Node.js v0.11.7 Manual & Documentation
Table of Contents
Modules#
Stability: 5 - Locked
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.
Note that exports
is a reference to module.exports
making it suitable
for augmentation only. If you are exporting a single item such as a
constructor you will want to use module.exports
directly instead.
function MyConstructor (opts) {
//...
}
// BROKEN: Does not modify exports
exports = MyConstructor;
// exports the constructor properly
module.exports = MyConstructor;
Variables
local to the module will be private. In this example the variable PI
is
private to circle.js
.
The module system is implemented in the require("module")
module.
Cycles#
When there are circular require()
calls, a module might not be
done being executed when it is returned.
Consider this situation:
a.js
:
console.log('a starting');
exports.done = false;
var 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;
var 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');
var a = require('./a.js');
var 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.
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
, .json
, and then .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 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.
If the given path does not exist, require()
will throw an Error with its
code
property set to 'MODULE_NOT_FOUND'
.
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 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.
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.
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.
The module
Object#
- {Object}
In each module, the module
free variable is a reference to the object
representing the current module. In particular
module.exports
is accessible via the exports
module-global.
module
isn't actually a global but rather local to each module.
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
. For example suppose we
were making a module called a.js
var EventEmitter = require('events').EventEmitter;
module.exports = new EventEmitter();
// Do some work, and after some time emit
// the 'ready' event from the module itself.
setTimeout(function() {
module.exports.emit('ready');
}, 1000);
Then in another file we could do
var a = require('./a');
a.on('ready', function() {
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(function() {
module.exports = { a: "hello" };
}, 0);
y.js:
var x = require('./x');
console.log(x.a);
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.
module.id#
- String
The identifier for the module. Typically this is the fully resolved filename.
module.filename#
- String
The fully resolved filename to the module.
module.loaded#
- Boolean
Whether or not the module is done loading, or is in the process of loading.
module.parent#
- Module Object
The module that required this one.
module.children#
- Array
The module objects required by this one.
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.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.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 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
c. let I = I - 1
6. return DIRS
Loading from the global folders#
If the NODE_PATH
environment variable is set to a colon-delimited list
of absolute paths, then node will search those paths for modules if they
are not found elsewhere. (Note: On Windows, NODE_PATH
is delimited by
semicolons instead of colons.)
Additionally, node 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'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.
Accessing the main module#
When a file is run directly from Node, 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'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 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 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.