# Modules

> Stability: 2 - Stable

<!--name=module-->

Node.js has a simple module loading system.  In Node.js, files and modules
are in one-to-one correspondence (each file is treated as a separate module).

As an 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](#modules_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).

Below, `bar.js` makes use of the `square` module, which exports a constructor:

```js
const square = require('./square.js');
const mySquare = square(2);
console.log(`The area of my square 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 = (width) => {
  return {
    area: () => width ** 2
  };
};
```

The module system is implemented in the `require('module')` module.

## 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')`.

Because `module` provides a `filename` property (normally equivalent to
`__filename`), the entry point of the current application can be obtained
by checking `require.main.filename`.

## Addenda: Package Manager Tips

<!-- type=misc -->

The semantics of Node.js's `require()` function were designed to be general
enough to support a number of 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.

Below 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.

Since Node.js looks up the `realpath` of any modules it loads (that is,
resolves symlinks), and then looks for their dependencies in the `node_modules`
folders as described [here](#modules_loading_from_node_modules_folders), this
situation is very simple to resolve 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.

## 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.resolve()` does:

```txt
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 be the filesystem root
3. If X begins with './' or '/' or '../'
   a. LOAD_AS_FILE(Y + X)
   b. LOAD_AS_DIRECTORY(Y + X)
4. LOAD_NODE_MODULES(X, dirname(Y))
5. THROW "not found"

LOAD_AS_FILE(X)
1. If X is a file, load X as JavaScript text.  STOP
2. If X.js is a file, load X.js as JavaScript text.  STOP
3. If X.json is a file, parse 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, load X/index.js as JavaScript text.  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. let M = X + (json main field)
   c. LOAD_AS_FILE(M)
   d. LOAD_INDEX(M)
2. LOAD_INDEX(X)

LOAD_NODE_MODULES(X, START)
1. let DIRS=NODE_MODULES_PATHS(START)
2. for each DIR in DIRS:
   a. LOAD_AS_FILE(DIR/X)
   b. 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" CONTINUE
   b. DIR = path join(PARTS[0 .. I] + "node_modules")
   c. DIRS = DIRS + DIR
   d. let I = I - 1
5. return DIRS
```

## 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.

Multiple calls to `require('foo')` may 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.

## Core Modules

<!--type=misc-->

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

The core modules are defined within Node.js's source and are located in the
`lib/` folder.

Core 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.

## 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:

```txt
$ 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`.

`.js` files are interpreted as JavaScript text files, and `.json` files are
parsed as JSON text files. `.node` files are interpreted as compiled addon
modules loaded with `dlopen`.

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 an [`Error`][] with its
`code` property set to `'MODULE_NOT_FOUND'`.

## Folders as Modules

<!--type=misc-->

It is convenient to organize programs and libraries into self-contained
directories, and then provide a single entry point to that library.
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`.

This is the extent of Node.js's awareness of package.json files.

*Note*: If the file specified by the `"main"` entry of `package.json` is
missing and can not be resolved, Node.js will report the entire module as
missing with the default error:

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

If there is no package.json file present in the directory, 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 above
example, then `require('./some-library')` would attempt to load:

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

## Loading from `node_modules` Folders

<!--type=misc-->

If the module identifier passed to `require()` is not a
[core](#modules_core_modules) module, and does not begin with `'/'`, `'../'`, or
`'./'`, then Node.js starts at the parent directory of the current module, and
adds `/node_modules`, and attempts to load the module from that location. Node
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.

*Note*: 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 frozen.

`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 locations:

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

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

These are mostly for historic reasons.

*Note*: 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=var -->

* {string}

The directory name of the current module. This 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=var -->

* {string}

The file name of the current module. This is the resolved absolute path of the
current module file.

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=var -->

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=var -->

* {Object}

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

<!-- type=var -->

* {Function}

To require modules.

#### require.cache
<!-- YAML
added: v0.3.0
-->

* {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. Note that
this does not apply to [native addons][], for which reloading will result in an
Error.

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

> Stability: 0 - Deprecated

* {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.

Since the module system is locked, this feature will probably never go
away.  However, it may have subtle bugs and complexities that are best
left untouched.

Note that the number of file system operations that the module system
has to perform in order to resolve a `require(...)` statement to a
filename scales linearly with the number of registered extensions.

In other words, adding extensions slows down the module loader and
should be discouraged.

#### require.resolve()
<!-- YAML
added: v0.3.0
-->

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

## The `module` Object
<!-- YAML
added: v0.1.16
-->

<!-- type=var -->
<!-- name=module -->

* {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
-->

* {Array}

The module objects required by this one.

### module.exports
<!-- YAML
added: v0.1.16
-->

* {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`. Note that 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('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');
});
```


Note that 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`, for example:

<!-- 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
-->

* {string}

The fully resolved filename to the module.

### module.id
<!-- YAML
added: v0.1.16
-->

* {string}

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

### module.loaded
<!-- YAML
added: v0.1.16
-->

* {boolean}

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

### module.parent
<!-- YAML
added: v0.1.16
-->

* {Object} Module object

The module that first required this one.

### module.paths
<!-- YAML
added: v0.4.0
-->

* {string[]}

The search paths for the module.

### module.require(id)
<!-- YAML
added: v0.5.1
-->

* `id` {string}
* Returns: {Object} `module.exports` from the resolved module

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

*Note*: 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.

[`__dirname`]: #modules_dirname
[`__filename`]: #modules_filename
[`Error`]: errors.html#errors_class_error
[`module` object]: #modules_the_module_object
[`path.dirname()`]: path.html#path_path_dirname_path
[exports shortcut]: #modules_exports_shortcut
[module resolution]: #modules_all_together
[native addons]: addons.html
