export function Button({ variant = 'primary', children, onClick }: ButtonProps) { return ( <button className={btn btn-${variant}} onClick={onClick}> {children} ); }


### Server Components (Default in React 19)

Server Components render on the server, reducing client JavaScript:

```tsx
// app/posts/page.tsx - Server Component (default)
async function PostList() {
  const posts = await db.posts.findMany();
  return (
    <ul>
      {posts.map(post => <li key={post.id}>{post.title}</li>)}
    </ul>
  );
}

Client Components

Mark interactive components with 'use client':

'use client';
import { useState } from 'react';

export function Counter() {
  const [count, setCount] = useState(0);
  return <button onClick={() => setCount(c => c + 1)}>{count}</button>;
}

React 19 Features

Actions & useActionState

Replace manual form handling with Actions:

'use client';
import { useActionState } from 'react';

async function submitForm(prev: State, formData: FormData) {
  'use server';
  const name = formData.get('name');
  await db.users.create({ name });
  return { success: true };
}

function Form() {
  const [state, action, pending] = useActionState(submitForm, { success: false });
  return (
    <form action={action}>
      <input name="name" />
      <button disabled={pending}>{pending ? 'Saving...' : 'Save'}</button>
    </form>
  );
}

use() Hook for Promises & Context

import { use } from 'react';

function UserProfile({ userPromise }) {
  const user = use(userPromise); // Suspends until resolved
  return <div>{user.name}</div>;
}

function ThemeButton() {
  const theme = use(ThemeContext); // Read context conditionally
  return <button style={{ color: theme.primary }}>Click</button>;
}

useOptimistic for Instant UI Updates

'use client';
import { useOptimistic } from 'react';

function TodoList({ todos, addTodo }) {
  const [optimisticTodos, addOptimisticTodo] = useOptimistic(
    todos,
    (state, newTodo) => [...state, { ...newTodo, pending: true }]
  );
  
  async function handleAdd(formData: FormData) {
    const text = formData.get('text');
    addOptimisticTodo({ text, id: Date.now() });
    await addTodo(text);
  }
  
  return (
    <form action={handleAdd}>
      <input name="text" />
      <ul>
        {optimisticTodos.map(todo => (
          <li key={todo.id} style={{ opacity: todo.pending ? 0.5 : 1 }}>
            {todo.text}
          </li>
        ))}
      </ul>
    </form>
  );
}

useFormStatus for Form State

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

function SubmitButton() {
  const { pending } = useFormStatus();
  return <button disabled={pending}>{pending ? 'Submitting...' : 'Submit'}</button>;
}

Project Structure

src/
├── app/                  # App Router (Next.js) or routes
├── components/
│   ├── ui/              # Reusable UI primitives
│   ├── features/        # Feature-specific components
│   └── layouts/         # Layout components
├── hooks/               # Custom hooks
├── lib/                 # Utilities, API clients
├── types/               # TypeScript types
└── styles/              # Global styles, tokens

Custom Hooks Pattern

function useAsync<T>(asyncFn: () => Promise<T>, deps: unknown[]) {
  const [state, setState] = useState<{
    data: T | null;
    loading: boolean;
    error: Error | null;
  }>({ data: null, loading: true, error: null });

  useEffect(() => {
    setState(s => ({ ...s, loading: true }));
    asyncFn()
      .then(data => setState({ data, loading: false, error: null }))
      .catch(error => setState({ data: null, loading: false, error }));
  }, deps);

  return state;
}

Performance Guidelines

Let React Compiler optimize - React 19's compiler auto-memoizes; avoid manual useMemo/useCallback unless profiling shows need
Use Server Components - Default to server rendering, add 'use client' only for interactivity
Lazy load routes - Use React.lazy() and Suspense for code splitting
Avoid prop drilling - Use Context or composition patterns

Related Skills

Atomic Design: Component hierarchy patterns → See references/atomic-integration.md
CSS Tokens: Styling with design tokens → See references/styling-patterns.md
Storybook: Component documentation → See references/storybook-setup.md

react-web

React Web Development (React 19+)

Core Patterns

Function Components with TypeScript