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From Require.js to Webpack - Part 2 (the how)

This is the follow up to a post I wrote recently called From Require.js to Webpack - Party 1 (the why) which was published in my personal blog.

In that post I talked about 3 main reasons for moving from require.js to webpack:

  1. Common JS support
  2. NPM support
  3. a healthy loader/plugin ecosystem.

Here I'll instead talk about some of the technical challenges that we faced during the migration. Despite the clear benefits in developer experience (DX) the setup was fairly difficult and I'd like to cover some of the challanges we faced to make the transition a bit easier.

From paths to alias to NPM

The first thing you do when you're converting from require.js to webpack is you take your whole require.js configuration file and convert it to a webpack.config.js file.

In practice for us this meant addressing three areas:

  1. Tell webpack where you keep your JS files
  2. The enormous paths list in our require.js config
  3. All of the shim config

Module Path Resolution

The first thing you need to do is tell webpack where your JS files are. Require.js could usually infer this based on the <script> tag you used to set it up or you might have configured it using the baseUrl option.

This is super easy to setup in webpack by adding the following to your config:

	resolve: {
		modulesDirectories: ['public/js']

If you forget to set this webpack will probably assume node_modules.

Migrating Require.js paths to webpack alias

Initially the conversion process is really straight forward.

Start with a config like this:

    paths: {
        "backbone": "lib/backbone-1.1.0",
        "jquery": "lib/jquery-1.10.2",
        "underscore": "lib/lodash.underscore-2.3.0",
        "jqueryUI": "lib/jquery-ui.min"

And it translates very straightforwardly into a webpack config of the following:

module.exports = {
	resolve: {
		alias: {
        		"backbone": "lib/backbone-1.1.0",
        		"jquery": "lib/jquery-1.10.2",
        		"underscore": "lib/lodash.underscore-2.3.0",
		        "jqueryUI": "lib/jquery-ui.min"

Hey that was pretty easy!

More info:

A Loader for Every Shim

The next thing, which took some special care, was fixing up our shim config. This was slightly harder than it looked, because it's easy to forget what the shim's are actually doing. Let's revisit that for a moment.

A require.js shim takes modules that are not AMD-compatible and makes them AMD comptible by wrapping them in a little bit of code which will pull in the appropriate dependencies.

Let'e examine exactly how that works with the follow simple example:

    shim: {
        "underscore": {
            exports: "_"
        "backbone": {
            deps: ["jquery", "underscore"],
            exports: "Backbone"

Here we are applying shims for both underscore and backbone*. For underscore the shim will wrap the library and then return the value of the _ variable for any scripts using it as a dependency. The backbone case is slightly more complicated:

  1. It wraps the library and exports the value of the Backbone variable.
  2. It makes sure that when evaluated backbone has access to both jquery and underscore

Let's see how we would get the same setup using webpack loaders:

  module: {
    loaders: [
      { test: /underscore/, loader: 'exports?_' }
      { test: /backbone/, loader: 'exports?Backbone!imports?underscore,jquery' }

A few things to note:

  1. The test is a regular expression which matches against the full file path, so be careful to be specific!
  2. In order to use these loaders you need to install them npm install exports-loader imports-loader

Pretty much all of the common use cases you might bump into are covered in the webpack guide to shimming modules.

The Fun Part: NPM

So now that you've migrated all of your shim and paths config, let me suggest something: delete it.

Now go to NPM and install underscore, jquery, backbone, react. Whatever else you're using is probably there and probably will work out of the box without any special loaders or aliasing.

That's the magic of webpack.

If you get your dependencies from NPM you don't need any of that. They will just work and you can go on building great apps not spending time carefully maintaining each dependency in your config files.

To add support for NPM to webpack, just make sure this is included in your webpack config:

	resolve: {
		modulesDirectories: ['public/js', 'node_modules']

Migrating the Router

The heart of our app is this fancy router. When a new route comes in we would dynamically pull in the view associated with that path, instantiate it and re-render the main app view with the new page. It works pretty well. The main nav and everything are part of the main app, but each tab in the navbar had its own separate JS bundle and we'd only pull that in on an as-needed basis*.

This was by far the most challenging piece, mostly because my misunderstanding of how different the splitting/bundling technique is between webpack and require.js.

* This has changed for us somewhat recently, but hopefully the lesson will still be valuable for you!

Goodbye Manual Splitting

So we've talked a lot about our require.js config. Now we're going to talk about our r.js config. All of the special information needed to create our r.js build. Here's the bulk of it:

	baseUrl: 'public/js',
	mainConfigFile: 'public/js/config.js',
	dir: '.build/js',
	modules: [
		{ name: 'config' },
		{ name: 'view/summary/index', exclude: ['config'] },
		{ name: 'view/activity/index', exclude: ['config'] },
		{ name: 'view/transfer/index', exclude: ['config'] },
		{ name: 'view/wallet/index', exclude: ['config'] },
		{ name: 'view/settings/index', exclude: ['config'] },

The bulk of this file was concerned with all of the different bundles we were making. With webpack that all goes away (mostly).

At first my instict was to replace this with the webpack entry concept. I spent a lot of time going down the wrong path worrying about entries.

There is another approach that worked better for us:

  • Using async require() to create split points in our app and then letting webpack create the bundles automatically.

The Routing Code: Old and New

Here is the gist of the old require.js code:

function handleRouteChange(path) {
    require(['/views/' + path], function(PageView) {
        app.setView(new PageView());

The first thing I tried to do when moving this to webpack was just leaving it. It seemed to work! The problem is that it only created a single bundle for all of the views. This means a signicantly larger payload for the browser.

Here's our improved webpack solution:

function loadPage(PageView) {
    app.setView(new PageView());
function handleRouteChange(path) {
    switch (path) {
        case 'settings':
            require(['/views/settings'], loadPage);
        case 'transfer':
            require(['/views/transfer'], loadPage);
        case 'wallet':
            require(['/views/wallet'], loadPage);
	    // you could either require everything here as a last resort or just leave it...

Each time you use the AMD-style require([]) or require.ensure() webpack will see what you passed in there and go and create a bundle with every possible match. That works great if you pass in a single file, but when you use a variable, it might end up bundling up your entire view folder. That's why you need to use something like a switch statement to make sure that you declare your split points on a route-by-route basis.

You should probably grumble about this a bit, but just remember because you do this you don't need any module or entry logic in your config. It will create all the bundles you need automatically, mostly automatically anyway :)

Entry or Async Require

Let me do my best to explain when you want an entry and when you want to rely on webpack to do the splitting for you:

  1. Use an entry if you want to include a new <script> tag in a page.
  2. Use require.ensure() or another form of async require if you want webpack to dynamically pull in additional modules.

I recommend reading Pete Hunt's excellent webpack how-to on the subject of async loading for more information.

The trouble with our CDN

One of the last things that got us was our CDN. Everything worked fine in local development mode, but when we got to our live testing environments we noticed that it was trying to pull additional bundles off of the webserver by default and not the CDN where our JS actually lives in production. require.js handles this automatically by parsing the URL from the <script> on the page. Webpack doesn't have this magic. If you have a non-standard place where you want it to pull down additional dynamic bundles you need to tell it:

__webpack_public_path__ = document.body.getAttribute('data-js-path') + '/apps/';

More info can be found here:

If we'd used webpack to handle our total build process we could let webpack apply the hash automatically, but since we're still only using it for our JS build, we need to handle this programmatically. Probably one of my least favorite things about the setup. Not terribly hard, just hard to remember and definitely threw us for a pretty big loop.

Build Size & Time

At this point we were pretty excited about everything, now that it was basically all working, but we noticed that the file sizes were a bit higher, maybe even a lot higher than our old require.js builds.

Enter Display Reasons

webpack --display-reasons is the most helpful things ever.

Run webpack with that flag and you'll see:

  1. Every module that was added to your bundle
  2. How much space it's taking up
  3. The filename and module format that included it

Here is the partial output from a recent build:


This is a goldmine of information. Which modules took up the most space? How did that get included in this bundle? All answered with this information!

Note: By default all of the files in NPM are excluded from the output. If you want to add that simply add the --display-modules flag to your query (as above) and now you can see exactly every file that requires jquery (node_modules is shortened to ~). It's pretty awesome and it helped us find a little issue with moment.js.

Moment.js and the Ignore Plugin

When we first started this process we noticed moment.js was pulling in 900kb of files. In the minified output it was still around 100kb. After poking around a bit we came across this stackoverflow article: How to prevent Moment.js from Loading Locales with Webpack.

Becuase you're now in a CommonJS environment Moment thinks it's safe to load all of its locale information. How should it know you're targetting someone's Windows XP box? Thankfully there's a webpack plugin that will strip out those extra requests prety easily.

	plugins: [
		new webpack.IgnorePlugin(/^\.\/locale$/, [/moment$/]), // saves ~100k from build

Source maps

These can cause your builds to appear huge. They are great and can really help with debugging, but make sure you know what you're doing with them and make sure you're not counting them against your total bundle size. Most browsers only download these if the developer console is open, so they don't affect your customers.

However, if you set your source-map type to eval or eval-source-map the source-maps are bundled directly into your JS. Do not use this type of source map in your production build.

More info here:


To add minification all you need to do is add this config:

    plugins: [
        new webpack.optimize.UglifyJsPlugin({minimize: true})


Webpack has been great to work with and I think the whole team basically loves it now. I hope this little guide helps your team avoid the hurldes we ran into. If you need any help feel free to leave a note or checkout the excellent webpack chat room on gitter.

-- Jamund

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One additional thing to maybe mention is how to get the minified version of react (and some other modules).

    plugins: [
        new webpack.DefinePlugin({'process.env.NODE_ENV': '"production"'})

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Not sure if I should include this or not

Require.js to Webpack Thesaurus

Webpack and require.js have a lot of similar concepts, but sometimes they don't use the same words to explain them. I'll do my best in this table to provide equivalent definitions.

Require.js Version Webpack Version
plugin loader
require.config() or config.js webpack.config.js
paths config alias config
shim config expose/export loaders
r.js webpack
define([]) module.exports
require(['abc']) require('abc')
require([viewName]) use with caution
modules entry

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Hi Jamund. Humongous thanks for writing this guide. I've been transitioning my current company to webpack, and this gist has helped a lot. I've decided to write a CLI tool that'll help automate a bit of the steps you outlined, specifically with the configuration file:

Again, thanks for all this!

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okbel commented Dec 22, 2015

Thank you!

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Great article! thanks for sharing :)

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gotbahn commented Feb 18, 2016

Thank you for great article. Did you have any troubles with require module.config? Did you rewrite define([]) to module.exports ?

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aflext commented Feb 19, 2016

Great For Sharing!

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I think aliases are not always that straightforward to translate.
I've found that aliases with "/" on them don't translate well to webpack:


"app/user/actions": "/the/directory/some/more/folders/the_file"

Compiler then complains when a module tries to require it, it looks like it does not resolve properly the path
Instead of resolving to :


It resolves to:


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what if a require text plugin?

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thanks for sharing

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skyl commented Jun 28, 2016

I too am wondering about module.config ...

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In RequireJS there is a plugin called requirejs-promise, that allow files to resolve themselves, using promises - see project on github: , how can we do this with Webpack???
require example: require(["promise!file-that-need-be-loaded-with-promise-plugin"], function(module){ ... });

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This is awesome. Am working through a lot of these issues now at work. Nicely done!

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Thanks for sharing

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@xjamundx Thanks for this guide. It's gold.

I would like to know if you could give some more hits regarding this:

Each time you use the AMD-style require([]) or require.ensure() webpack will see what you passed in there and go and create a bundle with every possible match. That works great if you pass in a single file, but when you use a variable, it might end up bundling up your entire view folder. That's why you need to use something like a switch statement to make sure that you declare your split points on a route-by-route basis.

We have too many routes in our app to implement a routing config per view. We are using a dynamic require where we calculate the name of the bundle to load with some static path and some info retrieved from the server.

You say that "it might end up bundling up your entire view folder". Do you know why this may happen? Do you know how we can avoid it?

Thanks a lot.

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homeslicesolutions-zz commented Apr 8, 2017

@hackerslizc what if a require text plugin?

I know this is a year later, but the solution for that is to get either the text-loader or the raw-loader then use resolveLoader option in your config. Like so:

  resolveLoader: {
    alias: {
      "text": "raw-loader"

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I am currently porting an old backbone web application that uses RequireJS to Webpack. Thank you 👍

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zeerain commented Jul 28, 2017

very nice , thank you! I just need it~

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reharik commented Sep 30, 2017

I can't tell you how happy I am now, after reading this. I recently started a job and while the code is a bit of a horror show, I knew that switching to webpack would be a good first step. I thought about switching the hundreds of files from require to es5 or commonjs, spent a whole day doing a one small part. The task seemed daunting, and I was really wondering if was going to be able to do it. If not I'd be looking for a new job cuz I can't work in this crap.
Your post covers a super similar situation to ours and i'm can't wait for monday. Obviously I have not tried it yet, but I'm very excited and am quite sure it will be a success.
Thanks again

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Thanks for sharing :)

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Thanks for this! :)

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Still pure gold. Thanks for such an in-depth writeup.

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Ever so useful! thanks a lot!

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benblayer commented Mar 4, 2020

Hi, thanks for the great article!
What about require-config-packages ? i.e. :
packages: [{
name: "...",
location: "...",
main: "..."

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Still having some confusions. I really loved being able to async load some modules not required on every page using require or define callback. Trying to migrate to webpack works fine and I noticed I can use import, but it will completly pre-compile all imports. Trying to use require or define results in frontend errors.

I've already been trying to pre-define resolveable paths to webpack like this:

module.exports = {
    resolve: {
        alias: {
            'jquery': path.resolve('Public/JavaScript/Vendor', 'jquery.min.js')

But still frontend javascript in web-browser complains:

Uncaught (in promise) TypeError: Failed to resolve module specifier 'jquery'

The path being used is confirmed to be valid! And also there is no AJAX action or something in background trying to load such lib as I know to be workin with requirejs.

How would I migrate such code being able to load multiple libs asynchronously?

define(['query', 'lib-a', 'lib-b']).then(($, la, lb) => {
    console.log('AMD Test', $, la, lb);

Thanks in advice.

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