React Server Components

# React Server Components

> Run components on the server to eliminate client JavaScript and enable direct data access

## When to Use

- Components that fetch data and render it without client-side interactivity
- Components that import large libraries (markdown parsers, date formatters) only needed for rendering
- You need direct database or filesystem access without an API layer
- Using Next.js App Router (RSC is the default) or another RSC-compatible framework

## Instructions

1. **Server Components are the default** in Next.js App Router. Do not add `'use client'` unless needed.
2. Server Components can:
   - `async/await` directly: `const data = await db.query(...)`
   - Import server-only libraries without impacting bundle size
   - Read environment variables and secrets directly
3. Add `'use client'` directive at the top of a file when the component needs:
   - `useState`, `useEffect`, or any hooks
   - Browser APIs (`window`, `document`)
   - Event handlers (`onClick`, `onChange`)
4. **Composition rule:** Server Components can render Client Components, but Client Components cannot render Server Components (only pass them as `children` props).
5. Pass data from Server to Client Components as serializable props (no functions, classes, or non-serializable objects).

```typescript
// Server Component (no 'use client')
async function ProductPage({ id }: { id: string }) {
  const product = await db.products.findById(id); // direct DB access
  return (
    <div>
      <h1>{product.name}</h1>
      <AddToCartButton productId={id} /> {/* Client Component */}
    </div>
  );
}

// Client Component
'use client';
function AddToCartButton({ productId }: { productId: string }) {
  return <button onClick={() => addToCart(productId)}>Add to Cart</button>;
}
```

## Details

RSC introduces a server/client component split at the file level. The framework serializes server-rendered output as a JSON-like payload (RSC payload) and sends it to the client for React to reconcile.

**What RSC eliminates:**

- API routes for data fetching in many cases (direct DB access in server components)
- Client-side waterfall requests (async server components fetch in parallel)
- Bundle cost for render-only dependencies (they never reach the browser)

**Common mistakes:**

- Adding `'use client'` to every file "to be safe" — this negates RSC benefits
- Trying to pass functions as props from server to client components — functions are not serializable
- Using `useEffect` for data fetching in client components when a server component would work

**Framework support (2024):** Next.js App Router is the primary production implementation. Remix, Waku, and other frameworks have RSC support in various stages.

## Source

https://patterns.dev/react/react-server-components

## Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

## Harness Integration

- **Type:** knowledge — this skill is a reference document, not a procedural workflow.
- **No tools or state** — consumed as context by other skills and agents.

## Success Criteria

- The patterns described in this document are applied correctly in the implementation.
- Edge cases and anti-patterns listed in this document are avoided.