JavaScript Modules: A Beginner’s Guide

The Module pattern is used to mimic the concept of classes so that we can store both public and private methods and variables inside a single object — similar to how classes are used in other programming languages like Java or Python. 

That allows us to create a public-facing API for the methods that we want to expose to the world, while still encapsulating private variables and methods in a closure scope.

• CommonJS is a volunteer working group that designs and implements JavaScript APIs for declaring modules. A CommonJS module is essentially a reusable piece of JavaScript that exports specific objects, making them available for other modules to require in their programs.
• Unlike CommonJS, AMD takes a browser-first approach alongside asynchronous behavior to get the job done. (Note, there are a lot of people who strongly believe that dynamically loading files piecemeal as you start to run code isn’t favorable, which we’ll explore more when in the next section on module-building).

For projects that require you to support both AMD and CommonJS features, there’s yet another format: Universal Module Definition (UMD).

UMD essentially creates a way to use either of the two, while also supporting the global variable definition. As a result, UMD modules are capable of working on both client and server.