w3resource
React Tutorial

Code-Splitting


Bundling

Most React apps will have their files "bundled" using tools like Webpack or Browserify. Bundling is the process of following imported files and merging them into a single file: a "bundle". This bundle can then be included on a webpage to load an entire app at once.

Example

App:

// app.js
import { add } from './math.js';

console.log(add(16, 26)); // 42
// math.js
export function add(a, b) {
  return a + b;
}

Bundle:

function add(a, b) {
  return a + b;
}
console.log(add(16, 26)); // 42

Note:

Your bundles may end up looking a lot different than this.

Tools like Create React App, Next.js, Gatsby, already have Webpack setup out of the box to bundle your app.

Code Splitting

Bundling is great, but as your app grows, your bundle will grow too. Especially if you are including large third-party libraries. You need to keep an eye on the code you are including in your bundle so that you don't accidentally make it so large that your app takes a long time to load.

To avoid winding up with a large bundle, it's good to get ahead of the problem and start "splitting" your bundle. Code-Splitting is a feature supported by bundlers like Webpack and Browserify (via factor-bundle) which can create multiple bundles that can be dynamically loaded at runtime.

Code-splitting your app can help you "lazy-load" just the things that are currently needed by the user, which can dramatically improve the performance of your app. While you haven't reduced the overall amount of code in your app, you've avoided loading code that the user may never need and reduced the amount of code needed during the initial load.

import()

The best way to introduce code-splitting into your app is through the dynamic import() syntax.

Before:

import { add } from './math';
console.log(add(16, 26));

After:

import("./math").then(math => {
  console.log(math.add(16, 26));
});

Note:

The dynamic import() syntax is a ECMAScript (JavaScript) proposal not currently part of the language standard. It is expected to be accepted in the future.

When Webpack comes across this syntax, it automatically starts code-splitting your app. If you're using Create React App, this is already configured for you and you can start using it immediately. It's also supported out of the box in Next.js.

React.lazy

Note:

React.lazy and Suspense are not yet available for server-side rendering. If you want to do code-splitting in a server rendered app, we recommend Loadable Components. It has a nice guide for bundle splitting with server-side rendering.

The React.lazy function lets you render a dynamic import as a regular component.

Before:

import OtherComponent from './OtherComponent';
function MyComponent() {
  return (
   <div>
      <OtherComponent />
    </div>
  );
}

After:

const OtherComponent = React.lazy(() => import('./OtherComponent'));
function MyComponent() {
  return (
   <div>
      <OtherComponent />
    </div>
  );
}

This will automatically load the bundle containing the OtherComponent when this component gets rendered.

React.lazy takes a function that must call a dynamic import(). This must return a Promise which resolves to a module with a default export containing a React component.