Relation between CommonJS, AMD and RequireJS?

RequireJS implements the AMD API (source).

CommonJS is a way of defining modules with the help of an exports object, that defines the module contents. Simply put, a CommonJS implementation might work like this:

// someModule.js
exports.doSomething = function() { return "foo"; };

//otherModule.js
var someModule = require('someModule'); // in the vein of node    
exports.doSomethingElse = function() { return someModule.doSomething() + "bar"; };

Basically, CommonJS specifies that you need to have a require() function to fetch dependencies, an exports variable to export module contents and a module identifier (which describes the location of the module in question in relation to this module) that is used to require the dependencies (source). CommonJS has various implementations, including Node.js, which you mentioned.

CommonJS was not particularly designed with browsers in mind, so it doesn't fit in the browser environment very well (*I really have no source for this--it just says so everywhere, including the RequireJS site.*) Apparently, this has something to do with asynchronous loading, etc.

On the other hand, RequireJS implements AMD, which is designed to suit the browser environment (source). Apparently, AMD started as a spinoff of the CommonJS Transport format and evolved into its own module definition API. Hence the similarities between the two. The new feature in AMD is the define() function that allows the module to declare its dependencies before being loaded. For example, the definition could be:

define('module/id/string', ['module', 'dependency', 'array'], 
function(module, factory function) {
  return ModuleContents;  
});

So, CommonJS and AMD are JavaScript module definition APIs that have different implementations, but both come from the same origins.

AMD is more suited for the browser, because it supports asynchronous loading of module dependencies.

RequireJS is an implementation of AMD, while at the same time trying to keep the spirit of CommonJS (mainly in the module identifiers).

To confuse you even more, RequireJS, while being an AMD implementation, offers a CommonJS wrapper so CommonJS modules can almost directly be imported for use with RequireJS.

define(function(require, exports, module) {
  var someModule = require('someModule'); // in the vein of node    
  exports.doSomethingElse = function() { return someModule.doSomething() + "bar"; };
});

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CommonJS is more than that - it's a project to define a common API and ecosystem for JavaScript. One part of CommonJS is the Module specification. Node.js and RingoJS are server-side JavaScript runtimes, and yes, both of them implement modules based on the CommonJS Module spec.

AMD (Asynchronous Module Definition) is another specification for modules. RequireJS is probably the most popular implementation of AMD. One major difference from CommonJS is that AMD specifies that modules are loaded asynchronously - that means modules are loaded in parallel, as opposed to blocking the execution by waiting for a load to finish.

AMD is generally more used in client-side (in-browser) JavaScript development due to this, and CommonJS Modules are generally used server-side. However, you can use either module spec in either environment - for example, RequireJS offers directions for running in Node.js and browserify is a CommonJS Module implementation that can run in the browser.

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The short answer would be:

CommonJS and AMD are specifications (or formats) on how modules and their dependencies should be declared in javascript applications.

RequireJS is a script loader library that is AMD compliant, curljs being another example.

CommonJS compliant:

Taken from Addy Osmani's book.

// package/lib is a dependency we require
var lib = require( "package/lib" );

// behavior for our module
function foo(){
    lib.log( "hello world!" );
}

// export (expose) foo to other modules as foobar
exports.foobar = foo;

AMD compliant:

// package/lib is a dependency we require
define(["package/lib"], function (lib) {

    // behavior for our module
    function foo() {
        lib.log( "hello world!" );
    }

    // export (expose) foo to other modules as foobar
    return {
        foobar: foo
    }
});

Somewhere else the module can be used with:

require(["package/myModule"], function(myModule) {
    myModule.foobar();
});

Some background:

Actually, CommonJS is much more than an API declaration and only a part of it deals with that. AMD started as a draft specification for the module format on the CommonJS list, but full consensus wasn't reached and further development of the format moved to the amdjs group. Arguments around which format is better state that CommonJS attempts to cover a broader set of concerns and that it's better suited for server side development given its synchronous nature, and that AMD is better suited for client side (browser) development given its asynchronous nature and the fact that it has its roots in Dojo's module declaration implementation.

Sources:

RequireJS - Why AMD?

Addy Osmani - Learning JavaScript Design Patterns - Modern Modular JavaScript Design Patterns

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Quoting

AMD:

One browser-first approach

Opting for asynchronous behavior and simplified backwards compatibility

It doesn't have any concept of File I/O.

It supports objects, functions, constructors, strings, JSON and many other types of modules.

CommonJS:

One server-first approach

Assuming synchronous behavior

Cover a broader set of concerns such as I/O, File system, Promises and more.

Supports unwrapped modules, it can feel a little more close to the ES.next/Harmony specifications, freeing you of the define() wrapper that AMD enforces.

Only support objects as modules.

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It is quite normal to organize JavaScript program modular into several files and to call child-modules from the main js module.

The thing is JavaScript doesn't provide this. Not even today in latest browser versions of Chrome and FF.

But, is there any keyword in JavaScript to call another JavaScript module?

This question may be a total collapse of the world for many because the answer is No.

In ES5 ( released in 2009 ) JavaScript had no keywords like import, include, or require.

ES6 saves the day ( released in 2015 ) proposing the import keyword ( https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Statements/import ), and now all modern browsers support this.

If you use Babel 6.18.0 and transpile with ES2015 option only

import myDefault from "my-module";

you will get require again.

"use strict";
var _myModule = require("my-module");
var _myModule2 = _interopRequireDefault(_myModule);
function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }

This is because require means the module will be loaded from Node.js. Node.js will handle everything from system level file read to wrapping functions into the module.

Because in JavaScript functions are the only wrappers to represent the modules.

I'm a lot confused about CommonJS and AMD?

Both CommonJS and AMD are just two different techniques how to overcome the JavaScript "defect" to load modules smart.

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AMD

introduced in JavaScript to scale JavaScript project into multiple files

mostly used in browser based application and libraries

popular implementation is RequireJS, Dojo Toolkit

CommonJS:

it is specification to handle large number of functions, files and modules of big project

initial name ServerJS introduced in January, 2009 by Mozilla

renamed in August, 2009 to CommonJS to show the broader applicability of the APIs

initially implementation were server, nodejs, desktop based libraries

Example

upper.js file

exports.uppercase = str => str.toUpperCase()

main.js file

const uppercaseModule = require('uppercase.js')
uppercaseModule.uppercase('test')

Summary

AMD – one of the most ancient module systems, initially implemented by the library require.js.

CommonJS – the module system created for Node.js server.

UMD – one more module system, suggested as a universal one, compatible with AMD and CommonJS.

Resources:

https://exploringjs.com/es6/ch_modules.html#sec_modules-in-javascript

Wikipedia

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