Introduction

In the previous lesson, we introduced ES6 modules (ESM) and npm. The introduction of ESM greatly mitigated some of the issues with managing individual script files and dependencies. However, downloading a large number of module files individually can reduce performance, especially as more third party files are imported.

Fortunately, more recent web technologies have greatly improved these aspects, but bundlers still provide us with a lot of power to process and optimize our code in various ways. This power, however, does come with the small cost of needing to configure a bundler. For now, our needs are few and simple, and we can look at the basic things one at a time.

Awareness of bundlers and basic experience with them is valuable. While in recent years, new build tools have come out that handle a lot of basic configuration for us, in the real world, you may not always get a chance to use these shiny new tools. It’s very reasonable to end up working with codebases that use tools that require more manual configuration. Even if you did get to work with tools that handle more things for you, it’s useful to understand what those tools are actually doing for you.

Lesson overview

This section contains a general overview of topics that you will learn in this lesson.

• Explain the purpose of bundlers and how they work.
• Configure Webpack to bundle JavaScript modules.
• Configure Webpack to handle non-JavaScript files during bundling, including using HtmlWebpackPlugin.
• Set up Webpack’s development server.

Bundling

In the previous lesson, we learned what an entry point is, what a dependency graph is, and how to add an entry point file to HTML as a module script. With bundling, the same concepts of entry points and dependency graphs apply: we provide the bundler with an entry point. It then builds a dependency graph from that file, combines all relevant files together, and then outputs a single file with all the necessary code included.

While it does this, we could also get it to do a whole bunch of other things, such as minifying our code, image optimizations, or even “tree shaking”. Most of these extra optimizations are out of the scope of this course; we will instead be focusing on basic bundling of JavaScript, and handling HTML, CSS, and images.

Webpack

Webpack is one of the most popular JavaScript bundlers, if not the most popular one, and has been for a long time. Let’s get started with bundling!

We’ll first need to make a new directory for our practice app, then create a package.json file in it for npm to record information about packages we use (like Webpack). Run the following in your terminal:

mkdir webpack-practice &&
cd webpack-practice &&
npm init -y

Once inside your new directory, we can go ahead and install Webpack, which involves two packages.

npm install --save-dev webpack webpack-cli

Note that we included the --save-dev flag (you can also use -D as a shortcut), which tells npm to record our two packages as development dependencies. We will only be using Webpack during development. The actual code that makes Webpack run will not be part of the code that the browser will run.

Also notice that when these finished installing, a node_modules directory and a package-lock.json got auto-generated. node_modules is where Webpack’s actual code (and a whole bunch of other stuff) lives, and package-lock.json is just another file npm uses to track package information.

Bundling JavaScript

Now that we’ve installed Webpack in our project directory, let’s create a src directory with two JavaScript files inside it: index.js and greeting.js.

mkdir src && touch src/index.js src/greeting.js

Inside our two JavaScript files, we’ll have the following:

// index.js
import { greeting } from "./greeting.js";

console.log(greeting);

// greeting.js
export const greeting = "Hello, Odinite!";

Great, now we have an index.js that imports from, and so depends on, greeting.js. In order to bundle this, we’ll also want a Webpack configuration file, which will contain all the details we need for bundling, such as the entry point, the output destination, and anything like plugins and loaders (which we will cover shortly).

Back in your project root (so outside of src), create a webpack.config.js file that contains the following:

// webpack.config.js
const path = require("path");

module.exports = {
  mode: "development",
  entry: "./src/index.js",
  output: {
    filename: "main.js",
    path: path.resolve(__dirname, "dist"),
    clean: true,
  },
};

Yes, you may have noticed this file uses CommonJS (CJS) syntax instead of ESM. That’s because this file (and Webpack itself) runs in NodeJS and not the browser. By default, NodeJS uses CJS syntax, and the configuration file also contains some CJS-specific things. We need not worry about this - this is just stuff we need for Webpack to do its thing.

You’ll notice the exported object contains a few key sections:

• mode: For now, we will just leave this in development mode, as it will be more useful to us. We will revisit this and production mode in a later lesson.
• entry: A file path from the config file to whichever file is our entry point, which in this case is src/index.js.
• output: An object containing information about the output bundle.
  • filename: The name of the output bundle - it can be anything you want.
  • path: The path to the output directory, in this case, dist. If this directory doesn’t already exist when we run Webpack, it will automatically create it for us as well. Don’t worry too much about why we have the path.resolve part - this is just the way Webpack recommends we specify the output directory.
  • clean: If we include this option and set it to true, then each time we run Webpack to bundle, it will empty the output directory first before bundling the files into it. This helps us keep dist clean, so it only contains the files produced by the most recent bundling.

With these files all in place, let’s run Webpack and see what happens!

npx webpack

You should see that Webpack has created a dist directory for us containing a main.js file! Inside this file is…a lot of stuff… Don’t worry, most of this stuff is just for development tools we will use later. If you go ahead and run this file with node dist/main.js, you should see Hello, Odinite! logged in the terminal.

Congratulations! You’ve just made your first bundle with Webpack!

Handling HTML

Let’s actually include some HTML. After all, we’re interested in making websites! Since HTML isn’t JavaScript, Webpack can’t just bundle it straight away, but there’s a nifty tool we can use called HtmlWebpackPlugin that’s just perfect for us.

Run the following command to install HtmlWebpackPlugin (also as a dev dependency):

npm install --save-dev html-webpack-plugin

We should also create a template.html inside src (you can name this file whatever you want) and fill that with the usual HTML boilerplate. We do not need to put a script tag in this file! HtmlWebpackPlugin will automatically add our output bundle as a script tag. We wouldn’t want to double up by including our own one as well! Inside our webpack.config.js, we can add a few little bits.

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

module.exports = {
  mode: "development",
  entry: "./src/index.js",
  output: {
    filename: "main.js",
    path: path.resolve(__dirname, "dist"),
    clean: true,
  },
  plugins: [
    new HtmlWebpackPlugin({
      template: "./src/template.html",
    }),
  ],
};

All we’re doing here is making sure our Webpack configuration has access to HtmlWebpackPlugin, then adding it as a plugin to the configuration object. Inside the HtmlWebpackPlugin constructor call, we pass in any options. For now, we’re only interested in the template option.

If we provide the path to our src/template.html file as a template, when we run npx webpack again, you’ll notice our dist directory not only contains a main.js file but an index.html file as well (it can’t combine them into one file). You’ll also notice that HtmlWebpackPlugin has automatically added a deferred script tag to our index.html file - what a darling! If you open this file in the browser and check the browser console, you should see our lovely "Hello, Odinite!" string logged.

We’ve now successfully configured Webpack to handle our HTML file and inject the appropriate script tag. Any changes to HTML we make, we can just rerun Webpack to generate fresh dist code.

Let’s see how we’d handle CSS.

Loading CSS

We don’t just need one new package for CSS, we need two. Gosh, what a greedy little thing… Let’s install them.

npm install --save-dev style-loader css-loader

css-loader will read any CSS files we import in a JavaScript file and store the result in a string. style-loader then takes that string and actually adds the JavaScript code that will apply those styles to the page. Therefore, we need both.

Back in our webpack.config.js, we need to add these loaders so Webpack knows what to do. Since these aren’t plugins, they go in a separate section:

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

module.exports = {
  mode: "development",
  entry: "./src/index.js",
  output: {
    filename: "main.js",
    path: path.resolve(__dirname, "dist"),
    clean: true,
  },
  plugins: [
    new HtmlWebpackPlugin({
      template: "./src/template.html",
    }),
  ],
  module: {
    rules: [
      {
        test: /\.css$/i,
        use: ["style-loader", "css-loader"],
      },
    ],
  },
};

All this does is tell Webpack that if it encounters an imported file ending with .css, it should use the listed loaders to process that CSS file.

Now that Webpack knows what to do with imported CSS files, let’s add some CSS! Create a src/styles.css with the following:

/* styles.css */
body {
  background-color: rebeccapurple;
}

You can now import your CSS file into one of your JavaScript files. src/index.js makes sense. We don’t need anything from the imported CSS file itself. Since our CSS and style loaders will handle all of that for us, we can just use a side effect import.

import "./styles.css";
import { greeting } from "./greeting.js";

console.log(greeting);

Once again, bundle with Webpack using npx webpack, then open dist/index.html and enjoy the beautiful purple screen!

What about link tags?

Notice how we don’t link our CSS file in our HTML template like we would’ve done before. While you could do this with one of the loaders from the next section, in the real world, projects often contain many moving parts and many modules. Eventually, it becomes easier to work with multiple smaller CSS files that you import in the modules they’re needed. There are even ways those files can be scoped only to those modules and not globally!

We’re only introducing the minimum to allow you to import your CSS into your JavaScript, but many build tools and more sophisticated bundler configurations will do a lot more to imported CSS than what we’re showing here.

Loading images

We’re nearly done with the main Webpack configuration! If we have any local image files we want to include within our website, they will also require a little extra configuration since they’re not JavaScript files.

There are three different ways you could be dealing with local image files:

1. Image files used in our CSS inside url()

   Lucky us! css-loader already handles this for us, so there’s nothing extra to do for image paths in CSS!

2. Image files we reference in our HTML template, e.g. as the src of an <img>

   We need to install and tell Webpack to use something called html-loader, which will detect image file paths in our HTML template and load the right image files for us. Without this, ./odin.png would just be a bit of text that will no longer reference the correct file once we run Webpack to build into dist. Let’s install it:

   npm install --save-dev html-loader
   

   Then, add the following object to the modules.rules array within webpack.config.js:

   {
     test: /\.html$/i,
     loader: "html-loader",
   }
   

3. Images we use in our JavaScript, where we will need to import the files

   If we need to use a local image file in our JavaScript (such as when manipulating the DOM to create or edit img elements and set their src attribute), we need to import the images into our JavaScript module. Since images aren’t JavaScript, we need to tell Webpack that these files will be assets by adding an asset/resource rule. No need to install anything here. Just add the following object to the modules.rules array within webpack.config.js:

   {
     test: /\.(png|svg|jpg|jpeg|gif)$/i,
     type: "asset/resource",
   }
   

   You can always edit the regex in the test property to remove any file extensions you don’t need or add any extensions you do need. What’s shown above is straight from Webpack’s Asset Management guide and will recognize most of the common image file extensions.

   Then, in whatever JavaScript module we want to use that image in, we just have to default import it.

   import odinImage from "./odin.png";
      
   const image = document.createElement("img");
   image.src = odinImage;
      
   document.body.appendChild(image);
   

   We have to import it so that the odinImage variable contains the correct file path, even when we bundle into dist. If we just wrote image.src = "./odin.png";, then the “file path” would just be a plain string. When we bundle into dist, Webpack will not magically recognize that this string in our JavaScript references a file and so will not include it in the bundle. When we import it and set the correct asset/resource rule, Webpack will recognize the import, include the image file when we bundle, and also make sure the imported variable contains the correct file path at the end.

After all that, if we added both html-loader and the image asset/resource rule, our webpack.config.js would look something like this:

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

module.exports = {
  mode: "development",
  entry: "./src/index.js",
  output: {
    filename: "main.js",
    path: path.resolve(__dirname, "dist"),
    clean: true,
  },
  plugins: [
    new HtmlWebpackPlugin({
      template: "./src/template.html",
    }),
  ],
  module: {
    rules: [
      {
        test: /\.css$/i,
        use: ["style-loader", "css-loader"],
      },
      {
        test: /\.html$/i,
        loader: "html-loader",
      },
      {
        test: /\.(png|svg|jpg|jpeg|gif)$/i,
        type: "asset/resource",
      },
    ],
  },
};

You might notice that when images are included when bundling, the output image file in dist has a different file name (it will likely be some jumble of numbers and letters). By default, Webpack gives your bundled image files a new name by hashing their contents. You do not need to know how this works, nor do you need to dig into the details of why, nor how to change it. You just need to be aware that this is expected behavior (it’s to do with preventing issues with the browser cache and matching file names).

Webpack dev server

During this lesson, did you get a bit annoyed with having to run npx webpack to rebundle with every change? Fortunately, there are multiple solutions for this, and we will focus on what we think is the most useful option: webpack-dev-server. Install it as follows:

npm install --save-dev webpack-dev-server

You may have used something like the Live Preview VSCode extension before, where it automatically refreshes your web page whenever you save a change. webpack-dev-server is very similar, meaning we won’t have to keep running npx webpack after each change we make.

It works by bundling your code behind the scenes (as if we ran npx webpack, but without saving the files to dist), and it does this every time you save a file that’s used in the bundle. We can also use something called a source map so that any error messages reference files and lines from our development code and not the jumbled mess inside our single bundled .js file!

Once installed, in our webpack.config.js, we only need to add a couple more properties somewhere in the configuration object (the order of these properties does not matter):

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

module.exports = {
  mode: "development",
  entry: "./src/index.js",
  output: {
    filename: "main.js",
    path: path.resolve(__dirname, "dist"),
    clean: true,
  },
  devtool: "eval-source-map",
  devServer: {
    watchFiles: ["./src/template.html"],
  },
  plugins: [
    new HtmlWebpackPlugin({
      template: "./src/template.html",
    }),
  ],
  module: {
    rules: [
      {
        test: /\.css$/i,
        use: ["style-loader", "css-loader"],
      },
      {
        test: /\.html$/i,
        loader: "html-loader",
      },
      {
        test: /\.(png|svg|jpg|jpeg|gif)$/i,
        type: "asset/resource",
      },
    ],
  },
};

Firstly, we add a source map by setting eval-source-map as a devtool option. If we don’t do this, any error messages we get won’t necessarily match up to the correct files and line numbers from our development code. In the devtools “Sources” tab, we also won’t be able to find our original untouched code, making the Chrome debugger harder to use. Adding this source map will solve both of these problems for us.

Secondly, by default, webpack-dev-server will only auto-restart when it detects any changes to files we import into our JavaScript bundle, so our HTML template will be ignored! All we need to do is add it to the dev server’s array of watched files - nice and simple!

Once set up, npx webpack serve will host our web page on http://localhost:8080/, which we can open in our browser and start working!

Rounding up

Yes, yes, this all might seem like a lot. You’ve gone from just having some basic HTML, CSS and JS files, and not needing much else to suddenly needing this loader, that plugin, this configuration file, etc. In the real world, as apps get more complex, we need tools that can improve our development experience while optimizing things in production. Even though we’re not using all of the features available to us right now, a general understanding of what these sorts of tools are doing for us is valuable. Later in the curriculum, you will use different tools that abstract a lot of these underlying mechanisms away from us. Using them and having no clue what they’re actually doing for you can make things harder for you when you eventually encounter a situation that actually needs some kind of manual configuration.

In a later lesson, we will introduce some extra things that can make setting up and working with Webpack much quicker and more straightforward. For now, however, it’s good to get a little manual practice in.

Assignment

Knowledge check

The following questions are an opportunity to reflect on key topics in this lesson. If you can’t answer a question, click on it to review the material, but keep in mind you are not expected to memorize or master this knowledge.

• What is a bundler?
• What is Webpack?
• How do you bundle JavaScript?
• How do you load CSS using Webpack?
• How do you automatically build HTML files in dist using Webpack?
• How would you handle assets like local image files?
• What Webpack tool could you use during development to view changes to your website live?
• How does using a source map help with development?

Additional resources

This section contains helpful links to related content. It isn’t required, so consider it supplemental.

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