React Component Architecture - RSC

Core Principle: RSC as Default

All components in Next.js App Router are Server Components by default.

Server Components

• Async functions
• Can fetch data directly
• Cannot use state or effects
• Cannot use event handlers
• Cannot use browser APIs
• Run only on server

```tsx // Server Component (no 'use client') export default async function PostPage({ params }) { const post = await fetchPost(params.id);

return ( {post.title} {post.content} ); } ```

Client Components

Add `'use client'` directive when component needs:

1. State: `useState`, `useReducer`
2. Effects: `useEffect`, `useLayoutEffect`
3. Event handlers: `onClick`, `onChange`
4. Browser APIs: `window`, `localStorage`
5. React Context consumers

```tsx 'use client';

import { useState } from 'react'; import { Button } from '@/components/ui/button';

export function LikeButton({ postId, initialLikes }) { const [likes, setLikes] = useState(initialLikes);

return ( <Button onClick={() => setLikes(likes + 1)}> 👍 {likes} </Button> ); } ```

Island Architecture Pattern

Keep Client Components small and at leaves:

```tsx // ✅ GOOD: Server parent, small Client leaf // app/posts/[id]/page.tsx (Server Component) export default async function PostPage({ params }) { const post = await fetchPost(params.id);

return ( {/* Server-rendered content */} {post.title} {post.content}

  {/* Small interactive island */}
  <LikeButton postId={post.id} initialLikes={post.likes} />
</article>

); }

// ❌ BAD: Entire page as Client Component 'use client'; export default function PostPage() { const [post, setPost] = useState(null);

useEffect(() => { fetchPost().then(setPost); }, []);

// All content client-rendered, larger bundle } ```

Data Flow: Server to Client

Pass data as serializable props:

```tsx // Server Component export default async function Page() { const data = await fetchData();

return <ClientComponent data={data} />; // ✅ Serializable }

// Cannot pass functions (unless Server Actions) <ClientComponent onClick={handleClick} /> // ❌ Function not serializable ```

Anti-Patterns

❌ `'use client'` on page.tsx/layout.tsx - Forces entire route client ❌ Data fetching in `useEffect` - Creates waterfall ❌ Large Client Components - Increases bundle size

✅ Server Components by default ✅ Extract only interactive parts to Client Components ✅ Fetch data in Server Components, pass as props

Token Estimate: ~3,000 tokens