# Modules

    Stability: 3 - Locked

<!--name=module-->

Node.js has a simple module loading system.  In Node.js, files and modules are
in one-to-one correspondence.  As an example, `foo.js` loads the module
`circle.js` in the same directory.

The contents of `foo.js`:

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

The contents of `circle.js`:

```js
const PI = Math.PI;

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

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

```

The module `circle.js` has exported the functions `area()` and
`circumference()`.  To add functions and objects to the root of your module,
you can add them to the special `exports` object.

Variables local to the module will be private, as though the module was wrapped
in a function. In this example the variable `PI` is private to `circle.js`.

If you want the root of your module's export to be a function (such as a
constructor) or if you want to export a complete object in one assignment
instead of building it one property at a time, assign it to `module.exports`
instead of `exports`.

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

```js
const square = require('./square.js');
var 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 * width
  };
}
```

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 you can determine whether a file has been run
directly by testing

```js
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`, you
may have to install a specific version of package `bar`.  The `bar` package
may itself have dependencies, and in some cases, these dependencies may even
collide or form cycles.

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:

```
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 './' or '/' or '../'
   a. LOAD_AS_FILE(Y + X)
   b. LOAD_AS_DIRECTORY(Y + X)
3. LOAD_NODE_MODULES(X, dirname(Y))
4. 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_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)
2. If X/index.js is a file, load X/index.js as JavaScript text.  STOP
3. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP
4. If X/index.node is a file, load X/index.node as binary addon.  STOP

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
   c. DIR = path join(PARTS[0 .. I] + "node_modules")
   b. DIRS = DIRS + DIR
   c. 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.

If you want to have a module execute code multiple times, then 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`:

```
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`:

```
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`:

```
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:

```
$ 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
```

If you have cyclic module dependencies in your program, make sure to
plan accordingly.

## 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:

```
{ "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:

```
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 native 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.

You can 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.  **You are highly encouraged
to place your dependencies locally in `node_modules` folders.**  They
will be loaded faster, and more reliably.

## The `module` Object

<!-- 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` isn't actually
a global but rather local to each module.

### module.children

* {Array}

The module objects required by this one.

### module.exports

* {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 you want to do.

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 alias

The `exports` variable that is available within a module starts as a reference
to `module.exports`. As with any variable, if you assign a new value to it, it
is no longer bound to the previous value.

To illustrate the behavior, imagine this hypothetical implementation of
`require()`:

```js
function require(...) {
  // ...
  ((module, exports) => {
    // Your module code here
    exports = some_func;        // re-assigns exports, exports is no longer
                                // a shortcut, and nothing is exported.
    module.exports = some_func; // makes your module export 0
  })(module, module.exports);
  return module;
}
```

As a guideline, if the relationship between `exports` and `module.exports`
seems like magic to you, ignore `exports` and only use `module.exports`.

### module.filename

* {String}

The fully resolved filename to the module.

### module.id

* {String}

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

### module.loaded

* {Boolean}

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

### module.parent

* {Object} Module object

The module that first required this one.

### module.require(id)

* `id` {String}
* Return: {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 that in order to do this, you must 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.

[`Error`]: errors.html#errors_class_error
[module resolution]: #modules_all_together
