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name: Next.js 16 App Router description: | Next.js 16 App Router patterns - pages, API routes, server components, client components, middleware.

Use when: building Next.js 16 applications with App Router.

Skill: nextjs

Scope

• Applies to: Next.js 16 App Router - Server Components, Client Components, API routes, Server Actions, middleware, caching
• Does NOT cover: Pages Router, deployment specifics, framework internals

Assumptions

• Next.js 16+
• Node.js 20.9+
• TypeScript 5.1+
• React 19+ (for full feature support)
• Turbopack (stable, default bundler)

Principles

• Use Server Components by default (async/await directly in components)
• Use Client Components ('use client') only for interactivity (hooks, event handlers)
• Use Server Actions for mutations ('use server')
• Use use cache directive for explicit component-level caching (Next.js 16)
• Use revalidatePath and revalidateTag for cache invalidation
• Use Suspense boundaries for async data loading
• Use useTransition for non-urgent updates
• Use useDeferredValue for deferring expensive computations
• Route handlers use route.ts files with named exports (GET, POST, etc.)

Constraints

MUST

• Use Server Components by default (no 'use client' unless needed)
• Mark Server Actions with 'use server' directive
• Use revalidatePath or revalidateTag after mutations
• Handle searchParams as Promise in Server Components (Next.js 15+)

SHOULD

• Use useTransition for heavy updates that shouldn't block UI
• Use useDeferredValue with useMemo only for expensive operations (1000+ items)
• Use Suspense boundaries for progressive loading
• Use route groups (group) for organization without affecting URLs

AVOID

• Unnecessary useMemo/useCallback (React 19 handles most optimizations)
• Client Components for static content
• Mixing Server and Client Component patterns incorrectly
• React Compiler (wait for better ecosystem support)

Interactions

• Works with drizzle-orm for database access
• Complements ai-sdk-ui for chat interfaces
• Uses typescript for type safety

Patterns

Server Component

Copy// app/page.tsx
export default async function Page() {
  const data = await fetchData()
  return <div>{data}</div>
}

Client Component

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

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

Server Action

Copy'use server'
import { revalidatePath } from 'next/cache'

export async function createItem(formData: FormData) {
  await createItemInDB(formData)
  revalidatePath('/items')
}

Route Handler

Copy// app/api/items/route.ts
export async function GET() {
  return Response.json({ items: [] })
}

export async function POST(request: Request) {
  const data = await request.json()
  return Response.json({ created: data })
}

Caching

Copy'use cache'
export async function CachedComponent() {
  const data = await fetchData() // Cached at component level
  return <div>{data}</div>
}

References

• Next.js Documentation - Complete App Router guide
• React Server Components - Server Component patterns