React Suspense: A Guide to Concurrent Rendering
React Suspense is one of the most powerful features introduced in React, designed to make asynchronous rendering more intuitive and improve user experience. It allows developers to build applications that handle asynchronous data fetching, code splitting, and more, with better performance and smoother transitions. This guide will delve into what React Suspense is, how it works, and how you can use it to implement concurrent rendering in your React applications.
M Zeeshan
8/16/20243 min read
What is React Suspense?
React Suspense is a mechanism that allows React components to “wait” for something (like data, images, or code) before they are rendered. Instead of blocking the entire rendering process while waiting, Suspense lets you display a fallback UI, such as a loading spinner, until the requested resource is ready.
Suspense plays a critical role in React’s concurrent rendering model, which is designed to keep your app responsive by rendering work in small chunks, interleaved with other tasks, rather than in one blocking operation.
Key Features of React Suspense:
Code Splitting: Load only the parts of your app that are needed, reducing initial load times.
Data Fetching: Wait for data to be fetched before rendering components.
Image Loading: Wait for images to load before displaying them.
Concurrent Rendering: Improve performance by managing how rendering work is broken down and executed.
How React Suspense Works
React Suspense is implemented using the Suspense component, which wraps around components that need to wait for some asynchronous operation to complete. When the child component is not yet ready to render, the Suspense component displays a fallback UI until the data or code is loaded.
Basic Example:
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import React, { Suspense } from 'react'; const LazyComponent = React.lazy(() => import('./LazyComponent')); function App() { return ( <Suspense fallback={<div>Loading...</div>}> <LazyComponent /> </Suspense> ); } export default App;
In this example, the LazyComponent is loaded lazily using React.lazy(). While the component is being fetched, the Suspense component displays a loading message (<div>Loading...</div>). Once the component is ready, it replaces the fallback with the loaded component.
Concurrent Rendering with React Suspense
Concurrent rendering is a new rendering paradigm introduced in React 18, where React can pause and resume rendering work. This allows the UI to remain responsive while heavy tasks are being processed.
Benefits of Concurrent Rendering:
Non-blocking Updates: User interactions like typing, clicking, or scrolling are prioritized, keeping the app responsive.
Interruptible Work: React can pause work on one task and switch to a more critical task, like updating the UI in response to user input.
Progressive Loading: Components can progressively load content, improving perceived performance.
React Suspense is a key part of enabling concurrent rendering, as it allows you to manage the loading states of your components gracefully.
Example:
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import { Suspense, useState, useTransition } from 'react'; function App() { const [isPending, startTransition] = useTransition(); const [resource, setResource] = useState(null); const fetchData = () => { startTransition(() => { // Simulate data fetching setResource(fetchDataFromAPI()); }); }; return ( <div> <button onClick={fetchData}>Fetch Data</button> {isPending ? <div>Loading...</div> : <div>Data Loaded</div>} <Suspense fallback={<div>Loading data...</div>}> {resource && <ResourceComponent resource={resource} />} </Suspense> </div> ); }
In this example, useTransition is used to defer the state update for fetching data. The Suspense component then handles the rendering of the ResourceComponent, showing a loading state until the data is ready. This pattern helps in keeping the UI responsive even when performing asynchronous operations.
Data Fetching with React Suspense
React Suspense can also be used for data fetching by pairing it with libraries like React Query, SWR, or custom hooks. The idea is to allow your components to “suspend” rendering until the data they need is available.
Using Suspense with a Custom Hook:
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function useData() { let dataPromise = fetchDataFromAPI(); if (!dataPromise) { throw dataPromise; } return dataPromise; } function DataComponent() { const data = useData(); return <div>{data}</div>; } function App() { return ( <Suspense fallback={<div>Loading data...</div>}> <DataComponent /> </Suspense> ); }
Here, useData is a custom hook that throws a promise when data is being fetched. The Suspense component catches this promise and shows a fallback UI until the promise resolves.
Advanced Use Cases
1. Suspense for Code Splitting
Suspense can be used to load different parts of your application dynamically. This is especially useful for large applications where loading everything upfront would lead to poor performance.
Example:
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const Home = React.lazy(() => import('./Home')); const About = React.lazy(() => import('./About')); function App() { return ( <Suspense fallback={<div>Loading...</div>}> <Switch> <Route path="/" component={Home} exact /> <Route path="/about" component={About} /> </Switch> </Suspense> ); }
This approach ensures that only the code necessary for the current route is loaded, which speeds up initial load times and reduces the app’s overall bundle size.
2. Error Boundaries with Suspense
Combining Suspense with error boundaries allows you to handle errors gracefully when loading components or fetching data.
Example:
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class ErrorBoundary extends React.Component { state = { hasError: false }; static getDerivedStateFromError() { return { hasError: true }; } render() { if (this.state.hasError) { return <div>Something went wrong.</div>; } return this.props.children; } } function App() { return ( <ErrorBoundary> <Suspense fallback={<div>Loading...</div>}> <LazyComponent /> </Suspense> </ErrorBoundary> ); }
This pattern helps in providing a better user experience by displaying a user-friendly message when something goes wrong during the loading of components.