Introduction to JavaScript Modules

There have been different approaches to splitting code into modules.

Reference

https://dev.to/iggredible/what-the-heck-are-cjs-amd-umd-and-esm-ikm

CJS

CJS is short for CommonJS. Here is what it looks like:

//importing 
const doSomething = require('./doSomething.js'); 

//exporting
module.exports = function doSomething(n) {
  // do something
}

• Some of you may immediately recognize CJS syntax from node. That's because node uses CJS module format.
• CJS imports module synchronously.
• You can import from a library node_modules or local dir. Either by const myLocalModule = require('./some/local/file.js') or var React = require('react'); works.
• When CJS imports, it will give you a copy of the imported object.
• CJS will not work in the browser. It will have to be transpiled and bundled.

AMD

AMD stands for Asynchronous Module Definition. Here is a sample code:

define(['dep1', 'dep2'], function (dep1, dep2) {
    //Define the module value by returning a value.
    return function () {};
});

or

// "simplified CommonJS wrapping" https://requirejs.org/docs/whyamd.html
define(function (require) {
    var dep1 = require('dep1'),
        dep2 = require('dep2');
    return function () {};
});

• AMD imports modules asynchronously (hence the name).
• AMD is made for frontend (when it was proposed) (while CJS backend).
• AMD syntax is less intuitive than CJS. I think of AMD as the exact opposite sibling of CJS.

UMD

UMD stands for Universal Module Definition. Here is what it may look like (source):

(function (root, factory) {
    if (typeof define === "function" && define.amd) {
        define(["jquery", "underscore"], factory);
    } else if (typeof exports === "object") {
        module.exports = factory(require("jquery"), require("underscore"));
    } else {
        root.Requester = factory(root.$, root._);
    }
}(this, function ($, _) {
    // this is where I defined my module implementation

    var Requester = { // ... };

    return Requester;
}));

• Works on front and back end (hence the name universal).
• Unlike CJS or AMD, UMD is more like a pattern to configure several module systems. Check here for more patterns.
• UMD is usually used as a fallback module when using bundler like Rollup/ Webpack

ESM

ESM stands for ES Modules. It is Javascript's proposal to implement a standard module system. I am sure many of you have seen this:

import React from 'react';

Other sightings in the wild:

import {foo, bar} from './myLib';

...

export default function() {
  // your Function
};
export const function1() {...};
export const function2() {...};

• Works in many modern browsers
• It has the best of both worlds: CJS-like simple syntax and AMD's async
• Tree-shakeable, due to ES6's static module structure
• ESM allows bundlers like Rollup to remove unnecessary code, allowing sites to ship less codes to get faster load.
• Can be called in HTML, just do:

<script type="module">
  import {func1} from 'my-lib';

  func1();
</script>

This may not work 100% in all browsers yet (source).

Summary

• ESM is the best module format thanks to its simple syntax, async nature, and tree-shakeability.
• UMD works everywhere and usually used as a fallback in case ESM does not work
• CJS is synchronous and good for back end.
• AMD is asynchronous and good for front end.

Thanks for reading, devs! In the future, I plan to write in depth about each module, especially ESM because it is packed with many awesomeness. Stay tuned!

Let me know if you notice any errors.