Node.js v21.0.0-v8-canary20231015b406551b75 documentation

Introduction#

A package is a folder tree described by a package.json file. The package consists of the folder containing the package.json file and all subfolders until the next folder containing another package.json file, or a folder named node_modules.

This page provides guidance for package authors writing package.json files along with a reference for the package.json fields defined by Node.js.

Determining module system#

Introduction#

Node.js will treat the following as ES modules when passed to node as the initial input, or when referenced by import statements or import() expressions:

• Files with an .mjs extension.

• Files with a .js extension when the nearest parent package.json file contains a top-level "type" field 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 the following as CommonJS when passed to node as the initial input, or when referenced by import statements or import() expressions:

• Files with a .cjs extension.

• Files with a .js extension 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.

Aside from these explicit cases, there are other cases where Node.js defaults to one module system or the other based on the value of the --experimental-default-type flag:

• Files ending in .js or with no extension, if there is no package.json file present in the same folder or any parent folder.

• Files ending in .js or with no extension, if the nearest parent package.json field lacks a "type" field; unless the folder is inside a node_modules folder. (Package scopes under node_modules are always treated as CommonJS when the package.json file lacks a "type" field, regardless of --experimental-default-type, for backward compatibility.)

• Strings passed in as an argument to --eval or piped to node via STDIN, when --input-type is unspecified.

This flag currently defaults to "commonjs", but it may change in the future to default to "module". For this reason it is best to be explicit wherever possible; in particular, package authors should always include the "type" field in their package.json files, 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.

Modules loaders#

Node.js has two systems for resolving a specifier and loading modules.

There is the CommonJS module loader:

• It is fully synchronous.
• It is responsible for handling require() calls.
• It is monkey patchable.
• It supports folders as modules.
• When resolving a specifier, if no exact match is found, it will try to add extensions (.js, .json, and finally .node) and then attempt to resolve folders as modules.
• It treats .json as JSON text files.
• .node files are interpreted as compiled addon modules loaded with process.dlopen().
• It treats all files that lack .json or .node extensions as JavaScript text files.
• It cannot be used to load ECMAScript modules (although it is possible to load ECMASCript modules from CommonJS modules). When used to load a JavaScript text file that is not an ECMAScript module, it loads it as a CommonJS module.

There is the ECMAScript module loader:

• It is asynchronous.
• It is responsible for handling import statements and import() expressions.
• It is not monkey patchable, can be customized using loader hooks.
• It does not support folders as modules, directory indexes (e.g. './startup/index.js') must be fully specified.
• It does no extension searching. A file extension must be provided when the specifier is a relative or absolute file URL.
• It can load JSON modules, but an import assertion is required.
• It accepts only .js, .mjs, and .cjs extensions for JavaScript text files.
• It can be used to load JavaScript CommonJS modules. Such modules are passed through the cjs-module-lexer to try to identify named exports, which are available if they can be determined through static analysis. Imported CommonJS modules have their URLs converted to absolute paths and are then loaded via the CommonJS module loader.

package.json and file extensions#

Within a package, the package.json "type" field defines how Node.js should interpret .js files. If a package.json file does not have a "type" field, .js files are treated as CommonJS.

A package.json "type" value of "module" tells Node.js to interpret .js files within that package as using ES module syntax.

The "type" field 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, treated as an ES module 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 "type" value 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". copy

Files ending with .mjs are always loaded as ES modules regardless of the nearest parent package.json.

Files ending with .cjs are always loaded as CommonJS regardless of the nearest parent package.json.

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. copy

The .mjs and .cjs extensions can be used to mix types within the same package:

• Within a "type": "module" package, 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).

• Within a "type": "commonjs" package, 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).

--input-type flag#

Strings passed in as an argument to --eval (or -e), or piped to node via STDIN, are treated as ES modules when the --input-type=module flag is set.

node --input-type=module --eval "import { sep } from 'node:path'; console.log(sep);"

echo "import { sep } from 'node:path'; console.log(sep);" | node --input-type=module copy

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.

Determining package manager#

While all Node.js projects are expected to be installable by all package managers once published, their development teams are often required to use one specific package manager. To make this process easier, Node.js ships with a tool called Corepack that aims to make all package managers transparently available in your environment - provided you have Node.js installed.

By default Corepack won't enforce any specific package manager and will use the generic "Last Known Good" versions associated with each Node.js release, but you can improve this experience by setting the "packageManager" field in your project's package.json.

Package entry points#

In a package's package.json file, two fields can define entry points for a package: "main" and "exports". Both fields apply to both ES module and CommonJS module entry points.

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" provides a modern alternative to "main" allowing multiple entry points to be defined, conditional entry resolution support between environments, and preventing any other entry points besides those defined in "exports". This encapsulation allows module authors to clearly define the public interface for their package.

For new packages targeting the currently supported versions of Node.js, the "exports" field is recommended. For packages supporting Node.js 10 and below, the "main" field is required. If both "exports" and "main" are defined, the "exports" field takes precedence over "main" in supported versions of Node.js.

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.

Existing packages introducing the "exports" field will prevent consumers of the 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 previously exported main, lib, feature, and the package.json could use the following package.exports:

{
  "name": "my-package",
  "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": "./feature/index.js",
    "./feature/index.js": "./feature/index.js",
    "./package.json": "./package.json"
  }
} copy

Alternatively a project could choose to export entire folders both with and without extensioned subpaths using export patterns:

{
  "name": "my-package",
  "exports": {
    ".": "./lib/index.js",
    "./lib": "./lib/index.js",
    "./lib/*": "./lib/*.js",
    "./lib/*.js": "./lib/*.js",
    "./feature": "./feature/index.js",
    "./feature/*": "./feature/*.js",
    "./feature/*.js": "./feature/*.js",
    "./package.json": "./package.json"
  }
} copy

With the above providing backwards-compatibility for any minor package versions, a future major change for the package can then properly restrict the exports to only the specific feature exports exposed:

{
  "name": "my-package",
  "exports": {
    ".": "./lib/index.js",
    "./feature/*.js": "./feature/*.js",
    "./feature/internal/*": null
  }
} copy

Main entry point export#

When writing a new package, it is recommended to use the "exports" field:

{
  "exports": "./index.js"
} copy

When the "exports" field is defined, all subpaths of the package are encapsulated and no longer available to importers. For example, require('pkg/subpath.js') throws an ERR_PACKAGE_PATH_NOT_EXPORTED error.

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.

All currently supported versions of Node.js and modern build tools support the "exports" field. For projects using an older version of Node.js or a related build tool, compatibility can be achieved by including the "main" field alongside "exports" pointing to the same module:

{
  "main": "./index.js",
  "exports": "./index.js"
} copy

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:

{
  "exports": {
    ".": "./index.js",
    "./submodule.js": "./src/submodule.js"
  }
} copy

Now only the defined subpath in "exports" can be imported by a consumer:

import submodule from 'es-module-package/submodule.js';
// Loads ./node_modules/es-module-package/src/submodule.js copy

While other subpaths will error:

import submodule from 'es-module-package/private-module.js';
// Throws ERR_PACKAGE_PATH_NOT_EXPORTED copy

Extensions in subpaths#

Package authors should provide either extensioned (import 'pkg/subpath.js') or extensionless (import 'pkg/subpath') subpaths in their exports. This ensures that there is only one subpath for each exported module so that all dependents import the same consistent specifier, keeping the package contract clear for consumers and simplifying package subpath completions.

Traditionally, packages tended to use the extensionless style, which has the benefits of readability and of masking the true path of the file within the package.

With import maps now providing a standard for package resolution in browsers and other JavaScript runtimes, using the extensionless style can result in bloated import map definitions. Explicit file extensions can avoid this issue by enabling the import map to utilize a packages folder mapping to map multiple subpaths where possible instead of a separate map entry per package subpath export. This also mirrors the requirement of using the full specifier path in relative and absolute import specifiers.

Exports sugar#

If the "." export is the only export, the "exports" field provides sugar for this case being the direct "exports" field value.

{
  "exports": {
    ".": "./index.js"
  }
} copy

can be written:

{
  "exports": "./index.js"
} copy

Subpath imports#

In addition to the "exports" field, there is a package "imports" field to create private mappings that only apply to import specifiers from within the package itself.

Entries in the "imports" field must always start with # to ensure they are disambiguated from external package specifiers.

For example, the imports field can be used to gain the benefits of conditional exports for internal modules:

// package.json
{
  "imports": {
    "#dep": {
      "node": "dep-node-native",
      "default": "./dep-polyfill.js"
    }
  },
  "dependencies": {
    "dep-node-native": "^1.0.0"
  }
} copy

where import '#dep' does not get the resolution of the external package dep-node-native (including its exports in turn), and instead gets the local file ./dep-polyfill.js relative to the package in other environments.

Unlike the "exports" field, the "imports" field permits mapping to external packages.

The resolution rules for the imports field are otherwise analogous to the exports field.

Subpath patterns#

For packages with a small number of exports or imports, we recommend explicitly listing each exports subpath entry. But for packages that have large numbers of subpaths, this might cause package.json bloat and maintenance issues.

For these use cases, subpath export patterns can be used instead:

// ./node_modules/es-module-package/package.json
{
  "exports": {
    "./features/*.js": "./src/features/*.js"
  },
  "imports": {
    "#internal/*.js": "./src/internal/*.js"
  }
} copy

* maps expose nested subpaths as it is a string replacement syntax only.

All instances of * on the right hand side will then be replaced with this value, including if it contains any / separators.

import featureX from 'es-module-package/features/x.js';
// Loads ./node_modules/es-module-package/src/features/x.js

import featureY from 'es-module-package/features/y/y.js';
// Loads ./node_modules/es-module-package/src/features/y/y.js

import internalZ from '#internal/z.js';
// Loads ./node_modules/es-module-package/src/internal/z.js copy

This is a direct static matching and replacement without any special handling for file extensions. Including the "*.js" on both sides of the mapping restricts the exposed package exports to only JS files.

The property of exports being statically enumerable is maintained with exports patterns since the individual exports for a package can be determined by treating the right hand side target pattern as a ** glob against the list of files within the package. Because node_modules paths are forbidden in exports targets, this expansion is dependent on only the files of the package itself.

To exclude private subfolders from patterns, null targets can be used:

// ./node_modules/es-module-package/package.json
{
  "exports": {
    "./features/*.js": "./src/features/*.js",
    "./features/private-internal/*": null
  }
} copy

import featureInternal from 'es-module-package/features/private-internal/m.js';
// Throws: ERR_PACKAGE_PATH_NOT_EXPORTED

import featureX from 'es-module-package/features/x.js';
// Loads ./node_modules/es-module-package/src/features/x.js copy

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
{
  "exports": {
    "import": "./index-module.js",
    "require": "./index-require.cjs"
  },
  "type": "module"
} copy

Node.js implements the following conditions, listed in order from most specific to least specific as conditions should be defined:

• "node-addons" - similar to "node" and matches for any Node.js environment. This condition can be used to provide an entry point which uses native C++ addons as opposed to an entry point which is more universal and doesn't rely on native addons. This condition can be disabled via the --no-addons flag.
• "node" - matches for any Node.js environment. Can be a CommonJS or ES module file. In most cases explicitly calling out the Node.js platform is not necessary.
• "import" - matches when the package is loaded via import or import(), or via any top-level import or resolve operation by the ECMAScript module loader. Applies regardless of the module format of the target file. Always mutually exclusive with "require".
• "require" - matches when the package is loaded via require(). The referenced file should be loadable with require() although the condition matches regardless of the module format of the target file. Expected formats include CommonJS, JSON, and native addons but not ES modules as require() doesn't support them. Always mutually exclusive with "import".
• "default" - the generic fallback that always matches. Can be a CommonJS or ES module file. This condition should always come last.

Within the "exports" object, key order is significant. During condition matching, earlier entries have higher priority and take precedence over later entries. The general rule is that conditions should be from most specific to least specific in object order.

Using the "import" and "require" conditions can lead to some hazards, which are further explained in the dual CommonJS/ES module packages section.

The "node-addons" condition can be used to provide an entry point which uses native C++ addons. However, this condition can be disabled via the --no-addons flag. When using "node-addons", it's recommended to treat "default" as an enhancement that provides a more universal entry point, e.g. using WebAssembly instead of a native addon.

Conditional exports can also be extended to exports subpaths, for example:

{
  "exports": {
    ".": "./index.js",
    "./feature.js": {
      "node": "./feature-node.js",
      "default": "./feature.js"
    }
  }
} copy

Defines a package where require('pkg/feature.js') and import 'pkg/feature.js' could provide different implementations between Node.js and other JS environments.

When using environment branches, always include a "default" condition where possible. Providing a "default" condition ensures that any unknown JS environments are able to use this universal implementation, which helps avoid these JS environments from having to pretend to be existing environments in order to support packages with conditional exports. For this reason, using "node" and "default" condition branches is usually preferable to using "node" and "browser" condition branches.

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:

{
  "exports": {
    "node": {
      "import": "./feature-node.mjs",
      "require": "./feature-node.cjs"
    },
    "default": "./feature.mjs"
  }
} copy

Conditions continue to be matched in order as with flat conditions. If a nested condition 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.

Resolving user conditions#

When running Node.js, custom user conditions can be added with the --conditions flag:

node --conditions=development index.js copy

which would then resolve the "development" condition in package imports and exports, while resolving the existing "node", "node-addons", "default", "import", and "require" conditions as appropriate.

Any number of custom conditions can be set with repeat flags.

Community Conditions Definitions#

Condition strings other than the "import", "require", "node", "node-addons" and "default" conditions implemented in Node.js core are ignored by default.

Other platforms may implement other conditions and user conditions can be enabled in Node.js via the --conditions / -C flag.

Since custom package conditions require clear definitions to ensure correct usage, a list of common known package conditions and their strict definitions is provided below to assist with ecosystem coordination.

• "types" - can be used by typing systems to resolve the typing file for the given export. This condition should always be included first.
• "browser" - any web browser environment.
• "development" - can be used to define a development-only environment entry point, for example to provide additional debugging context such as better error messages when running in a development mode. Must always be mutually exclusive with "production".
• "production" - can be used to define a production environment entry point. Must always be mutually exclusive with "development".

For other runtimes, platform-specific condition key definitions are maintained by the WinterCG in the Runtime Keys proposal specification.

New conditions definitions may be added to this list by creating a pull request to the Node.js documentation for this section. The requirements for listing a new condition definition here are that:

• The definition should be clear and unambiguous for all implementers.
• The use case for why the condition is needed should be clearly justified.
• There should exist sufficient existing implementation usage.
• The condition name should not conflict with another condition definition or condition in wide usage.
• The listing of the condition definition should provide a coordination benefit to the ecosystem that wouldn't otherwise be possible. For example, this would not necessarily be the case for company-specific or application-specific conditions.
• The condition should be such that a Node.js user would expect it to be in Node.js core documentation. The "types" condition is a good example: It doesn't really belong in the Runtime Keys proposal but is a good fit here in the Node.js docs.

The above definitions may be moved to a dedicated conditions registry in due course.

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": {
    ".": "./index.mjs",
    "./foo.js": "./foo.js"
  }
} copy

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 ./index.mjs. copy

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 previous package, 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'; copy

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.js'); // Loads from ./foo.js. copy

Finally, self-referencing also works with scoped packages. For example, this code will also work:

// package.json
{
  "name": "@my/package",
  "exports": "./index.js"
} copy

// ./index.js
module.exports = 42; copy

// ./other.js
console.log(require('@my/package')); copy

$ node other.js
42 copy

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 might instead be written where any version of Node.js receives only CommonJS sources, and any separate ES module sources the package might contain are 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 might 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",
  "exports": {
    "import": "./wrapper.mjs",
    "require": "./index.cjs"
  }
} copy

The preceding example uses explicit extensions .mjs and .cjs. If your files use the .js extension, "type": "module" will cause such files to be treated as ES modules, just as "type": "commonjs" would cause them to be treated as CommonJS. See Enabling.

// ./node_modules/pkg/index.cjs
exports.name = 'value'; copy

// ./node_modules/pkg/wrapper.mjs
import cjsModule from './index.cjs';
export const name = cjsModule.name; copy

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; copy

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",
  "exports": {
    ".": "./index.cjs",
    "./module": "./wrapper.mjs"
  }
} copy

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",
  "exports": {
    "import": "./index.mjs",
    "require": "./index.cjs"
  }
} copy

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 might 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 copy

   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; copy

   // ./node_modules/pkg/index.mjs
   import state from './state.cjs';
   export {
     state,
   }; copy

   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",
  "exports": {
    ".": "./index.cjs",
    "./module": "./index.mjs"
  }
} copy

Node.js package.json field definitions#

This section describes the fields used by the Node.js runtime. Other tools (such as npm) use additional fields which are ignored by Node.js and not documented here.

The following fields in package.json files are used in Node.js:

• "name" - Relevant when using named imports within a package. Also used by package managers as the name of the package.
• "main" - The default module when loading the package, if exports is not specified, and in versions of Node.js prior to the introduction of exports.
• "packageManager" - The package manager recommended when contributing to the package. Leveraged by the Corepack shims.
• "type" - The package type determining whether to load .js files as CommonJS or ES modules.
• "exports" - Package exports and conditional exports. When present, limits which submodules can be loaded from within the package.
• "imports" - Package imports, for use by modules within the package itself.

"name"#

• Type: <string>

{
  "name": "package-name"
} copy

The "name" field defines your package's name. Publishing to the npm registry requires a name that satisfies certain requirements.

The "name" field can be used in addition to the "exports" field to self-reference a package using its name.

"main"#

• Type: <string>

{
  "main": "./index.js"
} copy

The "main" field defines the entry point of a package when imported by name via a node_modules lookup. Its value is a path.

When a package has an "exports" field, this will take precedence over the "main" field when importing the package by name.

It also defines the script that is used when the package directory is loaded via require().

// This resolves to ./path/to/directory/index.js.
require('./path/to/directory'); copy

"packageManager"#

• Type: <string>

{
  "packageManager": "<package manager name>@<version>"
} copy

The "packageManager" field defines which package manager is expected to be used when working on the current project. It can be set to any of the supported package managers, and will ensure that your teams use the exact same package manager versions without having to install anything else other than Node.js.

This field is currently experimental and needs to be opted-in; check the Corepack page for details about the procedure.

"type"#

• Type: <string>

The "type" field defines the module format that Node.js uses for all .js files that have that package.json file as their nearest parent.

Files ending with .js are 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 a node_modules folder or the volume root is reached.

// package.json
{
  "type": "module"
} copy

# In same folder as preceding package.json
node my-app.js # Runs as ES module copy

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, .js files are treated as CommonJS.

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 copy

Regardless of the value of the "type" field, .mjs files are always treated as ES modules and .cjs files are always treated as CommonJS.

"exports"#

• Type: <Object> | <string> | <string[]>

{
  "exports": "./index.js"
} copy

The "exports" field allows defining the entry points of a package when imported by name loaded either via a node_modules lookup or a self-reference to its own name. It is supported in Node.js 12+ as an alternative to the "main" that can support defining subpath exports and conditional exports while encapsulating internal unexported modules.

Conditional Exports can also be used within "exports" to define different package entry points per environment, including whether the package is referenced via require or via import.

All paths defined in the "exports" must be relative file URLs starting with ./.

"imports"#

• Type: <Object>

// package.json
{
  "imports": {
    "#dep": {
      "node": "dep-node-native",
      "default": "./dep-polyfill.js"
    }
  },
  "dependencies": {
    "dep-node-native": "^1.0.0"
  }
} copy

Entries in the imports field must be strings starting with #.

Package imports permit mapping to external packages.

This field defines subpath imports for the current package.