Dynamic Import

Import parts of your code on demand


In our chat application, we have four key components: UserInfo, ChatList, ChatInput and EmojiPicker. However, only three of these components are used instantly on the initial page load: UserInfo, ChatList and ChatInput. The EmojiPicker isn't directly visible, and may not even be rendered at all if the user won't even click on the Emoji in order to toggle the EmojiPicker. This would mean that we unnecessarily added the EmojiPicker module to our initial bundle, which potentially increased the loading time!

In order to solve this, we can dynamically import the EmojiPicker component. Instead of statically importing it, we'll only import it when we want to show the EmojiPicker. An easy way to dynamically import components in React is by using React Suspense. The React.Suspense component receives the component that should be dynamically loaded, which makes it possible for the App component can render its contents faster by suspending the import of the EmojiPicker module! When the user clicks on the emoji, the EmojiPicker component gets rendered for the first time. The EmojiPicker component renders a Suspense component, which receives the lazily imported module: the EmojiPicker in this case. The Suspense component accepts a fallback prop, which receives the component that should get rendered while the suspended component is still loading!

Instead of unnecessarily adding EmojiPicker to the initial bundle

, we can split it up into its own bundle
and reduce the size of the initial bundle
!

A smaller initial bundle size means a faster initial load: the user doesn't have to stare at a blank loading screen for as long. The fallback component lets the user know that our application hasn't frozen: they simply need to wait a little while for the module to be processed and executed.

Asset                             Size         Chunks            Chunk Names
emoji-picker.bundle.js           1.48 KiB      1    [emitted]    emoji-picker
main.bundle.js                   1.33 MiB      main [emitted]    main
vendors~emoji-picker.bundle.js   171 KiB       2    [emitted]    vendors~emoji-picker

Whereas previously the initial bundle

was 1.5MiB, we've been able to reduce it to 1.33 MiB by suspending the import of the EmojiPicker!

In the console, you can see that the EmojiPicker doesn't get executed until we've toggled the EmojiPicker!

ChatInput.js
import React, { Suspense, lazy } from "react";
// import Send from "./icons/Send";
// import Emoji from "./icons/Emoji";
const Send = lazy(() =>
import(/*webpackChunkName: "send-icon" */ "./icons/Send")
);
const Emoji = lazy(() =>
import(/*webpackChunkName: "emoji-icon" */ "./icons/Emoji")
);
// Lazy load EmojiPicker when <EmojiPicker /> renders
const Picker = lazy(() =>
import(/*webpackChunkName: "emoji-picker" */ "./EmojiPicker")
);
const ChatInput = () => {
const [pickerOpen, togglePicker] = React.useReducer(state => !state, false);
return (
<Suspense fallback={<p id="loading">Loading...</p>}>
<div className="chat-input-container">
<input type="text" placeholder="Type a message..." />
<Emoji onClick={togglePicker} />
{pickerOpen && <Picker />}
<Send />
</div>
</Suspense>
);
};
console.log("ChatInput loaded", Date.now());
export default ChatInput;

When building the application, we can see the different bundles

that Webpack created.

By dynamically importing the EmojiPicker component, we managed to reduce the initial bundle

size from 1.5MiB to 1.33 MiB! Although the user may still have to wait a while until the EmojiPicker has been fully loaded, we have improved the user experience by making sure the application is rendered and interactive while the user waits for the component to load.


Loadable Components

Server-side rendering doesn't support React Suspense (yet). A good alternative to React Suspense is the loadable-components library, which can be used in SSR applications.

ChatInput.js
import React from "react";
import loadable from "@loadable/component";
import Send from "./icons/Send";
import Emoji from "./icons/Emoji";
const EmojiPicker = loadable(() => import("./EmojiPicker"), {
fallback: <div id="loading">Loading...</div>
});
const ChatInput = () => {
const [pickerOpen, togglePicker] = React.useReducer(state => !state, false);
return (
<div className="chat-input-container">
<input type="text" placeholder="Type a message..." />
<Emoji onClick={togglePicker} />
{pickerOpen && <EmojiPicker />}
<Send />
</div>
);
};
export default ChatInput;

Similar to React Suspense, we can pass the lazily imported module to the loadable, which will only import the module once the EmojiPicker module is being requested! While the module is being loaded, we can render a fallback component.

Although loadable components are a great alternative to React Suspense for SSR applications, they're also useful in CSR applications in order to suspend the import of modules.

ChatInput.js
import React from "react";
import Send from "./icons/Send";
import Emoji from "./icons/Emoji";
import loadable from "@loadable/component";
const EmojiPicker = loadable(() => import("./components/EmojiPicker"), {
fallback: <p id="loading">Loading...</p>
});
const ChatInput = () => {
const [pickerOpen, togglePicker] = React.useReducer(state => !state, false);
return (
<div className="chat-input-container">
<input type="text" placeholder="Type a message..." />
<Emoji onClick={togglePicker} />
{pickerOpen && <EmojiPicker />}
<Send />
</div>
);
};
console.log("ChatInput loaded", Date.now());
export default ChatInput;