doc/api/packages.md

# Modules: Packages

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## 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`.

* Code containing syntax only successfully parsed as [ES modules][], such as
  `import` or `export` statements or `import.meta`, with no explicit marker of
  how it should be interpreted. Explicit markers are `.mjs` or `.cjs`
  extensions, `package.json` `"type"` fields with either `"module"` or
  `"commonjs"` values, or the `--input-type` flag. Dynamic `import()`
  expressions are supported in either CommonJS or ES modules and would not force
  a file to be treated as an ES module. See [Syntax detection][].

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`.

* Files with a `.js` extension with no parent `package.json` file or where the
  nearest parent `package.json` file lacks a `type` field, and where the code
  can evaluate successfully as CommonJS. In other words, Node.js tries to run
  such "ambiguous" files as CommonJS first, and will retry evaluating them as ES
  modules if the evaluation as CommonJS fails because the parser found ES module
  syntax.

Writing ES module syntax in "ambiguous" files incurs a performance cost, and
therefore it is encouraged that authors 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.

### Syntax detection

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> Stability: 1.2 - Release candidate

Node.js will inspect the source code of ambiguous input to determine whether it
contains ES module syntax; if such syntax is detected, the input will be treated
as an ES module.

Ambiguous input is defined as:

* Files with a `.js` extension or no extension; and either no controlling
  `package.json` file or one that lacks a `type` field.
* String input (`--eval` or `STDIN`) when `--input-type`is not specified.

ES module syntax is defined as syntax that would throw when evaluated as
CommonJS. This includes the following:

* `import` statements (but _not_ `import()` expressions, which are valid in
  CommonJS).
* `export` statements.
* `import.meta` references.
* `await` at the top level of a module.
* Lexical redeclarations of the CommonJS wrapper variables (`require`, `module`,
  `exports`, `__dirname`, `__filename`).

### Module resolution and loading

Node.js has two types of module resolution and loading, chosen based on how the module is requested.

When a module is requested via `require()` (available by default in CommonJS modules,
and can be dynamically generated using `createRequire()` in both CommonJS and ES Modules):

* Resolution:
  * The resolution initiated by `require()` supports [folders as modules][].
  * When resolving a specifier, if no exact match is found, `require()` will try to add
    extensions (`.js`, `.json`, and finally `.node`) and then attempt to resolve
    [folders as modules][].
  * It does not support URLs as specifiers by default.
* Loading:
  * `.json` files are treated as JSON text files.
  * `.node` files are interpreted as compiled addon modules loaded with `process.dlopen()`.
  * `.ts`, `.mts` and `.cts` files are treated as [TypeScript][] text files.
  * Files with any other extension, or without extensions, are treated as JavaScript
    text files.
  * `require()` can only be used to [load ECMAScript modules from CommonJS modules][] if
    the [ECMAScript module][ES Module] _and its dependencies_ are synchronous
    (i.e. they do not contain top-level `await`).

When a module is requested via static `import` statements (only available in ES Modules)
or `import()` expressions (available in both CommonJS and ES Modules):

* Resolution:
  * The resolution of `import`/`import()` does not support folders as modules,
    directory indexes (e.g. `'./startup/index.js'`) must be fully specified.
  * It does not perform extension searching. A file extension must be provided
    when the specifier is a relative or absolute file URL.
  * It supports `file://` and `data:` URLs as specifiers by default.
* Loading:
  * `.json` files are treated as JSON text files. When importing JSON modules,
    an import type attribute is required (e.g.
    `import json from './data.json' with { type: 'json' }`).
  * `.node` files are interpreted as compiled addon modules loaded with
    `process.dlopen()`, if [`--experimental-addon-modules`][] is enabled.
  * `.ts`, `.mts` and `.cts` files are treated as [TypeScript][] text files.
  * It accepts only `.js`, `.mjs`, and `.cjs` extensions for JavaScript text
    files.
  * `.wasm` files are treated as [WebAssembly modules][].
  * Any other file extensions will result in a  [`ERR_UNKNOWN_FILE_EXTENSION`][] error.
    Additional file extensions can be facilitated via [customization hooks][].
  * `import`/`import()` can be used to load JavaScript [CommonJS modules][commonjs].
    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.

Regardless of how a module is requested, the resolution and loading process can be customized
using [customization hooks][].

### `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.

```js
// 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 of
the nearest parent `package.json`.

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

```js
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 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

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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.

```bash
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 running
string input as CommonJS. This is the default behavior if `--input-type` is
unspecified.

## 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`:

```json
{
  "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 and
without extensioned subpaths using export patterns:

```json
{
  "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 exports
to only the specific feature exports exposed:

```json
{
  "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:

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

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:

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

### Subpath exports

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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:

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

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

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

While other subpaths will error:

```js
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'`) 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.

#### Path Rules and Validation for Export Targets

When defining paths as targets in the [`"exports"`][] field, Node.js enforces
several rules to ensure security, predictability, and proper encapsulation.
Understanding these rules is crucial for authors publishing packages.

##### Targets must be relative URLs

All target paths in the [`"exports"`][] map (the values associated with export
keys) must be relative URL strings starting with `./`.

```json
// package.json
{
  "name": "my-package",
  "exports": {
    ".": "./dist/main.js",          // Correct
    "./feature": "./lib/feature.js", // Correct
    // "./origin-relative": "/dist/main.js", // Incorrect: Must start with ./
    // "./absolute": "file:///dev/null", // Incorrect: Must start with ./
    // "./outside": "../common/util.js" // Incorrect: Must start with ./
  }
}
```

Reasons for this behavior include:

* **Security:** Prevents exporting arbitrary files from outside the
  package's own directory.
* **Encapsulation:** Ensures all exported paths are resolved relative to
  the package root, making the package self-contained.

##### No path traversal or invalid segments

Export targets must not resolve to a location outside the package's root
directory. Additionally, path segments like `.` (single dot), `..` (double dot),
or `node_modules` (and their URL-encoded equivalents) are generally disallowed
within the `target` string after the initial `./` and in any `subpath` part
substituted into a target pattern.

```json
// package.json
{
  "name": "my-package",
  "exports": {
    // ".": "./dist/../../elsewhere/file.js", // Invalid: path traversal
    // ".": "././dist/main.js",             // Invalid: contains "." segment
    // ".": "./dist/../dist/main.js",       // Invalid: contains ".." segment
    // "./utils/./helper.js": "./utils/helper.js" // Key has invalid segment
  }
}
```

### Exports sugar

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

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

can be written:

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

### Subpath imports

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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:

```json
// 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 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

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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:

```json
// ./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 syntax
only.**

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

```js
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
```

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:

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

```js
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
```

### Conditional exports

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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:

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

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, native addons, and ES modules. _Always mutually
  exclusive with `"import"`._
* `"module-sync"` - matches no matter the package is loaded via `import`,
  `import()` or `require()`. The format is expected to be ES modules that does
  not contain top-level await in its module graph - if it does,
  `ERR_REQUIRE_ASYNC_MODULE` will be thrown when the module is `require()`-ed.
* `"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:

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

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:

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

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

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

```bash
node --conditions=development index.js
```

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.

Typical conditions should only contain alphanumerical characters,
using ":", "-", or "=" as separators if necessary. Anything else may run
into compability issues outside of node.

In node, conditions have very few restrictions, but specifically these include:

1. They must contain at least one character.
2. They cannot start with "." since they may appear in places that also
   allow relative paths.
3. They cannot contain "," since they may be parsed as a comma-separated
   list by some CLI tools.
4. They cannot be integer property keys like "10" since that can have
   unexpected effects on property key ordering for JS objects.

### Community Conditions Definitions

Condition strings other than the `"import"`, `"require"`, `"node"`, `"module-sync"`,
`"node-addons"` and `"default"` conditions
[implemented in Node.js core](#conditional-exports) 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

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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:

```json
// 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:

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

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

```js
// ./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:

```cjs
// ./a-module.js
const { something } = require('a-package/foo.js'); // Loads from ./foo.js.
```

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

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

```cjs
// ./index.js
module.exports = 42;
```

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

```console
$ node other.js
42
```

## Dual CommonJS/ES module packages

See [the package examples repository][] for details.

## Node.js `package.json` field definitions

This section describes the fields used by the Node.js runtime. Other tools (such
as [npm](https://docs.npmjs.com/cli/v8/configuring-npm/package-json)) 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.
* [`"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"`

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* Type: {string}

```json
{
  "name": "package-name"
}
```

The `"name"` field defines your package's name. Publishing to the
_npm_ registry requires a name that satisfies
[certain requirements](https://docs.npmjs.com/files/package.json#name).

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

### `"main"`

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* Type: {string}

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

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()`](modules.md#folders-as-modules).

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

### `"type"`

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* 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.

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

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

If the nearest parent `package.json` lacks a `"type"` field, or contains
`"type": "commonjs"`, `.js` files are treated as [CommonJS][]. If the volume
root is reached and no `package.json` is found, `.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"`.

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

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"`

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-->

* Type: {Object|string|string\[]}

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

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"`

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* Type: {Object}

```json
// 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.

[CommonJS]: modules.md
[Conditional exports]: #conditional-exports
[ES module]: esm.md
[ES modules]: esm.md
[Node.js documentation for this section]: https://github.com/nodejs/node/blob/HEAD/doc/api/packages.md#conditions-definitions
[Runtime Keys]: https://runtime-keys.proposal.wintercg.org/
[Syntax detection]: #syntax-detection
[TypeScript]: typescript.md
[WebAssembly modules]: esm.md#wasm-modules
[WinterCG]: https://wintercg.org/
[`"exports"`]: #exports
[`"imports"`]: #imports
[`"main"`]: #main
[`"name"`]: #name
[`"type"`]: #type
[`--conditions` / `-C` flag]: #resolving-user-conditions
[`--experimental-addon-modules`]: cli.md#--experimental-addon-modules
[`--no-addons` flag]: cli.md#--no-addons
[`ERR_PACKAGE_PATH_NOT_EXPORTED`]: errors.md#err_package_path_not_exported
[`ERR_UNKNOWN_FILE_EXTENSION`]: errors.md#err_unknown_file_extension
[`package.json`]: #nodejs-packagejson-field-definitions
[customization hooks]: module.md#customization-hooks
[entry points]: #package-entry-points
[folders as modules]: modules.md#folders-as-modules
[import maps]: https://github.com/WICG/import-maps
[load ECMAScript modules from CommonJS modules]: modules.md#loading-ecmascript-modules-using-require
[packages folder mapping]: https://github.com/WICG/import-maps#packages-via-trailing-slashes
[self-reference]: #self-referencing-a-package-using-its-name
[subpath exports]: #subpath-exports
[subpath imports]: #subpath-imports
[the dual CommonJS/ES module packages section]: #dual-commonjses-module-packages
[the full specifier path]: esm.md#mandatory-file-extensions
[the package examples repository]: https://github.com/nodejs/package-examples