What is the purpose of Node.js module.exports and how do you use it?
I can't seem to find any information on this, but it appears to be a rather important part of Node.js as I often see it in source code.
According to the Node.js documentation:
module
A reference to the current
module. In particular module.exports
is the same as the exports object. See
src/node.js for more information.
But this doesn't really help.
What exactly does module.exports do, and what would a simple example be?
module.exports is the object that's actually returned as the result of a require call.
The exports variable is initially set to that same object (i.e. it's a shorthand "alias"), so in the module code you would usually write something like this:
let myFunc1 = function() { ... };
let myFunc2 = function() { ... };
exports.myFunc1 = myFunc1;
exports.myFunc2 = myFunc2;
to export (or "expose") the internally scoped functions myFunc1 and myFunc2.
And in the calling code you would use:
const m = require('./mymodule');
m.myFunc1();
where the last line shows how the result of require is (usually) just a plain object whose properties may be accessed.
NB: if you overwrite exports then it will no longer refer to module.exports. So if you wish to assign a new object (or a function reference) to exports then you should also assign that new object to module.exports
It's worth noting that the name added to the exports object does not have to be the same as the module's internally scoped name for the value that you're adding, so you could have:
let myVeryLongInternalName = function() { ... };
exports.shortName = myVeryLongInternalName;
// add other objects, functions, as required
followed by:
const m = require('./mymodule');
m.shortName(); // invokes module.myVeryLongInternalName
This has already been answered but I wanted to add some clarification...
You can use both exports and module.exports to import code into your application like this:
var mycode = require('./path/to/mycode');
The basic use case you'll see (e.g. in ExpressJS example code) is that you set properties on the exports object in a .js file that you then import using require()
So in a simple counting example, you could have:
(counter.js):
var count = 1;
exports.increment = function() {
count++;
};
exports.getCount = function() {
return count;
};
... then in your application (web.js, or really any other .js file):
var counting = require('./counter.js');
console.log(counting.getCount()); // 1
counting.increment();
console.log(counting.getCount()); // 2
In simple terms, you can think of required files as functions that return a single object, and you can add properties (strings, numbers, arrays, functions, anything) to the object that's returned by setting them on exports.
Sometimes you'll want the object returned from a require() call to be a function you can call, rather than just an object with properties. In that case you need to also set module.exports, like this:
(sayhello.js):
module.exports = exports = function() {
console.log("Hello World!");
};
(app.js):
var sayHello = require('./sayhello.js');
sayHello(); // "Hello World!"
The difference between exports and module.exports is explained better in this answer here.
Note that the NodeJS module mechanism is based on CommonJS modules which are supported in many other implementations like RequireJS, but also SproutCore, CouchDB, Wakanda, OrientDB, ArangoDB, RingoJS, TeaJS, SilkJS, curl.js, or even Adobe Photoshop (via PSLib).
You can find the full list of known implementations here.
Unless your module use node specific features or module, I highly encourage you then using exports instead of module.exports which is not part of the CommonJS standard, and then mostly not supported by other implementations.
Another NodeJS specific feature is when you assign a reference to a new object to exports instead of just adding properties and methods to it like in the last example provided by Jed Watson in this thread. I would personally discourage this practice as this breaks the circular reference support of the CommonJS modules mechanism. It is then not supported by all implementations and Jed example should then be written this way (or a similar one) to provide a more universal module:
(sayhello.js):
exports.run = function() {
console.log("Hello World!");
}
(app.js):
var sayHello = require('./sayhello');
sayHello.run(); // "Hello World!"
Or using ES6 features
(sayhello.js):
Object.assign(exports, {
// Put all your public API here
sayhello() {
console.log("Hello World!");
}
});
(app.js):
const { sayHello } = require('./sayhello');
sayHello(); // "Hello World!"
PS: It looks like Appcelerator also implements CommonJS modules, but without the circular reference support (see: Appcelerator and CommonJS modules (caching and circular references))
Some few things you must take care if you assign a reference to a new object to exports and /or modules.exports:
1. All properties/methods previously attached to the original exports or module.exports are of course lost because the exported object will now reference another new one
This one is obvious, but if you add an exported method at the beginning of an existing module, be sure the native exported object is not referencing another object at the end
exports.method1 = function () {}; // exposed to the original exported object
exports.method2 = function () {}; // exposed to the original exported object
module.exports.method3 = function () {}; // exposed with method1 & method2
var otherAPI = {
// some properties and/or methods
}
exports = otherAPI; // replace the original API (works also with module.exports)
2. In case one of exports or module.exports reference a new value, they don't reference to the same object any more
exports = function AConstructor() {}; // override the original exported object
exports.method2 = function () {}; // exposed to the new exported object
// method added to the original exports object which not exposed any more
module.exports.method3 = function () {};
3. Tricky consequence. If you change the reference to both exports and module.exports, hard to say which API is exposed (it looks like module.exports wins)
// override the original exported object
module.exports = function AConstructor() {};
// try to override the original exported object
// but module.exports will be exposed instead
exports = function AnotherConstructor() {};
the module.exports property or the exports object allows a module to select what should be shared with the application
I have a video on module_export available here
When dividing your program code over multiple files, module.exports is used to publish variables and functions to the consumer of a module. The require() call in your source file is replaced with corresponding module.exports loaded from the module.
Remember when writing modules
Module loads are cached, only initial call evaluates JavaScript.
It's possible to use local variables and functions inside a module, not everything needs to be exported.
The module.exports object is also available as exports shorthand. But when returning a sole function, always use module.exports.
According to: "Modules Part 2 - Writing modules".
the refer link is like this:
exports = module.exports = function(){
//....
}
the properties of exports or module.exports ,such as functions or variables , will be exposed outside
there is something you must pay more attention : don't override exports .
why ?
because exports just the reference of module.exports , you can add the properties onto the exports ,but if you override the exports , the reference link will be broken .
good example :
exports.name = 'william';
exports.getName = function(){
console.log(this.name);
}
bad example :
exports = 'william';
exports = function(){
//...
}
If you just want to exposed only one function or variable , like this:
// test.js
var name = 'william';
module.exports = function(){
console.log(name);
}
// index.js
var test = require('./test');
test();
this module only exposed one function and the property of name is private for the outside .
There are some default or existing modules in node.js when you download and install node.js like http, sys etc.
Since they are already in node.js, when we want to use these modules we basically do like import modules, but why? because they are already present in the node.js. Importing is like taking them from node.js and putting them into your program. And then using them.
Whereas Exports is exactly the opposite, you are creating the module you want, let's say the module addition.js and putting that module into the node.js, you do it by exporting it.
Before I write anything here, remember, module.exports.additionTwo is same as exports.additionTwo
Huh, so that's the reason, we do like
exports.additionTwo = function(x)
{return x+2;};
Be careful with the path
Lets say you have created an addition.js module,
exports.additionTwo = function(x){
return x + 2;
};
When you run this on your NODE.JS command prompt:
node
var run = require('addition.js');
This will error out saying
Error: Cannot find module addition.js
This is because the node.js process is unable the addition.js since we didn't mention the path. So, we have can set the path by using NODE_PATH
set NODE_PATH = path/to/your/additon.js
Now, this should run successfully without any errors!!
One more thing, you can also run the addition.js file by not setting the NODE_PATH, back to your nodejs command prompt:
node
var run = require('./addition.js');
Since we are providing the path here by saying it's in the current directory ./ this should also run successfully.
A module encapsulates related code into a single unit of code. When creating a module, this can be interpreted as moving all related functions into a file.
Suppose there is a file Hello.js which include two functions
sayHelloInEnglish = function() {
return "Hello";
};
sayHelloInSpanish = function() {
return "Hola";
};
We write a function only when utility of the code is more than one call.
Suppose we want to increase utility of the function to a different file say World.js,in this case exporting a file comes into picture which can be obtained by module.exports.
You can just export both the function by the code given below
var anyVariable={
sayHelloInEnglish = function() {
return "Hello";
};
sayHelloInSpanish = function() {
return "Hola";
};
}
module.export=anyVariable;
Now you just need to require the file name into World.js inorder to use those functions
var world= require("./hello.js");
The intent is:
Modular programming is a software design technique that emphasizes
separating the functionality of a program into independent,
interchangeable modules, such that each contains everything necessary
to execute only one aspect of the desired functionality.
Wikipedia
I imagine it becomes difficult to write a large programs without modular / reusable code. In nodejs we can create modular programs utilising module.exports defining what we expose and compose our program with require.
Try this example:
fileLog.js
function log(string) { require('fs').appendFileSync('log.txt',string); }
module.exports = log;
stdoutLog.js
function log(string) { console.log(string); }
module.exports = log;
program.js
const log = require('./stdoutLog.js')
log('hello world!');
execute
$ node program.js
hello world!
Now try swapping ./stdoutLog.js for ./fileLog.js.
What is the purpose of a module system?
It accomplishes the following things:
Keeps our files from bloating to really big sizes. Having files with e.g. 5000 lines of code in it are usually real hard to deal with during development.
Enforces separation of concerns. Having our code split up into multiple files allows us to have appropriate file names for every file. This way we can easily identify what every module does and where to find it (assuming we made a logical directory structure which is still your responsibility).
Having modules makes it easier to find certain parts of code which makes our code more maintainable.
How does it work?
NodejS uses the CommomJS module system which works in the following manner:
If a file wants to export something it has to declare it using module.export syntax
If a file wants to import something it has to declare it using require('file') syntax
Example:
test1.js
const test2 = require('./test2'); // returns the module.exports object of a file
test2.Func1(); // logs func1
test2.Func2(); // logs func2
test2.js
module.exports.Func1 = () => {console.log('func1')};
exports.Func2 = () => {console.log('func2')};
Other useful things to know:
Modules are getting cached. When you are loading the same module in 2 different files the module only has to be loaded once. The second time a require() is called on the same module the is pulled from the cache.
Modules are loaded in synchronous. This behavior is required, if it was asynchronous we couldn't access the object retrieved from require() right away.
ECMAScript modules - 2022
From Node 14.0 ECMAScript modules are no longer experimental and you can use them instead of classic Node's CommonJS modules.
ECMAScript modules are the official standard format to package JavaScript code for reuse. Modules are defined using a variety of import and export statements.
You can define an ES module that exports a function:
// my-fun.mjs
function myFun(num) {
// do something
}
export { myFun };
Then, you can import the exported function from my-fun.mjs:
// app.mjs
import { myFun } from './my-fun.mjs';
myFun();
.mjs is the default extension for Node.js ECMAScript modules.
But you can configure the default modules extension to lookup when resolving modules using the package.json "type" field, or the --input-type flag in the CLI.
Recent versions of Node.js fully supports both ECMAScript and CommonJS modules. Moreover, it provides interoperability between them.
module.exports
ECMAScript and CommonJS modules have many differences but the most relevant difference - to this question - is that there are no more requires, no more exports, no more module.exports
In most cases, the ES module import can be used to load CommonJS modules.
If needed, a require function can be constructed within an ES module using module.createRequire().
ECMAScript modules releases history
Release
Changes
v15.3.0, v14.17.0, v12.22.0
Stabilized modules implementation
v14.13.0, v12.20.0
Support for detection of CommonJS named exports
v14.0.0, v13.14.0, v12.20.0
Remove experimental modules warning
v13.2.0, v12.17.0
Loading ECMAScript modules no longer requires a command-line flag
v12.0.0
Add support for ES modules using .js file extension via package.json "type" field
v8.5.0
Added initial ES modules implementation
You can find all the changelogs in Node.js repository
let test = function() {
return "Hello world"
};
exports.test = test;
I've run into a problem with RequireJS that pops up randomly in different areas over and over, after a long period (about a year) of working fine.
I declare my requireJS file like this:
define(['TestController'], function (TestController)
{
return {
oneFunction: function(callback)
{
//When I try to use "TestController" here, I get the
//"Error: Module name "TestController" has not been
//loaded yet for context" error...
TestController.test(); //ERROR
//I had been using the above for years, without changes,
//and it worked great. then out of the blue started not
// working. Ok, let's try something else:
if(typeof TestController == "undefined")
{
var TestController = require('TestController'); //ERROR
}
//The above method worked for a few months, then broke AGAIN
// out of the blue, with the same error. My last resort is one
// that always works, however it makes my code have about 20+
//layers of callbacks:
require(['TestController'], function(TestController){
TestController.test();
//WORKS, but what's the point of declaring it as a
//requirement at the top if it doesn't work and I have to
//wrap my code every time? :(
});
},
anotherFunction: function()
{
console.log("hello");
}
}
});
I am getting the "Error: Module name "TestController" has not been loaded yet for context" error over and over until I re-declare the dependency... My question is, what's the point of declaring 'TestController' at the top as a dependency if I have to keep re-declaring it as if I never listed it? What am I doing wrong here?
I declare 'TestController' in other files and it works great, but every once and a while, ONE of the declarations will fail...and it's always a different file (there are about 200-300)... I never know which one, and the only way to fix it is to re-declare it and wrap it.
Anyone see anything I'm doing wrong that could be causing this? I keep updating RequireJS to see if it fixes it and it doesn't :/
Version
RequireJS 2.1.22
jquery-1.12.1
node 4.2.6
As #Louis pointed out, it was circular dependencies that was causing the problem.
Circular Dependency Solution #1: 'exports'
Here's the solution straight from RequireJS's documentation:
If you define a circular dependency ("a" needs "b" and "b" needs "a"), then in this case when "b"'s module function is called, it will get an undefined value for "a". "b" can fetch "a" later after modules have been defined by using the require() method (be sure to specify require as a dependency so the right context is used to look up "a"):
//Inside b.js:
define(["require", "a"],
function(require, a) {
//"a" in this case will be null if "a" also asked for "b",
//a circular dependency.
return function(title) {
return require("a").doSomething();
}
}
);
If you are familiar with CommonJS modules, you could instead use exports to create an empty object for the module that is available immediately for reference by other modules.
//Inside b.js:
define(function(require, exports, module) {
//If "a" has used exports, then we have a real
//object reference here. However, we cannot use
//any of "a"'s properties until after "b" returns a value.
var a = require("a");
exports.foo = function () {
return a.bar();
};
});
Circular Dependency Solution #2: Visualize with madge
I came accross this npm module that will create a dependency graph for you : https://github.com/pahen/madge
I've decided to analyze my code with madge and remove the circular dependencies.
Here is how I used the tool:
cd <Client-Code-Location>
madge --image dep.png .
This gave me an image of the dependencies, however there were no circular dependencies found. So I decided to try another way:
cd <Client-Code-Location>
madge --image dep.png --format amd .
This way I was able to see where I had the circular dependency. :)
I am new into Node.js, and I am trying to create a server for a real time project of mine. I am a bit puzzled thought, as I have defined some constants in a module, that, although, I export they are not available in any other module referring at it. i.e.
the constants containing module is like:
--- constants.js
///// ---- CONSTANTS ---- /////
exports.MessageType = {
START_GAME: "001",
END_GAME: "002",
LIVE_STAT: "003",
PAUSE_GAME: "004"
};
while the main module has:
var MessageType= require('constants').MessageType
...
switch (msgType) {
case MessageType.START_GAME: ...
}
-- Running the above results in an errro:
TypeError: Cannot read property 'START_GAME' of undefined
Any clue?
require('constants') returns the Node.js built-in constants module.
To include a file in your directory, you need to pass a relative path:
require('./constants')
I'm trying to bring the Underscore.String library in to a Require.js project. The library is setup to support AMD, with the following code:
} else if (typeof define === 'function' && define.amd) {
// Register as a named module with AMD.
define('underscore.string', [], function() {
return _s;
});
But I have a problem: I don't keep the library in my root path, I keep it in "ext/underscore.string". This seems to make it impossible to require the library.
I have tried requiring both "ext/underscore.string" and "underscore.string", with and without defining a path (of "underscore.string": "ext/underscore.string"). When I don't have a path, and I require "underscore.string" the file (unsurprisingly) doesn't load, and in all other cases the file loads but the library doesn't get defined.
If I try to reference the library afterwards I get:
Error: Module name "underscore.string" has not been loaded yet for
context:
... even if I do so immediately after the define line (in the code above)! In other words, if I change the code to
define('underscore.string', [], function() {
return _s;
});
console.log(require('underscore.string'))
Require tells me that "underscore.string" hasn't been loaded yet!
Can anyone help me figure out how I can bring this library in to my codebase?
In your require configuration do:
var require = {
...
map: {
"*": {
"underscore.string": "path/to/file/disregarding/baseUrl"
}
}
};
NOTE: The path to file should include the baseUrl, so in your case and assuming baseUrl="scripts", it would be something like:
"scripts/ext/underscore.string.js"
NOTE 2: It needs the .js extension, i.e. it is exact file name.
Suppose I have a JS-library, neatly wrapped in a define('myModule', function(myModule) { return myModule.someObject; });
How could I bind the myModule.someObject to global scope (please don't ask why, I know modular programming has a lot of benefits over putting stuff on the global scope), without using requirejs or any other module handling framework?
The thing is: while developing, I'd like to use requirejs. The library that we build should be able to be included by somebody using requirejs (AMD, CommonJS, whatever), but should also be available as window.SomeObject for the people that don't want to use require just for the sake of being able to use our SomeObject. After the development phase, all code will be minified and obfuscated to a single JS file.
I think I'm just googling with the wrong search terms, because all I can find is an answer to the question how to include code that isn't wrapped in a requirejs friendly define function.
Any ideas on this would greatly be appreciated. Thanks!
--- EDIT ---
My file (before it all started) looked like:
(function(define, global) {
define([a,b,c],function(theA, theB, theC) {
return theA + theB + theC; // or something, it doesn't matter
});
})(define, this);
I'm thinking of something like this:
(function(define, global) {
// same as above
})(typeof define === 'function'
? define
: function(factory /* need more args? */) { /* solution here */ }, this);
But I'm not sure how to implement it properly...
I guess you need to wrap your modules so that they could be accessed without requirejs:
if ( typeof define === "function" && define.amd ) {
define( "mymodule", [], function () {
// do your logic
return mystuff;
} );
} else {
// do your logic
window.mystuff = mystuff;
}
Look at jQuery as an example.
I would refrain from giving your module an id if you can help it, it makes it less portable. jQuery is incredibly annoying that it forces you to set a jquery path option, but they did it for compatibility reasons. Always prefer anonymous modules if you can.
From the jQuery source
// Register as a named AMD module, since jQuery can be concatenated with other
// files that may use define, but not via a proper concatenation script that
// understands anonymous AMD modules. A named AMD is safest and most robust
// way to register. Lowercase jquery is used because AMD module names are
// derived from file names, and jQuery is normally delivered in a lowercase
// file name. Do this after creating the global so that if an AMD module wants
// to call noConflict to hide this version of jQuery, it will work.
James Burke goes into a little more detail here also.
I would instead use a more common example from the umdjs repository:
(function (root, factory) {
if (typeof define === 'function' && define.amd) {
// AMD. Register as an anonymous module.
define(['b'], factory);
} else {
// Browser globals
root.amdWeb = factory(root.b);
}
}(this, function (b) {
//use b in some fashion.
// Just return a value to define the module export.
// This example returns an object, but the module
// can return a function as the exported value.
return {};
}));
For another example that also supports CommonJS, check out the reqwest library:
!function (name, context, definition) {
if (typeof module != 'undefined' && module.exports) module.exports = definition()
else if (typeof define == 'function' && define.amd) define(definition)
else context[name] = definition()
}('reqwest', this, function () {
return {};
});
How can I provide a library to others that does not depend on RequireJS?
This allows you to ship code that does not ship with all of RequireJS, and allows you to export any kind of API that works on a plain web page without an AMD loader.
You need to make a build config file which uses wrap and almond.
It all feels pretty dirty, but I've had it working (by following the almond ReadMe) with exactly what you're describing.