Node.js v4.6.1 Documentation

Table of Contents

Buffer#

Stability: 2 - Stable

Prior to the introduction of TypedArray in ECMAScript 2015 (ES6), the JavaScript language had no mechanism for reading or manipulating streams of binary data. The Buffer class was introduced as part of the Node.js API to make it possible to interact with octet streams in the context of things like TCP streams and file system operations.

Now that TypedArray has been added in ES6, the Buffer class implements the Uint8Array API in a manner that is more optimized and suitable for Node.js' use cases.

Instances of the Buffer class are similar to arrays of integers but correspond to fixed-sized, raw memory allocations outside the V8 heap. The size of the Buffer is established when it is created and cannot be resized.

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

const buf1 = new Buffer(10);
  // creates a buffer of length 10
  // This is the same as Buffer.allocUnsafe(10), and the returned
  // Buffer instance might contain old data that needs to be
  // overwritten using either fill() or write().

const buf2 = new Buffer([1,2,3]);
  // creates a buffer containing [01, 02, 03]
  // This is the same as Buffer.from([1,2,3]).

const buf3 = new Buffer('test');
  // creates a buffer containing ASCII bytes [74, 65, 73, 74]
  // This is the same as Buffer.from('test').

const buf4 = new Buffer('tést', 'utf8');
  // creates a buffer containing UTF8 bytes [74, c3, a9, 73, 74]
  // This is the same as Buffer.from('tést', 'utf8').

const buf5 = Buffer.alloc(10);
  // Creates a zero-filled Buffer of length 10.

const buf6 = Buffer.alloc(10, 1);
  // Creates a Buffer of length 10, filled with 0x01.

const buf7 = Buffer.allocUnsafe(10);
  // 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 either fill() or write().

const buf8 = Buffer.from([1,2,3]);
  // Creates a Buffer containing [01, 02, 03].

const buf9 = Buffer.from('test');
  // Creates a Buffer containing ASCII bytes [74, 65, 73, 74].

const buf8 = Buffer.from('tést', 'utf8');
  // Creates a Buffer containing UTF8 bytes [74, c3, a9, 73, 74].

Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe()#

Historically, Buffer instances have been 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. The memory allocated for such Buffer instances is not initialized and can contain sensitive data. Such Buffer objects must be initialized manually by using either buf.fill(0) or by writing to the Buffer completely. 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.
  • Passing a string, array, or Buffer as the first argument copies the passed object's data into the Buffer.
  • Passing an ArrayBuffer returns a Buffer that shares allocated memory with the given ArrayBuffer.

Because the behavior of new Buffer() changes significantly based on the type of value passed as the first argument, applications that do not properly validate the input arguments passed to new Buffer(), or that fail to appropriately initialize newly allocated Buffer content, can inadvertently introduce security and reliability issues into their code.

To make the creation of Buffer objects more reliable and less error prone, new Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe() methods have been introduced as an alternative means of creating Buffer instances.

Developers should migrate all existing uses of the new Buffer() constructors to one of these new APIs.

Buffer instances returned by Buffer.allocUnsafe(size) 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(size) never use the shared internal memory pool.

What makes Buffer.allocUnsafe(size) and Buffer.allocUnsafeSlow(size) "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.

Buffers and Character Encodings#

Buffers are commonly used to represent sequences of encoded characters such as UTF8, UCS2, Base64 or even Hex-encoded data. It is possible to convert back and forth between Buffers and ordinary JavaScript string objects by using an explicit encoding method.

const buf = new Buffer('hello world', 'ascii');
console.log(buf.toString('hex'));
  // prints: 68656c6c6f20776f726c64
console.log(buf.toString('base64'));
  // prints: aGVsbG8gd29ybGQ=

The character encodings currently supported by Node.js include:

  • 'ascii' - for 7-bit ASCII data only. This encoding method is very fast and will strip the high bit if set.

  • 'utf8' - Multibyte encoded Unicode characters. Many web pages and other document formats use UTF-8.

  • 'utf16le' - 2 or 4 bytes, little-endian encoded Unicode characters. Surrogate pairs (U+10000 to U+10FFFF) are supported.

  • 'ucs2' - Alias of 'utf16le'.

  • 'base64' - Base64 string 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.

  • 'binary' - A way of encoding the buffer into a one-byte (latin-1) encoded string. The string 'latin-1' is not supported. Instead, pass 'binary' to use 'latin-1' encoding.

  • 'hex' - Encode each byte as two hexadecimal characters.

Buffers and TypedArray#

Buffers are also Uint8Array TypedArray instances. However, there are subtle incompatibilities with the TypedArray specification in ECMAScript 2015. For instance, while ArrayBuffer#slice() creates a copy of the slice, the implementation of Buffer#slice() creates a view over the existing Buffer without copying, making Buffer#slice() far more efficient.

It is also possible to create new TypedArray instances from a Buffer with the following caveats:

  1. The Buffer instances's memory is copied to the TypedArray, not shared.

  2. The Buffer's memory is interpreted as an array of distinct elements, and not as a byte array of the target type. That is, new Uint32Array(new Buffer([1,2,3,4])) creates a 4-element Uint32Array with elements [1,2,3,4], not a Uint32Array with a single element [0x1020304] or [0x4030201].

It is possible to create a new Buffer that shares the same allocated memory as a TypedArray instance by using the TypeArray objects .buffer property:

const arr = new Uint16Array(2);
arr[0] = 5000;
arr[1] = 4000;

const buf1 = new Buffer(arr); // copies the buffer
const buf2 = new Buffer(arr.buffer); // shares the memory with arr;

console.log(buf1);
  // Prints: <Buffer 88 a0>, copied buffer has only two elements
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>

Note that when creating a Buffer using the TypeArray's .buffer, it is not currently possible to use only a portion of the underlying ArrayBuffer. To create a Buffer that uses only a part of the ArrayBuffer, use the buf.slice() function after the Buffer is created:

const arr = new Uint16Array(20);
const buf = new Buffer(arr.buffer).slice(0, 16);
console.log(buf.length);
  // Prints: 16

The Buffer.from() and TypedArray.from() (e.g.Uint8Array.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 ES6 iteration#

Buffers can be iterated over using the ECMAScript 2015 (ES6) for..of syntax:

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

for (var 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.

The --zero-fill-buffers command line option#

Node.js can be started using the --zero-fill-buffers command line option to force all newly allocated Buffer and SlowBuffer instances created using either new Buffer(size) and new SlowBuffer(size) to be automatically zero-filled upon creation. Use of this flag changes the default behavior of these methods and can have a significant impact on performance. Use of the --zero-fill-buffers option is recommended only when absolutely necessary to enforce that newly allocated Buffer instances cannot contain potentially sensitive data.

$ node --zero-fill-buffers
> Buffer(5);
<Buffer 00 00 00 00 00>

Class: Buffer#

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

new Buffer(array)#

Allocates a new Buffer using an array of octets.

const buf = new Buffer([0x62,0x75,0x66,0x66,0x65,0x72]);
  // creates a new Buffer containing ASCII bytes
  // ['b','u','f','f','e','r']

new Buffer(buffer)#

Copies the passed buffer data onto a new Buffer instance.

const buf1 = new Buffer('buffer');
const buf2 = new Buffer(buf1);

buf1[0] = 0x61;
console.log(buf1.toString());
  // 'auffer'
console.log(buf2.toString());
  // 'buffer' (copy is not changed)

new Buffer(arrayBuffer)#

  • arrayBuffer - The .buffer property of a TypedArray or a new ArrayBuffer()

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;

const buf = new Buffer(arr.buffer); // shares the memory with arr;

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

// changing the TypdArray changes the Buffer also
arr[1] = 6000;

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

new Buffer(size)#

Allocates a new Buffer of size bytes. The size must be less than or equal to the value of require('buffer').kMaxLength (on 64-bit architectures, kMaxLength is (2^31)-1). Otherwise, a RangeError is thrown. If a size less than 0 is specified, a zero-length Buffer will be created.

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

const buf = new Buffer(5);
console.log(buf);
  // <Buffer 78 e0 82 02 01>
  // (octets will be different, every time)
buf.fill(0);
console.log(buf);
  // <Buffer 00 00 00 00 00>

new Buffer(str[, encoding])#

Creates a new Buffer containing the given JavaScript string str. If provided, the encoding parameter identifies the strings character encoding.

const buf1 = new Buffer('this is a tést');
console.log(buf1.toString());
  // prints: this is a tést
console.log(buf1.toString('ascii'));
  // prints: this is a tC)st

const buf2 = new Buffer('7468697320697320612074c3a97374', 'hex');
console.log(buf2.toString());
  // prints: this is a tést

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

Added in: v4.5.0

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);
  // <Buffer 00 00 00 00 00>

The size must be less than or equal to the value of require('buffer').kMaxLength (on 64-bit architectures, kMaxLength is (2^31)-1). Otherwise, a RangeError is thrown. If a size less than 0 is specified, a zero-length Buffer will be created.

If fill is specified, the allocated Buffer will be initialized by calling buf.fill(fill). See [buf.fill()][] for more information.

const buf = Buffer.alloc(5, 'a');
console.log(buf);
  // <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). For example:

const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64');
console.log(buf);
  // <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64>

Calling Buffer.alloc(size) can be significantly slower than the alternative Buffer.allocUnsafe(size) but ensures that the newly created Buffer instance contents will never contain sensitive data.

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

Class Method: Buffer.allocUnsafe(size)#

Added in: v4.5.0

Allocates a new non-zero-filled Buffer of size bytes. The size must be less than or equal to the value of require('buffer').kMaxLength (on 64-bit architectures, kMaxLength is (2^31)-1). Otherwise, a RangeError is thrown. If a size less than 0 is specified, a zero-length Buffer will be created.

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 to zeroes.

const buf = Buffer.allocUnsafe(5);
console.log(buf);
  // <Buffer 78 e0 82 02 01>
  // (octets will be different, every time)
buf.fill(0);
console.log(buf);
  // <Buffer 00 00 00 00 00>

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

Note that 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(size) (and the new Buffer(size) constructor) only when size is less than or equal to Buffer.poolSize >> 1 (floor of Buffer.poolSize divided by two). The default value of Buffer.poolSize is 8192 but can be modified.

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(size) provides.

Class Method: Buffer.allocUnsafeSlow(size)#

Added in: v4.5.0

Allocates a new non-zero-filled and non-pooled Buffer of size bytes. The size must be less than or equal to the value of require('buffer').kMaxLength (on 64-bit architectures, kMaxLength is (2^31)-1). Otherwise, a RangeError is thrown. If a size less than 0 is specified, a zero-length Buffer will be created.

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 to zeroes.

When using Buffer.allocUnsafe() to allocate new Buffer instances, allocations under 4KB are, by default, sliced from a single pre-allocated Buffer. This allows applications to avoid the garbage collection overhead of creating many individually allocated Buffers. This approach improves both performance and memory usage by eliminating the need to track and cleanup as many Persistent 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() then copy out the relevant bits.

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

socket.on('readable', () => {
  const data = socket.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);
});

Use of Buffer.allocUnsafeSlow() should be used only as a last resort after a developer has observed undue memory retention in their applications.

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

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

Returns the actual byte length of a string. This is not the same as String.prototype.length since that returns the number of characters in a string.

Example:

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

console.log(`${str}: ${str.length} characters, ` +
            `${Buffer.byteLength(str, 'utf8')} bytes`);

// ½ + ¼ = ¾: 9 characters, 12 bytes

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

Added in: v0.11.13

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

const arr = [Buffer('1234'), Buffer('0123')];
arr.sort(Buffer.compare);

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

Added in: v0.7.11
  • list <Array> List of Buffer objects to concat
  • totalLength <Number> Total length of the Buffers in the list when concatenated
  • Return: <Buffer>

Returns a new Buffer which is the result of concatenating all the Buffers 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 Buffers in the list. This, however, adds an additional loop to the function, so it is faster to provide the length explicitly.

Example: build a single Buffer from a list of three Buffers:

const buf1 = new Buffer(10).fill(0);
const buf2 = new Buffer(14).fill(0);
const buf3 = new Buffer(18).fill(0);
const totalLength = buf1.length + buf2.length + buf3.length;

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

// 42
// <Buffer 00 00 00 00 ...>
// 42

Class Method: Buffer.from(array)#

Added in: v3.0.0

Allocates a new Buffer using an array of octets.

const buf = Buffer.from([0x62,0x75,0x66,0x66,0x65,0x72]);
  // creates a new Buffer containing ASCII bytes
  // ['b','u','f','f','e','r']

A TypeError will be thrown if array is not an Array.

Class Method: Buffer.from(arrayBuffer)#

Added in: v4.5.0
  • arrayBuffer <ArrayBuffer> The .buffer property of a TypedArray or a new ArrayBuffer()

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;

const buf = Buffer.from(arr.buffer); // shares the memory with arr;

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

// changing the TypedArray changes the Buffer also
arr[1] = 6000;

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

A TypeError will be thrown if arrayBuffer is not an ArrayBuffer.

Class Method: Buffer.from(buffer)#

Added in: v3.0.0

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());
  // 'auffer'
console.log(buf2.toString());
  // 'buffer' (copy is not changed)

A TypeError will be thrown if buffer is not a Buffer.

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

Added in: v4.5.0
  • str <String> String to encode.
  • encoding <String> Encoding to use, Default: 'utf8'

Creates a new Buffer containing the given JavaScript string str. If provided, the encoding parameter identifies the character encoding. If not provided, encoding defaults to 'utf8'.

const buf1 = Buffer.from('this is a tést');
console.log(buf1.toString());
  // prints: this is a tést
console.log(buf1.toString('ascii'));
  // prints: this is a tC)st

const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex');
console.log(buf2.toString());
  // prints: this is a tést

A TypeError will be thrown if str is not a string.

Class Method: Buffer.isBuffer(obj)#

Returns 'true' if obj is a Buffer.

Class Method: Buffer.isEncoding(encoding)#

Added in: v0.9.1

Returns true if the encoding is a valid encoding argument, or false otherwise.

buf[index]#

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

Example: copy an ASCII string into a Buffer, one byte at a time:

const str = "Node.js";
const buf = new Buffer(str.length);

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

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

buf.compare(otherBuffer)#

Added in: v0.11.13

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

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

[buf1, buf2, buf3].sort(Buffer.compare);
  // produces sort order [buf1, buf3, buf2]

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

Copies data from a region of this Buffer to a region in the target Buffer even if the target memory region overlaps with the source.

Example: build two Buffers, then copy buf1 from byte 16 through byte 19 into buf2, starting at the 8th byte in buf2.

const buf1 = new Buffer(26);
const buf2 = new Buffer(26).fill('!');

for (var i = 0 ; i < 26 ; i++) {
  buf1[i] = i + 97; // 97 is ASCII a
}

buf1.copy(buf2, 8, 16, 20);
console.log(buf2.toString('ascii', 0, 25));
  // Prints: !!!!!!!!qrst!!!!!!!!!!!!!

Example: Build a single Buffer, then copy data from one region to an overlapping region in the same Buffer

const buf = new Buffer(26);

for (var i = 0 ; i < 26 ; i++) {
  buf[i] = i + 97; // 97 is ASCII a
}

buf.copy(buf, 0, 4, 10);
console.log(buf.toString());

// efghijghijklmnopqrstuvwxyz

buf.entries()#

Added in: v1.1.0
  • Return: <Iterator>

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

const buf = new Buffer('buffer');
for (var pair of buf.entries()) {
  console.log(pair);
}
// prints:
//   [0, 98]
//   [1, 117]
//   [2, 102]
//   [3, 102]
//   [4, 101]
//   [5, 114]

buf.equals(otherBuffer)#

Added in: v1.0.0

Returns a boolean indicating whether this and otherBuffer have exactly the same bytes.

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

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

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

Added in: v0.5.0

Fills the Buffer with the specified value. If the offset and end are not given it will fill the entire Buffer. The method returns a reference to the Buffer so calls can be chained.

const b = new Buffer(50).fill('h');
console.log(b.toString());
  // Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

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

Added in: v1.5.0

Operates similar to Array#indexOf() in that it returns either the starting index position of value in Buffer or -1 if the Buffer does not contain value. The value can be a String, Buffer or Number. Strings are by default interpreted as UTF8. Buffers will use the entire Buffer (to compare a partial Buffer use buf.slice()). Numbers can range from 0 to 255.

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

buf.indexOf('this');
  // returns 0
buf.indexOf('is');
  // returns 2
buf.indexOf(new Buffer('a buffer'));
  // returns 8
buf.indexOf(97); // ascii for 'a'
  // returns 8
buf.indexOf(new Buffer('a buffer example'));
  // returns -1
buf.indexOf(new Buffer('a buffer example').slice(0,8));
  // returns 8

const utf16Buffer = new Buffer('\u039a\u0391\u03a3\u03a3\u0395', 'ucs2');

utf16Buffer.indexOf('\u03a3',  0, 'ucs2');
  // returns 4
utf16Buffer.indexOf('\u03a3', -4, 'ucs2');
  // returns 6

buf.keys()#

Added in: v1.1.0
  • Return: <Iterator>

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

const buf = new Buffer('buffer');
for (var key of buf.keys()) {
  console.log(key);
}
// prints:
//   0
//   1
//   2
//   3
//   4
//   5

buf.length#

Returns the amount of memory allocated for the Buffer in number of bytes. Note that this does not necessarily reflect the amount of usable data within the Buffer. For instance, in the example below, a Buffer with 1234 bytes is allocated, but only 11 ASCII bytes are written.

const buf = new Buffer(1234);

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

buf.write('some string', 0, 'ascii');
console.log(buf.length);
  // Prints: 1234

While the length property is not immutable, changing the value of length can result in undefined and inconsistent behavior. Applications that wish to modify the length of a Buffer should therefore treat length as read-only and use buf.slice() to create a new Buffer.

var buf = new Buffer(10);
buf.write('abcdefghj', 0, 'ascii');
console.log(buf.length);
  // Prints: 10
buf = buf.slice(0,5);
console.log(buf.length);
  // Prints: 5

buf.readDoubleBE(offset[, noAssert])#

buf.readDoubleLE(offset[, noAssert])#

Reads a 64-bit double from the Buffer at the specified offset with specified endian format (readDoubleBE() returns big endian, readDoubleLE() returns little endian).

Setting noAssert to true skips validation of the offset. This allows the offset to be beyond the end of the Buffer.

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

buf.readDoubleBE();
  // Returns: 8.20788039913184e-304
buf.readDoubleLE();
  // Returns: 5.447603722011605e-270
buf.readDoubleLE(1);
  // throws RangeError: Index out of range

buf.readDoubleLE(1, true); // Warning: reads passed end of buffer!
  // Segmentation fault! don't do this!

buf.readFloatBE(offset[, noAssert])#

buf.readFloatLE(offset[, noAssert])#

Reads a 32-bit float from the Buffer at the specified offset with specified endian format (readFloatBE() returns big endian, readFloatLE() returns little endian).

Setting noAssert to true skips validation of the offset. This allows the offset to be beyond the end of the Buffer.

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

buf.readFloatBE();
  // Returns: 2.387939260590663e-38
buf.readFloatLE();
  // Returns: 1.539989614439558e-36
buf.readFloatLE(1);
  // throws RangeError: Index out of range

buf.readFloatLE(1, true); // Warning: reads passed end of buffer!
  // Segmentation fault! don't do this!

buf.readInt8(offset[, noAssert])#

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

Setting noAssert to true skips validation of the offset. This allows the offset to be beyond the end of the Buffer.

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

const buf = new Buffer([1,-2,3,4]);

buf.readInt8(0);
  // returns 1
buf.readInt8(1);
  // returns -2

buf.readInt16BE(offset[, noAssert])#

buf.readInt16LE(offset[, noAssert])#

Reads a signed 16-bit integer from the Buffer at the specified offset with the specified endian format (readInt16BE() returns big endian, readInt16LE() returns little endian).

Setting noAssert to true skips validation of the offset. This allows the offset to be beyond the end of the Buffer.

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

const buf = new Buffer([1,-2,3,4]);

buf.readInt16BE();
  // returns 510
buf.readInt16LE(1);
  // returns 1022

buf.readInt32BE(offset[, noAssert])#

buf.readInt32LE(offset[, noAssert])#

Reads a signed 32-bit integer from the Buffer at the specified offset with the specified endian format (readInt32BE() returns big endian, readInt32LE() returns little endian).

Setting noAssert to true skips validation of the offset. This allows the offset to be beyond the end of the Buffer.

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

const buf = new Buffer([1,-2,3,4]);

buf.readInt32BE();
  // returns 33424132
buf.readInt32LE();
  // returns 67370497
buf.readInt32LE(1);
  // throws RangeError: Index out of range

buf.readIntBE(offset, byteLength[, noAssert])#

buf.readIntLE(offset, byteLength[, noAssert])#

Added in: v1.0.0

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

const buf = new Buffer(6);
buf.writeUInt16LE(0x90ab, 0);
buf.writeUInt32LE(0x12345678, 2);
buf.readIntLE(0, 6).toString(16);  // Specify 6 bytes (48 bits)
// Returns: '1234567890ab'

buf.readIntBE(0, 6).toString(16);
// Returns: -546f87a9cbee

Setting noAssert to true skips validation of the offset. This allows the offset to be beyond the end of the Buffer.

buf.readUInt8(offset[, noAssert])#

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

Setting noAssert to true skips validation of the offset. This allows the offset to be beyond the end of the Buffer.

const buf = new Buffer([1,-2,3,4]);

buf.readUInt8(0);
  // returns 1
buf.readUInt8(1);
  // returns 254

buf.readUInt16BE(offset[, noAssert])#

buf.readUInt16LE(offset[, noAssert])#

Reads an unsigned 16-bit integer from the Buffer at the specified offset with specified endian format (readUInt16BE() returns big endian, readUInt16LE() returns little endian).

Setting noAssert to true skips validation of the offset. This allows the offset to be beyond the end of the Buffer.

Example:

const buf = new Buffer([0x3, 0x4, 0x23, 0x42]);

buf.readUInt16BE(0);
  // Returns: 0x0304
buf.readUInt16LE(0);
  // Returns: 0x0403
buf.readUInt16BE(1);
  // Returns: 0x0423
buf.readUInt16LE(1);
  // Returns: 0x2304
buf.readUInt16BE(2);
  // Returns: 0x2342
buf.readUInt16LE(2);
  // Returns: 0x4223

buf.readUInt32BE(offset[, noAssert])#

buf.readUInt32LE(offset[, noAssert])#

Reads an unsigned 32-bit integer from the Buffer at the specified offset with specified endian format (readUInt32BE() returns big endian, readUInt32LE() returns little endian).

Setting noAssert to true skips validation of the offset. This allows the offset to be beyond the end of the Buffer.

Example:

const buf = new Buffer([0x3, 0x4, 0x23, 0x42]);

buf.readUInt32BE(0);
  // Returns: 0x03042342
console.log(buf.readUInt32LE(0));
  // Returns: 0x42230403

buf.readUIntBE(offset, byteLength[, noAssert])#

buf.readUIntLE(offset, byteLength[, noAssert])#

Added in: v1.0.0

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

const buf = new Buffer(6);
buf.writeUInt16LE(0x90ab, 0);
buf.writeUInt32LE(0x12345678, 2);
buf.readUIntLE(0, 6).toString(16);  // Specify 6 bytes (48 bits)
// Returns: '1234567890ab'

buf.readUIntBE(0, 6).toString(16);
// Returns: ab9078563412

Setting noAssert to true skips validation of the offset. This allows the offset to be beyond the end of the Buffer.

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

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

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

Example: build a Buffer with the ASCII alphabet, take a slice, then modify one byte from the original Buffer.

const buf1 = new Buffer(26);

for (var i = 0 ; i < 26 ; i++) {
  buf1[i] = i + 97; // 97 is ASCII a
}

const buf2 = buf1.slice(0, 3);
buf2.toString('ascii', 0, buf2.length);
  // Returns: 'abc'
buf1[0] = 33;
buf2.toString('ascii', 0, buf2.length);
  // Returns : '!bc'

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

const buf = new Buffer('buffer');

buf.slice(-6, -1).toString();
  // Returns 'buffe', equivalent to buf.slice(0, 5)
buf.slice(-6, -2).toString();
  // Returns 'buff', equivalent to buf.slice(0, 4)
buf.slice(-5, -2).toString();
  // Returns 'uff', equivalent to buf.slice(1, 4)

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

Decodes and returns a string from the Buffer data using the specified character set encoding.

const buf = new Buffer(26);
for (var i = 0 ; i < 26 ; i++) {
  buf[i] = i + 97; // 97 is ASCII a
}
buf.toString('ascii');
  // Returns: 'abcdefghijklmnopqrstuvwxyz'
buf.toString('ascii',0,5);
  // Returns: 'abcde'
buf.toString('utf8',0,5);
  // Returns: 'abcde'
buf.toString(undefined,0,5);
  // Returns: 'abcde', encoding defaults to 'utf8'

buf.toJSON()#

Added in: v0.9.2

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

Example:

const buf = new Buffer('test');
const json = JSON.stringify(buf);

console.log(json);
// Prints: '{"type":"Buffer","data":[116,101,115,116]}'

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

console.log(copy.toString());
// Prints: 'test'

buf.values()#

Added in: v1.1.0
  • Return: <Iterator>

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

const buf = new Buffer('buffer');
for (var value of buf.values()) {
  console.log(value);
}
// prints:
//   98
//   117
//   102
//   102
//   101
//   114

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

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

Writes string to the Buffer at offset using the given encoding. The length parameter is the number of bytes to write. If the Buffer did not contain enough space to fit the entire string, only a partial amount of the string will be written however, it will not write only partially encoded characters.

const buf = new Buffer(256);
const len = buf.write('\u00bd + \u00bc = \u00be', 0);
console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);
  // Prints: 12 bytes: ½ + ¼ = ¾

buf.writeDoubleBE(value, offset[, noAssert])#

buf.writeDoubleLE(value, offset[, noAssert])#

  • value <Number> Bytes to be written to Buffer
  • offset <Number> 0 <= offset <= buf.length - 8
  • noAssert <Boolean> Default: false
  • Return: <Number> The offset plus the number of written bytes

Writes value to the Buffer at the specified offset with specified endian format (writeDoubleBE() writes big endian, writeDoubleLE() writes little endian). The value argument should be a valid 64-bit double. Behavior is not defined when value is anything other than a 64-bit double.

Set noAssert to true to skip validation of value and offset. This means that value may be too large for the specific function and offset may be beyond the end of the Buffer leading to the values being silently dropped. This should not be used unless you are certain of correctness.

Example:

const buf = new Buffer(8);
buf.writeDoubleBE(0xdeadbeefcafebabe, 0);

console.log(buf);
  // Prints: <Buffer 43 eb d5 b7 dd f9 5f d7>

buf.writeDoubleLE(0xdeadbeefcafebabe, 0);

console.log(buf);
  // Prints: <Buffer d7 5f f9 dd b7 d5 eb 43>

buf.writeFloatBE(value, offset[, noAssert])#

buf.writeFloatLE(value, offset[, noAssert])#

  • value <Number> Bytes to be written to Buffer
  • offset <Number> 0 <= offset <= buf.length - 4
  • noAssert <Boolean> Default: false
  • Return: <Number> The offset plus the number of written bytes

Writes value to the Buffer at the specified offset with specified endian format (writeFloatBE() writes big endian, writeFloatLE() writes little endian). Behavior is not defined when value is anything other than a 32-bit float.

Set noAssert to true to skip validation of value and offset. This means that value may be too large for the specific function and offset may be beyond the end of the Buffer leading to the values being silently dropped. This should not be used unless you are certain of correctness.

Example:

const buf = new Buffer(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[, noAssert])#

  • value <Number> Bytes to be written to Buffer
  • offset <Number> 0 <= offset <= buf.length - 1
  • noAssert <Boolean> Default: false
  • Return: <Number> The offset plus the number of written bytes

Writes value to the Buffer at the specified offset. The value should be a valid signed 8-bit integer. Behavior is not defined when value is anything other than a signed 8-bit integer.

Set noAssert to true to skip validation of value and offset. This means that value may be too large for the specific function and offset may be beyond the end of the Buffer leading to the values being silently dropped. This should not be used unless you are certain of correctness.

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

const buf = new Buffer(2);
buf.writeInt8(2, 0);
buf.writeInt8(-2, 1);
console.log(buf);
  // Prints: <Buffer 02 fe>

buf.writeInt16BE(value, offset[, noAssert])#

buf.writeInt16LE(value, offset[, noAssert])#

  • value <Number> Bytes to be written to Buffer
  • offset <Number> 0 <= offset <= buf.length - 2
  • noAssert <Boolean> Default: false
  • Return: <Number> The offset plus the number of written bytes

Writes value to the Buffer at the specified offset with specified endian format (writeInt16BE() writes big endian, writeInt16LE() writes little endian). The value should be a valid signed 16-bit integer. Behavior is not defined when value is anything other than a signed 16-bit integer.

Set noAssert to true to skip validation of value and offset. This means that value may be too large for the specific function and offset may be beyond the end of the Buffer leading to the values being silently dropped. This should not be used unless you are certain of correctness.

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

const buf = new Buffer(4);
buf.writeInt16BE(0x0102,0);
buf.writeInt16LE(0x0304,2);
console.log(buf);
  // Prints: <Buffer 01 02 04 03>

buf.writeInt32BE(value, offset[, noAssert])#

buf.writeInt32LE(value, offset[, noAssert])#

  • value <Number> Bytes to be written to Buffer
  • offset <Number> 0 <= offset <= buf.length - 4
  • noAssert <Boolean> Default: false
  • Return: <Number> The offset plus the number of written bytes

Writes value to the Buffer at the specified offset with specified endian format (writeInt32BE() writes big endian, writeInt32LE() writes little endian). The value should be a valid signed 32-bit integer. Behavior is not defined when value is anything other than a signed 32-bit integer.

Set noAssert to true to skip validation of value and offset. This means that value may be too large for the specific function and offset may be beyond the end of the Buffer leading to the values being silently dropped. This should not be used unless you are certain of correctness.

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

const buf = new Buffer(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[, noAssert])#

buf.writeIntLE(value, offset, byteLength[, noAssert])#

Added in: v1.0.0
  • value <Number> Bytes to be written to Buffer
  • offset <Number> 0 <= offset <= buf.length - byteLength
  • byteLength <Number> 0 < byteLength <= 6
  • noAssert <Boolean> Default: false
  • Return: <Number> The offset plus the number of written bytes

Writes value to the Buffer at the specified offset and byteLength. Supports up to 48 bits of accuracy. For example:

const buf1 = new Buffer(6);
buf1.writeUIntBE(0x1234567890ab, 0, 6);
console.log(buf1);
  // Prints: <Buffer 12 34 56 78 90 ab>

const buf2 = new Buffer(6);
buf2.writeUIntLE(0x1234567890ab, 0, 6);
console.log(buf2);
  // Prints: <Buffer ab 90 78 56 34 12>

Set noAssert to true to skip validation of value and offset. This means that value may be too large for the specific function and offset may be beyond the end of the Buffer leading to the values being silently dropped. This should not be used unless you are certain of correctness.

Behavior is not defined when value is anything other than an integer.

buf.writeUInt8(value, offset[, noAssert])#

  • value <Number> Bytes to be written to Buffer
  • offset <Number> 0 <= offset <= buf.length - 1
  • noAssert <Boolean> Default: false
  • Return: <Number> The offset plus the number of written bytes

Writes value to the Buffer at the specified offset. The value should be a valid unsigned 8-bit integer. Behavior is not defined when value is anything other than an unsigned 8-bit integer.

Set noAssert to true to skip validation of value and offset. This means that value may be too large for the specific function and offset may be beyond the end of the Buffer leading to the values being silently dropped. This should not be used unless you are certain of correctness.

Example:

const buf = new Buffer(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[, noAssert])#

buf.writeUInt16LE(value, offset[, noAssert])#

  • value <Number> Bytes to be written to Buffer
  • offset <Number> 0 <= offset <= buf.length - 2
  • noAssert <Boolean> Default: false
  • Return: <Number> The offset plus the number of written bytes

Writes value to the Buffer at the specified offset with specified endian format (writeUInt16BE() writes big endian, writeUInt16LE() writes little endian). The value should be a valid unsigned 16-bit integer. Behavior is not defined when value is anything other than an unsigned 16-bit integer.

Set noAssert to true to skip validation of value and offset. This means that value may be too large for the specific function and offset may be beyond the end of the Buffer leading to the values being silently dropped. This should not be used unless you are certain of correctness.

Example:

const buf = new Buffer(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[, noAssert])#

buf.writeUInt32LE(value, offset[, noAssert])#

  • value <Number> Bytes to be written to Buffer
  • offset <Number> 0 <= offset <= buf.length - 4
  • noAssert <Boolean> Default: false
  • Return: <Number> The offset plus the number of written bytes

Writes value to the Buffer at the specified offset with specified endian format (writeUInt32BE() writes big endian, writeUInt32LE() writes little endian). The value should be a valid unsigned 32-bit integer. Behavior is not defined when value is anything other than an unsigned 32-bit integer.

Set noAssert to true to skip validation of value and offset. This means that value may be too large for the specific function and offset may be beyond the end of the Buffer leading to the values being silently dropped. This should not be used unless you are certain of correctness.

Example:

const buf = new Buffer(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[, noAssert])#

buf.writeUIntLE(value, offset, byteLength[, noAssert])#

  • value <Number> Bytes to be written to Buffer
  • offset <Number> 0 <= offset <= buf.length - byteLength
  • byteLength <Number> 0 < byteLength <= 6
  • noAssert <Boolean> Default: false
  • Return: <Number> The offset plus the number of written bytes

Writes value to the Buffer at the specified offset and byteLength. Supports up to 48 bits of accuracy. For example:

const buf = new Buffer(6);
buf.writeUIntBE(0x1234567890ab, 0, 6);
console.log(buf);
  // Prints: <Buffer 12 34 56 78 90 ab>

Set noAssert to true to skip validation of value and offset. This means that value may be too large for the specific function and offset may be beyond the end of the Buffer leading to the values being silently dropped. This should not be used unless you are certain of correctness.

Behavior is not defined when value is anything other than an unsigned integer.

buffer.INSPECT_MAX_BYTES#

Returns the maximum number of bytes that will be returned when buffer.inspect() is called. This can be overridden by user modules. See util.inspect() for more details on buffer.inspect() behavior.

Note that this is a property on the buffer module as returned by require('buffer'), not on the Buffer global or a Buffer instance.

Class: SlowBuffer#

Returns an un-pooled Buffer.

In order to avoid the garbage collection overhead of creating many individually allocated Buffers, by default allocations under 4KB are sliced from a single larger allocated object. This approach improves both performance and memory usage since v8 does not need to track and cleanup as many Persistent objects.

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 SlowBuffer then copy out the relevant bits.

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

socket.on('readable', () => {
  var data = socket.read();
  // allocate for retained data
  var sb = new SlowBuffer(10);
  // copy the data into the new allocation
  data.copy(sb, 0, 0, 10);
  store.push(sb);
});

Use of SlowBuffer should be used only as a last resort after a developer has observed undue memory retention in their applications.

new SlowBuffer(size)#

  • size Number

Allocates a new SlowBuffer of size bytes. The size must be less than or equal to the value of require('buffer').kMaxLength (on 64-bit architectures, kMaxLength is (2^31)-1). Otherwise, a RangeError is thrown. If a size less than 0 is specified, a zero-length SlowBuffer will be created.

The underlying memory for SlowBuffer instances is not initialized. The contents of a newly created SlowBuffer are unknown and could contain sensitive data. Use buf.fill(0) to initialize a SlowBuffer to zeroes.

const SlowBuffer = require('buffer').SlowBuffer;
const buf = new SlowBuffer(5);
console.log(buf);
  // <Buffer 78 e0 82 02 01>
  // (octets will be different, every time)
buf.fill(0);
console.log(buf);
  // <Buffer 00 00 00 00 00>