{ "source": "doc/api/modules.markdown", "modules": [ { "textRaw": "Modules", "name": "module", "stability": 5, "stabilityText": "Locked", "desc": "
Node has a simple module loading system. In Node, files and modules are in\none-to-one correspondence. As an example, foo.js
loads the module\ncircle.js
in the same directory.\n\n
The contents of foo.js
:\n\n
var circle = require('./circle.js');\nconsole.log( 'The area of a circle of radius 4 is '\n + circle.area(4));
\nThe contents of circle.js
:\n\n
var PI = Math.PI;\n\nexports.area = function (r) {\n return PI * r * r;\n};\n\nexports.circumference = function (r) {\n return 2 * PI * r;\n};
\nThe module circle.js
has exported the functions area()
and\ncircumference()
. To add functions and objects to the root of your module,\nyou can add them to the special exports
object.\n\n
Variables local to the module will be private, as though the module was wrapped\nin a function. In this example the variable PI
is private to circle.js
.\n\n
If you want the root of your module's export to be a function (such as a\nconstructor) or if you want to export a complete object in one assignment\ninstead of building it one property at a time, assign it to module.exports
\ninstead of exports
.\n\n
Below, bar.js
makes use of the square
module, which exports a constructor:\n\n
var square = require('./square.js');\nvar mySquare = square(2);\nconsole.log('The area of my square is ' + mySquare.area());
\nThe square
module is defined in square.js
:\n\n
// assigning to exports will not modify module, must use module.exports\nmodule.exports = function(width) {\n return {\n area: function() {\n return width * width;\n }\n };\n}
\nThe module system is implemented in the require("module")
module.\n\n
When there are circular require()
calls, a module might not have finished\nexecuting when it is returned.\n\n
Consider this situation:\n\n
\na.js
:\n\n
console.log('a starting');\nexports.done = false;\nvar b = require('./b.js');\nconsole.log('in a, b.done = %j', b.done);\nexports.done = true;\nconsole.log('a done');
\nb.js
:\n\n
console.log('b starting');\nexports.done = false;\nvar a = require('./a.js');\nconsole.log('in b, a.done = %j', a.done);\nexports.done = true;\nconsole.log('b done');
\nmain.js
:\n\n
console.log('main starting');\nvar a = require('./a.js');\nvar b = require('./b.js');\nconsole.log('in main, a.done=%j, b.done=%j', a.done, b.done);
\nWhen main.js
loads a.js
, then a.js
in turn loads b.js
. At that\npoint, b.js
tries to load a.js
. In order to prevent an infinite\nloop, an unfinished copy of the a.js
exports object is returned to the\nb.js
module. b.js
then finishes loading, and its exports
object is\nprovided to the a.js
module.\n\n
By the time main.js
has loaded both modules, they're both finished.\nThe output of this program would thus be:\n\n
$ node main.js\nmain starting\na starting\nb starting\nin b, a.done = false\nb done\nin a, b.done = true\na done\nin main, a.done=true, b.done=true
\nIf you have cyclic module dependencies in your program, make sure to\nplan accordingly.\n\n
\n" }, { "textRaw": "Core Modules", "name": "Core Modules", "type": "misc", "desc": "Node has several modules compiled into the binary. These modules are\ndescribed in greater detail elsewhere in this documentation.\n\n
\nThe core modules are defined in node's source in the lib/
folder.\n\n
Core modules are always preferentially loaded if their identifier is\npassed to require()
. For instance, require('http')
will always\nreturn the built in HTTP module, even if there is a file by that name.\n\n
If the exact filename is not found, then node will attempt to load the\nrequired filename with the added extension of .js
, .json
, and then .node
.\n\n
.js
files are interpreted as JavaScript text files, and .json
files are\nparsed as JSON text files. .node
files are interpreted as compiled addon\nmodules loaded with dlopen
.\n\n
A module prefixed with '/'
is an absolute path to the file. For\nexample, require('/home/marco/foo.js')
will load the file at\n/home/marco/foo.js
.\n\n
A module prefixed with './'
is relative to the file calling require()
.\nThat is, circle.js
must be in the same directory as foo.js
for\nrequire('./circle')
to find it.\n\n
Without a leading '/' or './' to indicate a file, the module is either a\n"core module" or is loaded from a node_modules
folder.\n\n
If the given path does not exist, require()
will throw an Error with its\ncode
property set to 'MODULE_NOT_FOUND'
.\n\n
If the module identifier passed to require()
is not a native module,\nand does not begin with '/'
, '../'
, or './'
, then node starts at the\nparent directory of the current module, and adds /node_modules
, and\nattempts to load the module from that location.\n\n
If it is not found there, then it moves to the parent directory, and so\non, until the root of the file system is reached.\n\n
\nFor example, if the file at '/home/ry/projects/foo.js'
called\nrequire('bar.js')
, then node would look in the following locations, in\nthis order:\n\n
/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\nclash.\n\n
\nYou can require specific files or sub modules distributed with a module by\nincluding a path suffix after the module name. For instance\nrequire('example-module/path/to/file')
would resolve path/to/file
\nrelative to where example-module
is located. The suffixed path follows the\nsame module resolution semantics.\n\n
It is convenient to organize programs and libraries into self-contained\ndirectories, and then provide a single entry point to that library.\nThere are three ways in which a folder may be passed to require()
as\nan argument.\n\n
The first is to create a package.json
file in the root of the folder,\nwhich specifies a main
module. An example package.json file might\nlook like this:\n\n
{ "name" : "some-library",\n "main" : "./lib/some-library.js" }
\nIf this was in a folder at ./some-library
, then\nrequire('./some-library')
would attempt to load\n./some-library/lib/some-library.js
.\n\n
This is the extent of Node's awareness of package.json files.\n\n
\nIf there is no package.json file present in the directory, then node\nwill attempt to load an index.js
or index.node
file out of that\ndirectory. For example, if there was no package.json file in the above\nexample, then require('./some-library')
would attempt to load:\n\n
./some-library/index.js
./some-library/index.node
Modules are cached after the first time they are loaded. This means\n(among other things) that every call to require('foo')
will get\nexactly the same object returned, if it would resolve to the same file.\n\n
Multiple calls to require('foo')
may not cause the module code to be\nexecuted multiple times. This is an important feature. With it,\n"partially done" objects can be returned, thus allowing transitive\ndependencies to be loaded even when they would cause cycles.\n\n
If you want to have a module execute code multiple times, then export a\nfunction, and call that function.\n\n
\n", "miscs": [ { "textRaw": "Module Caching Caveats", "name": "Module Caching Caveats", "type": "misc", "desc": "Modules are cached based on their resolved filename. Since modules may\nresolve to a different filename based on the location of the calling\nmodule (loading from node_modules
folders), it is not a guarantee\nthat require('foo')
will always return the exact same object, if it\nwould resolve to different files.\n\n
To get the exact filename that will be loaded when require()
is called, use\nthe require.resolve()
function.\n\n
Putting together all of the above, here is the high-level algorithm\nin pseudocode of what require.resolve does:\n\n
\nrequire(X) from module at path Y\n1. If X is a core module,\n a. return the core module\n b. STOP\n2. If X begins with './' or '/' or '../'\n a. LOAD_AS_FILE(Y + X)\n b. LOAD_AS_DIRECTORY(Y + X)\n3. LOAD_NODE_MODULES(X, dirname(Y))\n4. THROW "not found"\n\nLOAD_AS_FILE(X)\n1. If X is a file, load X as JavaScript text. STOP\n2. If X.js is a file, load X.js as JavaScript text. STOP\n3. If X.json is a file, parse X.json to a JavaScript Object. STOP\n4. If X.node is a file, load X.node as binary addon. STOP\n\nLOAD_AS_DIRECTORY(X)\n1. If X/package.json is a file,\n a. Parse X/package.json, and look for "main" field.\n b. let M = X + (json main field)\n c. LOAD_AS_FILE(M)\n2. If X/index.js is a file, load X/index.js as JavaScript text. STOP\n3. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP\n4. If X/index.node is a file, load X/index.node as binary addon. STOP\n\nLOAD_NODE_MODULES(X, START)\n1. let DIRS=NODE_MODULES_PATHS(START)\n2. for each DIR in DIRS:\n a. LOAD_AS_FILE(DIR/X)\n b. LOAD_AS_DIRECTORY(DIR/X)\n\nNODE_MODULES_PATHS(START)\n1. let PARTS = path split(START)\n2. let I = count of PARTS - 1\n3. let DIRS = []\n4. while I >= 0,\n a. if PARTS[I] = "node_modules" CONTINUE\n c. DIR = path join(PARTS[0 .. I] + "node_modules")\n b. DIRS = DIRS + DIR\n c. let I = I - 1\n5. return DIRS
\n"
},
{
"textRaw": "Loading from the global folders",
"name": "Loading from the global folders",
"type": "misc",
"desc": "If the NODE_PATH
environment variable is set to a colon-delimited list\nof absolute paths, then node will search those paths for modules if they\nare not found elsewhere. (Note: On Windows, NODE_PATH
is delimited by\nsemicolons instead of colons.)\n\n
Additionally, node will search in the following locations:\n\n
\n$HOME/.node_modules
$HOME/.node_libraries
$PREFIX/lib/node
Where $HOME
is the user's home directory, and $PREFIX
is node's\nconfigured node_prefix
.\n\n
These are mostly for historic reasons. You are highly encouraged to\nplace your dependencies locally in node_modules
folders. They will be\nloaded faster, and more reliably.\n\n
When a file is run directly from Node, require.main
is set to its\nmodule
. That means that you can determine whether a file has been run\ndirectly by testing\n\n
require.main === module
\nFor a file foo.js
, this will be true
if run via node foo.js
, but\nfalse
if run by require('./foo')
.\n\n
Because module
provides a filename
property (normally equivalent to\n__filename
), the entry point of the current application can be obtained\nby checking require.main.filename
.\n\n
The semantics of Node's require()
function were designed to be general\nenough to support a number of sane directory structures. Package manager\nprograms such as dpkg
, rpm
, and npm
will hopefully find it possible to\nbuild native packages from Node modules without modification.\n\n
Below we give a suggested directory structure that could work:\n\n
\nLet's say that we wanted to have the folder at\n/usr/lib/node/<some-package>/<some-version>
hold the contents of a\nspecific version of a package.\n\n
Packages can depend on one another. In order to install package foo
, you\nmay have to install a specific version of package bar
. The bar
package\nmay itself have dependencies, and in some cases, these dependencies may even\ncollide or form cycles.\n\n
Since Node looks up the realpath
of any modules it loads (that is,\nresolves symlinks), and then looks for their dependencies in the\nnode_modules
folders as described above, this situation is very simple to\nresolve with the following architecture:\n\n
/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
\ndepends on./usr/lib/node/foo/1.2.3/node_modules/bar
- Symbolic link to\n/usr/lib/node/bar/4.3.2/
./usr/lib/node/bar/4.3.2/node_modules/*
- Symbolic links to the packages\nthat bar
depends on.Thus, even if a cycle is encountered, or if there are dependency\nconflicts, every module will be able to get a version of its dependency\nthat it can use.\n\n
\nWhen the code in the foo
package does require('bar')
, it will get the\nversion that is symlinked into /usr/lib/node/foo/1.2.3/node_modules/bar
.\nThen, when the code in the bar
package calls require('quux')
, it'll get\nthe version that is symlinked into\n/usr/lib/node/bar/4.3.2/node_modules/quux
.\n\n
Furthermore, to make the module lookup process even more optimal, rather\nthan putting packages directly in /usr/lib/node
, we could put them in\n/usr/lib/node_modules/<name>/<version>
. Then node will not bother\nlooking for missing dependencies in /usr/node_modules
or /node_modules
.\n\n
In order to make modules available to the node REPL, it might be useful to\nalso add the /usr/lib/node_modules
folder to the $NODE_PATH
environment\nvariable. Since the module lookups using node_modules
folders are all\nrelative, and based on the real path of the files making the calls to\nrequire()
, the packages themselves can be anywhere.\n
In each module, the module
free variable is a reference to the object\nrepresenting the current module. For convenience, module.exports
is\nalso accessible via the exports
module-global. module
isn't actually\na global but rather local to each module.\n\n
The module.exports
object is created by the Module system. Sometimes this is not\nacceptable; many want their module to be an instance of some class. To do this,\nassign the desired export object to module.exports
. Note that assigning the\ndesired object to exports
will simply rebind the local exports
variable,\nwhich is probably not what you want to do.\n\n
For example suppose we were making a module called a.js
\n\n
var EventEmitter = require('events').EventEmitter;\n\nmodule.exports = new EventEmitter();\n\n// Do some work, and after some time emit\n// the 'ready' event from the module itself.\nsetTimeout(function() {\n module.exports.emit('ready');\n}, 1000);
\nThen in another file we could do\n\n
\nvar a = require('./a');\na.on('ready', function() {\n console.log('module a is ready');\n});
\nNote that assignment to module.exports
must be done immediately. It cannot be\ndone in any callbacks. This does not work:\n\n
x.js:\n\n
\nsetTimeout(function() {\n module.exports = { a: "hello" };\n}, 0);
\ny.js:\n\n
\nvar x = require('./x');\nconsole.log(x.a);
\n",
"modules": [
{
"textRaw": "exports alias",
"name": "exports_alias",
"desc": "The exports
variable that is available within a module starts as a reference\nto module.exports
. As with any variable, if you assign a new value to it, it\nis no longer bound to the previous value.\n\n
To illustrate the behaviour, imagine this hypothetical implementation of\nrequire()
:\n\n
function require(...) {\n // ...\n function (module, exports) {\n // Your module code here\n exports = some_func; // re-assigns exports, exports is no longer\n // a shortcut, and nothing is exported.\n module.exports = some_func; // makes your module export 0\n } (module, module.exports);\n return module;\n}
\nAs a guideline, if the relationship between exports
and module.exports
\nseems like magic to you, ignore exports
and only use module.exports
.\n\n
The identifier for the module. Typically this is the fully resolved\nfilename.\n\n\n
\n" }, { "textRaw": "`filename` {String} ", "name": "filename", "desc": "The fully resolved filename to the module.\n\n\n
\n" }, { "textRaw": "`loaded` {Boolean} ", "name": "loaded", "desc": "Whether or not the module is done loading, or is in the process of\nloading.\n\n\n
\n" }, { "textRaw": "`parent` {Module Object} ", "name": "parent", "desc": "The module that required this one.\n\n\n
\n" }, { "textRaw": "`children` {Array} ", "name": "children", "desc": "The module objects required by this one.\n\n\n\n
\n" } ], "methods": [ { "textRaw": "module.require(id)", "type": "method", "name": "require", "signatures": [ { "return": { "textRaw": "Return: {Object} `module.exports` from the resolved module ", "name": "return", "type": "Object", "desc": "`module.exports` from the resolved module" }, "params": [ { "textRaw": "`id` {String} ", "name": "id", "type": "String" } ] }, { "params": [ { "name": "id" } ] } ], "desc": "The module.require
method provides a way to load a module as if\nrequire()
was called from the original module.\n\n
Note that in order to do this, you must get a reference to the module
\nobject. Since require()
returns the module.exports
, and the module
is\ntypically only available within a specific module's code, it must be\nexplicitly exported in order to be used.\n\n\n