What Changed in React 19

React 19 is not a minor update but a paradigm shift. Stable Server Components, new hooks, and the React Compiler fundamentally change how developers write applications.

Stable Server Components

React Server Components (RSC) are now production-ready. By rendering on the server and sending HTML to the client, they reduce bundle size significantly.

// app/page.jsx — Server Component (default)
export default async function Home() {
  const posts = await db.query('SELECT * FROM posts');
  return (
    <ul>
      {posts.map(post => (
        <li key={post.id}>{post.title}</li>
      ))}
    </ul>
  );
}

Components using useState or useEffect must declare “use client” at the top.

The use() Hook

The most notable new feature is the use() hook, which lets you resolve promises directly inside components.

function Comments({ commentsPromise }) {
  const comments = use(commentsPromise);
  return comments.map(c => <p key={c.id}>{c.text}</p>);
}

Unlike traditional hooks, use() works inside conditionals and loops.

Server Actions for Forms

Server Actions let you handle form submissions without writing client JavaScript.

export default function CreatePost() {
  async function handleSubmit(formData) {
    'use server';
    const title = formData.get('title');
    await db.execute('INSERT INTO posts (title) VALUES (?)', [title]);
    revalidatePath('/');
  }

  return (
    <form action={handleSubmit}>
      <input name="title" required />
      <button type="submit">Create</button>
    </form>
  );
}

React Compiler

The React Compiler (formerly React Forget) automatically inserts useMemo and useCallback calls. Manual optimization hooks are no longer necessary in v19-compatible codebases.

Deprecated APIs

• Legacy lifecycle methods like componentWillMount are fully removed
• ReactDOM.render() is replaced entirely by createRoot()
• PropTypes official support ends

Migration from React 18 typically takes one day to one week depending on project size.