import { useActionState } from "react";

function UpdateNameForm() {
  const [error, submitAction, isPending] = useActionState(
    async (prevState, formData) => {
      const name = formData.get("name");
      const error = await updateName(name);
      if (error) {
        return error;
      }
      redirect("/profile");
      return null;
    },
    null
  );

  return (
    <form action={submitAction}>
      <input type="text" name="name" />
      <button type="submit" disabled={isPending}>
        Update
      </button>
      {error && <p>{error}</p>}
    </form>
  );
}

Here, useActionState manages the submission state and error handling for you, making the code cleaner and easier to maintain.

2. Optimistic Updates with useOptimistic

Optimistic UI updates let users see changes immediately while an async request is in progress. The new useOptimistic hook makes this pattern straightforward.

Example: Optimistic Name Change

import { useOptimistic } from "react";

function ChangeName({ currentName, onUpdateName }) {
  const [optimisticName, setOptimisticName] = useOptimistic(currentName);

  const submitAction = async (formData) => {
    const newName = formData.get("name");
    setOptimisticName(newName); // Show optimistic state
    const updatedName = await updateName(newName); // Wait for the async request
    onUpdateName(updatedName); // Update the actual state
  };

  return (
    <form action={submitAction}>
      <p>Your name: {optimisticName}</p>
      <input type="text" name="name" />
      <button type="submit">Change Name</button>
    </form>
  );
}

useOptimistic ensures a seamless user experience by showing updates even before the server responds.

3. Enhanced Error Reporting for Hydration Mismatches

React 19 improves error handling, particularly for hydration errors. Instead of vague errors, you now get detailed diffs of mismatched content between the server and client.

Example: Hydration Error Diff

Uncaught Error: Hydration failed because the server-rendered HTML didn’t match the client.
Tree mismatch:
+ Client: <span>Welcome</span>
- Server: <span>Hello</span>

These clear messages help developers debug issues quickly and efficiently.

4. Server Components and Server Actions

React Server Components (RSCs) allow components to be rendered on the server, improving performance. Server Actions enable calling async functions on the server directly from Client Components.

Example: Using Server Actions

// Server Component
export const fetchComments = async () => {
  const response = await fetch("/api/comments");
  return await response.json();
};

// Client Component
import { use } from "react";

function Comments({ commentsPromise }) {
  const comments = use(commentsPromise); // Suspends until resolved
  return (
    <ul>
      {comments.map((comment) => (
        <li key={comment.id}>{comment.text}</li>
      ))}
    </ul>
  );
}

// Usage
function App() {
  return (
    <Suspense fallback={<p>Loading comments...</p>}>
      <Comments commentsPromise={fetchComments()} />
    </Suspense>
  );
}

Server Actions streamline fetching and rendering server-side data within Client Components.

5. Native Metadata and Stylesheet Management

React 19 now supports