Modules: Packages | Node.js v22.3.0 Documentation (2024)

Introduction#

A package is a folder tree described by a package.json file. The packageconsists of the folder containing the package.json file and all subfoldersuntil the next folder containing another package.json file, or a foldernamed node_modules.

This page provides guidance for package authors writing package.json filesalong 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 theinitial 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 filecontains 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.

• When using --experimental-detect-module, code containing syntax onlysuccessfully parsed as ES modules, such as import or exportstatements or import.meta, having no explicit marker of how it should beinterpreted. Explicit markers are .mjs or .cjs extensions, package.json"type" fields with either "module" or "commonjs" values, or--input-type or --experimental-default-type flags. Dynamic import()expressions are supported in either CommonJS or ES modules and would notcause a file to be treated as an ES module.

Node.js will treat the following as CommonJS when passed to node as theinitial 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 filecontains a top-level field "type" with a value of "commonjs".

• Strings passed in as an argument to --eval or --print, or piped to nodevia STDIN, with the flag --input-type=commonjs.

Aside from these explicit cases, there are other cases where Node.js defaults toone 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 filepresent in the same folder or any parent folder.

• Files ending in .js or with no extension, if the nearest parentpackage.json field lacks a "type" field; unless the folder is inside anode_modules folder. (Package scopes under node_modules are always treatedas CommonJS when the package.json file lacks a "type" field, regardlessof --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 todefault to "module". For this reason it is best to be explicit whereverpossible; in particular, package authors should always include the "type"field in their package.json files, even in packages where all sources areCommonJS. Being explicit about the type of the package will future-proof thepackage in case the default type of Node.js ever changes, and it will also makethings easier for build tools and loaders to determine how the files in thepackage 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 addextensions (.js, .json, and finally .node) and then attempt to resolvefolders as modules.
• It treats .json as JSON text files.
• .node files are interpreted as compiled addon modules loaded withprocess.dlopen().
• It treats all files that lack .json or .node extensions as JavaScripttext files.
• It can only be used to load ECMASCript modules from CommonJS modules ifthe module graph is synchronous (that contains no top-level await) when--experimental-require-module is enabled.When used to load a JavaScript text file that is not an ECMAScript module,the file will be loaded as a CommonJS module.

There is the ECMAScript module loader:

• It is asynchronous, unless it's being used to load modules for require().
• 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 providedwhen the specifier is a relative or absolute file URL.
• It can load JSON modules, but an import type attribute is required.
• It accepts only .js, .mjs, and .cjs extensions for JavaScript textfiles.
• It can be used to load JavaScript CommonJS modules. Such modulesare 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 absolutepaths and are then loaded via the CommonJS module loader.

package.json and file extensions#

Within a package, the package.json "type" field defines howNode.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 .jsfiles 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". 

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

Files ending with .cjs are always loaded as CommonJS regardless of thenearest 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. 

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

• Within a "type": "module" package, Node.js can be instructed tointerpret a particular file as CommonJS by naming it with a .cjsextension (since both .js and .mjs files are treated as ES modules withina "module" package).

• Within a "type": "commonjs" package, Node.js can be instructed tointerpret a particular file as an ES module by naming it with an .mjsextension (since both .js and .cjs files are treated as CommonJS within a"commonjs" package).

--input-type flag#

Added in: v12.0.0

Strings passed in as an argument to --eval (or -e), or piped to node viaSTDIN, are treated as ES modules when the --input-type=module flagis 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 

For completeness there is also --input-type=commonjs, for explicitly runningstring input as CommonJS. This is the default behavior if --input-type isunspecified.

Determining package manager#

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

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

Package entry points#

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

The "main" field is supported in all versions of Node.js, but itscapabilities are limited: it only defines the main entry point of the package.

The "exports" provides a modern alternative to "main" allowingmultiple entry points to be defined, conditional entry resolution supportbetween environments, and preventing any other entry points besides thosedefined in "exports". This encapsulation allows module authors toclearly 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 andbelow, 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 differentpackage entry points per environment, including whether the package isreferenced via require or via import. For more information about supportingboth CommonJS and ES modules in a single package please consultthe dual CommonJS/ES module packages section.

Existing packages introducing the "exports" field will prevent consumersof the package from using any entry points that are not defined, including thepackage.json (e.g. require('your-package/package.json')). This willlikely be a breaking change.

To make the introduction of "exports" non-breaking, ensure that everypreviously supported entry point is exported. It is best to explicitly specifyentry 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" }} 

Alternatively a project could choose to export entire folders both with andwithout 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" }} 

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

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

Main entry point export#

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

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

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

This encapsulation of exports provides more reliable guaranteesabout package interfaces for tools and when handling semver upgrades for apackage. It is not a strong encapsulation since a direct require of anyabsolute subpath of the package such asrequire('/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 relatedbuild tool, compatibility can be achieved by including the "main" fieldalongside "exports" pointing to the same module:

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

Subpath exports#

Added in: v12.7.0

When using the "exports" field, custom subpaths can be defined alongwith the main entry point by treating the main entry point as the"." subpath:

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

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 

While other subpaths will error:

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

Extensions in subpaths#

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

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

With import maps now providing a standard for package resolution in browsersand other JavaScript runtimes, using the extensionless style can result inbloated import map definitions. Explicit file extensions can avoid this issue byenabling the import map to utilize a packages folder mapping to map multiplesubpaths where possible instead of a separate map entry per package subpathexport. This also mirrors the requirement of using the full specifier pathin relative and absolute import specifiers.

Exports sugar#

Added in: v12.11.0

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

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

can be written:

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

Subpath imports#

Added in: v14.6.0, v12.19.0

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

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

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

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

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

Unlike the "exports" field, the "imports" field permits mapping to externalpackages.

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

Subpath patterns#

For packages with a small number of exports or imports, we recommendexplicitly listing each exports subpath entry. But for packages that havelarge numbers of subpaths, this might cause package.json bloat andmaintenance 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" }} 

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

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

import featureX from 'es-module-package/features/x.js';// Loads ./node_modules/es-module-package/src/features/x.jsimport featureY from 'es-module-package/features/y/y.js';// Loads ./node_modules/es-module-package/src/features/y/y.jsimport internalZ from '#internal/z.js';// Loads ./node_modules/es-module-package/src/internal/z.js 

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

The property of exports being statically enumerable is maintained with exportspatterns since the individual exports for a package can be determined bytreating the right hand side target pattern as a ** glob against the list offiles within the package. Because node_modules paths are forbidden in exportstargets, 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 }} 

import featureInternal from 'es-module-package/features/private-internal/m.js';// Throws: ERR_PACKAGE_PATH_NOT_EXPORTEDimport featureX from 'es-module-package/features/x.js';// Loads ./node_modules/es-module-package/src/features/x.js 

Conditional exports#

Conditional exports provide a way to map to different paths depending oncertain conditions. They are supported for both CommonJS and ES module imports.

For example, a package that wants to provide different ES module exports forrequire() and import can be written:

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

Node.js implements the following conditions, listed in order from mostspecific 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 relyon native addons. This condition can be disabled via the--no-addons flag.
• "node" - matches for any Node.js environment. Can be a CommonJS or ESmodule file. In most cases explicitly calling out the Node.js platform isnot necessary.
• "import" - matches when the package is loaded via import orimport(), or via any top-level import or resolve operation by theECMAScript module loader. Applies regardless of the module format of thetarget file. Always mutually exclusive with "require".
• "require" - matches when the package is loaded via require(). Thereferenced file should be loadable with require() although the conditionmatches regardless of the module format of the target file. Expectedformats include CommonJS, JSON, native addons, and ES modulesif --experimental-require-module is enabled. Always mutuallyexclusive with "import".
• "default" - the generic fallback that always matches. Can be a CommonJSor ES module file. This condition should always come last.

Within the "exports" object, key order is significant. During conditionmatching, earlier entries have higher priority and take precedence over laterentries. The general rule is that conditions should be from most specific toleast 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 whichuses 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" } }} 

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

When using environment branches, always include a "default" condition wherepossible. Providing a "default" condition ensures that any unknown JSenvironments are able to use this universal implementation, which helps avoidthese JS environments from having to pretend to be existing environments inorder 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 foruse in Node.js but not the browser:

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

Conditions continue to be matched in order as with flat conditions. Ifa nested condition does not have any mapping it will continue checkingthe remaining conditions of the parent condition. In this way nestedconditions behave analogously to nested JavaScript if statements.

Resolving user conditions#

Added in: v14.9.0, v12.19.0

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

node --conditions=development index.js 

which would then resolve the "development" condition in package imports andexports, 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" conditionsimplemented in Node.js core are ignored by default.

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

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

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

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

New conditions definitions may be added to this list by creating a pull requestto the Node.js documentation for this section. The requirements for listinga 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 orcondition in wide usage.
• The listing of the condition definition should provide a coordinationbenefit to the ecosystem that wouldn't otherwise be possible. For example,this would not necessarily be the case for company-specific orapplication-specific conditions.
• The condition should be such that a Node.js user would expect it to be inNode.js core documentation. The "types" condition is a good example: Itdoesn't really belong in the Runtime Keys proposal but is a good fithere in the Node.js docs.

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

Self-referencing a package using its name#

Within a package, the values defined in the package'spackage.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" }} 

Then any module in that package can reference an export in the package itself:

// ./a-module.mjsimport { something } from 'a-package'; // Imports "something" from ./index.mjs. 

Self-referencing is available only if package.json has "exports", andwill allow importing only what that "exports" (in the package.json)allows. So the code below, given the previous package, will generate a runtimeerror:

// ./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.jsconst { something } = require('a-package/foo.js'); // Loads from ./foo.js. 

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

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

// ./index.jsmodule.exports = 42; 

// ./other.jsconsole.log(require('@my/package')); 

$ node other.js42 

Dual CommonJS/ES module packages#

Prior to the introduction of support for ES modules in Node.js, it was a commonpattern for package authors to include both CommonJS and ES module JavaScriptsources in their package, with package.json "main" specifying theCommonJS entry point and package.json "module" specifying the ES moduleentry point.This enabled Node.js to run the CommonJS entry point while build tools such asbundlers used the ES module entry point, since Node.js ignored (and stillignores) the top-level "module" field.

Node.js can now run ES module entry points, and a package can contain bothCommonJS and ES module entry points (either via separate specifiers such as'pkg' and 'pkg/es-module', or both at the same specifier via Conditionalexports). Unlike in the scenario where "module" is only used by bundlers,or ES module files are transpiled into CommonJS on the fly before evaluation byNode.js, the files referenced by the ES module entry point are evaluated as ESmodules.

Dual package hazard#

When an application is using a package that provides both CommonJS and ES modulesources, there is a risk of certain bugs if both versions of the package getloaded. This potential comes from the fact that the pkgInstance created byconst pkgInstance = require('pkg') is not the same as the pkgInstancecreated by import pkgInstance from 'pkg' (or an alternative main path like'pkg/module'). This is the “dual package hazard,” where two versions of thesame package can be loaded within the same runtime environment. While it isunlikely that an application or package would intentionally load both versionsdirectly, it is common for an application to load one version while a dependencyof the application loads the other version. This hazard can happen becauseNode.js supports intermixing CommonJS and ES modules, and can lead to unexpectedbehavior.

If the package main export is a constructor, an instanceof comparison ofinstances created by the two versions returns false, and if the export is anobject, properties added to one (like pkgInstance.foo = 3) are not present onthe other. This differs from how import and require statements work inall-CommonJS or all-ES module environments, respectively, and therefore issurprising to users. It also differs from the behavior users are familiar withwhen 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 packagecontains both CommonJS and ES module sources and both sources are provided foruse in Node.js, either via separate main entry points or exported paths. Apackage might instead be written where any version of Node.js receives onlyCommonJS sources, and any separate ES module sources the package might containare intended only for other environments such as browsers. Such a packagewould be usable by any version of Node.js, since import can refer to CommonJSfiles; 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 breakingchange version bump. This has the disadvantage that thenewest version of the package would only be usable in ES module-supportingversions of Node.js.

Every pattern has tradeoffs, but there are two broad approaches that satisfy thefollowing 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.jsthat lack support for ES modules.
3. The package main entry point, e.g. 'pkg' can be used by both require toresolve 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' ratherthan import pkg from 'pkg'; pkg.name.
5. The package is potentially usable in other ES module environments such asbrowsers.
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, andcreate an ES module wrapper file that defines the named exports. UsingConditional exports, the ES module wrapper is used for import and theCommonJS entry point for require.

// ./node_modules/pkg/package.json{ "type": "module", "exports": { "import": "./wrapper.mjs", "require": "./index.cjs" }} 

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

// ./node_modules/pkg/index.cjsexports.name = 'value'; 

// ./node_modules/pkg/wrapper.mjsimport cjsModule from './index.cjs';export const name = cjsModule.name; 

In this example, the name from import { name } from 'pkg' is the samesingleton as the name from const { name } = require('pkg'). Therefore ===returns true when comparing the two names and the divergent specifier hazardis avoided.

If the module is not simply a list of named exports, but rather contains aunique 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 optionallyalong 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 notto refactor it into ES module syntax, but wishes to provide named exports forES module consumers.
• The package has other packages that depend on it, and the end user mightinstall both this package and those other packages. For example a utilitiespackage is used directly in an application, and a utilities-plus packageadds a few more functions to utilities. Because the wrapper exportsunderlying CommonJS files, it doesn't matter if utilities-plus is written inCommonJS or ES module syntax; it will work either way.
• The package stores internal state, and the package author would prefer not torefactor the package to isolate its state management. See the next section.

A variant of this approach not requiring conditional exports for consumers couldbe to add an export, e.g. "./module", to point to an all-ES module-syntaxversion of the package. This could be used via import 'pkg/module' by userswho are certain that the CommonJS version will not be loaded anywhere in theapplication, such as by dependencies; or if the CommonJS version can be loadedbut doesn't affect the ES module version (for example, because the package isstateless):

// ./node_modules/pkg/package.json{ "type": "module", "exports": { ".": "./index.cjs", "./module": "./wrapper.mjs" }} 

Approach #2: Isolate state#

A package.json file can define the separate CommonJS and ES module entrypoints directly:

// ./node_modules/pkg/package.json{ "type": "module", "exports": { "import": "./index.mjs", "require": "./index.cjs" }} 

This can be done if both the CommonJS and ES module versions of the package areequivalent, for example because one is the transpiled output of the other; andthe package's management of state is carefully isolated (or the package isstateless).

The reason that state is an issue is because both the CommonJS and ES moduleversions of the package might get used within an application; for example, theuser's application code could import the ES module version while a dependencyrequires the CommonJS version. If that were to occur, two copies of thepackage would be loaded in memory and therefore two separate states would bepresent. 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 aresome ways to isolate state so that it's shared between the potentially loadedCommonJS and ES module instances of the package:

1. If possible, contain all state within an instantiated object. JavaScript'sDate, for example, needs to be instantiated to contain state; if it were apackage, 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 newobject, or modify a passed-in object, to keep the state external to thepackage.

2. Isolate the state in one or more CommonJS files that are shared between theCommonJS and ES module versions of the package. For example, if the CommonJSand ES module entry points are index.cjs and index.mjs, respectively:

   // ./node_modules/pkg/index.cjsconst state = require('./state.cjs');module.exports.state = state; 

   // ./node_modules/pkg/index.mjsimport state from './state.cjs';export { state,}; 

   Even if pkg is used via both require and import in an application (forexample, via import in application code and via require by a dependency)each reference of pkg will contain the same state; and modifying thatstate from either module system will apply to both.

Any plugins that attach to the package's singleton would need to separatelyattach 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 authorwants that version to be used wherever such syntax is supported.
• The package is stateless or its state can be isolated without too muchdifficulty.
• The package is unlikely to have other public packages that depend on it, or ifit does, the package is stateless or has state that need not be shared betweendependencies or with the overall application.

Even with isolated state, there is still the cost of possible extra codeexecution between the CommonJS and ES module versions of a package.

As with the previous approach, a variant of this approach not requiringconditional 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" }} 

Node.js package.json field definitions#

This section describes the fields used by the Node.js runtime. Other tools (suchas npm) useadditional 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 usedby package managers as the name of the package.
• "main" - The default module when loading the package, if exports is notspecified, and in versions of Node.js prior to the introduction of exports.
• "packageManager" - The package manager recommended when contributing tothe package. Leveraged by the Corepack shims.
• "type" - The package type determining whether to load .js files asCommonJS 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 packageitself.

"name"#

• Type: <string>

{ "name": "package-name"} 

The "name" field defines your package's name. Publishing to thenpm registry requires a name that satisfiescertain requirements.

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

"main"#

Added in: v0.4.0

• Type: <string>

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

The "main" field defines the entry point of a package when imported by namevia 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 loadedvia require().

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

"packageManager"#

Added in: v16.9.0, v14.19.0

• Type: <string>

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

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

This field is currently experimental and needs to be opted-in; check theCorepack 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 parentpackage.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 foundwhen searching in the current folder, that folder's parent, and so on upuntil a node_modules folder or the volume root is reached.

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

# In same folder as preceding package.jsonnode 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 volumeroot is reached and no package.json is found, .js files are treated asCommonJS.

import statements of .js files are treated as ES modules if the nearestparent package.json contains "type": "module".

// my-app.js, part of the same example as aboveimport './startup.js'; // Loaded as ES module because of package.json 

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

"exports"#

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

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

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

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

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

"imports"#

Added in: v14.6.0, v12.19.0

• Type: <Object>

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

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.