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CommonJS modules

Table des matières

Ajouté en: v0.10.0

Stabilité: 2 - Stable

CommonJS modules are the original way to package JavaScript code for Node.js. Node.js also supports the ECMAScript modules standard used by browsers and other JavaScript runtimes.

In Node.js, each file is treated as a separate module. For example, consider a file named foo.js:

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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:

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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:

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The square module is defined in square.js:

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The CommonJS module system is implemented in the module core module.

Enabling

Node.js has two module systems: CommonJS modules and ECMAScript modules.

By default, Node.js will treat the following as CommonJS modules:

  • Files with a .cjs extension;

  • Files with a .js extension when the nearest parent package.json file contains a top-level field "type" with a value of "commonjs".

  • Files with a .js extension when the nearest parent package.json file doesn't contain a top-level field "type". Package authors should include the "type" field, even in packages where all sources are CommonJS. Being explicit about the type of the package will make things easier for build tools and loaders to determine how the files in the package should be interpreted.

  • Files with an extension that is not .mjs, .cjs, .json, .node, or .js (when the nearest parent package.json file contains a top-level field "type" with a value of "module", those files will be recognized as CommonJS modules only if they are being required, not when used as the command-line entry point of the program).

See Determining module system for more details.

Calling require() always use the CommonJS module loader. Calling import() always use the ECMAScript module loader.

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').

When the entry point is not a CommonJS module, require.main is undefined, and the main module is out of reach.

Package manager tips

The semantics of the Node.js require() function were designed to be general enough to support 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.

Because Node.js looks up the realpath of any modules it loads (that is, it resolves symlinks) and then looks for their dependencies in node_modules folders, this situation can be resolved 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.

The .mjs extension

Due to the synchronous nature of require(), it is not possible to use it to load ECMAScript module files. Attempting to do so will throw a ERR_REQUIRE_ESM error. Use import() instead.

The .mjs extension is reserved for ECMAScript Modules which cannot be loaded via require(). See Determining module system section for more info regarding which files are parsed as ECMAScript modules.

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() does:

TEXT

b.js:

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main.js:

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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:

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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. When loading a file that has a different extension (e.g. .cjs), its full name must be passed to require(), including its file extension (e.g. require('./file.cjs')).

.json files are parsed as JSON text files, .node files are interpreted as compiled addon modules loaded with process.dlopen(). Files using any other extension (or no extension at all) are parsed as JavaScript text files. Refer to the Determining module system section to understand what parse goal will be used.

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 a MODULE_NOT_FOUND error.

Folders as modules

Stabilité: 3 - Legacy: Use subpath exports][] or [subpath imports instead.

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:

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If this was in a folder at ./some-library, then require('./some-library') would attempt to load ./some-library/lib/some-library.js.

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 previous 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:

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In all three above cases, an import('./some-library') call would result in a ERR_UNSUPPORTED_DIR_IMPORT error. Using package subpath exports or subpath imports can provide the same containment organization benefits as folders as modules, and work for both require and import.

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 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 defined.

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 the Node.js 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:

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By doing this, Node.js achieves a few things:

  • It keeps top-level variables (defined with var, const, or let) 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 and exports 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 module scope

M __dirname

Ajouté en: v0.1.27

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

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M __filename

Ajouté en: v0.0.1

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

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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.

M exports

Ajouté en: v0.1.12

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.

M module

Ajouté en: v0.1.16

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().

M require(id)

Ajouté en: v0.1.13

  • id string module name or path
  • Returns: any exported module content

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. The relative paths of POSIX style are resolved in an OS independent fashion, meaning that the examples above will work on Windows in the same way they would on Unix systems.

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M require.cache

Ajouté en: v0.3.0

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. This does not apply to native addons, for which reloading will result in an error.

Adding or replacing entries is also possible. This cache is checked before built-in modules and if a name matching a built-in module is added to the cache, only node:-prefixed require calls are going to receive the built-in module. Use with care!

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M require.extensions

Déprécié en: v0.10.6

Stabilité: 0 - Deprecated

Instruct require on how to handle certain file extensions.

Process files with the extension .sjs as .js:

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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.

Avoid using require.extensions. Use could cause subtle bugs and resolving the extensions gets slower with each registered extension.

M require.main

Ajouté en: v0.1.17

The Module object representing the entry script loaded when the Node.js process launched, or undefined if the entry point of the program is not a CommonJS module. See "Accessing the main module".

In entry.js script:

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BASH
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M require.resolve(request[, options])
Historique
VersionChangements
v8.9.0The `paths` option is now supported.
v0.3.0Ajouté en: v0.3.0
  • 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. Each of these paths is used as a starting point for the module resolution algorithm, meaning that the node_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.

If the module can not be found, a MODULE_NOT_FOUND error is thrown.

require.resolve.paths(request)

Ajouté en: v8.9.0

  • request string The module path whose lookup paths are being retrieved.
  • Returns: 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

Ajouté en: v0.1.16

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.

M module.children

Ajouté en: v0.1.16

  • module[]

The module objects required for the first time by this one.

M module.exports

Ajouté en: v0.1.16

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. 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:

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Then in another file we could do:

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Assignment to module.exports must be done immediately. It cannot be done in any callbacks. This does not work:

x.js:

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y.js:

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M exports shortcut

Ajouté en: v0.1.16

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:

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When the module.exports property is being completely replaced by a new object, it is common to also reassign exports:

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To illustrate the behavior, imagine this hypothetical implementation of require(), which is quite similar to what is actually done by require():

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M module.filename

Ajouté en: v0.1.16

The fully resolved filename of the module.

M module.id

Ajouté en: v0.1.16

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

M module.isPreloading

Ajouté en: v15.4.0, v14.17.0

  • Type: boolean true if the module is running during the Node.js preload phase.

M module.loaded

Ajouté en: v0.1.16

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

M module.parent

Déprécié en: v14.6.0, v12.19.0

Stabilité: 0 - Deprecated: Please use `require.main`][] and [`module.children` instead.

The module that first required this one, or null if the current module is the entry point of the current process, or undefined if the module was loaded by something that is not a CommonJS module (E.G.: REPL or import).

M module.path

Ajouté en: v11.14.0

The directory name of the module. This is usually the same as the path.dirname() of the module.id.

M module.paths

Ajouté en: v0.4.0

  • string[]

The search paths for the module.

M module.require(id)

Ajouté en: v0.5.1

  • id string
  • Returns: any exported module content

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

This section was moved to Modules: module core module.

Source map v3 support

This section was moved to Modules: module core module.

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