React 19

React 19

Overview of New Features

Actions

Actions are async functions used in transitions to handle mutations:

import { useTransition } from 'react';

function UpdateProfile() {
  const [isPending, startTransition] = useTransition();

  async function handleSubmit(formData: FormData) {
    startTransition(async () => {
      const result = await updateProfile(formData);
      if (result.error) {
        showToast(result.error);
      }
    });
  }

  return (
    <form action={handleSubmit}>
      <input name="name" type="text" />
      <button type="submit" disabled={isPending}>
        {isPending ? 'Saving...' : 'Save'}
      </button>
    </form>
  );
}

useActionState

Replaces the experimental useFormState. Manages form action state including pending status:

import { useActionState } from 'react';

interface FormState {
  message: string | null;
  errors: Record<string, string[]>;
}

async function createPost(prevState: FormState, formData: FormData): Promise<FormState> {
  const title = formData.get('title') as string;

  if (!title || title.length < 3) {
    return { message: null, errors: { title: ['Title must be at least 3 characters'] } };
  }

  try {
    await savePost({ title });
    return { message: 'Post created!', errors: {} };
  } catch (error) {
    return { message: 'Failed to create post', errors: {} };
  }
}

function CreatePostForm() {
  const [state, formAction, isPending] = useActionState(createPost, {
    message: null,
    errors: {}
  });

  return (
    <form action={formAction}>
      <input name="title" type="text" />
      {state.errors.title && <p className="error">{state.errors.title[0]}</p>}
      <button type="submit" disabled={isPending}>
        {isPending ? 'Creating...' : 'Create Post'}
      </button>
    </form>
  );
}

useFormStatus

Access the status of a parent <form> from any child component:

import { useFormStatus } from 'react-dom';

function SubmitButton() {
  const { pending, data, method, action } = useFormStatus();

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

// Usage - must be child of <form>
function ContactForm() {
  return (
    <form action={submitContact}>
      <input name="email" type="email" required />
      <SubmitButton />
    </form>
  );
}

Reusable Form Components

'use client';
import { useFormStatus } from 'react-dom';

export function FormButton({ children, pendingText = 'Submitting...', ...props }) {
  const { pending } = useFormStatus();

  return (
    <button {...props} disabled={pending || props.disabled}>
      {pending ? pendingText : children}
    </button>
  );
}

Full Reference: See hooks.md for useOptimistic, use() hook, Ref handling, Document Metadata.

Server Components & Server Actions

// app/users/page.tsx (Server Component by default)
export default async function UsersPage() {
  const users = await fetchUsers();
  return <UserList users={users} />;
}

// app/actions.ts
'use server';
export async function createPost(formData: FormData) {
  const title = formData.get('title') as string;
  await db.post.create({ data: { title } });
  revalidatePath('/posts');
  redirect(`/posts`);
}

Full Reference: See server.md for Server Components patterns, React Compiler, Migration Guide.

Best Practices

• ✅ Use Actions for form submissions and mutations
• ✅ Use useOptimistic for immediate feedback
• ✅ Use use() instead of useEffect for data fetching
• ✅ Let React Compiler handle memoization
• ✅ Use Server Components for static content
• ✅ Use Server Actions for secure mutations
• ❌ Don't create Promises inside components for use()
• ❌ Don't mix client/server code without proper boundaries

When NOT to Use This Skill

• React 18 and earlier features - Use react skill instead
• General form handling - Use react-forms or react-hook-form skills
• Performance optimization - Use react-performance skill

Anti-Patterns

Quick Troubleshooting

Reference Files

External Documentation

• React 19 Blog
• React 19 Upgrade Guide