React What's New, Why It Matters, and Migration Tips

Here we go again—another post about new features in React ?. But honestly, this one’s worth it. React 19 has officially moved from its Release Candidate (RC), launched in April 2024, to its stable release, packed with powerful updates! From performance improvements to new hooks and tools, React 19 offers something for everyone, whether building small apps or enterprise-grade solutions.

Let’s dive into what’s new, with code examples and migration tips to help you seamlessly upgrade your projects.

What’s New in React 19

1. Performance Optimizations

React 19 introduces:

• Selective Hydration Enhancements: Optimizes hydration for server-rendered apps by only hydrating parts of the UI as needed.
• Smaller Bundle Size: React 19’s library is lighter, leading to faster load times and reduced network usage.

Example: Efficient Hydration

import { hydrateRoot } from 'react-dom/client';
import App from './App';

const container = document.getElementById('root');
hydrateRoot(container, <App />);

2. New and Enhanced Hooks

a. useFormStatus: Simplified Form State Management

Forms are easier to manage with the new useFormStatus hook. It tracks a form’s pending state without requiring custom context or prop drilling.

import { useFormStatus } from 'react-dom';

function SubmitButton() {
  const { pending } = useFormStatus();
  return <button type="submit" disabled={pending}>Submit</button>;
}

b. useDeferredValue: The initialValue Option

The updated useDeferredValue hook ensures smoother UI transitions by rendering an initial fallback value while processing deferred data.

import { useDeferredValue } from 'react';

function Search({ deferredValue }) {
  const value = useDeferredValue(deferredValue, '');

  return <Results query={value} />;
}

c. useOptimistic: Handle Optimistic Updates

The new useOptimistic hook makes optimistic UI updates straightforward.

import { useOptimistic } from 'react';

function LikeButton() {
  const [optimisticLikes, setOptimisticLikes] = useOptimistic(0);

  function handleLike() {
    setOptimisticLikes(prev => prev + 1);
    fetch('/api/like', { method: 'POST' });
  }

  return <button onClick={handleLike}>Likes: {optimisticLikes}</button>;
}

3. Document Metadata Hoisting

React 19 automatically hoists metadata like