CommonJS is a project with the goal of specifying an ecosystem for JavaScript outside the browser (for example, on the server or for native desktop applications).
Server side JavaScript has been around for a long time, and potentially offers some unique and interesting advantages over other languages because the same language is spoken by both client and server.
Unfortunately, though, server side JavaScript is very fragmented. A script that accesses files can’t be used without modification on both rhino and V8. Spidermonkey and JavaScriptCore can’t both load in additional modules in the same way. A JavaScript web framework is very much tied to its interpreter and is often forced to create a bunch of APIs that Python, Ruby and Java programmers take for granted.
The goal for this project is to create a standard library that will ultimately allow web developers to choose among any number of web frameworks and tools and run that code on the platform that makes the most sense for their application.
The project was started by Mozilla engineer Kevin Dangoor in January 2009 and initially named ServerJS.
What I’m describing here is not a technical problem. It’s a matter of people getting together and making a decision to step forward and start building up something bigger and cooler together. — Kevin Dangoor
In August 2009, the project was renamed CommonJS to show the broader applicability of the APIs. Specifications are created and approved in an open process. A specification is only considered final after it has been finished by multiple implementations. CommonJS is not affiliated with the ECMA International group TC39 working on ECMAScript, but some members of TC39 participate in the project.
In May 2013, Isaac Z. Schlueter, the author of npm, the package manager for Node.js, said CommonJS is being made obsolete by Node.js, and is avoided by the core Node.js developers.
As an example, foo.js loads the module circle.js in the same directory.
The contents of foo.js:
const circle = require('./circle.js');
console.log( `The area of a circle of radius 4 is ${circle.area(4)}`);The contents of circle.js:
const PI = Math.PI;
exports.area = function (r) {
return PI * r * r;
};
exports.circumference = function (r) {
return 2 * PI * r;
};The module circle.js has exported the functions area(..) and circumference(..). To add functions and objects to the root of your module, you can add them to the special exports object.
Variables local to the module will be private, as though the module was wrapped in a function. In this example the variable PI is private to circle.js.
If you want the root of your module’s export to be a function (such as a constructor) or if you want to export a complete object in one assignment instead of building it one property at a time, assign it to module.exports instead of exports.
Below, bar.js makes use of the square module, which exports a constructor:
const square = require('./square.js');
var mySquare = square(2);
console.log(`The area of my square is ${mySquare.area()}`);The square module is defined in square.js:
// Assigning to exports will not modify module, must use module.exports
module.exports = function (width) {
return {
area: function () {
return (width * width);
}
};
}The module system is implemented in the require('module') module.
This section was taken from Node.js documentation site.