react-component-architecture-rsc
React Server Components vs Client Components decision framework with composition patterns. PROACTIVELY activate for: (1) deciding when to use 'use client' directive, (2) implementing island architecture with small client leaves, (3) passing server data to client components. Triggers: "server component", "client component", "use client"
What this skill does
# 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 (
<article>
<h1>{post.title}</h1>
<p>{post.content}</p>
</article>
);
}
```
### 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)}>
Like {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 (
<article>
{/* Server-rendered content */}
<h1>{post.title}</h1>
<p>{post.content}</p>
{/* 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
```
## Composition Pattern
Server Components can be passed as children to Client Components:
```tsx
// ClientWrapper.tsx (Client Component)
'use client'
export function ClientWrapper({ children }: { children: React.ReactNode }) {
const [isOpen, setIsOpen] = useState(false);
return (
<div>
<button onClick={() => setIsOpen(!isOpen)}>Toggle</button>
{isOpen && children}
</div>
);
}
// page.tsx (Server Component)
export default async function Page() {
const data = await fetchServerData();
return (
<ClientWrapper>
{/* This renders on the server! */}
<ServerContent data={data} />
</ClientWrapper>
);
}
```
## Decision Checklist
| Need | Component Type |
|------|----------------|
| `useState`, `useReducer` | Client |
| `useEffect`, `useLayoutEffect` | Client |
| `onClick`, `onChange`, etc. | Client |
| `window`, `localStorage`, `document` | Client |
| `useContext` (consuming) | Client |
| `async/await` data fetching | Server |
| Direct database access | Server |
| Static content | Server |
## 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
## Best Practices
- Server Components by default
- Extract only interactive parts to Client Components
- Fetch data in Server Components, pass as props
- Use Server Actions for mutations from Client Components
---
**Related Skills**: `rsc-composition-patterns`, `nextjs-app-router-data-fetching`
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