by Jake Wiesler

Tree-shaking ES6 Modules in webpack 2

Webpack 2 was just released from beta last week. It brings with it a variety of anticipated features, including native support for ES6 modules.

Instead of using the var module = require('module') syntax, webpack 2 supports ES6 imports and exports. This opens the door for code optimizations like tree-shaking.

What is tree-shaking?

Popularized by Rich Harris’ Rollup.js module bundler, tree-shaking is the ability to only include code in your bundle that is being used.

When I first played around with Rollup, I was amazed at how well it worked with ES6 modules. The development experience just felt…right. I could create separate modules written in “future JavaScript,” then include them anywhere in my code. Any code that goes unused doesn’t make it into my bundle. Genius!

What problem does it solve?

If you’re writing JavaScript in 2017 and understand (see: JavaScript fatigue) the various tools around, your development experience probably feels pretty fluid. This is important, but what’s also important is user experience. A lot of these modern tools ends up bloating web applications with massive JavaScript files, resulting in slower performance.

What I love about Rollup is that it takes a stab at this issue, and brings a solution to the forefront of the JavaScript community. Now big names like webpack are attempting to iterate on it.

Tree-shaking may not be “the solution to end all solutions” but it is an important piece in the larger pie.

A simple example

Before you get started I want to provide you with a trivial example of tree-shaking. Your application is made up of 2 files, index.js and module.js.

Inside of module.js you export 2 named arrow functions:

// module.js 

export const sayHello = name => `Hello ${name}!`; 

export const sayBye = name => `Bye ${name}!`

Only sayHello is imported into index.js file:

// index.js 

import { sayHello } from './module'; 

sayHello('World');

sayBye is exported but never imported. Anywhere. Therefore, due to tree-shaking, it won't be included in your bundle:

// bundle.js 

const sayHello = name => `Hello ${name}!`; 

sayHello('World');

Depending on the bundler used, the output file above may look different. It’s just a simplified version, but you get the idea.

Recently I read an article written by Roman Liutikov, and he made a great analogy in order to visualize the concept of tree-shaking:

“If you wonder why it’s called tree-shaking: think of your application as a dependency graph, this is a tree, and each export is a branch. So if you shake the tree, the dead branches will fall.” — Roman Liutikov

Tree-shaking in webpack 2

Unfortunately for those of us using webpack, tree-shaking is “behind a switch,” if you will. Unlike Rollup, some configuration needs to be done before you can get the functionality you’re looking for. The “behind a switch” part might confuse some people. I’ll explain.

Step 1: Project setup

I’m going to assume that you understand webpack basics and can find your way around a basic webpack configuration file.

Let’s start by creating a new directory:

mkdir webpack-tree-shaking && cd webpack-tree-shaking

Once inside, let’s initialize a new npm project:

npm init -y

The -y option generates the package.json quickly without requiring you to answer a bunch of questions.

Next, let’s install a few project dependencies:

npm i --save-dev [email protected] html-webpack-plugin

The command above will install the latest beta version of webpack 2 locally in your project as well as a useful plugin named html-webpack-plugin. The latter is not necessary for the goal of this walkthrough but will make things a bit quicker.

Note: The command npm i --save-dev [email protected] is still recommended by the webpack team at the time of writing. [email protected] will eventually be phased out in favor of the webpack latest command. Check out the How To Download? section of webpack’s latest release post for more details.

Open up package.json and make sure they've been installed as devDependencies.

Step 2: Create JS files

In order to see tree-shaking in action you need to have some JavaScript to play around with. In your project’s root, create a src folder with 2 files inside:

mkdir src && cd src 

touch index.js 

touch module.js

Note: The touch command creates a new file through the terminal.

Copy the code below into the correct files:

// module.js 

export const sayHello = name => `Hello ${name}!`; 

export const sayBye = name => `Bye ${name}!`;
// index.js 

import { sayHello } from './module'; 

const element = document.createElement('h1'); 

element.innerHTML = sayHello('World'); 

document.body.appendChild(element);

If you’ve gotten this far, your folder structure should look like this:

/ 
| - node_modules/ 
| - src/ 
|    | - index.js 
|    | - module.js 
| - package.json

Step 3: Webpack from the CLI

Since you have no configuration file created for your project, the only way to get webpack to do any work at the moment is through the webpack CLI. Let’s perform a quick test.

In your terminal, run the following command in your project’s root:

node_modules/.bin/webpack

After running this command, you should see output like this:

No configuration file found and no output filename configured via CLI option. A configuration file could be named 'webpack.config.js' in the current directory. Use --help to display the CLI options.

The command doesn’t do anything, and the webpack CLI confirms this. You haven’t given webpack any information about what files you want to bundle up. You could provide this information via the command line or a configuration file. Let’s choose the former just to test that everything is working:

node_modules/.bin/webpack src/index.js dist/bundle.js

What you’ve done now is pass webpack an entry file and an output file via the CLI. This information tells webpack, "go to src/index.js and bundle up all the necessary code into dist/bundle.js". And it does just that. You'll notice that you now have a dist directory containing bundle.js.

Open it up and check it out. There’s some extra javascript in the bundle necessary for webpack to do its thing, but at the bottom of the file you should see your own code as well.

Step 4: Create a webpack configuration file

Webpack can handle a lot of things. I’ve spent a good chunk of my free time diving into this bundler and I still have barely scratched the surface. Once you’ve move passed trivial examples its best to leave the CLI behind and create a configuration file to handle the heavy lifting.

In your project’s root, create a webpack.config.js file:

touch webpack.config.js

This file can be as complicated as you make it. We’ll keep it light for the sake of this post:

// webpack.config.js
const HtmlWebpackPlugin = require('html-webpack-plugin');

module.exports = {
  entry: './src/index.js',
  output: {
    filename: 'bundle.js',
    path: 'dist'
  },
  plugins: [
    new HtmlWebpackPlugin({ title: 'Tree-shaking' })
  ]
}

This file provides webpack with the same information you gave to the CLI earlier. You’ve defined index.js as your entry file and bundle.js as your output file. You've also added your html-webpack-plugin which will generate an html file in your dist directory. Convenient.

Go ahead and test this to make sure it’s still working. Remove your dist directory, and in the command line type:

webpack

If everything went smoothly, you can open up dist/index.html and see "Hello World!".

Note: The use of a configuration file gives us the convenience of typing webpack instead of node_modules/.bin/webpack. Small wins.

Step 5: Babel

I mentioned earlier that webpack 2 brings native support for ES6 modules. This is all true, but it doesn’t change the fact that ES6 is not fully supported across all browsers. Because of this, you’re required to transform your ES6 code into readily acceptable JavaScript using a tool like Babel. In conjunction with webpack, Babel gives us the ability to write your “future JavaScript” without worrying about the implications of unsupported browsers.

Let’s go ahead and install Babel in your project:

npm i --save-dev babel-core babel-loader babel-preset-es2015

Take note of the babel-preset-es2015 package. This little guy is the reason I sat down to write all of this up.

Step 6: babel-loader

Webpack can be configured to transform specific files into modules via loaders. Once they are transformed, they are added to a dependency graph. Webpack uses the graph to resolve dependencies and includes only what is needed into the final bundle. This is the basis for how webpack works.

You can now configure webpack to use babel-loader to transform all of your .js files:

// webpack.config.js
const HtmlWebpackPlugin = require('html-webpack-plugin');

module.exports = {
  entry: './src/index.js',
  output: { filename: 'bundle.js', path: 'dist' },
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: 'babel-loader',
        options: { 
          presets: [ 
            'es2015' 
          ] 
        }
      }
    ]
  },
  plugins: [ 
    new HtmlWebpackPlugin({ title: 'Tree-shaking' }) 
  ]
};

The module property provides a set of instructions for webpack. It says, "take any files ending in .js and transform them using babel-loader, but don't transform any files inside of node_modules!"

We’re also passing the babel-preset-es2015 package as an option to babel-loader. This just tells babel-loader how to transform the JavaScript.

Run webpack again to make sure everything is good. Yes? Great! What we've done is bundled up your JavaScript files while compiling them down to JavaScript thats readily supported across browsers.

The underlying problem

The package babel-preset-es2015 contains another package named babel-plugin-transform-es2015-modules-commonjs that turns all of your ES6 modules into CommonJS modules. This isn't ideal, and here's why.

Javascript bundlers such as webpack and Rollup can only perform tree-shaking on modules that have a static structure. If a module is static, then the bundler can determine its structure at build time, safely removing code that isn’t being imported anywhere.

CommonJS modules do not have a static structure. Because of this, webpack won’t be able to tree-shake unused code from the final bundle. Luckily, Babel has alleviated this issue by providing developers with an option that you can pass to your presets array along with babel-preset-es2015:

options: { presets: [ [ 'es2015', { modules: false } ] ] }

According to Babel’s documentation:

“modules - Enable transformation of ES6 module syntax to another module type (Enabled by default to "commonjs"). Can be false to not transform modules, or one of ["amd", "umd", "systemjs", "commonjs"]".

Slide that extra bit of code into your configuration and you’ll be cooking with peanut oil.

The final state of webpack.config.js looks like this:

// webpack.config.js
const HtmlWebpackPlugin = require('html-webpack-plugin');

module.exports = {
  entry: './src/index.js',
  output: { filename: 'bundle.js', path: 'dist' },
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        loader: 'babel-loader',
        options: { 
          presets: [ 
            [ 'es2015', { modules: false } ] 
          ] 
        }
      }
    ]
  },
  plugins: [ new HtmlWebpackPlugin({ title: 'Tree-shaking' }) ]
};

The Grand Finale

Run webpack again and pop open your bundle.js file. You won't notice any difference. Before you go crazy, know this! It's ok. We've been running webpack in development mode this whole time. Webpack knows that you have unused exports in your code. Even though it's included in the final bundle, sayBye will never make it to production.

If you still don’t believe me, run webpack -p in your terminal. The -p option stands for production. Webpack will perform a few extra performance optimizations, including minification, removing any unused code along the way.

Open up bundle.js. Since it's minified, go ahead and search for Hello. It should be there. Search for Bye. It shouldn't.

Voila! You now have a working implementation of tree-shaking in webpack 2!

For the curious, I’ve been slowly iterating over my own lightweight webpack configuration in a GitHub Repo:

jake-wies/webpack-hotplate
webpack-hotplate - A webpack boilerplate for personal projects
github.com

It’s not meant to be overly verbose and bloated. It’s focused on being an approachable boilerplate with walkthroughs at every turn. If you’re interested, check it out!

If you have any questions, feel free to reach out on Twitter!