Code Splitting | webpack

Output Management chapter.

Code splitting is one of the most compelling features of webpack. This feature allows you to split your code into various bundles which can then be loaded on demand or in parallel. It can be used to achieve smaller bundles and control resource load prioritization which, if used correctly, can have a major impact on load time.

There are three general approaches to code splitting available:

Entry Points: Manually split code using entry configuration.
Prevent Duplication: Use Entry dependencies or SplitChunksPlugin to dedupe and split chunks.
Dynamic Imports: Split code via inline function calls within modules.

Entry Points

This is by far the easiest and most intuitive way to split code. However, it is more manual and has some pitfalls we will go over. Let's take a look at how we might split another module from the main bundle:

project

webpack-demo
|- package.json
|- package-lock.json
|- webpack.config.js
|- /dist
|- /src
  |- index.js
+ |- another-module.js
|- /node_modules

another-module.js

import _ from 'lodash';

console.log(_.join(['Another', 'module', 'loaded!'], ' ');

webpack.config.js

 const path = require('path');

 module.exports = {
-  entry: './src/index.js',
+  mode: 'development',
+  entry: {
+    index: './src/index.js',
+    another: './src/another-module.js',
+  },
   output: {
-    filename: 'main.js',
+    filename: '[name].bundle.js',
     path: path.resolve(__dirname, 'dist'),
   },
 };

This will yield the following build result:

...
[webpack-cli] Compilation finished
asset index.bundle.js 553 KiB [emitted] (name: index)
asset another.bundle.js 553 KiB [emitted] (name: another)
runtime modules 2.49 KiB 12 modules
cacheable modules 530 KiB
  ./src/index.js 257 bytes [built] [code generated]
  ./src/another-module.js 84 bytes [built] [code generated]
  ./node_modules/lodash/lodash.js 530 KiB [built] [code generated]
webpack 5.4.0 compiled successfully in 245 ms

As mentioned there are some pitfalls to this approach:

If there are any duplicated modules between entry chunks they will be included in both bundles.
It isn't as flexible and can't be used to dynamically split code with the core application logic.

The first of these two points is definitely an issue for our example, as lodash is also imported within ./src/index.js and will thus be duplicated in both bundles. Let's remove this duplication in next section.

Prevent Duplication

Entry dependencies

The dependOn option allows to share the modules between the chunks:

webpack.config.js

 const path = require('path');

 module.exports = {
   mode: 'development',
   entry: {
-    index: './src/index.js',
-    another: './src/another-module.js',
+    index: {
+      import: './src/index.js',
+      dependOn: 'shared',
+    },
+    another: {
+      import: './src/another-module.js',
+      dependOn: 'shared',
+    },
+    shared: 'lodash',
   },
   output: {
     filename: '[name].bundle.js',
     path: path.resolve(__dirname, 'dist'),
   },
 };

If we're going to use multiple entry points on a single HTML page, optimization.runtimeChunk: 'single' is needed too, otherwise we could get into trouble described here.

webpack.config.js

 const path = require('path');

 module.exports = {
   mode: 'development',
   entry: {
     index: {
       import: './src/index.js',
       dependOn: 'shared',
     },
     another: {
       import: './src/another-module.js',
       dependOn: 'shared',
     },
     shared: 'lodash',
   },
   output: {
     filename: '[name].bundle.js',
     path: path.resolve(__dirname, 'dist'),
   },
+  optimization: {
+    runtimeChunk: 'single',
+  },
 };

And here's the result of build:

...
[webpack-cli] Compilation finished
asset shared.bundle.js 549 KiB [compared for emit] (name: shared)
asset runtime.bundle.js 7.79 KiB [compared for emit] (name: runtime)
asset index.bundle.js 1.77 KiB [compared for emit] (name: index)
asset another.bundle.js 1.65 KiB [compared for emit] (name: another)
Entrypoint index 1.77 KiB = index.bundle.js
Entrypoint another 1.65 KiB = another.bundle.js
Entrypoint shared 557 KiB = runtime.bundle.js 7.79 KiB shared.bundle.js 549 KiB
runtime modules 3.76 KiB 7 modules
cacheable modules 530 KiB
  ./node_modules/lodash/lodash.js 530 KiB [built] [code generated]
  ./src/another-module.js 84 bytes [built] [code generated]
  ./src/index.js 257 bytes [built] [code generated]
webpack 5.4.0 compiled successfully in 249 ms

As you can see there's another runtime.bundle.js file generated besides shared.bundle.js, index.bundle.js and another.bundle.js.

Although using multiple entry points per page is allowed in webpack, it should be avoided when possible in favor of an entry point with multiple imports: entry: { page: ['./analytics', './app'] }. This results in a better optimization and consistent execution order when using async script tags.

SplitChunksPlugin

The SplitChunksPlugin allows us to extract common dependencies into an existing entry chunk or an entirely new chunk. Let's use this to de-duplicate the lodash dependency from the previous example:

webpack.config.js

  const path = require('path');

  module.exports = {
    mode: 'development',
    entry: {
      index: './src/index.js',
      another: './src/another-module.js',
    },
    output: {
      filename: '[name].bundle.js',
      path: path.resolve(__dirname, 'dist'),
    },
+   optimization: {
+     splitChunks: {
+       chunks: 'all',
+     },
+   },
  };

With the optimization.splitChunks configuration option in place, we should now see the duplicate dependency removed from our index.bundle.js and another.bundle.js. The plugin should notice that we've separated lodash out to a separate chunk and remove the dead weight from our main bundle. However, it's important to note that common dependencies are only extracted into a separate chunk if they meet the size thresholds specified by webpack.

Let's do an npm run build to see if it worked:

...
[webpack-cli] Compilation finished
asset vendors-node_modules_lodash_lodash_js.bundle.js 549 KiB [compared for emit] (id hint: vendors)
asset index.bundle.js 8.92 KiB [compared for emit] (name: index)
asset another.bundle.js 8.8 KiB [compared for emit] (name: another)
Entrypoint index 558 KiB = vendors-node_modules_lodash_lodash_js.bundle.js 549 KiB index.bundle.js 8.92 KiB
Entrypoint another 558 KiB = vendors-node_modules_lodash_lodash_js.bundle.js 549 KiB another.bundle.js 8.8 KiB
runtime modules 7.64 KiB 14 modules
cacheable modules 530 KiB
  ./src/index.js 257 bytes [built] [code generated]
  ./src/another-module.js 84 bytes [built] [code generated]
  ./node_modules/lodash/lodash.js 530 KiB [built] [code generated]
webpack 5.4.0 compiled successfully in 241 ms

Here are some other useful plugins and loaders provided by the community for splitting code:

mini-css-extract-plugin: Useful for splitting CSS out from the main application.

Dynamic Imports

Two similar techniques are supported by webpack when it comes to dynamic code splitting. The first and recommended approach is to use the require.ensure. Let's try using the first of these two approaches...

Before we start, let's remove the extra entry and optimization.splitChunks from our configuration in the above example as they won't be needed for this next demonstration:

webpack.config.js

 const path = require('path');

 module.exports = {
   mode: 'development',
   entry: {
     index: './src/index.js',
-    another: './src/another-module.js',
   },
   output: {
     filename: '[name].bundle.js',
     path: path.resolve(__dirname, 'dist'),
   },
-  optimization: {
-    splitChunks: {
-      chunks: 'all',
-    },
-  },
 };

We'll also update our project to remove the now unused files:

project

webpack-demo
|- package.json
|- package-lock.json
|- webpack.config.js
|- /dist
|- /src
  |- index.js
- |- another-module.js
|- /node_modules

Now, instead of statically importing lodash, we'll use dynamic importing to separate a chunk:

src/index.js

-import _ from 'lodash';
-
-function component() {
+function getComponent() {
-  const element = document.createElement('div');

-  / Lodash, now imported by this script
-  element.innerHTML = _.join(['Hello', 'webpack'], ' ');
+  return import('lodash')
+    .then(({ default: _ }) => {
+      const element = document.createElement('div');
+
+      element.innerHTML = _.join(['Hello', 'webpack'], ' ');

-  return element;
+      return element;
+    })
+    .catch((error) => 'An error occurred while loading the component');
 }

-document.body.appendChild(component());
+getComponent().then((component) => {
+  document.body.appendChild(component);
+});

The reason we need default is that since webpack 4, when importing a CommonJS module, the import will no longer resolve to the value of module.exports, it will instead create an artificial namespace object for the CommonJS module. For more information on the reason behind this, read webpack 4: import() and CommonJs.

Let's run webpack to see lodash separated out to a separate bundle:

...
[webpack-cli] Compilation finished
asset vendors-node_modules_lodash_lodash_js.bundle.js 549 KiB [compared for emit] (id hint: vendors)
asset index.bundle.js 13.5 KiB [compared for emit] (name: index)
runtime modules 7.37 KiB 11 modules
cacheable modules 530 KiB
  ./src/index.js 434 bytes [built] [code generated]
  ./node_modules/lodash/lodash.js 530 KiB [built] [code generated]
webpack 5.4.0 compiled successfully in 268 ms

As import() returns a promise, it can be used with async functions. Here's how it would simplify the code:

src/index.js

-function getComponent() {
+async function getComponent() {
+  const element = document.createElement('div');
+  const { default: _ } = await import('lodash');

-  return import('lodash')
-    .then(({ default: _ }) => {
-      const element = document.createElement('div');
+  element.innerHTML = _.join(['Hello', 'webpack'], ' ');

-      element.innerHTML = _.join(['Hello', 'webpack'], ' ');
-
-      return element;
-    })
-    .catch((error) => 'An error occurred while loading the component');
+  return element;
 }

 getComponent().then((component) => {
   document.body.appendChild(component);
 });

Prefetching/Preloading modules

Webpack 4.6.0+ adds support for prefetching and preloading.

Using these inline directives while declaring your imports allows webpack to output “Resource Hint” which tells the browser that for:

prefetch: resource is probably needed for some navigation in the future
preload: resource will also be needed during the current navigation

An example of this is having a HomePage component, which renders a LoginButton component which then on demand loads a LoginModal component after being clicked.

LoginButton.js

/...
import(/* webpackPrefetch: true */ './path/to/LoginModal.js');

This will result in <link rel="prefetch" href="login-modal-chunk.js"> being appended in the head of the page, which will instruct the browser to prefetch in idle time the login-modal-chunk.js file.

Preload directive has a bunch of differences compared to prefetch:

A preloaded chunk starts loading in parallel to the parent chunk. A prefetched chunk starts after the parent chunk finishes loading.
A preloaded chunk has medium priority and is instantly downloaded. A prefetched chunk is downloaded while the browser is idle.
A preloaded chunk should be instantly requested by the parent chunk. A prefetched chunk can be used anytime in the future.
Browser support is different.

An example of this can be having a Component which always depends on a big library that should be in a separate chunk.

Let's imagine a component ChartComponent which needs a huge ChartingLibrary. It displays a LoadingIndicator when rendered and instantly does an on demand import of ChartingLibrary:

ChartComponent.js

/...
import(/* webpackPreload: true */ 'ChartingLibrary');

When a page which uses the ChartComponent is requested, the charting-library-chunk is also requested via <link rel="preload">. Assuming the page-chunk is smaller and finishes faster, the page will be displayed with a LoadingIndicator, until the already requested charting-library-chunk finishes. This will give a little load time boost since it only needs one round-trip instead of two. Especially in high-latency environments.

Sometimes you need to have your own control over preload. For example, preload of any dynamic import can be done via async script. This can be useful in case of streaming server side rendering.

const lazyComp = () =>
  import('DynamicComponent').catch(error) => {
    / Do something with the error.
    / For example, we can retry the request in case of any net error
  });

If the script loading will fail before webpack starts loading of that script by itself (webpack creates a script tag to load its code, if that script is not on a page), that catch handler won't start till chunkLoadTimeout is not passed. This behavior can be unexpected. But it's explainable — webpack can not throw any error, cause webpack doesn't know, that script failed. Webpack will add onerror handler to the script right after the error has happen.

To prevent such problem you can add your own onerror handler, which removes the script in case of any error:

<script
  src="https://example.com/dist/dynamicComponent.js"
  async
  onerror="this.remove()"
></script>

In that case, errored script will be removed. Webpack will create its own script and any error will be processed without any timeouts.

Bundle Analysis

Once you start splitting your code, it can be useful to analyze the output to check where modules have ended up. The official analyze tool is a good place to start. There are some other community-supported options out there as well:

webpack-chart: Interactive pie chart for webpack stats.
webpack-visualizer: Visualize and analyze your bundles to see which modules are taking up space and which might be duplicates.
webpack-bundle-analyzer: A plugin and CLI utility that represents bundle content as a convenient interactive zoomable treemap.
webpack bundle optimize helper: This tool will analyze your bundle and give you actionable suggestions on what to improve to reduce your bundle size.
bundle-stats: Generate a bundle report(bundle size, assets, modules) and compare the results between different builds.
webpack-stats-viewer: A plugin with build for webpack stats. Show more information about webpack bundle detail.

Next Steps

See Lazy Loading for a more concrete example of how import() can be used in a real application and Caching to learn how to split code more effectively.

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