React 19 Tutorial: Modernizing Your Applications

React 19 is a significant release that introduces built-in compiler optimizations and massive render improvements. This tutorial walks you through the exciting new features, enhanced APIs, and how to upgrade your existing applications seamlessly.

Why React 19 Matters

React 19 is the first version to ship with a built-in compiler, offering optimized performance out of the box. This release focuses on improving the developer experience, simplifying state management, and enhancing rendering efficiency.

1. Actions API: Simplify Async Transitions

React 19 introduces the Actions API, making asynchronous transitions and state management more straightforward.

Using startTransition

The startTransition function now handles pending, error, and optimistic states for you:

function Example() {
  const [data, setData] = useState(null);

  const handleTransition = startTransition(async () => {
    const response = await fetch('https://api.example.com/data');
    const newData = await response.json();
    setData(newData);
  });

  return (
    <div>
      <button onClick={handleTransition} disabled={pending}>
        {pending ? 'Loading...' : 'Fetch Data'}
      </button>
    </div>
  );
}

Using form Action Attribute

Pass an async function to the action attribute of a <form> element and React handles the rest:

function MyForm() {
  const handleSubmit = async (formData) => {
    const response = await fetch('https://api.example.com/submit', {
      method: 'POST',
      body: new URLSearchParams(formData),
    });
    if (!response.ok) throw new Error('Failed to submit');
  };

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

2. Optimistic UI with useOptimistic and useActionState

Say goodbye to custom useState and useEffect hooks for optimistic updates. React 19 introduces two new hooks:

useOptimistic

Coordinates optimistic UI updates with server-side mutations.

useActionState

Simplifies handling pending states and errors for asynchronous operations.

function SaveProfile() {
  const [error, submit, pending] = useActionState(
    async (_, formData) => save(formData), 
    null
  );

  return (
    <form action={submit}>
      <input name="name" />
      <button disabled={pending}>
        {pending ? 'Saving...' : 'Save'}
      </button>
      {error && <p>{error}</p>}
    </form>
  );
}

3. useFormStatus: Context-Free Form Handling

The new useFormStatus hook surfaces the status of any <form> element to its children:

function SubmitButton() {
  const status = useFormStatus();

  return (
    <button type="submit" disabled={status.pending}>
      {status.pending ? 'Submitting...' : 'Submit'}
    </button>
  );
}

4. Suspense Pre-Warming

React 19 introduces Suspense pre-warming, which pre-renders components while data is being fetched. This makes your UI appear instantly when data resolves.

function DataComponent() {
  const data = useAsyncResourcefetchData();

  if (!data) return <Suspense fallback={<Loading Spinner />} />;
  return <div>Data: {data}</div>;
}

5. DOM Static APIs

The new ReactDOM flushSync and createRoot functions come with stability improvements:

• ReactDOM.flushSync: Ensures synchronous updates are applied immediately.
• createRoot: Now more stable and efficient.

6. How to Upgrade to React 19

Upgrading your existing React applications to version 19 is streamlined:

1. Install React and ReactDOM 19:

npm i react@19 react-dom@19

1. Run the official codemod tool:

npx react-codemod react-19

1. Search for legacy startTransition patterns and follow the official upgrade guide to clean up deprecated APIs.

Final Thoughts

React 19 is a leap forward in modern JavaScript application development. With its built-in compiler optimizations, simplified async state management, and enhanced developer tools, you can build faster, more efficient applications with less code. Dive in and embrace the future of React!

For more details, check out the official React 19 documentation. Happy coding!