Code-split JavaScript

Loading large JavaScript resources impacts page speed significantly. Splitting your JavaScript into smaller chunks and only downloading what is necessary for a page to function during startup can greatly improve your page's load responsiveness, which in turn can improve your page's Interaction to Next Paint (INP).

As a page downloads, parses, and compiles large JavaScript files, it can become unresponsive for periods of time. The page elements are visible, as they are a part of a page's initial HTML and styled by CSS. However, because the JavaScript required to power those interactive elements—as well as other scripts loaded by the page—may be parsing and executing the JavaScript for them to function. The result is that the user may feel as though the interaction was significantly delayed, or even altogether broken.

This often happens because the main thread is blocked, as JavaScript is parsed and compiled on the main thread. If this process takes too long, interactive page elements may not respond quickly enough to user input. One remedy for this is to load only the JavaScript you need for the page to function, while deferring other JavaScript to load later on through a technique known as code splitting. This module focuses on the latter of these two techniques.

Reduce JavaScript parsing and execution during startup through code splitting

Lighthouse throws a warning when JavaScript execution takes longer than 2 seconds, and fails when it takes more than 3.5 seconds. Excessive JavaScript parsing and execution is a potential problem at any point in the page lifecycle, as it has the potential to increase an interaction's input delay if the time at which the user interacts with the page coincides with the moment the main thread tasks responsible for processing and executing JavaScript are running.

More than that, excessive JavaScript execution and parsing is particularly problematic during the initial page load, as this is the point in the page lifecycle that users are quite likely to interact with the page. In fact, Total Blocking Time (TBT)—a load responsiveness metric—is highly correlated with INP, suggesting that users have a high tendency to attempt interactions during the initial page load.

The Lighthouse audit that reports the time spent executing each JavaScript file that your page requests is useful in that it can help you identify exactly which scripts may be candidates for code splitting. You can then go further by using the coverage tool in Chrome DevTools to identify exactly which parts of a page's JavaScript go unused during page load.

Code splitting is a useful technique that can reduce a page's initial JavaScript payloads. It lets you split a JavaScript bundle into two parts:

  • The JavaScript needed at page load, and therefore can't be loaded at any other time.
  • Remaining JavaScript that can be loaded at a later point in time, most often at the point in which the user interacts with a given interactive element on the page.

Code splitting can be done by using the dynamic import() syntax. This syntax—unlike <script> elements which requests a given JavaScript resource during startup—makes a request for a JavaScript resource at a later point during the page lifecycle.

document.querySelectorAll('#myForm input').addEventListener('blur', async () => {
  // Get the form validation named export from the module through destructuring:
  const { validateForm } = await import('/validate-form.mjs');

  // Validate the form:
}, { once: true });

In the preceding JavaScript snippet, the validate-form.mjs module is downloaded, parsed, and executed only when a user blurs any of a form's <input> fields. In this situation, the JavaScript resource responsible for driving the form's validation logic is only ever involved with the page when it is most likely to be actually used.

JavaScript bundlers like webpack, Parcel, Rollup, and esbuild can be configured to split JavaScript bundles into smaller chunks whenever they encounter a dynamic import() call in your source code. Most of these tools do this automatically, but esbuild in particular requires you to opt into this optimization.

Helpful notes on code splitting

While code splitting is an effective method of reducing main thread contention during the initial page load, it pays to keep a few things in mind if you decide to go audit your JavaScript source code for code splitting opportunities.

Use a bundler if you can

It's common practice for developers to use JavaScript modules during the development process. It's an excellent developer experience improvement that improves code readability and maintainability. However, there are some suboptimal performance characteristics that can result when shipping JavaScript modules to production.

Most importantly, you should use a bundler to process and optimize your source code, including modules you intend to code split. Bundlers are very effective at not just applying optimizations to JavaScript source code, but they are also quite effective at balancing performance considerations such as bundle size against compression ratio. Compression effectiveness increases with bundle size, but bundlers also try to ensure that bundles aren't so large that they incur long tasks due to script evaluation.

Bundlers also avoid the problem of shipping a large number of unbundled modules over the network. Architectures that use JavaScript modules tend to have large, complex module trees. When module trees are unbundled, each module represents a separate HTTP request, and interactivity in your web app may be delayed if you don't bundle modules. While it's possible to use the <link rel="modulepreload"> resource hint to load large module trees as early as possible, JavaScript bundles are still preferable from a loading performance standpoint.

Don't inadvertently disable streaming compilation

Chromium's V8 JavaScript engine offers a number of optimizations out of the box to ensure that your production JavaScript code loads as efficiently as possible. One of these optimizations is known as streaming compilation which—like the incremental parsing of HTML streamed to the browser—compiles streamed chunks of JavaScript as they arrive from the network.

You have a couple of ways to ensure that streaming compilation occurs for your web application in Chromium:

  • Transform your production code to avoid using JavaScript modules. Bundlers can transform your JavaScript source code based on a compilation target, and the target is often specific to a given environment. V8 will apply streaming compilation to any JavaScript code that doesn't use modules, and you can configure your bundler to transform your JavaScript module code into a syntax that doesn't use JavaScript modules and their features.
  • If you want to ship JavaScript modules to production, use the .mjs extension. Whether or not your production JavaScript uses modules, there's no special content type for JavaScript that uses modules versus JavaScript that does not. Where V8 is concerned, you effectively opt out of streaming compilation when you ship JavaScript modules in production using the .js extension. If you use the .mjs extension for JavaScript modules, V8 can ensure that streaming compilation for module-based JavaScript code isn't broken.

Don't let these considerations dissuade you from using code splitting. Code splitting is an effective way of reducing initial JavaScript payloads to users, but by using a bundler and knowing how you can preserve V8's streaming compilation behavior, you can ensure that your production JavaScript code is as fast for users as it can be.

Dynamic import demo


webpack ships with a plugin named SplitChunksPlugin, which lets you configure how the bundler splits JavaScript files. webpack recognizes both the dynamic import() and static import statements. The behavior of SplitChunksPlugin can be modified by specifying the chunks option in its configuration:

  • chunks: async is the default value, and refers to dynamic import() calls.
  • chunks: initial refers to static import calls.
  • chunks: all covers both dynamic import() and static imports, allowing you to share chunks between async and initial imports.

By default, whenever webpack encounters a dynamic import() statement. it creates a separate chunk for that module:

/* main.js */

// An application-specific chunk required during the initial page load:
import myFunction from './my-function.js';

myFunction('Hello world!');

// If a specific condition is met, a separate chunk is downloaded on demand,
// rather than being bundled with the initial chunk:
if (condition) {
  // Assumes top-level await is available. More info:
  await import('/form-validation.js');

The default webpack configuration for the preceding code snippet results in two separate chunks:

  • The main.js chunk—which webpack classifies as an initial chunk—that includes main.js and ./my-function.js module.
  • The async chunk, which includes only form-validation.js (containing a file hash in the resource name if configured). This chunk is only downloaded if and when condition is truthy.

This configuration lets you defer loading the form-validation.js chunk until it's actually needed. This can improve load responsiveness by reducing script evaluation time during the initial page load. Script download and evaluation for the form-validation.js chunk occurs when a specified condition is met, in which case, the dynamically imported module is downloaded. One example may be a condition where a polyfill is only downloaded for a particular browser, or—as in the earlier example—the imported module is necessary for a user interaction.

On the other hand, changing the SplitChunksPlugin configuration to specify chunks: initial ensures that code is split only on initial chunks. These are chunks such as those statically imported, or listed in webpack's entry property. Looking at the preceding example, the resulting chunk would be a combination of form-validation.js and main.js in a single script file, resulting in potentially worse initial page load performance.

The options for SplitChunksPlugin can also be configured to separate larger scripts into multiple smaller ones—for example by using the maxSize option to instruct webpack to split chunks into separate files if they exceed what is specified by maxSize. Dividing large script files into smaller files can improve load responsiveness, as in some cases CPU-intensive script evaluation work is divided into smaller tasks, which are less likely to block the main thread for longer periods of time.

Additionally, generating larger JavaScript files also means that scripts are more likely to suffer from cache invalidation. For example, if you ship a very large script with both framework and first-party application code, the entire bundle can be invalidated if only the framework is updated, but nothing else in the bundled resource.

On the other hand, smaller script files increases the likelihood that a return visitor retrieves resources from the cache, resulting in faster page loads on repeat visits. However, smaller files benefit less from compression than larger ones, and may increase network round-trip time on page loads with an unprimed browser cache. Care must be taken to strike a balance between caching efficiency, compression effectiveness, and script evaluation time.

webpack demo

webpack SplitChunksPlugin demo.

Test your knowledge

Which type of import statement is used when performing code splitting?

Dynamic import().
Static import.
Try again.

Which type of import statement must be at the top of a JavaScript module, and in no other location?

Dynamic import().
Try again.
Static import.

When using SplitChunksPlugin in webpack, what is the difference between an async chunk and an initial chunk?

async chunks are loaded using dynamic import() and initial chunks are loaded using static import.
async chunks are loaded using static import and initial chunks are loaded using dynamic import().
Try again.

Up next: Lazy load images and <iframe> elements

Though it tends to be a fairly expensive type of resource, JavaScript isn't the only resource type you can defer the loading of. Image and <iframe> elements are potentially costly resources in their own right. Similar to JavaScript, you can defer the loading of images and <iframe> element by lazy loading them, which is explained in the next module of this course.