## Modules

Node uses the CommonJS module system.

Node has a simple module loading system.  In Node, 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`:

    var circle = require('./circle.js');
    console.log( 'The area of a circle of radius 4 is '
               + circle.area(4));

The contents of `circle.js`:

    var PI = Math.PI;

    exports.area = function (r) {
      return PI * r * r;
    };

    exports.circumference = function (r) {
      return 2 * PI * r;
    };

The module `circle.js` has exported the functions `area()` and
`circumference()`.  To export an object, add to the special `exports`
object.

Variables
local to the module will be private. In this example the variable `PI` is
private to `circle.js`.

### Core Modules

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

The core modules are defined in node's source 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.

### File Modules

If the exact filename is not found, then node will attempt to load the
required filename with the added extension of `.js`, and then `.node`.

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

A 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 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 is either a
"core module" or is loaded from a `node_modules` folder.

### Loading from `node_modules` Folders

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

If it is not found there, then it moves to the parent directory, and so
on, until either the module is found, or the root of the tree is
reached.

For example, if the file at `'/home/ry/projects/foo.js'` called
`require('bar.js')`, then node 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.

#### Optimizations to the `node_modules` Lookup Process

When there are many levels of nested dependencies, it is possible for
these file trees to get fairly long.  The following optimizations are thus
made to the process.

First, `/node_modules` is never appended to a folder already ending in
`/node_modules`.

Second, if the file calling `require()` is already inside a `node_modules`
hierarchy, then the top-most `node_modules` folder is treated as the
root of the search tree.

For example, if the file at
`'/home/ry/projects/foo/node_modules/bar/node_modules/baz/quux.js'`
called `require('asdf.js')`, then node would search the following
locations:

* `/home/ry/projects/foo/node_modules/bar/node_modules/baz/node_modules/asdf.js`
* `/home/ry/projects/foo/node_modules/bar/node_modules/asdf.js`
* `/home/ry/projects/foo/node_modules/asdf.js`

### Folders as Modules

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's awareness of package.json files.

If there is no package.json file present in the directory, then node
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`

### Caching

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.

### All Together...

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)
    1. If X is a core module,
       a. return the core module
       b. STOP
    2. If X begins with `./` 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.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. LOAD_AS_FILE(X/index)

    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 ROOT = index of first instance of "node_modules" in PARTS, or 0
    3. let I = count of PARTS - 1
    4. let DIRS = []
    5. while I > ROOT,
       a. if PARTS[I] = "node_modules" CONTINUE
       c. DIR = path join(PARTS[0 .. I] + "node_modules")
       b. DIRS = DIRS + DIR
    6. return DIRS

### Loading from the `require.paths` Folders

In node, `require.paths` is an array of strings that represent paths to
be searched for modules when they are not prefixed with `'/'`, `'./'`, or
`'../'`.  For example, if require.paths were set to:

    [ '/home/micheil/.node_modules',
      '/usr/local/lib/node_modules' ]

Then calling `require('bar/baz.js')` would search the following
locations:

* 1: `'/home/micheil/.node_modules/bar/baz.js'`
* 2: `'/usr/local/lib/node_modules/bar/baz.js'`

The `require.paths` array can be mutated at run time to alter this
behavior.

It is set initially from the `NODE_PATH` environment variable, which is
a colon-delimited list of absolute paths.  In the previous example,
the `NODE_PATH` environment variable might have been set to:

    /home/micheil/.node_modules:/usr/local/lib/node_modules

Loading from the `require.paths` locations is only performed if the
module could not be found using the `node_modules` algorithm above.
Global modules are lower priority than bundled dependencies.

#### **Note:** Please Avoid Modifying `require.paths`

For compatibility reasons, `require.paths` is still given first priority
in the module lookup process.  However, it may disappear in a future
release.

While it seemed like a good idea at the time, and enabled a lot of
useful experimentation, in practice a mutable `require.paths` list is
often a troublesome source of confusion and headaches.

##### Setting `require.paths` to some other value does nothing.

This does not do what one might expect:

    require.paths = [ '/usr/lib/node' ];

All that does is lose the reference to the *actual* node module lookup
paths, and create a new reference to some other thing that isn't used
for anything.

##### Putting relative paths in `require.paths` is... weird.

If you do this:

    require.paths.push('./lib');

then it does *not* add the full resolved path to where `./lib`
is on the filesystem.  Instead, it literally adds `'./lib'`,
meaning that if you do `require('y.js')` in `/a/b/x.js`, then it'll look
in `/a/b/lib/y.js`.  If you then did `require('y.js')` in
`/l/m/n/o/p.js`, then it'd look in `/l/m/n/o/lib/y.js`.

In practice, people have used this as an ad hoc way to bundle
dependencies, but this technique is brittle.

##### Zero Isolation

There is (by regrettable design), only one `require.paths` array used by
all modules.

As a result, if one node program comes to rely on this behavior, it may
permanently and subtly alter the behavior of all other node programs in
the same process.  As the application stack grows, we tend to assemble
functionality, and it is a problem with those parts interact in ways
that are difficult to predict.

## Addenda: Package Manager Tips

The semantics of Node's `require()` function were designed to be general
enough to support a number of sane directory structures. Package manager
programs such as `dpkg`, `rpm`, and `npm` will hopefully find it possible to
build native packages from Node 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 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 above, 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 will not bother
looking for missing dependencies in `/usr/node_modules` or `/node_modules`.

In order to make modules available to the node 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.