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Node.js v10.17.0-test201912065dae3ef88b Documentation
Table of Contents
Modules#
In the Node.js module system, each file is treated as a separate module. For
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;
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).
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 Square class:
const Square = require('./square.js');
const mySquare = new Square(2);
console.log(`The area of mySquare is ${mySquare.area()}`);
The square
module is defined in square.js
:
// assigning to exports will not modify module, must use module.exports
module.exports = class Square {
constructor(width) {
this.width = width;
}
area() {
return this.width ** 2;
}
};
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 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#
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, 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 '/'
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 = [GLOBAL_FOLDERS]
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.
Provided require.cache
is not modified, multiple calls to
require('foo')
will 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#
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
Careful planning is required to allow cyclic module dependencies to work correctly within an application.
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.
If there is no package.json
file present in the directory, or if the
'main'
entry is missing or cannot be resolved, 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
If these attempts fail, then Node.js will report the entire module as missing with the default error:
Error: Cannot find module 'some-library'
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.js 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#
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.
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 list of GLOBAL_FOLDERS:
- 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.
It is strongly encouraged to place dependencies in the local node_modules
folder. These 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) {
// 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 scope#
__dirname#
The directory name of the current module. This is the same as the
path.dirname()
of the __filename
.
Example: running node example.js
from /Users/mjr
console.log(__dirname);
// Prints: /Users/mjr
console.log(path.dirname(__filename));
// Prints: /Users/mjr
__filename#
The file name of the current module. This is the current module file's absolute path with symlinks resolved.
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
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#
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#
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()#
Used to import modules, JSON
, and local files. Modules can be imported
from node_modules
. Local modules and JSON files can be imported using
a relative path (e.g. ./
, ./foo
, ./bar/baz
, ../foo
) that will be
resolved against the directory named by __dirname
(if defined) or
the current working directory.
// Importing a local module:
const myLocalModule = require('./path/myLocalModule');
// Importing a JSON file:
const jsonData = require('./path/filename.json');
// Importing a module from node_modules or Node.js built-in module:
const crypto = require('crypto');
require.cache#
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#
Instruct require
on how to handle certain file extensions.
Process files with the extension .sjs
as .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.main#
The Module
object representing the entry script loaded when the Node.js
process launched.
See "Accessing the main module".
In entry.js
script:
console.log(require.main);
node entry.js
Module {
id: '.',
exports: {},
parent: null,
filename: '/absolute/path/to/entry.js',
loaded: false,
children: [],
paths:
[ '/absolute/path/to/node_modules',
'/absolute/path/node_modules',
'/absolute/node_modules',
'/node_modules' ] }
require.resolve(request[, options])#
request
<string> The module path to resolve.-
options
<Object>paths
<string[]> Paths to resolve module location from. If present, these paths are used instead of the default resolution paths, with the exception of GLOBAL_FOLDERS like$HOME/.node_modules
, which are always included. Note that each of these paths is used as a starting point for the module resolution algorithm, meaning that thenode_modules
hierarchy is checked from this location.
- Returns: <string>
Use the internal require()
machinery to look up the location of a module,
but rather than loading the module, just return the resolved filename.
require.resolve.paths(request)#
request
<string> The module path whose lookup paths are being retrieved.- Returns: <string[]> | <null>
Returns an array containing the paths searched during resolution of request
or
null
if the request
string references a core module, for example http
or
fs
.
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 for the first time 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 is desired.
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
:
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(/* ... */) {
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#
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#
The module that first required this one.
module.paths#
The search paths for the module.
module.require(id)#
The module.require
method provides a way to load a module as if
require()
was called from the original module.
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.
The Module
Object#
Provides general utility methods when interacting with instances of
Module
— the module
variable often seen in file modules. Accessed
via require('module')
.
module.builtinModules#
A list of the names of all modules provided by Node.js. Can be used to verify if a module is maintained by a third party or not.
Note that module
in this context isn't the same object that's provided
by the module wrapper. To access it, require the Module
module:
const builtin = require('module').builtinModules;
module.createRequireFromPath(filename)#
filename
<string> Filename to be used to construct the relative require function.- Returns: {
require
} Require function
const { createRequireFromPath } = require('module');
const requireUtil = createRequireFromPath('../src/utils');
// require `../src/utils/some-tool`
requireUtil('./some-tool');