React 18: Key Features You Need to Know

React 18 represents one of the most significant updates to the React library, introducing a new concurrent rendering engine and several powerful new features. Let's explore the key improvements and how they can benefit your applications.

Concurrent Rendering

The cornerstone of React 18 is its new concurrent rendering engine. But what exactly is concurrent rendering, and why should you care?

Concurrent rendering allows React to prepare multiple versions of your UI at the same time without blocking the main thread. This is a fundamental shift in how React works, enabling new features that weren't possible before.

The beauty of concurrent React is that it's largely transparent to developers. You don't need to explicitly opt-in to benefit from many of its improvements:

// The same component code works with both the legacy and concurrent renderers
function Profile() {
  const [data, setData] = useState(null);
  
  useEffect(() => {
    fetchUserData().then(userData => {
      setData(userData);
    });
  }, []);
  
  if (!data) return <Spinner />;
  return <ProfileDetails data={data} />;
}

Automatic Batching

React 18 introduces automatic batching for improved performance. Batching is when React groups multiple state updates into a single re-render for better performance.

Previously, React only batched updates inside React event handlers:

// Before React 18: only batched inside React events
function handleClick() {
  setCount(c => c + 1); // Does not re-render yet
  setFlag(f => !f);     // Does not re-render yet
  // React will only re-render once at the end (batching)
}

But updates inside promises, setTimeout, native event handlers, or any other event were not batched:

// Before React 18: no batching outside of React events
setTimeout(() => {
  setCount(c => c + 1); // Causes a re-render
  setFlag(f => !f);     // Causes a re-render
}, 1000);

With React 18, all updates will be automatically batched, regardless of where they originate:

// React 18: all updates are batched
setTimeout(() => {
  setCount(c => c + 1); // Does not re-render yet
  setFlag(f => !f);     // Does not re-render yet
  // React will only re-render once at the end (batching)
}, 1000);

New APIs

React 18 introduces several new APIs that leverage the power of concurrent rendering:

useId

A new hook for generating unique IDs that work in server and client components:

function AccessibleField() {
  const id = useId();
  return (
    <>
      <label htmlFor={id}>Email</label>
      <input id={id} type="email" />
    </>
  );
}

useDeferredValue

This hook lets you defer updating a non-critical part of the UI:

function SearchResults({ query }) {
  const deferredQuery = useDeferredValue(query);
  
  // This component will re-render when deferredQuery changes,
  // which may lag behind the query itself
  return <ExpensiveSearchResults query={deferredQuery} />;
}

useTransition

The useTransition hook allows you to mark state updates as non-urgent:

function App() {
  const [isPending, startTransition] = useTransition();
  const [count, setCount] = useState(0);
  
  function handleClick() {
    startTransition(() => {
      // This state update is marked as a transition
      setCount(c => c + 1);
    });
  }

  return (
    <div>
      {isPending && <Spinner />}
      <button onClick={handleClick}>Add</button>
      <ExpensiveComponent count={count} />
    </div>
  );
}

Suspense on the Server

React 18 enables Suspense on the server through a new streaming server renderer:

// Server component with Suspense
<Layout>
  <NavBar />
  <Suspense fallback={<Spinner />}>
    <Comments />
  </Suspense>
</Layout>

This allows for streaming HTML from the server and selective hydration on the client, improving both perceived and actual loading performance.

Transition to the New Root API

React 18 introduces a new root API that supports concurrent features:

// Before React 18
import { render } from 'react-dom';
const container = document.getElementById('root');
render(<App />, container);

// React 18
import { createRoot } from 'react-dom/client';
const container = document.getElementById('root');
const root = createRoot(container);
root.render(<App />);

Conclusion

React 18 brings powerful improvements that make your applications more responsive and user-friendly. While many of these features work out of the box, understanding them will help you make the most of what React 18 has to offer.

The transition to concurrent rendering represents a significant evolution in React's architecture, setting the stage for future innovations. Start exploring these features today to build better user experiences for your applications.

Happy coding!