commonjs

The CommonJS group defined a module format to solve JavaScript scope issues by making sure each module is executed in its own namespace.

This is achieved by forcing modules to explicitly export those variables it wants to expose to the "universe", and also by defining those other modules required to properly work.

To achieve this CommonJS give you two tools:

1. the require() function, which allows to import a given module into the current scope.
2. the module object, which allows to export something from the current scope.

The mandatory hello world example:

Plain Simple JavaScript

Here is an examples without CommonJS:

We will define a value in a script file named salute.js. This script will contain just a value that will be used in other scripts:

// salute.js
var MySalute = "Hello";

Now, in a second file named world.js, we are going to use the value defined in salute.js.

// world.js
var Result = MySalute + " world!";

Module definitions

As it is, world.js will not work as MySalute is not defined. We need to define each script as a module:

// salute.js
var MySalute = "Hello";
module.exports = MySalute;

// world.js
var Result = MySalute + "world!";
module.exports = Result;

Here we make use of the special object module and place a reference of our variable into module.exports so the CommonJS module system nows this is the object of our module we want to show to the world. salute.js discloses MySalute, and world.js discloses Result.

Module dependency

We're near but there's still a step missing: dependency definition. We've already defined every script as an independent module, but world.js still needs to now who defines MySalute:

//salute.js
var MySalute = "Hello";
module.exports = MySalute;

//world.js
var MySalute = require("./salute");
var Result = MySalute + "world!";
module.exports = Result;

Note that we didn't use the full filename salute.js but ./salute when calling require, so you must omit the extension of your scripts. ./ means that the saltue module is in the same directory as the world module.

Functions example

// moduleA.js
module.exports = function( value ){
    return value*2;
}

// moduleB.js
var multiplyBy2 = require('moduleA');
var result = multiplyBy2( 4 );

Differences between CommonJS and RequireJS

There are two main differences between CommonJS and RequireJS.

The first one is how modules are defined. While CommonJS uses its own method (seen above), RequireJS implements the AMD (Asynchronous Module Definitions) specification.

The second difference is how dependencies are loaded. While CommonJS expects require calls to behave synchronously, RequireJS loads its modules asynchronously, behaving more accordingly as how the browser works. This heavily marks where to use each of these two module systems, CommonJS is used mainly in server JavaScript implementations (Nodejs), while RequireJS is headed to be used in the browser.

Advanced

// require a module
var dependency = require("./dependency");
// each module is only excuted once and the same
//  exports are returned
// require("./dependency") === require("./dependency")

// export stuff
exports.stuff = 42;
// exports is a object that is returned by the require
//  function when require this module.

// choose to not use the exports object
module.exports = function doStuff() {};
// the value of module.exports is returned by the require
//  function when require this module.

//// Advanded stuff ////

// unload module
var id = require.resolve("./dependency");
delete require.cache[id];
var newDependency = require("./dependency");
// newDependency !== dependency;

if(typeof require !== "function") {
  // This code is removed when bundled and minimized
}