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History and Background of JavaScript Module Loaders

Published: July 8, 2022

Browserify set out to allow use of CommonJS formatted modules in the browser. Consequently, Browserify isn’t as much a module loader as a module bundler: Browserify is entirely a build-time tool, producing a bundle of code which can then be loaded client-side.
Start with a build machine that has node & npm installed, and get the package:
npm install -g –save-dev browserify
Write your modules in CommonJS format, and when happy, issue the command to bundle:
browserify entry-point.js -o bundle-name.js
Browserify recursively finds all dependencies of entry-point and assembles them into a single file:
<script src=”bundle-name.js”></script>
Adapted from server-side patterns, Browserify does demand some changes in approach. With AMD, you might minify and concat “core” code, and then allow optional modules to be loaded a la carte. With Browserify, all modules have to be bundled; but specifying an entry point allows bundles to be organized based on related chunks of functionality, which makes sense both for bandwidth concerns and modular programming.
Launched in 2011, Browserify is going strong.

Webpack follows Browserify’s lead as a module bundler, but adds enough functionality to replace your build system. Expanding beyond CJS, Webpack supports not only AMD and ES6 formats, but non-script assets such as stylesheets and HTML templates.
Webpack runs on a concept called ‘loaders’, which are plugins registered to handle a file type. For example, a loader can handle ES6 transpilation (Webpack 2.0 handles ES6 natively), or SCSS compilation.
Loaders feed data into a “chunk”, which starts from an entry point – conceptually similar to a Browserify bundle. Once Webpack is set up, chunks are regenerated automatically as assets  change. This can be very powerful, as you don’t have to remember to edit chunks.
The feature that has everybody really excited is hot module replacement. Once Webpack is in charge of your chunks, while running webpack-dev-server, it knows enough to modify code in the browser as you change the source. While similar to other source watchers, webpack-dev-server doesn’t require a browser reload, so it falls into the category of productivity tools that shave milliseconds off your dev process.
Basic usage is beyond simple. Install Webpack like Browserify:
npm install -g –save-dev webpack
And pass the command an entry point and an output file:
webpack ./entry-point.js bundle-name.js
If you’re limiting use to Webpack’s impressive set of defaults, that command power always comes at a cost though. On one project, our team had several difficult problems – transpiled ES6 didn’t work after Webpack chunked it, and then SCSS worked locally but failed to compile in the cloud. In addition, Webpack’s loader plugin syntax overloads the argument to require(), so it won’t work outside of Webpack without modification (meaning you won’t be able to share code between client and server side).
Webpack has its sights set on the being next-generation compiler for the web, but maybe wait for the next version.

Wikipedia defines a polyfill as “additional code which provides facilities that are not built into a web browser”, but the ES6 Module Loader Polyfill which SystemJS extends goes beyond the browser. An excellent example of how agnostic modern Javascript has become about the environment it runs in,  the ES6 Module Loader Polyfill can also be used via npm in a Node environment.

SystemJS can be thought of as the browser interface to the ES6 Module Loader Polyfill. Its implementation is similar to RequireJS: include SystemJS on the page via a script tag, set options on a configuration object, and then call System.import() to load modules: