Understanding React Concurrent Mode

React Concurrent Mode is a set of new features in React designed to improve the responsiveness and user experience of React applications by allowing React to work in a more efficient, interruptible, and asynchronous way. It helps React apps remain fast and responsive, even when rendering large or complex UIs.

Unlike the traditional blocking rendering mode, Concurrent Mode allows React to split rendering work into chunks, prioritize updates, and keep the app responsive by pausing and resuming work when needed.

In this guide, we will dive into what Concurrent Mode is, why it matters, and how to enable and use it in React applications.

1. What is React Concurrent Mode?

React Concurrent Mode is an experimental feature that enables React to work in the background, without blocking the main thread, to prepare updates for the UI in a non-blocking, asynchronous manner.

In traditional React, rendering is a synchronous operation, meaning React must finish rendering one component before it can proceed to the next one. If there are large updates or computations, this can lead to janky UIs or slow performance, especially for users with low-powered devices or slow networks.

Concurrent Mode allows React to:

Interrupt rendering when necessary (e.g., when there’s user input).
Prioritize updates and render critical updates first (e.g., user interactions).
Pause and resume rendering to keep the app responsive and avoid blocking the main thread.

2. Key Features of Concurrent Mode

Concurrent Mode introduces several key features that enhance performance and user experience:

a. Interruptible Rendering

React can pause the rendering of one component to allow higher-priority updates (such as user interactions) to be processed first. Once the higher-priority update is complete, React can resume the interrupted work.

b. Prioritization

Updates can be assigned different priority levels. React ensures that critical updates, such as user input or animations, are processed before less important tasks, like background data fetching or UI updates.

c. Suspense for Data Fetching

Concurrent Mode works seamlessly with Suspense for managing asynchronous data loading. It allows components to “wait” for data before rendering, but in a non-blocking way. Suspense enables React to keep the UI responsive and display loading states for components that are waiting for data.

d. Concurrent Rendering

Instead of waiting for the entire component tree to render, React can break the rendering work into smaller chunks and spread it out over multiple frames. This means that even during large UI updates, React can continue to respond to user interactions.

3. How Does Concurrent Mode Work?

React’s default rendering behavior is synchronous: React processes the entire component tree, and only after it’s done does it update the DOM.

In Concurrent Mode, rendering is divided into units of work that can be scheduled and prioritized. For example, if a user types into an input field, React can pause a large render (such as a list of items) and process the user’s input immediately.

React can pause work, continue rendering, and even abandon work if it’s no longer needed (e.g., if the user navigates away from the component that is being rendered).

4. Enabling Concurrent Mode

As of now, Concurrent Mode is an experimental feature and needs to be explicitly enabled. To use Concurrent Mode, you need to use the ReactDOM.createRoot() API (available in React 18 and later). This replaces the traditional ReactDOM.render() API.

Example of Enabling Concurrent Mode

import React from 'react';
import ReactDOM from 'react-dom';

// Enable Concurrent Mode with createRoot
const root = ReactDOM.createRoot(document.getElementById('root'));

root.render(<App />);

Important Notes:

Concurrent Mode is not the default mode in React; it must be enabled explicitly.
As of React 18, Concurrent Mode is still experimental, so it may be subject to changes in future releases.

5. Suspense and Concurrent Mode

Suspense is a core feature of React Concurrent Mode that allows components to wait for data before rendering, without blocking the entire UI. In a traditional React app, loading data asynchronously can cause “flashes” or layout shifts while the data is fetched. With Suspense, React can show a loading indicator until the data is ready, keeping the app responsive and providing a smoother user experience.

For example:

Example: Using Suspense with Concurrent Mode

import React, { Suspense } from 'react';

const DataFetchingComponent = React.lazy(() => import('./DataFetchingComponent'));

const App = () => {
  return (
    <div>
      <h1>Concurrent Mode with Suspense</h1>
      {/* Suspense wraps the lazy-loaded component */}
      <Suspense fallback={<div>Loading...</div>}>
        <DataFetchingComponent />
      </Suspense>
    </div>
  );
};

export default App;

Explanation:

React.lazy() is used to dynamically import the DataFetchingComponent.
The Suspense component wraps the lazy-loaded component and shows a loading indicator (<div>Loading...</div>) while the component is being fetched.
In Concurrent Mode, React can pause the rendering of other components while DataFetchingComponent is loading, ensuring a smoother UI.

6. Benefits of Concurrent Mode

Improved User Experience: React apps can remain responsive even during large or complex updates. High-priority tasks, like user interactions, are always prioritized.
Faster Initial Load: By splitting work into chunks, React can prioritize rendering the first meaningful paint, making your app feel faster.
Better Handling of Data Fetching: Suspense works with Concurrent Mode to ensure that data fetching does not block the UI, improving loading states.
Enhanced Animations: Concurrent Mode allows for smoother animations by prioritizing them over other updates.

7. Challenges of Concurrent Mode

While Concurrent Mode offers numerous advantages, it also comes with a few challenges:

Complexity: It may require you to rethink how you structure your components and data fetching. Components that work well in synchronous mode might need to be refactored to work well with Concurrent Mode.
Experimental Features: Since Concurrent Mode is still experimental, it is not recommended for production apps unless you are prepared for potential breaking changes or issues.
Third-party Libraries: Some third-party libraries might not be compatible with Concurrent Mode, so be mindful of how your dependencies interact with the new behavior.

8. Concurrent Mode Features to Look Out For

startTransition API: Allows you to mark updates as low-priority, letting React interrupt them if necessary.
Concurrent Suspense: Improves the experience of async data fetching by allowing React to show loading states while deferring non-urgent updates.
Concurrent Rendering for Large Apps: React can handle large, complex UI updates in smaller chunks, improving responsiveness.

9. When to Use Concurrent Mode?

While Concurrent Mode is experimental, it’s a powerful tool for improving user experience and performance. You might consider using Concurrent Mode if:

You are building a large-scale React application where performance is critical.
You need smooth user interactions (e.g., form inputs, animations) during heavy computations or data-fetching.
You are ready to handle the complexity and potential bugs associated with experimental features.