React Concurrent Mode: What It Is and How to Use It

React Concurrent Mode is an experimental feature designed to improve the performance and responsiveness of React applications by allowing React to interrupt and prioritize updates. Although it is still evolving, Concurrent Mode introduces powerful capabilities that can make your React applications faster and more fluid. In this post, we’ll explore what Concurrent Mode is, how it works, and how to use it effectively in your applications.

Table of Contents

1. Introduction to Concurrent Mode
2. Key Features and Benefits
   • Concurrent Rendering
   • Suspense for Data Fetching
   • Prioritizing Updates
3. Setting Up Concurrent Mode
4. How to Use Concurrent Mode
   • Basic Usage
   • Advanced Usage
5. Best Practices and Considerations
6. Testing and Debugging with Concurrent Mode
7. Conclusion

---

1. Introduction to Concurrent Mode

Concurrent Mode is a set of new features in React that allows React to interrupt and resume work on tasks, prioritizing the most important updates. This capability aims to make applications more responsive and provide a smoother user experience by managing how and when updates are rendered.

Why Concurrent Mode?

Traditional React rendering is synchronous, meaning that React processes updates in a blocking manner. This can lead to performance issues, particularly in complex applications where multiple updates may occur simultaneously. Concurrent Mode addresses these issues by enabling React to work on multiple tasks concurrently and prioritize rendering based on the importance of updates.

2. Key Features and Benefits

Concurrent Rendering

Concurrent Rendering allows React to pause, resume, or discard work based on the priority of updates. This means React can start rendering higher-priority updates (e.g., user interactions) before lower-priority ones (e.g., data fetching).

Example: Concurrent Rendering

import { createRoot } from ‘react-dom/client’;
import App from ‘./App’;

// Enable Concurrent Mode
const root = createRoot(document.getElementById(‘root’));
root.render();

With Concurrent Mode enabled, React can manage updates more efficiently, improving the perceived performance of your application.

Suspense for Data Fetching

Suspense, when used with Concurrent Mode, allows you to declaratively handle asynchronous operations such as data fetching. By wrapping components with <Suspense>, you can provide fallback UIs while data is being loaded.

Example: Suspense for Data Fetching

import React, { Suspense, lazy } from ‘react’;

// Lazy load a component
const LazyComponent = lazy(() => import(‘./LazyComponent’));

const App = () => (
Loading…}>
);

export default App;

In this example, <LazyComponent> will be loaded asynchronously, and a fallback UI is displayed until the component is ready.

Prioritizing Updates

Concurrent Mode allows React to prioritize updates based on their urgency. For instance, React can prioritize user interactions like clicks and typing over less critical updates like background data fetching.

Example: Prioritizing Updates

import { useTransition } from ‘react’;

const MyComponent = () => {
const [isPending, startTransition] = useTransition();

const handleClick = () => {
startTransition(() => {
// Perform a state update that is less urgent
// e.g., updating a large list
});
};

return (
Update List

{isPending && Loading…}
);
};

export default MyComponent;

useTransition helps you mark certain updates as non-urgent, allowing React to prioritize more critical updates.

3. Setting Up Concurrent Mode

Concurrent Mode is still considered experimental, and enabling it requires some configuration. Here’s how to set it up:

Install React

Make sure you have the latest versions of React and ReactDOM:

npm install react react-dom

Enable Concurrent Mode

Use createRoot from the react-dom/client package to enable Concurrent Mode:

import { createRoot } from ‘react-dom/client’;
import App from ‘./App’;

const root = createRoot(document.getElementById(‘root’));
root.render();

This enables Concurrent Mode, allowing React to handle updates in a more sophisticated manner.

4. How to Use Concurrent Mode

Basic Usage

Start by enabling Concurrent Mode as shown above. Then, use features like Suspense and useTransition to manage asynchronous operations and prioritize updates.

Advanced Usage

For more advanced scenarios, you can leverage features like useDeferredValue and useTransition:

• useDeferredValue: Allows you to defer updates to non-critical state, improving performance.

import React, { useDeferredValue, useState } from ‘react’;

const MyComponent = () => {
const [input, setInput] = useState(”);
const deferredInput = useDeferredValue(input);

return (

setInput(e.target.value)} />
);
};

export default MyComponent;

useTransition: Helps manage transitions for state updates that can be deferred.

import React, { useTransition } from ‘react’;

const MyComponent = () => {
const [isPending, startTransition] = useTransition();

const handleClick = () => {
startTransition(() => {
// Update state in a non-urgent manner
});
};

return (
Start Transition

{isPending && Loading…}
);
};

export default MyComponent;

5. Best Practices and Considerations

• Feature Flags: Since Concurrent Mode is experimental, use feature flags to toggle it on and off in your application.
• Incremental Adoption: Gradually introduce Concurrent Mode features into your application to manage complexity.
• Monitor Performance: Regularly monitor and test the performance impacts of Concurrent Mode features.

6. Testing and Debugging with Concurrent Mode

Testing components using Concurrent Mode can be challenging. Tools like React Testing Library and React DevTools can help:

• React Testing Library: Write tests that simulate user interactions and validate behavior under Concurrent Mode.
• React DevTools: Use React DevTools to inspect the state and behavior of Concurrent Mode features.

7. Conclusion

React Concurrent Mode is a groundbreaking feature that enhances the performance and responsiveness of React applications by managing updates more effectively. By enabling Concurrent Mode, utilizing features like Suspense, useTransition, and useDeferredValue, you can build more fluid and efficient applications.

As Concurrent Mode evolves, it will continue to offer new possibilities for optimizing React applications. Stay updated with the latest React releases and best practices to make the most of these powerful features. Happy coding!