doc/api/modules.md

# Modules: CommonJS modules

<!--introduced_in=v0.10.0-->

> Stability: 2 - Stable

<!--name=module-->

CommonJS modules are the original way to package JavaScript code for Node.js.
Node.js also supports the [ECMAScript modules][] standard used by browsers
and other JavaScript runtimes.

In Node.js, each file is treated as a separate module. For
example, consider a file named `foo.js`:

```js
const circle = require('./circle.js');
console.log(`The area of a circle of radius 4 is ${circle.area(4)}`);
```

On the first line, `foo.js` loads the module `circle.js` that is in the same
directory as `foo.js`.

Here are the contents of `circle.js`:

```js
const { PI } = Math;

exports.area = (r) => PI * r ** 2;

exports.circumference = (r) => 2 * PI * r;
```

The module `circle.js` has exported the functions `area()` and
`circumference()`. Functions and objects are added to the root of a module
by specifying additional properties on the special `exports` object.

Variables local to the module will be private, because the module is wrapped
in a function by Node.js (see [module wrapper](#the-module-wrapper)).
In this example, the variable `PI` is private to `circle.js`.

The `module.exports` property can be assigned a new value (such as a function
or object).

In the following code, `bar.js` makes use of the `square` module, which exports
a Square class:

```js
const Square = require('./square.js');
const mySquare = new Square(2);
console.log(`The area of mySquare is ${mySquare.area()}`);
```

The `square` module is defined in `square.js`:

```js
// Assigning to exports will not modify module, must use module.exports
module.exports = class Square {
  constructor(width) {
    this.width = width;
  }

  area() {
    return this.width ** 2;
  }
};
```

The CommonJS module system is implemented in the [`module` core module][].

## Enabling

<!-- type=misc -->

Node.js has two module systems: CommonJS modules and [ECMAScript modules][].

By default, Node.js will treat the following as CommonJS modules:

* 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"`.

* Files with a `.js` extension or without an extension, when the nearest parent
  `package.json` file doesn't contain a top-level field [`"type"`][] or there is
  no `package.json` in any parent folder; unless the file contains syntax that
  errors unless it is evaluated as an ES module. Package authors should include
  the [`"type"`][] field, even in packages where all sources are CommonJS. Being
  explicit about the `type` of the package will make things easier for build
  tools and loaders to determine how the files in the package should be
  interpreted.

* Files with an extension that is not `.mjs`, `.cjs`, `.json`, `.node`, or `.js`
  (when the nearest parent `package.json` file contains a top-level field
  [`"type"`][] with a value of `"module"`, those files will be recognized as
  CommonJS modules only if they are being included via `require()`, not when
  used as the command-line entry point of the program).

See [Determining module system][] for more details.

Calling `require()` always use the CommonJS module loader. Calling `import()`
always use the ECMAScript module loader.

## Accessing the main module

<!-- type=misc -->

When a file is run directly from Node.js, `require.main` is set to its
`module`. That means that it is possible to determine whether a file has been
run directly by testing `require.main === module`.

For a file `foo.js`, this will be `true` if run via `node foo.js`, but
`false` if run by `require('./foo')`.

When the entry point is not a CommonJS module, `require.main` is `undefined`,
and the main module is out of reach.

## Package manager tips

<!-- type=misc -->

The semantics of the Node.js `require()` function were designed to be general
enough to support reasonable directory structures. Package manager programs
such as `dpkg`, `rpm`, and `npm` will hopefully find it possible to build
native packages from Node.js modules without modification.

In the following, we give a suggested directory structure that could work:

Let's say that we wanted to have the folder at
`/usr/lib/node/<some-package>/<some-version>` hold the contents of a
specific version of a package.

Packages can depend on one another. In order to install package `foo`, it
may be necessary to install a specific version of package `bar`. The `bar`
package may itself have dependencies, and in some cases, these may even collide
or form cyclic dependencies.

Because Node.js looks up the `realpath` of any modules it loads (that is, it
resolves symlinks) and then [looks for their dependencies in `node_modules` folders](#loading-from-node_modules-folders),
this situation can be resolved with the following architecture:

* `/usr/lib/node/foo/1.2.3/`: Contents of the `foo` package, version 1.2.3.
* `/usr/lib/node/bar/4.3.2/`: Contents of the `bar` package that `foo` depends
  on.
* `/usr/lib/node/foo/1.2.3/node_modules/bar`: Symbolic link to
  `/usr/lib/node/bar/4.3.2/`.
* `/usr/lib/node/bar/4.3.2/node_modules/*`: Symbolic links to the packages that
  `bar` depends on.

Thus, even if a cycle is encountered, or if there are dependency
conflicts, every module will be able to get a version of its dependency
that it can use.

When the code in the `foo` package does `require('bar')`, it will get the
version that is symlinked into `/usr/lib/node/foo/1.2.3/node_modules/bar`.
Then, when the code in the `bar` package calls `require('quux')`, it'll get
the version that is symlinked into
`/usr/lib/node/bar/4.3.2/node_modules/quux`.

Furthermore, to make the module lookup process even more optimal, rather
than putting packages directly in `/usr/lib/node`, we could put them in
`/usr/lib/node_modules/<name>/<version>`. Then Node.js will not bother
looking for missing dependencies in `/usr/node_modules` or `/node_modules`.

In order to make modules available to the Node.js REPL, it might be useful to
also add the `/usr/lib/node_modules` folder to the `$NODE_PATH` environment
variable. Since the module lookups using `node_modules` folders are all
relative, and based on the real path of the files making the calls to
`require()`, the packages themselves can be anywhere.

## Loading ECMAScript modules using `require()`

<!-- YAML
added:
  - v22.0.0
  - v20.17.0
changes:
  - version:
    - v23.5.0
    - v22.13.0
    - v20.19.0
    pr-url: https://github.com/nodejs/node/pull/56194
    description: This feature no longer emits an experimental warning by default,
                 though the warning can still be emitted by --trace-require-module.
  - version:
    - v23.0.0
    - v22.12.0
    - v20.19.0
    pr-url: https://github.com/nodejs/node/pull/55085
    description: This feature is no longer behind the `--experimental-require-module` CLI flag.
  - version:
    - v23.0.0
    - v22.12.0
    pr-url: https://github.com/nodejs/node/pull/54563
    description: Support `'module.exports'` interop export in `require(esm)`.
-->

> Stability: 1.2 - Release candidate

The `.mjs` extension is reserved for [ECMAScript Modules][].
See [Determining module system][] section for more info
regarding which files are parsed as ECMAScript modules.

`require()` only supports loading ECMAScript modules that meet the following requirements:

* The module is fully synchronous (contains no top-level `await`); and
* One of these conditions are met:
  1. The file has a `.mjs` extension.
  2. The file has a `.js` extension, and the closest `package.json` contains `"type": "module"`
  3. The file has a `.js` extension, the closest `package.json` does not contain
     `"type": "commonjs"`, and the module contains ES module syntax.

If the ES Module being loaded meets the requirements, `require()` can load it and
return the [module namespace object][]. In this case it is similar to dynamic
`import()` but is run synchronously and returns the name space object
directly.

With the following ES Modules:

```mjs
// distance.mjs
export function distance(a, b) { return Math.sqrt((b.x - a.x) ** 2 + (b.y - a.y) ** 2); }
```

```mjs
// point.mjs
export default class Point {
  constructor(x, y) { this.x = x; this.y = y; }
}
```

A CommonJS module can load them with `require()`:

```cjs
const distance = require('./distance.mjs');
console.log(distance);
// [Module: null prototype] {
//   distance: [Function: distance]
// }

const point = require('./point.mjs');
console.log(point);
// [Module: null prototype] {
//   default: [class Point],
//   __esModule: true,
// }
```

For interoperability with existing tools that convert ES Modules into CommonJS,
which could then load real ES Modules through `require()`, the returned namespace
would contain a `__esModule: true` property if it has a `default` export so that
consuming code generated by tools can recognize the default exports in real
ES Modules. If the namespace already defines `__esModule`, this would not be added.
This property is experimental and can change in the future. It should only be used
by tools converting ES modules into CommonJS modules, following existing ecosystem
conventions. Code authored directly in CommonJS should avoid depending on it.

The result returned by `require()` is the [module namespace object][], which places
the default export in the `.default` property, similar to the results returned by `import()`.
To customize what should be returned by `require(esm)` directly, the ES Module can export the
desired value using the string name `"module.exports"`.

<!-- eslint-disable @stylistic/js/semi -->

```mjs
// point.mjs
export default class Point {
  constructor(x, y) { this.x = x; this.y = y; }
}

// `distance` is lost to CommonJS consumers of this module, unless it's
// added to `Point` as a static property.
export function distance(a, b) { return Math.sqrt((b.x - a.x) ** 2 + (b.y - a.y) ** 2); }
export { Point as 'module.exports' }
```

<!-- eslint-disable node-core/no-duplicate-requires -->

```cjs
const Point = require('./point.mjs');
console.log(Point); // [class Point]

// Named exports are lost when 'module.exports' is used
const { distance } = require('./point.mjs');
console.log(distance); // undefined
```

Notice in the example above, when the `module.exports` export name is used, named exports
will be lost to CommonJS consumers. To allow  CommonJS consumers to continue accessing
named exports, the module can make sure that the default export is an object with the
named exports attached to it as properties. For example with the example above,
`distance` can be attached to the default export, the `Point` class, as a static method.

<!-- eslint-disable @stylistic/js/semi -->

```mjs
export function distance(a, b) { return Math.sqrt((b.x - a.x) ** 2 + (b.y - a.y) ** 2); }

export default class Point {
  constructor(x, y) { this.x = x; this.y = y; }
  static distance = distance;
}

export { Point as 'module.exports' }
```

<!-- eslint-disable node-core/no-duplicate-requires -->

```cjs
const Point = require('./point.mjs');
console.log(Point); // [class Point]

const { distance } = require('./point.mjs');
console.log(distance); // [Function: distance]
```

If the module being `require()`'d contains top-level `await`, or the module
graph it `import`s contains top-level `await`,
[`ERR_REQUIRE_ASYNC_MODULE`][] will be thrown. In this case, users should
load the asynchronous module using [`import()`][].

If `--experimental-print-required-tla` is enabled, instead of throwing
`ERR_REQUIRE_ASYNC_MODULE` before evaluation, Node.js will evaluate the
module, try to locate the top-level awaits, and print their location to
help users fix them.

Support for loading ES modules using `require()` is currently
experimental and can be disabled using `--no-experimental-require-module`.
To print where this feature is used, use [`--trace-require-module`][].

This feature can be detected by checking if
[`process.features.require_module`][] is `true`.

## All together

<!-- type=misc -->

To get the exact filename that will be loaded when `require()` is called, use
the `require.resolve()` function.

Putting together all of the above, here is the high-level algorithm
in pseudocode of what `require()` does:

```text
require(X) from module at path Y
1. If X is a core module,
   a. return the core module
   b. STOP
2. If X begins with '/'
   a. set Y to the file system root
3. If X is equal to '.', or X begins with './', '/' or '../'
   a. LOAD_AS_FILE(Y + X)
   b. LOAD_AS_DIRECTORY(Y + X)
   c. THROW "not found"
4. If X begins with '#'
   a. LOAD_PACKAGE_IMPORTS(X, dirname(Y))
5. LOAD_PACKAGE_SELF(X, dirname(Y))
6. LOAD_NODE_MODULES(X, dirname(Y))
7. THROW "not found"

MAYBE_DETECT_AND_LOAD(X)
1. If X parses as a CommonJS module, load X as a CommonJS module. STOP.
2. Else, if the source code of X can be parsed as ECMAScript module using
  <a href="esm.md#resolver-algorithm-specification">DETECT_MODULE_SYNTAX defined in
  the ESM resolver</a>,
  a. Load X as an ECMAScript module. STOP.
3. THROW the SyntaxError from attempting to parse X as CommonJS in 1. STOP.

LOAD_AS_FILE(X)
1. If X is a file, load X as its file extension format. STOP
2. If X.js is a file,
    a. Find the closest package scope SCOPE to X.
    b. If no scope was found
      1. MAYBE_DETECT_AND_LOAD(X.js)
    c. If the SCOPE/package.json contains "type" field,
      1. If the "type" field is "module", load X.js as an ECMAScript module. STOP.
      2. If the "type" field is "commonjs", load X.js as a CommonJS module. STOP.
    d. MAYBE_DETECT_AND_LOAD(X.js)
3. If X.json is a file, load X.json to a JavaScript Object. STOP
4. If X.node is a file, load X.node as binary addon. STOP

LOAD_INDEX(X)
1. If X/index.js is a file
    a. Find the closest package scope SCOPE to X.
    b. If no scope was found, load X/index.js as a CommonJS module. STOP.
    c. If the SCOPE/package.json contains "type" field,
      1. If the "type" field is "module", load X/index.js as an ECMAScript module. STOP.
      2. Else, load X/index.js as a CommonJS module. STOP.
2. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP
3. If X/index.node is a file, load X/index.node as binary addon. STOP

LOAD_AS_DIRECTORY(X)
1. If X/package.json is a file,
   a. Parse X/package.json, and look for "main" field.
   b. If "main" is a falsy value, GOTO 2.
   c. let M = X + (json main field)
   d. LOAD_AS_FILE(M)
   e. LOAD_INDEX(M)
   f. LOAD_INDEX(X) DEPRECATED
   g. THROW "not found"
2. LOAD_INDEX(X)

LOAD_NODE_MODULES(X, START)
1. let DIRS = NODE_MODULES_PATHS(START)
2. for each DIR in DIRS:
   a. LOAD_PACKAGE_EXPORTS(X, DIR)
   b. LOAD_AS_FILE(DIR/X)
   c. LOAD_AS_DIRECTORY(DIR/X)

NODE_MODULES_PATHS(START)
1. let PARTS = path split(START)
2. let I = count of PARTS - 1
3. let DIRS = []
4. while I >= 0,
   a. if PARTS[I] = "node_modules", GOTO d.
   b. DIR = path join(PARTS[0 .. I] + "node_modules")
   c. DIRS = DIRS + DIR
   d. let I = I - 1
5. return DIRS + GLOBAL_FOLDERS

LOAD_PACKAGE_IMPORTS(X, DIR)
1. Find the closest package scope SCOPE to DIR.
2. If no scope was found, return.
3. If the SCOPE/package.json "imports" is null or undefined, return.
4. If `--experimental-require-module` is enabled
  a. let CONDITIONS = ["node", "require", "module-sync"]
  b. Else, let CONDITIONS = ["node", "require"]
5. let MATCH = PACKAGE_IMPORTS_RESOLVE(X, pathToFileURL(SCOPE),
  CONDITIONS) <a href="esm.md#resolver-algorithm-specification">defined in the ESM resolver</a>.
6. RESOLVE_ESM_MATCH(MATCH).

LOAD_PACKAGE_EXPORTS(X, DIR)
1. Try to interpret X as a combination of NAME and SUBPATH where the name
   may have a @scope/ prefix and the subpath begins with a slash (`/`).
2. If X does not match this pattern or DIR/NAME/package.json is not a file,
   return.
3. Parse DIR/NAME/package.json, and look for "exports" field.
4. If "exports" is null or undefined, return.
5. If `--experimental-require-module` is enabled
  a. let CONDITIONS = ["node", "require", "module-sync"]
  b. Else, let CONDITIONS = ["node", "require"]
6. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(DIR/NAME), "." + SUBPATH,
   `package.json` "exports", CONDITIONS) <a href="esm.md#resolver-algorithm-specification">defined in the ESM resolver</a>.
7. RESOLVE_ESM_MATCH(MATCH)

LOAD_PACKAGE_SELF(X, DIR)
1. Find the closest package scope SCOPE to DIR.
2. If no scope was found, return.
3. If the SCOPE/package.json "exports" is null or undefined, return.
4. If the SCOPE/package.json "name" is not the first segment of X, return.
5. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(SCOPE),
   "." + X.slice("name".length), `package.json` "exports", ["node", "require"])
   <a href="esm.md#resolver-algorithm-specification">defined in the ESM resolver</a>.
6. RESOLVE_ESM_MATCH(MATCH)

RESOLVE_ESM_MATCH(MATCH)
1. let RESOLVED_PATH = fileURLToPath(MATCH)
2. If the file at RESOLVED_PATH exists, load RESOLVED_PATH as its extension
   format. STOP
3. THROW "not found"
```

## Caching

<!--type=misc-->

Modules are cached after the first time they are loaded. This means (among other
things) that every call to `require('foo')` will get exactly the same object
returned, if it would resolve to the same file.

Provided `require.cache` is not modified, multiple calls to `require('foo')`
will not cause the module code to be executed multiple times. This is an
important feature. With it, "partially done" objects can be returned, thus
allowing transitive dependencies to be loaded even when they would cause cycles.

To have a module execute code multiple times, export a function, and call that
function.

### Module caching caveats

<!--type=misc-->

Modules are cached based on their resolved filename. Since modules may resolve
to a different filename based on the location of the calling module (loading
from `node_modules` folders), it is not a _guarantee_ that `require('foo')` will
always return the exact same object, if it would resolve to different files.

Additionally, on case-insensitive file systems or operating systems, different
resolved filenames can point to the same file, but the cache will still treat
them as different modules and will reload the file multiple times. For example,
`require('./foo')` and `require('./FOO')` return two different objects,
irrespective of whether or not `./foo` and `./FOO` are the same file.

## Built-in modules

<!--type=misc-->

<!-- YAML
changes:
  - version:
      - v16.0.0
      - v14.18.0
    pr-url: https://github.com/nodejs/node/pull/37246
    description: Added `node:` import support to `require(...)`.
-->

Node.js has several modules compiled into the binary. These modules are
described in greater detail elsewhere in this documentation.

The built-in modules are defined within the Node.js source and are located in the
`lib/` folder.

Built-in modules can be identified using the `node:` prefix, in which case
it bypasses the `require` cache. For instance, `require('node:http')` will
always return the built in HTTP module, even if there is `require.cache` entry
by that name.

Some built-in modules are always preferentially loaded if their identifier is
passed to `require()`. For instance, `require('http')` will always
return the built-in HTTP module, even if there is a file by that name.

The list of all the built-in modules can be retrieved from [`module.builtinModules`][].
The modules being all listed without the `node:` prefix, except those that mandate such
prefix (as explained in the next section).

### Built-in modules with mandatory `node:` prefix

When being loaded by `require()`, some built-in modules must be requested with the
`node:` prefix. This requirement exists to prevent newly introduced built-in
modules from having a conflict with user land packages that already have
taken the name. Currently the built-in modules that requires the `node:` prefix are:

* [`node:sea`][]
* [`node:sqlite`][]
* [`node:test`][]
* [`node:test/reporters`][]

The list of these modules is exposed in [`module.builtinModules`][], including the prefix.

## Cycles

<!--type=misc-->

When there are circular `require()` calls, a module might not have finished
executing when it is returned.

Consider this situation:

`a.js`:

```js
console.log('a starting');
exports.done = false;
const b = require('./b.js');
console.log('in a, b.done = %j', b.done);
exports.done = true;
console.log('a done');
```

`b.js`:

```js
console.log('b starting');
exports.done = false;
const a = require('./a.js');
console.log('in b, a.done = %j', a.done);
exports.done = true;
console.log('b done');
```

`main.js`:

```js
console.log('main starting');
const a = require('./a.js');
const b = require('./b.js');
console.log('in main, a.done = %j, b.done = %j', a.done, b.done);
```

When `main.js` loads `a.js`, then `a.js` in turn loads `b.js`. At that
point, `b.js` tries to load `a.js`. In order to prevent an infinite
loop, an **unfinished copy** of the `a.js` exports object is returned to the
`b.js` module. `b.js` then finishes loading, and its `exports` object is
provided to the `a.js` module.

By the time `main.js` has loaded both modules, they're both finished.
The output of this program would thus be:

```console
$ node main.js
main starting
a starting
b starting
in b, a.done = false
b done
in a, b.done = true
a done
in main, a.done = true, b.done = true
```

Careful planning is required to allow cyclic module dependencies to work
correctly within an application.

## File modules

<!--type=misc-->

If the exact filename is not found, then Node.js will attempt to load the
required filename with the added extensions: `.js`, `.json`, and finally
`.node`. When loading a file that has a different extension (e.g. `.cjs`), its
full name must be passed to `require()`, including its file extension (e.g.
`require('./file.cjs')`).

`.json` files are parsed as JSON text files, `.node` files are interpreted as
compiled addon modules loaded with `process.dlopen()`. Files using any other
extension (or no extension at all) are parsed as JavaScript text files. Refer to
the [Determining module system][] section to understand what parse goal will be
used.

A required module prefixed with `'/'` is an absolute path to the file. For
example, `require('/home/marco/foo.js')` will load the file at
`/home/marco/foo.js`.

A required module prefixed with `'./'` is relative to the file calling
`require()`. That is, `circle.js` must be in the same directory as `foo.js` for
`require('./circle')` to find it.

Without a leading `'/'`, `'./'`, or `'../'` to indicate a file, the module must
either be a core module or is loaded from a `node_modules` folder.

If the given path does not exist, `require()` will throw a
[`MODULE_NOT_FOUND`][] error.

## Folders as modules

<!--type=misc-->

> Stability: 3 - Legacy: Use [subpath exports][] or [subpath imports][] instead.

There are three ways in which a folder may be passed to `require()` as
an argument.

The first is to create a [`package.json`][] file in the root of the folder,
which specifies a `main` module. An example [`package.json`][] file might
look like this:

```json
{ "name" : "some-library",
  "main" : "./lib/some-library.js" }
```

If this was in a folder at `./some-library`, then
`require('./some-library')` would attempt to load
`./some-library/lib/some-library.js`.

If there is no [`package.json`][] file present in the directory, or if the
[`"main"`][] entry is missing or cannot be resolved, then Node.js
will attempt to load an `index.js` or `index.node` file out of that
directory. For example, if there was no [`package.json`][] file in the previous
example, then `require('./some-library')` would attempt to load:

* `./some-library/index.js`
* `./some-library/index.node`

If these attempts fail, then Node.js will report the entire module as missing
with the default error:

```console
Error: Cannot find module 'some-library'
```

In all three above cases, an `import('./some-library')` call would result in a
[`ERR_UNSUPPORTED_DIR_IMPORT`][] error. Using package [subpath exports][] or
[subpath imports][] can provide the same containment organization benefits as
folders as modules, and work for both `require` and `import`.

## Loading from `node_modules` folders

<!--type=misc-->

If the module identifier passed to `require()` is not a
[built-in](#built-in-modules) module, and does not begin with `'/'`, `'../'`, or
`'./'`, then Node.js starts at the directory of the current module, and
adds `/node_modules`, and attempts to load the module from that location.
Node.js will not append `node_modules` to a path already ending in
`node_modules`.

If it is not found there, then it moves to the parent directory, and so
on, until the root of the file system is reached.

For example, if the file at `'/home/ry/projects/foo.js'` called
`require('bar.js')`, then Node.js would look in the following locations, in
this order:

* `/home/ry/projects/node_modules/bar.js`
* `/home/ry/node_modules/bar.js`
* `/home/node_modules/bar.js`
* `/node_modules/bar.js`

This allows programs to localize their dependencies, so that they do not
clash.

It is possible to require specific files or sub modules distributed with a
module by including a path suffix after the module name. For instance
`require('example-module/path/to/file')` would resolve `path/to/file`
relative to where `example-module` is located. The suffixed path follows the
same module resolution semantics.

## Loading from the global folders

<!-- type=misc -->

If the `NODE_PATH` environment variable is set to a colon-delimited list
of absolute paths, then Node.js will search those paths for modules if they
are not found elsewhere.

On Windows, `NODE_PATH` is delimited by semicolons (`;`) instead of colons.

`NODE_PATH` was originally created to support loading modules from
varying paths before the current [module resolution][] algorithm was defined.

`NODE_PATH` is still supported, but is less necessary now that the Node.js
ecosystem has settled on a convention for locating dependent modules.
Sometimes deployments that rely on `NODE_PATH` show surprising behavior
when people are unaware that `NODE_PATH` must be set. Sometimes a
module's dependencies change, causing a different version (or even a
different module) to be loaded as the `NODE_PATH` is searched.

Additionally, Node.js will search in the following list of GLOBAL\_FOLDERS:

* 1: `$HOME/.node_modules`
* 2: `$HOME/.node_libraries`
* 3: `$PREFIX/lib/node`

Where `$HOME` is the user's home directory, and `$PREFIX` is the Node.js
configured `node_prefix`.

These are mostly for historic reasons.

It is strongly encouraged to place dependencies in the local `node_modules`
folder. These will be loaded faster, and more reliably.

## The module wrapper

<!-- type=misc -->

Before a module's code is executed, Node.js will wrap it with a function
wrapper that looks like the following:

```js
(function(exports, require, module, __filename, __dirname) {
// Module code actually lives in here
});
```

By doing this, Node.js achieves a few things:

* It keeps top-level variables (defined with `var`, `const`, or `let`) scoped to
  the module rather than the global object.
* It helps to provide some global-looking variables that are actually specific
  to the module, such as:
  * The `module` and `exports` objects that the implementor can use to export
    values from the module.
  * The convenience variables `__filename` and `__dirname`, containing the
    module's absolute filename and directory path.

## The module scope

### `__dirname`

<!-- YAML
added: v0.1.27
-->

* Type: {string}

The directory name of the current module. This is the same as the
[`path.dirname()`][] of the [`__filename`][].

Example: running `node example.js` from `/Users/mjr`

```js
console.log(__dirname);
// Prints: /Users/mjr
console.log(path.dirname(__filename));
// Prints: /Users/mjr
```

### `__filename`

<!-- YAML
added: v0.0.1
-->

* Type: {string}

The file name of the current module. This is the current module file's absolute
path with symlinks resolved.

For a main program this is not necessarily the same as the file name used in the
command line.

See [`__dirname`][] for the directory name of the current module.

Examples:

Running `node example.js` from `/Users/mjr`

```js
console.log(__filename);
// Prints: /Users/mjr/example.js
console.log(__dirname);
// Prints: /Users/mjr
```

Given two modules: `a` and `b`, where `b` is a dependency of
`a` and there is a directory structure of:

* `/Users/mjr/app/a.js`
* `/Users/mjr/app/node_modules/b/b.js`

References to `__filename` within `b.js` will return
`/Users/mjr/app/node_modules/b/b.js` while references to `__filename` within
`a.js` will return `/Users/mjr/app/a.js`.

### `exports`

<!-- YAML
added: v0.1.12
-->

* Type: {Object}

A reference to the `module.exports` that is shorter to type.
See the section about the [exports shortcut][] for details on when to use
`exports` and when to use `module.exports`.

### `module`

<!-- YAML
added: v0.1.16
-->

* Type: {module}

A reference to the current module, see the section about the
[`module` object][]. In particular, `module.exports` is used for defining what
a module exports and makes available through `require()`.

### `require(id)`

<!-- YAML
added: v0.1.13
-->

* `id` {string} module name or path
* Returns: {any} exported module content

Used to import modules, `JSON`, and local files. Modules can be imported
from `node_modules`. Local modules and JSON files can be imported using
a relative path (e.g. `./`, `./foo`, `./bar/baz`, `../foo`) that will be
resolved against the directory named by [`__dirname`][] (if defined) or
the current working directory. The relative paths of POSIX style are resolved
in an OS independent fashion, meaning that the examples above will work on
Windows in the same way they would on Unix systems.

```js
// Importing a local module with a path relative to the `__dirname` or current
// working directory. (On Windows, this would resolve to .\path\myLocalModule.)
const myLocalModule = require('./path/myLocalModule');

// Importing a JSON file:
const jsonData = require('./path/filename.json');

// Importing a module from node_modules or Node.js built-in module:
const crypto = require('node:crypto');
```

#### `require.cache`

<!-- YAML
added: v0.3.0
-->

* Type: {Object}

Modules are cached in this object when they are required. By deleting a key
value from this object, the next `require` will reload the module.
This does not apply to [native addons][], for which reloading will result in an
error.

Adding or replacing entries is also possible. This cache is checked before
built-in modules and if a name matching a built-in module is added to the cache,
only `node:`-prefixed require calls are going to receive the built-in module.
Use with care!

<!-- eslint-disable node-core/no-duplicate-requires, no-restricted-syntax -->

```js
const assert = require('node:assert');
const realFs = require('node:fs');

const fakeFs = {};
require.cache.fs = { exports: fakeFs };

assert.strictEqual(require('fs'), fakeFs);
assert.strictEqual(require('node:fs'), realFs);
```

#### `require.extensions`

<!-- YAML
added: v0.3.0
deprecated: v0.10.6
-->

> Stability: 0 - Deprecated

* Type: {Object}

Instruct `require` on how to handle certain file extensions.

Process files with the extension `.sjs` as `.js`:

```js
require.extensions['.sjs'] = require.extensions['.js'];
```

**Deprecated.** In the past, this list has been used to load non-JavaScript
modules into Node.js by compiling them on-demand. However, in practice, there
are much better ways to do this, such as loading modules via some other Node.js
program, or compiling them to JavaScript ahead of time.

Avoid using `require.extensions`. Use could cause subtle bugs and resolving the
extensions gets slower with each registered extension.

#### `require.main`

<!-- YAML
added: v0.1.17
-->

* Type: {module | undefined}

The `Module` object representing the entry script loaded when the Node.js
process launched, or `undefined` if the entry point of the program is not a
CommonJS module.
See ["Accessing the main module"](#accessing-the-main-module).

In `entry.js` script:

```js
console.log(require.main);
```

```bash
node entry.js
```

<!-- eslint-skip -->

```js
Module {
  id: '.',
  path: '/absolute/path/to',
  exports: {},
  filename: '/absolute/path/to/entry.js',
  loaded: false,
  children: [],
  paths:
   [ '/absolute/path/to/node_modules',
     '/absolute/path/node_modules',
     '/absolute/node_modules',
     '/node_modules' ] }
```

#### `require.resolve(request[, options])`

<!-- YAML
added: v0.3.0
changes:
  - version: v8.9.0
    pr-url: https://github.com/nodejs/node/pull/16397
    description: The `paths` option is now supported.
-->

* `request` {string} The module path to resolve.
* `options` {Object}
  * `paths` {string\[]} Paths to resolve module location from. If present, these
    paths are used instead of the default resolution paths, with the exception
    of [GLOBAL\_FOLDERS][GLOBAL_FOLDERS] like `$HOME/.node_modules`, which are
    always included. Each of these paths is used as a starting point for
    the module resolution algorithm, meaning that the `node_modules` hierarchy
    is checked from this location.
* Returns: {string}

Use the internal `require()` machinery to look up the location of a module,
but rather than loading the module, just return the resolved filename.

If the module can not be found, a `MODULE_NOT_FOUND` error is thrown.

##### `require.resolve.paths(request)`

<!-- YAML
added: v8.9.0
-->

* `request` {string} The module path whose lookup paths are being retrieved.
* Returns: {string\[]|null}

Returns an array containing the paths searched during resolution of `request` or
`null` if the `request` string references a core module, for example `http` or
`fs`.

## The `module` object

<!-- YAML
added: v0.1.16
-->

<!-- name=module -->

* Type: {Object}

In each module, the `module` free variable is a reference to the object
representing the current module. For convenience, `module.exports` is
also accessible via the `exports` module-global. `module` is not actually
a global but rather local to each module.

### `module.children`

<!-- YAML
added: v0.1.16
-->

* Type: {module\[]}

The module objects required for the first time by this one.

### `module.exports`

<!-- YAML
added: v0.1.16
-->

* Type: {Object}

The `module.exports` object is created by the `Module` system. Sometimes this is
not acceptable; many want their module to be an instance of some class. To do
this, assign the desired export object to `module.exports`. Assigning
the desired object to `exports` will simply rebind the local `exports` variable,
which is probably not what is desired.

For example, suppose we were making a module called `a.js`:

```js
const EventEmitter = require('node:events');

module.exports = new EventEmitter();

// Do some work, and after some time emit
// the 'ready' event from the module itself.
setTimeout(() => {
  module.exports.emit('ready');
}, 1000);
```

Then in another file we could do:

```js
const a = require('./a');
a.on('ready', () => {
  console.log('module "a" is ready');
});
```

Assignment to `module.exports` must be done immediately. It cannot be
done in any callbacks. This does not work:

`x.js`:

```js
setTimeout(() => {
  module.exports = { a: 'hello' };
}, 0);
```

`y.js`:

```js
const x = require('./x');
console.log(x.a);
```

#### `exports` shortcut

<!-- YAML
added: v0.1.16
-->

The `exports` variable is available within a module's file-level scope, and is
assigned the value of `module.exports` before the module is evaluated.

It allows a shortcut, so that `module.exports.f = ...` can be written more
succinctly as `exports.f = ...`. However, be aware that like any variable, if a
new value is assigned to `exports`, it is no longer bound to `module.exports`:

```js
module.exports.hello = true; // Exported from require of module
exports = { hello: false };  // Not exported, only available in the module
```

When the `module.exports` property is being completely replaced by a new
object, it is common to also reassign `exports`:

<!-- eslint-disable func-name-matching -->

```js
module.exports = exports = function Constructor() {
  // ... etc.
};
```

To illustrate the behavior, imagine this hypothetical implementation of
`require()`, which is quite similar to what is actually done by `require()`:

```js
function require(/* ... */) {
  const module = { exports: {} };
  ((module, exports) => {
    // Module code here. In this example, define a function.
    function someFunc() {}
    exports = someFunc;
    // At this point, exports is no longer a shortcut to module.exports, and
    // this module will still export an empty default object.
    module.exports = someFunc;
    // At this point, the module will now export someFunc, instead of the
    // default object.
  })(module, module.exports);
  return module.exports;
}
```

### `module.filename`

<!-- YAML
added: v0.1.16
-->

* Type: {string}

The fully resolved filename of the module.

### `module.id`

<!-- YAML
added: v0.1.16
-->

* Type: {string}

The identifier for the module. Typically this is the fully resolved
filename.

### `module.isPreloading`

<!-- YAML
added:
  - v15.4.0
  - v14.17.0
-->

* Type: {boolean} `true` if the module is running during the Node.js preload
  phase.

### `module.loaded`

<!-- YAML
added: v0.1.16
-->

* Type: {boolean}

Whether or not the module is done loading, or is in the process of
loading.

### `module.parent`

<!-- YAML
added: v0.1.16
deprecated:
  - v14.6.0
  - v12.19.0
-->

> Stability: 0 - Deprecated: Please use [`require.main`][] and
> [`module.children`][] instead.

* Type: {module | null | undefined}

The module that first required this one, or `null` if the current module is the
entry point of the current process, or `undefined` if the module was loaded by
something that is not a CommonJS module (E.G.: REPL or `import`).

### `module.path`

<!-- YAML
added: v11.14.0
-->

* Type: {string}

The directory name of the module. This is usually the same as the
[`path.dirname()`][] of the [`module.id`][].

### `module.paths`

<!-- YAML
added: v0.4.0
-->

* Type: {string\[]}

The search paths for the module.

### `module.require(id)`

<!-- YAML
added: v0.5.1
-->

* `id` {string}
* Returns: {any} exported module content

The `module.require()` method provides a way to load a module as if
`require()` was called from the original module.

In order to do this, it is necessary to get a reference to the `module` object.
Since `require()` returns the `module.exports`, and the `module` is typically
_only_ available within a specific module's code, it must be explicitly exported
in order to be used.

## The `Module` object

This section was moved to
[Modules: `module` core module](module.md#the-module-object).

<!-- Anchors to make sure old links find a target -->

* <a id="modules_module_builtinmodules" href="module.html#modulebuiltinmodules">`module.builtinModules`</a>
* <a id="modules_module_createrequire_filename" href="module.html#modulecreaterequirefilename">`module.createRequire(filename)`</a>
* <a id="modules_module_syncbuiltinesmexports" href="module.html#modulesyncbuiltinesmexports">`module.syncBuiltinESMExports()`</a>

## Source map v3 support

This section was moved to
[Modules: `module` core module](module.md#source-map-support).

<!-- Anchors to make sure old links find a target -->

* <a id="modules_module_findsourcemap_path_error" href="module.html#modulefindsourcemappath">`module.findSourceMap(path)`</a>
* <a id="modules_class_module_sourcemap" href="module.html#class-modulesourcemap">Class: `module.SourceMap`</a>
  * <a id="modules_new_sourcemap_payload" href="module.html#new-sourcemappayload--linelengths-">`new SourceMap(payload)`</a>
  * <a id="modules_sourcemap_payload" href="module.html#sourcemappayload">`sourceMap.payload`</a>
  * <a id="modules_sourcemap_findentry_linenumber_columnnumber" href="module.html#sourcemapfindentrylineoffset-columnoffset">`sourceMap.findEntry(lineNumber, columnNumber)`</a>

[Determining module system]: packages.md#determining-module-system
[ECMAScript Modules]: esm.md
[GLOBAL_FOLDERS]: #loading-from-the-global-folders
[`"main"`]: packages.md#main
[`"type"`]: packages.md#type
[`--trace-require-module`]: cli.md#--trace-require-modulemode
[`ERR_REQUIRE_ASYNC_MODULE`]: errors.md#err_require_async_module
[`ERR_UNSUPPORTED_DIR_IMPORT`]: errors.md#err_unsupported_dir_import
[`MODULE_NOT_FOUND`]: errors.md#module_not_found
[`__dirname`]: #__dirname
[`__filename`]: #__filename
[`import()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import
[`module.builtinModules`]: module.md#modulebuiltinmodules
[`module.children`]: #modulechildren
[`module.id`]: #moduleid
[`module` core module]: module.md
[`module` object]: #the-module-object
[`node:sea`]: single-executable-applications.md#single-executable-application-api
[`node:sqlite`]: sqlite.md
[`node:test/reporters`]: test.md#test-reporters
[`node:test`]: test.md
[`package.json`]: packages.md#nodejs-packagejson-field-definitions
[`path.dirname()`]: path.md#pathdirnamepath
[`process.features.require_module`]: process.md#processfeaturesrequire_module
[`require.main`]: #requiremain
[exports shortcut]: #exports-shortcut
[module namespace object]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import#module_namespace_object
[module resolution]: #all-together
[native addons]: addons.md
[subpath exports]: packages.md#subpath-exports
[subpath imports]: packages.md#subpath-imports