In the beginning, Javascript did not have a way to import/export modules. This is a problem. Imagine writing your app in just one file - it would be nightmarish!

Then, people much, much smarter than me attempted to add modularity to Javascript. Some of them are CJS, AMD, UMD, and ESM. You may have heard some of them (there are other methods, but these are the big players).

I will cover high-level information: syntax, purpose, and basic behaviors. My goal is to help readers recognize when they see them in the wild 💡.

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 () {};
});

// "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.

What Are CJS, AMD, UMD, and ESM in Javascript?

CJS

AMD

UMD

ESM

Summary

Resources: