lazy

lazy lets you defer loading component's code until it is rendered for the first time.

const SomeComponent = lazy(load)

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Reference

lazy(load)

Call lazy outside your components to declare a lazy-loaded React component:

import { lazy } from 'react';

const MarkdownPreview = lazy(() => import('./MarkdownPreview.js'));

See more examples below.

Parameters

• load: A function that returns a Promise or another thenable (a Promise-like object with a then method). React will not call load until the first time you attempt to render the returned component. After React first calls load, it will wait for it to resolve, and then render the resolved value's .default as a React component. Both the returned Promise and the Promise's resolved value will be cached, so React will not call load more than once. If the Promise rejects, React will throw the rejection reason for the nearest Error Boundary to handle.

Returns

lazy returns a React component you can render in your tree. While the code for the lazy component is still loading, attempting to render it will suspend. Use <Suspense> to display a loading indicator while it's loading.

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load function

Parameters

load receives no parameters.

Returns

You need to return a Promise or some other thenable (a Promise-like object with a then method). It needs to eventually resolve to an object whose .default property is a valid React component type, such as a function, memo, or a forwardRef component.

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Usage

Lazy-loading components with Suspense

Usually, you import components with the static import declaration:

import MarkdownPreview from './MarkdownPreview.js';

To defer loading this component's code until it's rendered for the first time, replace this import with:

import { lazy } from 'react';

const MarkdownPreview = lazy(() => import('./MarkdownPreview.js'));

This code relies on dynamic import(), which might require support from your bundler or framework. Using this pattern requires that the lazy component you're importing was exported as the default export.

Now that your component's code loads on demand, you also need to specify what should be displayed while it is loading. You can do this by wrapping the lazy component or any of its parents into a <Suspense> boundary:

<Suspense fallback={<Loading />}>
  <h2>Preview</h2>
  <MarkdownPreview />
 </Suspense>

In this example, the code for MarkdownPreview won't be loaded until you attempt to render it. If MarkdownPreview hasn't loaded yet, Loading will be shown in its place. Try ticking the checkbox:

import { useState, Suspense, lazy } from 'react';
import Loading from './Loading.js';

const MarkdownPreview = lazy(() => delayForDemo(import('./MarkdownPreview.js')));

export default function MarkdownEditor() {
  const [showPreview, setShowPreview] = useState(false);
  const [markdown, setMarkdown] = useState('Hello, **world**!');
  return (
    <>
      <textarea value={markdown} onChange={e => setMarkdown(e.target.value)} />
      <label>
        <input type="checkbox" checked={showPreview} onChange={e => setShowPreview(e.target.checked)} />
        Show preview
      </label>
      <hr />
      {showPreview && (
        <Suspense fallback={<Loading />}>
          <h2>Preview</h2>
          <MarkdownPreview markdown={markdown} />
        </Suspense>
      )}
    </>
  );
}

// Add a fixed delay so you can see the loading state
function delayForDemo(promise) {
  return new Promise(resolve => {
    setTimeout(resolve, 2000);
  }).then(() => promise);
}

export default function Loading() {
  return <p><i>Loading...</i></p>;
}

import { Remarkable } from 'remarkable';

const md = new Remarkable();

export default function MarkdownPreview({ markdown }) {
  return (
    <div
      className="content"
      dangerouslySetInnerHTML={{__html: md.render(markdown)}}
    />
  );
}

{
  "dependencies": {
    "immer": "1.7.3",
    "react": "latest",
    "react-dom": "latest",
    "react-scripts": "latest",
    "remarkable": "2.0.1"
  },
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test --env=jsdom",
    "eject": "react-scripts eject"
  }
}

label {
  display: block;
}

input, textarea {
  margin-bottom: 10px;
}

body {
  min-height: 200px;
}

This demo loads with an artificial delay. The next time you untick and tick the checkbox, Preview will be cached, so there will be no loading state. To see the loading state again, click "Reset" on the sandbox.

Learn more about managing loading states with Suspense.

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Troubleshooting

My lazy component's state gets reset unexpectedly

Do not declare lazy components inside other components:

import { lazy } from 'react';

function Editor() {
  // 🔴 Bad: This will cause all state to be reset on re-renders
  const MarkdownPreview = lazy(() => import('./MarkdownPreview.js'));
  // ...
}

Instead, always declare them at the top level of your module:

import { lazy } from 'react';

// ✅ Good: Declare lazy components outside of your components
const MarkdownPreview = lazy(() => import('./MarkdownPreview.js'));

function Editor() {
  // ...
}