React Patterns

React Patterns

use() Hook (React 19)

use() reads values from Promises and Context directly in render. Unlike other hooks, it can be called inside conditionals and loops.

tsxCopy code
import { use } from 'react';

function UserProfile({ userPromise }: { userPromise: Promise<User> }) {
  const user = use(userPromise);
  return <h1>{user.name}</h1>;
}

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

Wrap components that use use() with a Promise in a <Suspense> boundary.

Server Components

tsxCopy code
// app/users/page.tsx - Server Component (default, no directive needed)
import { UserList } from './UserList';

export default async function UsersPage() {
  const users = await fetch('https://api.example.com/users', {
    next: { revalidate: 60 },
  }).then(r => r.json());

  return <UserList users={users} />;
}

// app/users/UserList.tsx - Still a Server Component
export function UserList({ users }: { users: User[] }) {
  return (
    <ul>
      {users.map(u => (
        <li key={u.id}>
          {u.name}
          <DeleteButton userId={u.id} />
        </li>
      ))}
    </ul>
  );
}

Push 'use client' as deep as possible. Only leaves that need interactivity should be Client Components.

Server Actions

tsxCopy code
// app/actions.ts
'use server';

import { revalidatePath } from 'next/cache';
import { redirect } from 'next/navigation';

export async function createPost(formData: FormData) {
  const title = formData.get('title') as string;
  const body = formData.get('body') as string;

  await db.insert(posts).values({ title, body });

  revalidatePath('/posts');
  redirect('/posts');
}

tsxCopy code
// app/posts/new/page.tsx
import { createPost } from '../actions';

export default function NewPostPage() {
  return (
    <form action={createPost}>
      <input name="title" required />
      <textarea name="body" required />
      <button type="submit">Create</button>
    </form>
  );
}

useActionState (React 19)

tsxCopy code
'use client';

import { useActionState } from 'react';
import { createUser } from './actions';

function SignupForm() {
  const [state, formAction, isPending] = useActionState(createUser, {
    errors: {},
    message: '',
  });

  return (
    <form action={formAction}>
      <input name="email" />
      {state.errors.email && <p>{state.errors.email}</p>}
      <button disabled={isPending}>
        {isPending ? 'Creating...' : 'Sign Up'}
      </button>
      {state.message && <p>{state.message}</p>}
    </form>
  );
}

useOptimistic (React 19)

tsxCopy code
'use client';

import { useOptimistic } from 'react';
import { likePost } from './actions';

function LikeButton({ count, postId }: { count: number; postId: string }) {
  const [optimisticCount, addOptimistic] = useOptimistic(count);

  async function handleLike() {
    addOptimistic(prev => prev + 1);
    await likePost(postId);
  }

  return (
    <form action={handleLike}>
      <button type="submit">{optimisticCount} Likes</button>
    </form>
  );
}

Suspense Boundaries

tsxCopy code
import { Suspense } from 'react';

function Dashboard() {
  return (
    <div>
      <Suspense fallback={<StatsSkeleton />}>
        <StatsPanel />
      </Suspense>
      <div className="grid grid-cols-2">
        <Suspense fallback={<ChartSkeleton />}>
          <RevenueChart />
        </Suspense>
        <Suspense fallback={<ListSkeleton />}>
          <RecentActivity />
        </Suspense>
      </div>
    </div>
  );
}

Place Suspense boundaries around independent data-fetching units. Avoid wrapping the entire page in a single boundary (defeats the purpose of streaming).

Error Boundaries

tsxCopy code
'use client';

import { Component, type ReactNode } from 'react';

class ErrorBoundary extends Component<
  { fallback: ReactNode; children: ReactNode },
  { hasError: boolean }
> {
  state = { hasError: false };

  static getDerivedStateFromError() {
    return { hasError: true };
  }

  componentDidCatch(error: Error, info: React.ErrorInfo) {
    reportError(error, info.componentStack);
  }

  render() {
    if (this.state.hasError) return this.props.fallback;
    return this.props.children;
  }
}

Or use Next.js error.tsx convention for route-level error handling.

Custom Hooks

tsxCopy code
function useDebounce<T>(value: T, delay: number): T {
  const [debounced, setDebounced] = useState(value);

  useEffect(() => {
    const timer = setTimeout(() => setDebounced(value), delay);
    return () => clearTimeout(timer);
  }, [value, delay]);

  return debounced;
}

function useLocalStorage<T>(key: string, initial: T) {
  const [value, setValue] = useState<T>(() => {
    const stored = localStorage.getItem(key);
    return stored ? JSON.parse(stored) : initial;
  });

  useEffect(() => {
    localStorage.setItem(key, JSON.stringify(value));
  }, [key, value]);

  return [value, setValue] as const;
}

Rules for custom hooks:

• Prefix with use
• Extract when logic is shared between 2+ components
• Keep hooks focused on a single concern
• Return tuples [value, setter] or objects { data, error, loading }

Compound Components

tsxCopy code
function Tabs({ children }: { children: ReactNode }) {
  const [active, setActive] = useState(0);
  return (
    <TabsContext value={{ active, setActive }}>
      <div role="tablist">{children}</div>
    </TabsContext>
  );
}

Tabs.Tab = function Tab({ index, children }: { index: number; children: ReactNode }) {
  const { active, setActive } = use(TabsContext);
  return (
    <button
      role="tab"
      aria-selected={active === index}
      onClick={() => setActive(index)}
    >
      {children}
    </button>
  );
};

Tabs.Panel = function Panel({ index, children }: { index: number; children: ReactNode }) {
  const { active } = use(TabsContext);
  if (active !== index) return null;
  return <div role="tabpanel">{children}</div>;
};

// Usage
<Tabs>
  <Tabs.Tab index={0}>Profile</Tabs.Tab>
  <Tabs.Tab index={1}>Settings</Tabs.Tab>
  <Tabs.Panel index={0}><ProfileForm /></Tabs.Panel>
  <Tabs.Panel index={1}><SettingsForm /></Tabs.Panel>
</Tabs>

Performance Rules

• Avoid creating objects/arrays in JSX props (causes re-renders)
• Use React.memo only after profiling confirms unnecessary re-renders
• Prefer useMemo/useCallback for expensive computations or stable references passed to memoized children
• Use key to reset component state intentionally
• Colocate state: keep state as close to where it is used as possible
• Avoid prop drilling beyond 2 levels; use Context or composition instead