err);
    }
  };

  // Note: This is not a conformant accept-encoding parser.
  // See https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
  if (/\bdeflate\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'deflate' });
    pipeline(raw, zlib.createDeflate(), response, onError);
  } else if (/\bgzip\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'gzip' });
    pipeline(raw, zlib.createGzip(), response, onError);
  } else if (/\bbr\b/.test(acceptEncoding)) {
    response.writeHead(200, { 'Content-Encoding': 'br' });
    pipeline(raw, zlib.createBrotliCompress(), response, onError);
  } else {
    response.writeHead(200, {});
    pipeline(raw, response, onError);
  }
}).listen(1337);

By default, the zlib methods will throw an error when decompressing truncated data. However, if it is known that the data is incomplete, or the desire is to inspect only the beginning of a compressed file, it is possible to suppress the default error handling by changing the flushing method that is used to decompress the last chunk of input data:

// This is a truncated version of the buffer from the above examples
const buffer = Buffer.from('eJzT0yMA', 'base64');

zlib.unzip(
  buffer,
  // For Brotli, the equivalent is zlib.constants.BROTLI_OPERATION_FLUSH.
  { finishFlush: zlib.constants.Z_SYNC_FLUSH },
  (err, buffer) => {
    if (err) {
      console.error('An error occurred:', err);
      process.exitCode = 1;
    }
    console.log(buffer.toString());
  });

This will not change the behavior in other error-throwing situations, e.g. when the input data has an invalid format. Using this method, it will not be possible to determine whether the input ended prematurely or lacks the integrity checks, making it necessary to manually check that the decompressed result is valid.

Memory Usage Tuning#

For zlib-based streams#

From zlib/zconf.h, modified for Node.js usage:

The memory requirements for deflate are (in bytes):

(1 << (windowBits + 2)) + (1 << (memLevel + 9))

That is: 128K for windowBits = 15 + 128K for memLevel = 8 (default values) plus a few kilobytes for small objects.

For example, to reduce the default memory requirements from 256K to 128K, the options should be set to:

const options = { windowBits: 14, memLevel: 7 };

This will, however, generally degrade compression.

The memory requirements for inflate are (in bytes) 1 << windowBits. That is, 32K for windowBits = 15 (default value) plus a few kilobytes for small objects.

This is in addition to a single internal output slab buffer of size chunkSize, which defaults to 16K.

The speed of zlib compression is affected most dramatically by the level setting. A higher level will result in better compression, but will take longer to complete. A lower level will result in less compression, but will be much faster.

In general, greater memory usage options will mean that Node.js has to make fewer calls to zlib because it will be able to process more data on each write operation. So, this is another factor that affects the speed, at the cost of memory usage.

For Brotli-based streams#

There are equivalents to the zlib options for Brotli-based streams, although these options have different ranges than the zlib ones:

  • zlib’s level option matches Brotli’s BROTLI_PARAM_QUALITY option.
  • zlib’s windowBits option matches Brotli’s BROTLI_PARAM_LGWIN option.

See below for more details on Brotli-specific options.

Flushing#

Calling .flush() on a compression stream will make zlib return as much output as currently possible. This may come at the cost of degraded compression quality, but can be useful when data needs to be available as soon as possible.

In the following example, flush() is used to write a compressed partial HTTP response to the client:

const zlib = require('zlib');
const http = require('http');
const { pipeline } = require('stream');

http.createServer((request, response) => {
  // For the sake of simplicity, the Accept-Encoding checks are omitted.
  response.writeHead(200, { 'content-encoding': 'gzip' });
  const output = zlib.createGzip();
  let i;

  pipeline(output, response, (err) => {
    if (err) {
      // If an error occurs, there's not much we can do because
      // the server has already sent the 200 response code and
      // some amount of data has already been sent to the client.
      // The best we can do is terminate the response immediately
      // and log the error.
      clearInterval(i);
      response.end();
      console.error('An error occurred:', err);
    }
  });

  i = setInterval(() => {
    output.write(`The current time is ${Date()}\n`, () => {
      // The data has been passed to zlib, but the compression algorithm may
      // have decided to buffer the data for more efficient compression.
      // Calling .flush() will make the data available as soon as the client
      // is ready to receive it.
      output.flush();
    });
  }, 1000);
}).listen(1337);

Constants#

Added in: v0.5.8

zlib constants#

All of the constants defined in zlib.h are also defined on require('zlib').constants. In the normal course of operations, it will not be necessary to use these constants. They are documented so that their presence is not surprising. This section is taken almost directly from the zlib documentation.

Previously, the constants were available directly from require('zlib'), for instance zlib.Z_NO_FLUSH. Accessing the constants directly from the module is currently still possible but is deprecated.

Allowed flush values.

  • zlib.constants.Z_NO_FLUSH
  • zlib.constants.Z_PARTIAL_FLUSH
  • zlib.constants.Z_SYNC_FLUSH
  • zlib.constants.Z_FULL_FLUSH
  • zlib.constants.Z_FINISH
  • zlib.constants.Z_BLOCK
  • zlib.constants.Z_TREES

Return codes for the compression/decompression functions. Negative values are errors, positive values are used for special but normal events.

  • zlib.constants.Z_OK
  • zlib.constants.Z_STREAM_END
  • zlib.constants.Z_NEED_DICT
  • zlib.constants.Z_ERRNO
  • zlib.constants.Z_STREAM_ERROR
  • zlib.constants.Z_DATA_ERROR
  • zlib.constants.Z_MEM_ERROR
  • zlib.constants.Z_BUF_ERROR
  • zlib.constants.Z_VERSION_ERROR

Compression levels.

  • zlib.constants.Z_NO_COMPRESSION
  • zlib.constants.Z_BEST_SPEED
  • zlib.constants.Z_BEST_COMPRESSION
  • zlib.constants.Z_DEFAULT_COMPRESSION

Compression strategy.

  • zlib.constants.Z_FILTERED
  • zlib.constants.Z_HUFFMAN_ONLY
  • zlib.constants.Z_RLE
  • zlib.constants.Z_FIXED
  • zlib.constants.Z_DEFAULT_STRATEGY

Brotli constants#

Added in: v11.7.0, v10.16.0

There are several options and other constants available for Brotli-based streams:

Flush operations#

The following values are valid flush operations for Brotli-based streams:

  • zlib.constants.BROTLI_OPERATION_PROCESS (default for all operations)
  • zlib.constants.BROTLI_OPERATION_FLUSH (default when calling .flush())
  • zlib.constants.BROTLI_OPERATION_FINISH (default for the last chunk)

  • zlib.constants.BROTLI_OPERATION_EMIT_METADATA

    • This particular operation may be hard to use in a Node.js context, as the streaming layer makes it hard to know which data will end up in this frame. Also, there is currently no way to consume this data through the Node.js API.

Compressor options#

There are several options that can be set on Brotli encoders, affecting compression efficiency and speed. Both the keys and the values can be accessed as properties of the zlib.constants object.

The most important options are:

  • BROTLI_PARAM_MODE

    • BROTLI_MODE_GENERIC (default)
    • BROTLI_MODE_TEXT, adjusted for UTF-8 text
    • BROTLI_MODE_FONT, adjusted for WOFF 2.0 fonts

  • BROTLI_PARAM_QUALITY

    • Ranges from BROTLI_MIN_QUALITY to BROTLI_MAX_QUALITY, with a default of BROTLI_DEFAULT_QUALITY.

  • BROTLI_PARAM_SIZE_HINT

    • Integer value representing the expected input size; defaults to 0 for an unknown input size.

The following flags can be set for advanced control over the compression algorithm and memory usage tuning:

  • BROTLI_PARAM_LGWIN

    • Ranges from BROTLI_MIN_WINDOW_BITS to BROTLI_MAX_WINDOW_BITS, with a default of BROTLI_DEFAULT_WINDOW, or up to BROTLI_LARGE_MAX_WINDOW_BITS if the BROTLI_PARAM_LARGE_WINDOW flag is set.

  • BROTLI_PARAM_LGBLOCK

    • Ranges from BROTLI_MIN_INPUT_BLOCK_BITS to BROTLI_MAX_INPUT_BLOCK_BITS.

  • BROTLI_PARAM_DISABLE_LITERAL_CONTEXT_MODELING

    • Boolean flag that decreases compression ratio in favour of decompression speed.

  • BROTLI_PARAM_LARGE_WINDOW

    • Boolean flag enabling “Large Window Brotli” mode (not compatible with the Brotli format as standardized in RFC 7932).

  • BROTLI_PARAM_NPOSTFIX

    • Ranges from 0 to BROTLI_MAX_NPOSTFIX.

  • BROTLI_PARAM_NDIRECT

    • Ranges from 0 to 15 << NPOSTFIX in steps of 1 << NPOSTFIX.

Decompressor options#

These advanced options are available for controlling decompression:

  • BROTLI_DECODER_PARAM_DISABLE_RING_BUFFER_REALLOCATION

    • Boolean flag that affects internal memory allocation patterns.

  • BROTLI_DECODER_PARAM_LARGE_WINDOW

    • Boolean flag enabling “Large Window Brotli” mode (not compatible with the Brotli format as standardized in RFC 7932).

Class: Options#

History
VersionChanges
v9.4.0

The dictionary option can be an ArrayBuffer.

v8.0.0

The dictionary option can be an Uint8Array now.

v5.11.0

The finishFlush option is supported now.

v0.11.1

Added in: v0.11.1

Each zlib-based class takes an options object. No options are required.

Some options are only relevant when compressing and are ignored by the decompression classes.

See the deflateInit2 and inflateInit2 documentation for more information.

Class: BrotliOptions#

Added in: v11.7.0

Each Brotli-based class takes an options object. All options are optional.

For example:

const stream = zlib.createBrotliCompress({
  chunkSize: 32 * 1024,
  params: {
    [zlib.constants.BROTLI_PARAM_MODE]: zlib.constants.BROTLI_MODE_TEXT,
    [zlib.constants.BROTLI_PARAM_QUALITY]: 4,
    [zlib.constants.BROTLI_PARAM_SIZE_HINT]: fs.statSync(inputFile).size
  }
});

Class: zlib.BrotliCompress#

Added in: v11.7.0, v10.16.0

Compress data using the Brotli algorithm.

Class: zlib.BrotliDecompress#

Added in: v11.7.0, v10.16.0

Decompress data using the Brotli algorithm.

Class: zlib.Deflate#

Added in: v0.5.8

Compress data using deflate.

Class: zlib.DeflateRaw#

Added in: v0.5.8

Compress data using deflate, and do not append a zlib header.

Class: zlib.Gunzip#

History
VersionChanges
v6.0.0

Trailing garbage at the end of the input stream will now result in an 'error' event.

v5.9.0

Multiple concatenated gzip file members are supported now.

v5.0.0

A truncated input stream will now result in an 'error' event.

v0.5.8

Added in: v0.5.8

Decompress a gzip stream.

Class: zlib.Gzip#

Added in: v0.5.8

Compress data using gzip.

Class: zlib.Inflate#

History
VersionChanges
v5.0.0

A truncated input stream will now result in an 'error' event.

v0.5.8

Added in: v0.5.8

Decompress a deflate stream.

Class: zlib.InflateRaw#

History
VersionChanges
v6.8.0

Custom dictionaries are now supported by InflateRaw.

v5.0.0

A truncated input stream will now result in an 'error' event.

v0.5.8

Added in: v0.5.8

Decompress a raw deflate stream.

Class: zlib.Unzip#

Added in: v0.5.8

Decompress either a Gzip- or Deflate-compressed stream by auto-detecting the header.

Class: zlib.ZlibBase#

History
VersionChanges
v11.7.0, v10.16.0

This class was renamed from Zlib to ZlibBase.

v0.5.8

Added in: v0.5.8

Not exported by the zlib module. It is documented here because it is the base class of the compressor/decompressor classes.

This class inherits from stream.Transform, allowing zlib objects to be used in pipes and similar stream operations.

zlib.bytesRead#

Added in: v8.1.0Deprecated since: v10.0.0

Stability: 0 - Deprecated: Use zlib.bytesWritten instead.

Deprecated alias for zlib.bytesWritten. This original name was chosen because it also made sense to interpret the value as the number of bytes read by the engine, but is inconsistent with other streams in Node.js that expose values under these names.

zlib.bytesWritten#

Added in: v10.0.0

The zlib.bytesWritten property specifies the number of bytes written to the engine, before the bytes are processed (compressed or decompressed, as appropriate for the derived class).

zlib.close([callback])#

Added in: v0.9.4

Close the underlying handle.

zlib.flush([kind, ]callback)#

Added in: v0.5.8
  • kind Default: zlib.constants.Z_FULL_FLUSH for zlib-based streams, zlib.constants.BROTLI_OPERATION_FLUSH for Brotli-based streams.
  • callback <Function>

Flush pending data. Don't call this frivolously, premature flushes negatively impact the effectiveness of the compression algorithm.

Calling this only flushes data from the internal zlib state, and does not perform flushing of any kind on the streams level. Rather, it behaves like a normal call to .write(), i.e. it will be queued up behind other pending writes and will only produce output when data is being read from the stream.

zlib.params(level, strategy, callback)#

Added in: v0.11.4

This function is only available for zlib-based streams, i.e. not Brotli.

Dynamically update the compression level and compression strategy. Only applicable to deflate algorithm.

zlib.reset()#

Added in: v0.7.0

Reset the compressor/decompressor to factory defaults. Only applicable to the inflate and deflate algorithms.

zlib.constants#

Added in: v7.0.0

Provides an object enumerating Zlib-related constants.

zlib.createBrotliCompress([options])#

Added in: v11.7.0, v10.16.0

Creates and returns a new BrotliCompress object.

zlib.createBrotliDecompress([options])#

Added in: v11.7.0, v10.16.0

Creates and returns a new BrotliDecompress object.

zlib.createDeflate([options])#

Added in: v0.5.8

Creates and returns a new Deflate object.

zlib.createDeflateRaw([options])#

Added in: v0.5.8

Creates and returns a new DeflateRaw object.

An upgrade of zlib from 1.2.8 to 1.2.11 changed behavior when windowBits is set to 8 for raw deflate streams. zlib would automatically set windowBits to 9 if was initially set to 8. Newer versions of zlib will throw an exception, so Node.js restored the original behavior of upgrading a value of 8 to 9, since passing windowBits = 9 to zlib actually results in a compressed stream that effectively uses an 8-bit window only.

zlib.createGunzip([options])#

Added in: v0.5.8

Creates and returns a new Gunzip object.

zlib.createGzip([options])#

Added in: v0.5.8

Creates and returns a new Gzip object. See example.

zlib.createInflate([options])#

Added in: v0.5.8

Creates and returns a new Inflate object.

zlib.createInflateRaw([options])#

Added in: v0.5.8

Creates and returns a new InflateRaw object.

zlib.createUnzip([options])#

Added in: v0.5.8

Creates and returns a new Unzip object.

Convenience Methods#

All of these take a Buffer, TypedArray, DataView, ArrayBuffer or string as the first argument, an optional second argument to supply options to the zlib classes and will call the supplied callback with callback(error, result).

Every method has a *Sync counterpart, which accept the same arguments, but without a callback.

zlib.brotliCompress(buffer[, options], callback)#

Added in: v11.7.0, v10.16.0

zlib.brotliCompressSync(buffer[, options])#

Added in: v11.7.0, v10.16.0

Compress a chunk of data with BrotliCompress.

zlib.brotliDecompress(buffer[, options], callback)#

Added in: v11.7.0, v10.16.0

zlib.brotliDecompressSync(buffer[, options])#

Added in: v11.7.0, v10.16.0

Decompress a chunk of data with BrotliDecompress.

zlib.deflate(buffer[, options], callback)#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.6.0

Added in: v0.6.0

zlib.deflateSync(buffer[, options])#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.11.12

Added in: v0.11.12

Compress a chunk of data with Deflate.

zlib.deflateRaw(buffer[, options], callback)#

History
VersionChanges
v8.0.0

The buffer parameter can be any TypedArray or DataView.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.6.0

Added in: v0.6.0

zlib.deflateRawSync(buffer[, options])#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.11.12

Added in: v0.11.12

Compress a chunk of data with DeflateRaw.

zlib.gunzip(buffer[, options], callback)#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.6.0

Added in: v0.6.0

zlib.gunzipSync(buffer[, options])#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.11.12

Added in: v0.11.12

Decompress a chunk of data with Gunzip.

zlib.gzip(buffer[, options], callback)#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.6.0

Added in: v0.6.0

zlib.gzipSync(buffer[, options])#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.11.12

Added in: v0.11.12

Compress a chunk of data with Gzip.

zlib.inflate(buffer[, options], callback)#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.6.0

Added in: v0.6.0

zlib.inflateSync(buffer[, options])#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.11.12

Added in: v0.11.12

Decompress a chunk of data with Inflate.

zlib.inflateRaw(buffer[, options], callback)#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.6.0

Added in: v0.6.0

zlib.inflateRawSync(buffer[, options])#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.11.12

Added in: v0.11.12

Decompress a chunk of data with InflateRaw.

zlib.unzip(buffer[, options], callback)#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.6.0

Added in: v0.6.0

zlib.unzipSync(buffer[, options])#

History
VersionChanges
v9.4.0

The buffer parameter can be an ArrayBuffer.

v8.0.0

The buffer parameter can be any TypedArray or DataView.

v8.0.0

The buffer parameter can be an Uint8Array now.

v0.11.12

Added in: v0.11.12

Decompress a chunk of data with Unzip.