React 18 New Features – Concurrent Rendering, Automatic Batching, and More

React 18, released in March 2022, delivered some of the most significant updates to the popular JavaScript library in years. This latest version focuses on enhancing performance and revamping React‘s rendering engine to power a new generation of UI development.

While React 18 packs numerous enhancements, a few headline features stand out: concurrent rendering, automatic batching, transitions, suspense on the server, and strict mode updates. These additions aim to optimize how React handles rendering and state management under the hood, paving the way for smoother, more responsive user experiences.

As an experienced full-stack developer who has worked extensively with React, I‘m excited to dive into the technical details and practical applications of React 18. In this comprehensive guide, we‘ll explore what makes this release so groundbreaking and how you can start leveraging its powerful new capabilities in your projects. Let‘s get started!

Concurrent Rendering: A Game-Changer for Performance

The crown jewel of React 18 is undoubtedly concurrent rendering. In a nutshell, concurrent rendering allows React to work on multiple state updates simultaneously, pausing and resuming render work as needed to avoid blocking the main thread.

To appreciate why this is such a big deal, we need to understand how rendering worked before React 18. Previously, once React started rendering an update, it couldn‘t be interrupted. Even if the user tried to interact with the app, they‘d be stuck waiting for the current render to finish. This model was simple to reason about but could lead to frustrating delays and unresponsive UIs.

Concurrent rendering takes a different approach, treating rendering as an interruptible process. Now, React can start working on an update, pause midway through to handle a more urgent interaction, then pick up where it left off. Imagine you‘re streaming a movie while cooking dinner. If the doorbell rings, you can pause the video, answer the door, then resume right where you left off. No need to wait for the credits to roll before attending to something more pressing.

Here‘s a simplified example to illustrate the difference. Say we have a search component that fetches results from an API as the user types:

function SearchResults({ query }) {
  const results = useSearch(query);
  return (
    <ul>
      {results.map(result => (
        <li key={result.id}>{result.title}</li>
      ))}
    </ul>
  );
}

With synchronous rendering, each keystroke would trigger a new render that couldn‘t be interrupted. If the user typed quickly or the API was slow, lag would start to creep in, degrading the experience.

Concurrent rendering allows React to work on these updates in the background, prioritizing more urgent interactions like keystrokes or clicks. Less critical work, like re-rendering the search results, can be paused and resumed as needed to keep things snappy.

Best of all, we don‘t need to drastically change our code to take advantage of concurrent rendering. By simply upgrading to React 18 and using the new createRoot API, we can unlock its benefits:

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

const container = document.getElementById(‘app‘);
const root = createRoot(container);
root.render(<App />);

While concurrent rendering is a major technological shift, the React team has taken care to make the upgrade path as smooth as possible. Most existing code should work without modification, though you may need to make some adjustments if you rely on certain lifecycle methods or external libraries.

Automatic Batching: Streamlining State Updates

Another key feature in React 18 is automatic batching. Batching refers to grouping multiple state updates into a single re-render to minimize wasted work. This technique is essential for keeping React fast and efficient, especially as apps grow in size and complexity.

In previous versions of React, batching was automatic inside event handlers but not in other contexts like promises, timeouts, or native event handlers. This inconsistency could lead to unnecessary re-renders and performance bottlenecks.

To illustrate, consider this example:

function MyComponent() {
  const [count, setCount] = useState(0);
  const [flag, setFlag] = useState(false);

  function handleClick() {
    fetchSomething().then(() => {
      // React 17: triggers 2 re-renders
      setCount(c => c + 1); 
      setFlag(f => !f);

      // React 18: triggers 1 re-render
      setCount(c => c + 1);
      setFlag(f => !f);
    });
  }

  return (
    // ...
  );
}

In React 17, the state updates inside the promise would trigger two separate re-renders, even though they could be safely combined. React 18 fixes this by automatically batching all state updates, regardless of where they originate.

This means that in the vast majority of cases, React 18 will automatically do the right thing, grouping state updates together for better performance. If you need to opt out of this behavior for some reason, you can use the new flushSync API to force synchronous updates:

import { flushSync } from ‘react-dom‘;

function handleClick() {
  flushSync(() => {
    setCount(c => c + 1);
  });
  // React has updated the DOM by now
  flushSync(() => {
    setFlag(f => !f);
  });
  // React has updated the DOM by now
}

However, this should be a last resort. In general, it‘s best to let React handle batching automatically to ensure consistent and predictable performance.

Transitions: Separating Urgent and Non-Urgent Updates

Speaking of predictable performance, React 18 also introduces a new concept called transitions. Transitions let you mark certain state updates as non-urgent, allowing React to work on them in the background without blocking more critical interactions.

A common use case for transitions is sorting or filtering a large dataset. While the user is typing into a search input or toggling a filter, we want to provide immediate feedback (like updating the input value or activating a loading spinner). However, re-rendering the full filtered list can be expensive, leading to lag and unresponsiveness if not managed carefully.

With React 18‘s startTransition API, we can wrap the non-urgent state update in a transition, signaling to React that it‘s okay to defer the work if more pressing updates come in:

import { startTransition } from ‘react‘;

function SearchResults({ query }) {
  const [isPending, startTransition] = useTransition();
  const [filterTerm, setFilterTerm] = useState(‘‘);

  const results = useSearch(query, filterTerm);

  function handleFilterChange(event) {
    // Urgent update: show what was typed
    setFilterTerm(event.target.value);

    // Transition: defer re-rendering the results
    startTransition(() => {
      setFilterTerm(event.target.value);
    });
  }

  return (
    <div>
      <input
        value={filterTerm}
        onChange={handleFilterChange}
      />
      {isPending ? (
        <div>Loading...</div>  
      ) : (
        <ul>
          {results.map(result => (
            <li key={result.id}>{result.title}</li>
          ))}
        </ul>
      )}
    </div>
  );
}

Here, updating the input value happens immediately on every keystroke, keeping the UI responsive. Meanwhile, the expensive filtering operation is wrapped in a transition, allowing React to prioritize the input updates and potentially delay the re-render if the user is typing quickly.

The useTransition hook also gives us an isPending flag to indicate when a transition is in progress. We can use this to display a loading state or optimize our rendering logic, giving the user a smoother experience even as complex updates are happening behind the scenes.

It‘s worth noting that transitions aren‘t a silver bullet for performance issues. Like any optimization, they should be applied judiciously and tested thoroughly to ensure they‘re actually improving the user experience. However, for many common scenarios like data fetching, sorting, and filtering, transitions can be a powerful tool for building fast, responsive UIs.

Suspense on the Server: Streaming Rendering and Selective Hydration

Next up, let‘s talk about some exciting developments in server-side rendering with React 18. Two key features, streaming rendering and selective hydration, are now possible thanks to the new Suspense component and related APIs.

Streaming rendering allows the server to send HTML to the client in chunks, rather than waiting for the entire page to render before responding. This can dramatically improve perceived performance, as the user starts seeing content sooner while the rest of the page loads in progressively.

What‘s more, with suspense boundaries, we can declaratively specify which parts of the UI should be included in the initial HTML payload and which can be deferred and lazily loaded on the client. This granular control over rendering and hydration can help optimize bundle sizes and reduce time-to-interactive.

Here‘s a basic example of how you might use Suspense on the server:

import { Suspense } from ‘react‘;

function App() {
  return (
    <div>

      <Suspense fallback={<div>Loading feed...</div>}>
        <SocialFeed />
      </Suspense>
      <Suspense fallback={<div>Loading chat...</div>}>
        <ChatWidget />
      </Suspense>
    </div>
  );
}

On the server, React will send the HTML for the outer div and the fallback elements inside the Suspense boundaries first. This allows the user to see the page skeleton and basic loading states as soon as possible.

Meanwhile, React will continue rendering the SocialFeed and ChatWidget components on the server, streaming their HTML to the client as it becomes available. When the client receives this content, it can progressively replace the fallback elements with the actual UI, without waiting for the entire page to finish loading.

This approach can be a game-changer for complex, data-heavy pages that may have previously required extensive preloading or resulted in long initial load times. By breaking up the rendering and hydration process into smaller, more manageable chunks, we can deliver a faster, more responsive experience to users on a wide range of devices and network conditions.

Of course, as with any performance optimization, it‘s important to profile and measure the actual impact of these techniques in your specific application. Suspense and streaming rendering are powerful tools, but they may not be necessary or appropriate for every use case.

Strict Mode: Preparing for the Future

Finally, let‘s take a quick look at some changes to strict mode in React 18. Strict mode is an opt-in feature that helps identify potential problems and prepare your code for future versions of React. In 18, it‘s getting some notable updates to better support concurrent rendering and other advanced features.

One significant change is that React will now automatically mount and unmount components wrapped in strict mode, even if they don‘t have any observable side effects. This helps catch issues related to missing cleanup logic, like not properly unsubscribing from event listeners or timers.

Additionally, React 18‘s strict mode will simulate multiple renderings of the same component, with slight differences in the order of operations. This can surface subtle bugs related to race conditions or improper memoization, which might otherwise go unnoticed in development.

While these changes may cause some existing code to break or log additional warnings, they‘re ultimately designed to help you write more robust, future-proof React components. By opting into strict mode and addressing any issues it flags, you can ensure your app is ready to take full advantage of React‘s evolving feature set.

To enable strict mode, simply wrap your app‘s root component like so:

import { StrictMode } from ‘react‘;

ReactDOM.createRoot(document.getElementById(‘root‘)).render(
  <StrictMode>
    <App />
  </StrictMode>
);

Keep in mind that strict mode is a development-only feature and does not impact the production build of your app. It‘s purely a tool for identifying potential issues and encouraging best practices during development.

Embracing the Future of React

React 18 represents a major milestone for the library, introducing powerful new features and setting the stage for even more ambitious UI development in the future. By embracing concurrent rendering, automatic batching, transitions, suspense, and strict mode, we can build React applications that are faster, more responsive, and more resilient than ever before.

Of course, as with any major update, there may be some learning curves and migration challenges along the way. However, the React team has gone to great lengths to ensure a smooth adoption path, with comprehensive documentation, codemods, and gradual opt-in mechanisms for many of the new features.

As a seasoned developer who has weathered many framework and library updates over the years, my advice is to approach React 18 with an open mind and a willingness to explore. Take the time to read the release notes, experiment with the new APIs, and see how they can benefit your specific projects and use cases.

While some of the concepts, like concurrent rendering and suspense, may seem daunting at first, remember that you don‘t need to master them all at once. Start with the basics, like updating your rendering logic to use createRoot, and gradually work your way up to more advanced techniques as you gain confidence and experience.

Most importantly, don‘t be afraid to ask for help or guidance along the way. The React community is incredibly active and supportive, with countless resources, tutorials, and forums where you can learn from and collaborate with other developers.

In conclusion, React 18 is a powerful and exciting update that pushes the boundaries of what‘s possible with JavaScript UI development. By embracing its new features and best practices, we can build faster, more engaging web applications that delight users and stand the test of time. Happy coding!