CommonJS modules are the original way to package JavaScript code for Node.js.\nNode.js also supports the ECMAScript modules standard used by browsers\nand other JavaScript runtimes.
\nIn Node.js, each file is treated as a separate module. For\nexample, consider a file named foo.js:
const circle = require('./circle.js');\nconsole.log(`The area of a circle of radius 4 is ${circle.area(4)}`);\nOn the first line, foo.js loads the module circle.js that is in the same\ndirectory as foo.js.
Here are the contents of circle.js:
const { PI } = Math;\n\nexports.area = (r) => PI * r ** 2;\n\nexports.circumference = (r) => 2 * PI * r;\nThe module circle.js has exported the functions area() and\ncircumference(). Functions and objects are added to the root of a module\nby specifying additional properties on the special exports object.
Variables local to the module will be private, because the module is wrapped\nin a function by Node.js (see module wrapper).\nIn this example, the variable PI is private to circle.js.
The module.exports property can be assigned a new value (such as a function\nor object).
Below, bar.js makes use of the square module, which exports a Square class:
const Square = require('./square.js');\nconst mySquare = new Square(2);\nconsole.log(`The area of mySquare is ${mySquare.area()}`);\nThe square module is defined in square.js:
// Assigning to exports will not modify module, must use module.exports\nmodule.exports = class Square {\n constructor(width) {\n this.width = width;\n }\n\n area() {\n return this.width ** 2;\n }\n};\nThe CommonJS module system is implemented in the module core module.
Node.js has two module systems: CommonJS modules and ECMAScript modules.
\nBy default, Node.js will treat the following as CommonJS modules:
\nFiles with a .cjs extension;
Files with a .js extension when the nearest parent package.json file\ncontains a top-level field \"type\" with a value of \"commonjs\".
Files with a .js extension when the nearest parent package.json file\ndoesn't contain a top-level field \"type\". Package authors should include\nthe \"type\" field, even in packages where all sources are CommonJS. Being\nexplicit about the type of the package will make things easier for build\ntools and loaders to determine how the files in the package should be\ninterpreted.
Files with an extension that is not .mjs, .cjs, .json, .node, or .js\n(when the nearest parent package.json file contains a top-level field\n\"type\" with a value of \"module\", those files will be recognized as\nCommonJS modules only if they are being included via require(), not when\nused as the command-line entry point of the program).
See Determining module system for more details.
\nCalling require() always use the CommonJS module loader. Calling import()\nalways use the ECMAScript module loader.
When a file is run directly from Node.js, require.main is set to its\nmodule. That means that it is possible to determine whether a file has been\nrun directly by testing require.main === module.
For a file foo.js, this will be true if run via node foo.js, but\nfalse if run by require('./foo').
When the entry point is not a CommonJS module, require.main is undefined,\nand the main module is out of reach.
The semantics of the Node.js require() function were designed to be general\nenough to support reasonable directory structures. Package manager programs\nsuch as dpkg, rpm, and npm will hopefully find it possible to build\nnative packages from Node.js modules without modification.
Below we give a suggested directory structure that could work:
\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.
Packages can depend on one another. In order to install package foo, it\nmay be necessary to install a specific version of package bar. The bar\npackage may itself have dependencies, and in some cases, these may even collide\nor form cyclic dependencies.
Because Node.js looks up the realpath of any modules it loads (that is, it\nresolves symlinks) and then looks for their dependencies in node_modules folders,\nthis 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\non./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 that\nbar 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.
\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.
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.js will not bother\nlooking 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\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.
To get the exact filename that will be loaded when require() is called, use\nthe require.resolve() function.
Putting together all of the above, here is the high-level algorithm\nin pseudocode of what require() does:
\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 '/'\n a. set Y to be the file system root\n3. If X begins with './' or '/' or '../'\n a. LOAD_AS_FILE(Y + X)\n b. LOAD_AS_DIRECTORY(Y + X)\n c. THROW \"not found\"\n4. If X begins with '#'\n a. LOAD_PACKAGE_IMPORTS(X, dirname(Y))\n5. LOAD_PACKAGE_SELF(X, dirname(Y))\n6. LOAD_NODE_MODULES(X, dirname(Y))\n7. THROW \"not found\"\n\nLOAD_AS_FILE(X)\n1. If X is a file, load X as its file extension format. 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_INDEX(X)\n1. If X/index.js is a file, load X/index.js as JavaScript text. STOP\n2. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP\n3. If X/index.node is a file, load X/index.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. If \"main\" is a falsy value, GOTO 2.\n c. let M = X + (json main field)\n d. LOAD_AS_FILE(M)\n e. LOAD_INDEX(M)\n f. LOAD_INDEX(X) DEPRECATED\n g. THROW \"not found\"\n2. LOAD_INDEX(X)\n\nLOAD_NODE_MODULES(X, START)\n1. let DIRS = NODE_MODULES_PATHS(START)\n2. for each DIR in DIRS:\n a. LOAD_PACKAGE_EXPORTS(X, DIR)\n b. LOAD_AS_FILE(DIR/X)\n c. 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 b. DIR = path join(PARTS[0 .. I] + \"node_modules\")\n c. DIRS = DIR + DIRS\n d. let I = I - 1\n5. return DIRS + GLOBAL_FOLDERS\n\nLOAD_PACKAGE_IMPORTS(X, DIR)\n1. Find the closest package scope SCOPE to DIR.\n2. If no scope was found, return.\n3. If the SCOPE/package.json \"imports\" is null or undefined, return.\n4. let MATCH = PACKAGE_IMPORTS_RESOLVE(X, pathToFileURL(SCOPE),\n [\"node\", \"require\"]) defined in the ESM resolver.\n5. RESOLVE_ESM_MATCH(MATCH).\n\nLOAD_PACKAGE_EXPORTS(X, DIR)\n1. Try to interpret X as a combination of NAME and SUBPATH where the name\n may have a @scope/ prefix and the subpath begins with a slash (`/`).\n2. If X does not match this pattern or DIR/NAME/package.json is not a file,\n return.\n3. Parse DIR/NAME/package.json, and look for \"exports\" field.\n4. If \"exports\" is null or undefined, return.\n5. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(DIR/NAME), \".\" + SUBPATH,\n `package.json` \"exports\", [\"node\", \"require\"]) defined in the ESM resolver.\n6. RESOLVE_ESM_MATCH(MATCH)\n\nLOAD_PACKAGE_SELF(X, DIR)\n1. Find the closest package scope SCOPE to DIR.\n2. If no scope was found, return.\n3. If the SCOPE/package.json \"exports\" is null or undefined, return.\n4. If the SCOPE/package.json \"name\" is not the first segment of X, return.\n5. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(SCOPE),\n \".\" + X.slice(\"name\".length), `package.json` \"exports\", [\"node\", \"require\"])\n defined in the ESM resolver.\n6. RESOLVE_ESM_MATCH(MATCH)\n\nRESOLVE_ESM_MATCH(MATCH)\n1. let RESOLVED_PATH = fileURLToPath(MATCH)\n2. If the file at RESOLVED_PATH exists, load RESOLVED_PATH as its extension\n format. STOP\n3. THROW \"not found\"\n" }, { "textRaw": "Caching", "name": "Caching", "type": "misc", "desc": "
Modules are cached after the first time they are loaded. This means (among other\nthings) that every call to require('foo') will get exactly the same object\nreturned, if it would resolve to the same file.
Provided require.cache is not modified, multiple calls to require('foo')\nwill not cause the module code to be executed multiple times. This is an\nimportant feature. With it, \"partially done\" objects can be returned, thus\nallowing 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\nfunction.
", "miscs": [ { "textRaw": "Module caching caveats", "name": "Module caching caveats", "type": "misc", "desc": "Modules are cached based on their resolved filename. Since modules may resolve\nto a different filename based on the location of the calling module (loading\nfrom node_modules folders), it is not a guarantee that require('foo') will\nalways return the exact same object, if it would resolve to different files.
Additionally, on case-insensitive file systems or operating systems, different\nresolved filenames can point to the same file, but the cache will still treat\nthem as different modules and will reload the file multiple times. For example,\nrequire('./foo') and require('./FOO') return two different objects,\nirrespective of whether or not ./foo and ./FOO are the same file.
Node.js has several modules compiled into the binary. These modules are\ndescribed in greater detail elsewhere in this documentation.
\nThe core modules are defined within the Node.js source and are located in the\nlib/ folder.
Core modules can be identified using the node: prefix, in which case\nit bypasses the require cache. For instance, require('node:http') will\nalways return the built in HTTP module, even if there is require.cache entry\nby that name.
Some 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. The list\nof core modules that can be loaded without using the node: prefix is exposed\nas module.builtinModules.
When there are circular require() calls, a module might not have finished\nexecuting when it is returned.
Consider this situation:
\na.js:
console.log('a starting');\nexports.done = false;\nconst b = require('./b.js');\nconsole.log('in a, b.done = %j', b.done);\nexports.done = true;\nconsole.log('a done');\nb.js:
console.log('b starting');\nexports.done = false;\nconst a = require('./a.js');\nconsole.log('in b, a.done = %j', a.done);\nexports.done = true;\nconsole.log('b done');\nmain.js:
console.log('main starting');\nconst a = require('./a.js');\nconst 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.
By the time main.js has loaded both modules, they're both finished.\nThe output of this program would thus be:
$ 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\nCareful planning is required to allow cyclic module dependencies to work\ncorrectly within an application.
" }, { "textRaw": "File modules", "name": "File modules", "type": "misc", "desc": "If the exact filename is not found, then Node.js will attempt to load the\nrequired filename with the added extensions: .js, .json, and finally\n.node. When loading a file that has a different extension (e.g. .cjs), its\nfull name must be passed to require(), including its file extension (e.g.\nrequire('./file.cjs')).
.json files are parsed as JSON text files, .node files are interpreted as\ncompiled addon modules loaded with process.dlopen(). Files using any other\nextension (or no extension at all) are parsed as JavaScript text files. Refer to\nthe Determining module system section to understand what parse goal will be\nused.
A required 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.
A required module prefixed with './' is relative to the file calling\nrequire(). That is, circle.js must be in the same directory as foo.js for\nrequire('./circle') to find it.
Without a leading '/', './', or '../' to indicate a file, the module must\neither be a core module or is loaded from a node_modules folder.
If the given path does not exist, require() will throw a\nMODULE_NOT_FOUND error.
There are three ways in which a folder may be passed to require() as\nan argument.
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:
{ \"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.
If there is no package.json file present in the directory, or if the\n\"main\" entry is missing or cannot be resolved, then Node.js\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 previous\nexample, then require('./some-library') would attempt to load:
./some-library/index.js./some-library/index.nodeIf these attempts fail, then Node.js will report the entire module as missing\nwith the default error:
\nError: Cannot find module 'some-library'\nIn all three above cases, an import('./some-library') call would result in a\nERR_UNSUPPORTED_DIR_IMPORT error. Using package subpath exports or\nsubpath imports can provide the same containment organization benefits as\nfolders as modules, and work for both require and import.
If the module identifier passed to require() is not a\ncore module, and does not begin with '/', '../', or\n'./', then Node.js starts at the directory of the current module, and\nadds /node_modules, and attempts to load the module from that location.\nNode.js will not append node_modules to a path already ending in\nnode_modules.
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.
\nFor example, if the file at '/home/ry/projects/foo.js' called\nrequire('bar.js'), then Node.js would look in the following locations, in\nthis order:
/home/ry/projects/node_modules/bar.js/home/ry/node_modules/bar.js/home/node_modules/bar.js/node_modules/bar.jsThis allows programs to localize their dependencies, so that they do not\nclash.
\nIt is possible to require specific files or sub modules distributed with a\nmodule by including 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.
If the NODE_PATH environment variable is set to a colon-delimited list\nof absolute paths, then Node.js will search those paths for modules if they\nare not found elsewhere.
On Windows, NODE_PATH is delimited by semicolons (;) instead of colons.
NODE_PATH was originally created to support loading modules from\nvarying paths before the current module resolution algorithm was defined.
NODE_PATH is still supported, but is less necessary now that the Node.js\necosystem has settled on a convention for locating dependent modules.\nSometimes deployments that rely on NODE_PATH show surprising behavior\nwhen people are unaware that NODE_PATH must be set. Sometimes a\nmodule's dependencies change, causing a different version (or even a\ndifferent module) to be loaded as the NODE_PATH is searched.
Additionally, Node.js will search in the following list of GLOBAL_FOLDERS:
\n$HOME/.node_modules$HOME/.node_libraries$PREFIX/lib/nodeWhere $HOME is the user's home directory, and $PREFIX is the Node.js\nconfigured node_prefix.
These are mostly for historic reasons.
\nIt is strongly encouraged to place dependencies in the local node_modules\nfolder. These will be loaded faster, and more reliably.
Before a module's code is executed, Node.js will wrap it with a function\nwrapper that looks like the following:
\n(function(exports, require, module, __filename, __dirname) {\n// Module code actually lives in here\n});\nBy doing this, Node.js achieves a few things:
\nvar, const, or let) scoped to\nthe module rather than the global object.module and exports objects that the implementor can use to export\nvalues from the module.__filename and __dirname, containing the\nmodule's absolute filename and directory path.Due to the synchronous nature of require(), it is not possible to use it to\nload ECMAScript module files. Attempting to do so will throw a\nERR_REQUIRE_ESM error. Use import() instead.
The .mjs extension is reserved for