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Node.js v7.5.1-nightly2017020113a024d531 Documentation
Table of Contents
Modules#
Stability: 3 - Locked
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
:
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
:
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, because the module is wrapped
in a function by Node.js (see module wrapper).
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:
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
:
// 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#
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
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#
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, this
situation is very simple to resolve with the following architecture:
/usr/lib/node/foo/1.2.3/
- Contents of thefoo
package, version 1.2.3./usr/lib/node/bar/4.3.2/
- Contents of thebar
package thatfoo
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 thatbar
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...#
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
b. DIR = path join(PARTS[0 .. I] + "node_modules")
c. DIRS = DIRS + DIR
d. let I = I - 1
5. return DIRS
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.
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#
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#
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#
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#
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#
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#
If the module identifier passed to require()
is not a
core 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#
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 wrapper#
Before a module's code is executed, Node.js will wrap it with a function wrapper that looks like the following:
(function (exports, require, module, __filename, __dirname) {
// Your module code actually lives in here
});
By doing this, Node.js achieves a few things:
- It keeps top-level variables (defined with
var
,const
orlet
) 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
andexports
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
The 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#
The module objects required by this one.
module.exports#
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
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
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:
setTimeout(() => {
module.exports = { a: 'hello' };
}, 0);
y.js:
const x = require('./x');
console.log(x.a);
exports shortcut#
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
:
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:
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()
:
function require(...) {
var module = { exports: {} };
((module, exports) => {
// Your module code here. In this example, define a function.
function some_func() {};
exports = some_func;
// At this point, exports is no longer a shortcut to module.exports, and
// this module will still export an empty default object.
module.exports = some_func;
// At this point, the module will now export some_func, instead of the
// default object.
})(module, module.exports);
return module.exports;
}
module.filename#
The fully resolved filename to the module.
module.id#
The identifier for the module. Typically this is the fully resolved filename.
module.loaded#
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)#
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.