Understanding Concurrent Mode: The Future of Rendering in React

Published at: January 21, 2021

One of the most exciting and anticipated features in React is Concurrent Mode. This feature revolutionizes the way React renders and updates the user interface, making applications faster, more responsive, and more efficient. In this post, we’ll dive into what Concurrent Mode is, how it works, and how it improves the performance and responsiveness of React apps.

What is Concurrent Mode?

Concurrent Mode is a set of new features in React that enables React to render content in the background without blocking user interactions. In traditional rendering, React updates the UI synchronously, meaning that all changes need to be processed immediately, potentially causing delays or blocking the user interface (UI) while complex updates are being processed.

With Concurrent Mode, React can interrupt rendering to give priority to more urgent tasks, such as responding to user interactions like clicks or keystrokes, and then resume rendering the less urgent work in the background.

It’s as if React has gained the ability to multitask: it can do the heavy lifting (like rendering a complex component) while still keeping the app interactive, prioritizing user actions and making the app feel snappy and responsive.

How Concurrent Mode Works

To enable Concurrent Mode, React allows developers to prioritize different tasks and operations. The concept of “rendering work” is now handled differently, allowing React to split it up and work on it incrementally. It is also important to note that Concurrent Mode is opt-in, meaning that developers can enable it as needed, rather than it being on by default.

Key features of Concurrent Mode include:

  • Interruptible Rendering: React can pause work on one part of the UI (for example, rendering a list of items) and switch to another part (such as a button click) that requires immediate attention. This ensures that user interactions are never blocked by ongoing renders.
  • Prioritizing User Interactions: React gives high priority to updates that come from user interactions, like typing or clicking, and low priority to updates like lazy loading or non-urgent updates.
  • Incremental Rendering: React can break up the rendering process into smaller chunks, allowing for better performance and smoother updates.

How Concurrent Mode Improves Responsiveness and Performance

1. Reduced Blocking
In the past, React would block rendering updates to perform heavy computations. This could lead to long periods of unresponsiveness, like when a page was loading or when the app was fetching large amounts of data. With Concurrent Mode, React can handle this work in the background, ensuring that the user interface remains responsive and interactive.

For example, when an app is fetching a large dataset, React can still respond to user inputs, such as typing or clicking, without delay. The data can be rendered incrementally as it becomes available, which reduces the “jank” (lag or stuttering) users experience when interacting with the page.

2. Smarter Updates with Prioritization
By allowing React to decide which updates are the most important (like a user pressing a button) and which can be delayed (like rendering a list of items), Concurrent Mode ensures that important updates are processed first. It eliminates unnecessary delays caused by low-priority tasks and helps keep the app feeling faster.

For example, if a user starts typing into a search bar, Concurrent Mode ensures that typing remains responsive, even if React is in the middle of rendering another part of the app (like a list of results).

3. Better Handling of Complex UIs
For large, complex UIs, Concurrent Mode allows React to prioritize which parts of the UI are updated first, improving the overall perceived performance. In traditional rendering, React would try to update the whole UI at once, but Concurrent Mode allows React to break the update process into smaller chunks.

This chunked rendering allows the UI to remain responsive while React works on rendering parts of the page incrementally, so users don’t experience long loading times or freezes.

4. Seamless Transitions Between States
Concurrent Mode makes transitions between different states of the UI smoother by allowing React to pause and resume rendering. This is especially useful when transitioning between pages or when showing animations.

For example, when navigating between different parts of an app or switching tabs, Concurrent Mode ensures that the transition feels seamless, without any noticeable delay or jankiness.


Example of Enabling Concurrent Mode

Enabling Concurrent Mode is easy in React 18. Here’s how you can do it:

  1. First, you need to wrap your app in the ConcurrentMode root.
import ReactDOM from 'react-dom';
import App from './App';

ReactDOM.createRoot(document.getElementById('root')).render(
  <React.StrictMode>
    <App />
  </React.StrictMode>
);
  1. Once you’ve enabled it, React will automatically begin running in Concurrent Mode for the components in your app. The feature is opt-in, so you can gradually enable it to see how it affects performance.

Conclusion

Concurrent Mode is set to redefine the way React handles rendering, offering a more responsive and performant experience. It allows React to handle complex rendering tasks without blocking critical updates and ensures that apps feel faster and smoother, even during heavy computations.

By enabling Concurrent Mode, developers gain the ability to fine-tune their apps for better user experiences, especially in larger, more complex React applications.

If you haven’t already, now is a great time to start exploring Concurrent Mode in React 18. As more React features are introduced, we’ll see even more ways to take advantage of this revolutionary approach to rendering.

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