@ -57,6 +57,169 @@ The `square` module is defined in `square.js`:
The module system is implemented in the `require("module")` module.
## Accessing the main module
<!-- type=misc -->
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
<!-- type=misc -->
The semantics of Node.js'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.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 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.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...
<!-- type=misc -->
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
c. DIR = path join(PARTS[0 .. I] + "node_modules")
b. DIRS = DIRS + DIR
c. let I = I - 1
5. return DIRS
## Caching
<!-- type=misc -->
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
<!-- type=misc -->
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.
## Core Modules
<!-- type=misc -->
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
<!-- type=misc -->
@ -113,20 +276,6 @@ The output of this program would thus be:
If you have cyclic module dependencies in your program, make sure to
plan accordingly.
## Core Modules
<!-- type=misc -->
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.
## File Modules
<!-- type=misc -->
@ -153,36 +302,6 @@ 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'` .
## Loading from `node_modules` Folders
<!-- type=misc -->
If the module identifier passed to `require()` is not a native 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.
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.
## Folders as Modules
<!-- type=misc -->
@ -213,31 +332,69 @@ example, then `require('./some-library')` would attempt to load:
* `./some-library/index.js`
* `./some-library/index.node`
## Caching
## Loading from `node_modules` Folders
<!-- type=misc -->
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.
If the module identifier passed to `require()` is not a native 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.
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 it is not found there, then it moves to the parent directory, and so
on, until the root of the file system is reached.
If you want to have a module execute code multiple times, then export a
function, and call that function.
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:
### Module Caching Caveats
* `/home/ry/projects/node_modules/bar.js`
* `/home/ry/node_modules/bar.js`
* `/home/node_modules/bar.js`
* `/node_modules/bar.js`
<!-- type=misc -->
This allows programs to localize their dependencies, so that they do not
clash.
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.
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
<!-- type=misc -->
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 ](https://nodejs.org/api/modules.html#modules_all_together )
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` Object
@ -251,6 +408,12 @@ representing the current module. For convenience, `module.exports` is
also accessible via the `exports` module-global. `module` isn't actually
a global but rather local to each module.
### module.children
* {Array}
The module objects required by this one.
### module.exports
* {Object}
@ -318,19 +481,11 @@ To illustrate the behavior, imagine this hypothetical implementation of
As a guideline, if the relationship between `exports` and `module.exports`
seems like magic to you, ignore `exports` and only use `module.exports` .
### 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.
### module.filename
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.
* {String}
The fully resolved filename to the module.
### module.id
@ -339,14 +494,6 @@ explicitly exported in order to be used.
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}
@ -354,174 +501,21 @@ The fully resolved filename to the module.
Whether or not the module is done loading, or is in the process of
loading.
### module.parent
* {Module Object}
The module that first required this one.
### module.children
* {Array}
The module objects required by this one.
## All Together...
<!-- type=misc -->
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
c. DIR = path join(PARTS[0 .. I] + "node_modules")
b. DIRS = DIRS + DIR
c. let I = I - 1
5. return DIRS
## Loading from the global folders
<!-- type=misc -->
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 ](https://nodejs.org/api/modules.html#modules_all_together )
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.
## Accessing the main module
<!-- type=misc -->
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
<!-- type=misc -->
The semantics of Node.js'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.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 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.
### module.require(id)
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` .
* `id` {String}
* Return: {Object} `module.exports` from the resolved module
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` .
The `module.require` method provides a way to load a module as if
`require()` was called from the original module.
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.
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.
Tristian Flanagan
9 years ago
James M Snell