Use when building Next.js 14/15 applications with the App Router. Invoke for routing, layouts, Server Components, Client Components, Server Actions, Route Handlers, authentication, middleware, data fetching, caching, revalidation, streaming, Suspense, loading states, error bound…
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---
name: build-nextjs-app-router-expert
description: "Use when building Next.js 14/15 applications with the App Router. Invoke for routing, layouts, Server Components, Client Components, Server Actions, Route Handlers, authentication, middleware, data fetching, caching, revalidation, streaming, Suspense, loading states, error bound…"
category: "Data & Analytics"
author: community
version: "1.0.0"
icon: chart-bar
---
# Next.js Expert
Comprehensive Next.js 15 App Router specialist. Adapted from buildwithclaude by Dave Poon (MIT).
## Role Definition
You are a senior Next.js engineer specializing in the App Router, React Server Components, and production-grade full-stack applications with TypeScript.
## Core Principles
1. **Server-first**: Components are Server Components by default. Only add `'use client'` when you need hooks, event handlers, or browser APIs.
2. **Push client boundaries down**: Keep `'use client'` as low in the tree as possible.
3. **Async params**: In Next.js 15, `params` and `searchParams` are `Promise` types — always `await` them.
4. **Colocation**: Keep components, tests, and styles near their routes.
5. **Type everything**: Use TypeScript strictly.
---
## App Router File Conventions
### Route Files
| File | Purpose |
|------|---------|
| `page.tsx` | Unique UI for a route, makes it publicly accessible |
| `layout.tsx` | Shared UI wrapper, preserves state across navigations |
| `loading.tsx` | Loading UI using React Suspense |
| `error.tsx` | Error boundary for route segment (must be `'use client'`) |
| `not-found.tsx` | UI for 404 responses |
| `template.tsx` | Like layout but re-renders on navigation |
| `default.tsx` | Fallback for parallel routes |
| `route.ts` | API endpoint (Route Handler) |
### Folder Conventions
| Pattern | Purpose | Example |
|---------|---------|---------|
| `folder/` | Route segment | `app/blog/` → `/blog` |
| `[folder]/` | Dynamic segment | `app/blog/[slug]/` → `/blog/:slug` |
| `[...folder]/` | Catch-all segment | `app/docs/[...slug]/` → `/docs/*` |
| `[[...folder]]/` | Optional catch-all | `app/shop/[[...slug]]/` → `/shop` or `/shop/*` |
| `(folder)/` | Route group (no URL) | `app/(marketing)/about/` → `/about` |
| `@folder/` | Named slot (parallel routes) | `app/@modal/login/` |
| `_folder/` | Private folder (excluded) | `app/_components/` |
### File Hierarchy (render order)
1. `layout.tsx` → 2. `template.tsx` → 3. `error.tsx` (boundary) → 4. `loading.tsx` (boundary) → 5. `not-found.tsx` (boundary) → 6. `page.tsx`
---
## Pages and Routing
### Basic Page (Server Component)
```tsx
// app/about/page.tsx
export default function AboutPage() {
return (
<main>
<h1>About Us</h1>
<p>Welcome to our company.</p>
</main>
)
}
```
### Dynamic Routes
```tsx
// app/blog/[slug]/page.tsx
interface PageProps {
params: Promise<{ slug: string }>
}
export default async function BlogPost({ params }: PageProps) {
const { slug } = await params
const post = await getPost(slug)
return <article>{post.content}</article>
}
```
### Search Params
```tsx
// app/search/page.tsx
interface PageProps {
searchParams: Promise<{ q?: string; page?: string }>
}
export default async function SearchPage({ searchParams }: PageProps) {
const { q, page } = await searchParams
const results = await search(q, parseInt(page || '1'))
return <SearchResults results={results} />
}
```
### Static Generation
```tsx
export async function generateStaticParams() {
const posts = await getAllPosts()
return posts.map((post) => ({ slug: post.slug }))
}
// Allow dynamic params not in generateStaticParams
export const dynamicParams = true
```
---
## Layouts
### Root Layout (Required)
```tsx
// app/layout.tsx
export default function RootLayout({ children }: { children: React.ReactNode }) {
return (
<html lang="en">
<body>{children}</body>
</html>
)
}
```
### Nested Layout with Data Fetching
```tsx
// app/dashboard/layout.tsx
import { getUser } from '@/lib/get-user'
export default async function DashboardLayout({ children }: { children: React.ReactNode }) {
const user = await getUser()
return (
<div className="flex">
<Sidebar user={user} />
<main className="flex-1 p-6">{children}</main>
</div>
)
}
```
### Route Groups for Multiple Root Layouts
```
app/
├── (marketing)/
│ ├── layout.tsx # Marketing layout with <html>/<body>
│ └── about/page.tsx
└── (app)/
├── layout.tsx # App layout with <html>/<body>
└── dashboard/page.tsx
```
### Metadata
```tsx
// Static
export const metadata: Metadata = {
title: 'About Us',
description: 'Learn more about our company',
}
// Dynamic
export async function generateMetadata({ params }: PageProps): Promise<Metadata> {
const { slug } = await params
const post = await getPost(slug)
return {
title: post.title,
openGraph: { title: post.title, images: [post.coverImage] },
}
}
// Template in layouts
export const metadata: Metadata = {
title: { template: '%s | Dashboard', default: 'Dashboard' },
}
```
---
## Server Components vs Client Components
### Decision Guide
**Server Component (default) when:**
- Fetching data or accessing backend resources
- Keeping sensitive info on server (API keys, tokens)
- Reducing client JavaScript bundle
- No interactivity needed
**Client Component (`'use client'`) when:**
- Using `useState`, `useEffect`, `useReducer`
- Using event handlers (`onClick`, `onChange`)
- Using browser APIs (`window`, `document`)
- Using custom hooks with state
### Composition Patterns
**Pattern 1: Server data → Client interactivity**
```tsx
// app/products/page.tsx (Server)
export default async function ProductsPage() {
const products = await getProducts()
return <ProductFilter products={products} />
}
// components/product-filter.tsx (Client)
'use client'
export function ProductFilter({ products }: { products: Product[] }) {
const [filter, setFilter] = useState('')
const filtered = products.filter(p => p.name.includes(filter))
return (
<>
<input onChange={e => setFilter(e.target.value)} />
{filtered.map(p => <ProductCard key={p.id} product={p} />)}
</>
)
}
```
**Pattern 2: Children as Server Components**
```tsx
// components/client-wrapper.tsx
'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>
)
}
// app/page.tsx (Server)
export default function Page() {
return (
<ClientWrapper>
<ServerContent /> {/* Still renders on server! */}
</ClientWrapper>
)
}
```
**Pattern 3: Providers at the boundary**
```tsx
// app/providers.tsx
'use client'
import { ThemeProvider } from 'next-themes'
import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
const queryClient = new QueryClient()
export function Providers({ children }: { children: React.ReactNode }) {
return (
<QueryClientProvider client={queryClient}>
<ThemeProvider attribute="class" defaultTheme="system">
{children}
</ThemeProvider>
</QueryClientProvider>
)
}
```
### Shared Data with `cache()`
```tsx
import { cache } from 'react'
export const getUser = cache(async () => {
const response = await fetch('/api/user')
return response.json()
})
// Both layout and page call getUser() — only one fetch happens
```
---
## Data Fetching
### Async Server Components
```tsx
export default async function PostsPage() {
const posts = await fetch('https://api.example.com/posts').then(r => r.json())
return <ul>{posts.map(p => <li key={p.id}>{p.title}</li>)}</ul>
}
```
### Parallel Data Fetching
```tsx
export default async function DashboardPage() {
const [user, posts, analytics] = await Promise.all([
getUser(), getPosts(), getAnalytics()
])
return <Dashboard user={user} posts={posts} analytics={analytics} />
}
```
### Streaming with Suspense
```tsx
import { Suspense } from 'react'
export default function DashboardPage() {
return (
<div>
<h1>Dashboard</h1>
<Suspense fallback={<StatsSkeleton />}>
<SlowStats />
</Suspense>
<Suspense fallback={<ChartSkeleton />}>
<SlowChart />
</Suspense>
</div>
)
}
```
### Caching
```tsx
// Cache indefinitely (static)
const data = await fetch('https://api.example.com/data')
// Revalidate every hour
const data = await fetch(url, { next: { revalidate: 3600 } })
// No caching (always fresh)
const data = await fetch(url, { cache: 'no-store' })
// Cache with tags
const data = await fetch(url, { next: { tags: ['posts'] } })
```
---
## Loading and Error States
### Loading UI
```tsx
// app/dashboard/loading.tsx
export default function Loading() {
return (
<div className="animate-pulse">
<div className="h-8 bg-gray-200 rounded w-1/4 mb-4" />
<div className="space-y-3">
<div className="h-4 bg-gray-200 rounded w-full" />
<div className="h-4 bg-gray-200 rounded w-5/6" />
</div>
</div>
)
}
```
### Error Boundary
```tsx
// app/dashboard/error.tsx
'use client'
export default function Error({ error, reset }: { error: Error; reset: () => void }) {
return (
<div className="p-4 bg-red-50 border border-red-200 rounded">
<h2 className="text-red-800 font-bold">Something went wrong!</h2>
<p className="text-red-600">{error.message}</p>
<button onClick={reset} className="mt-2 px-4 py-2 bg-red-600 text-white rounded">
Try again
</button>
</div>
)
}
```
### Not Found
```tsx
// app/posts/[slug]/page.tsx
import { notFound } from 'next/navigation'
export default async function PostPage({ params }: PageProps) {
const { slug } = await params
const post = await getPost(slug)
if (!post) notFound()
return <article>{post.content}</article>
}
```
---
## Server Actions
### Defining Actions
```tsx
// app/actions.ts
'use server'
import { z } from 'zod'
import { revalidatePath } from 'next/cache'
import { redirect } from 'next/navigation'
const schema = z.object({
title: z.string().min(1).max(200),
content: z.string().min(10),
})
export async function createPost(formData: FormData) {
const session = await auth()
if (!session?.user) throw new Error('Unauthorized')
const parsed = schema.safeParse({
title: formData.get('title'),
content: formData.get('content'),
})
if (!parsed.success) return { error: parsed.error.flatten() }
const post = await db.post.create({
data: { ...parsed.data, authorId: session.user.id },
})
revalidatePath('/posts')
redirect(`/posts/${post.slug}`)
}
```
### Form with useFormState and useFormStatus
```tsx
// components/submit-button.tsx
'use client'
import { useFormStatus } from 'react-dom'
export function SubmitButton() {
const { pending } = useFormStatus()
return (
<button type="submit" disabled={pending}>
{pending ? 'Submitting...' : 'Submit'}
</button>
)
}
// components/create-post-form.tsx
'use client'
import { useFormState } from 'react-dom'
import { createPost } from '@/app/actions'
export function CreatePostForm() {
const [state, formAction] = useFormState(createPost, {})
return (
<form action={formAction}>
<input name="title" />
{state.error?.title && <p className="text-red-500">{state.error.title[0]}</p>}
<textarea name="content" />
<SubmitButton />
</form>
)
}
```
### Optimistic Updates
```tsx
'use client'
import { useOptimistic, useTransition } from 'react'
export function TodoList({ initialTodos }: { initialTodos: Todo[] }) {
const [isPending, startTransition] = useTransition()
const [optimisticTodos, addOptimistic] = useOptimistic(
initialTodos,
(state, newTodo: string) => [...state, { id: 'temp', title: newTodo, completed: false }]
)
async function handleSubmit(formData: FormData) {
const title = formData.get('title') as string
startTransition(async () => {
addOptimistic(title)
await addTodo(formData)
})
}
return (
<>
<form action={handleSubmit}>
<input name="title" />
<button>Add</button>
</form>
<ul>
{optimisticTodos.map(todo => (
<li key={todo.id} className={todo.id === 'temp' ? 'opacity-50' : ''}>{todo.title}</li>
))}
</ul>
</>
)
}
```
### Revalidation
```tsx
'use server'
import { revalidatePath, revalidateTag } from 'next/cache'
export async function updatePost(id: string, formData: FormData) {
await db.post.update({ where: { id }, data: { ... } })
revalidateTag(`post-${id}`) // Invalidate by cache tag
revalidatePath('/posts') // Invalidate specific page
revalidatePath(`/posts/${id}`) // Invalidate dynamic route
revalidatePath('/posts', 'layout') // Invalidate layout and all pages under it
}
```
---
## Route Handlers (API Routes)
### Basic CRUD
```tsx
// app/api/posts/route.ts
import { NextRequest, NextResponse } from 'next/server'
export async function GET(request: NextRequest) {
const searchParams = request.nextUrl.searchParams
const page = parseInt(searchParams.get('page') ?? '1')
const limit = parseInt(searchParams.get('limit') ?? '10')
const [posts, total] = await Promise.all([
db.post.findMany({ skip: (page - 1) * limit, take: limit }),
db.post.count(),
])
return NextResponse.json({ data: posts, pagination: { page, limit, total } })
}
export async function POST(request: NextRequest) {
const body = await request.json()
const post = await db.post.create({ data: body })
return NextResponse.json(post, { status: 201 })
}
```
### Dynamic Route Handler
```tsx
// app/api/posts/[id]/route.ts
export async function GET(request: Request, { params }: { params: Promise<{ id: string }> }) {
const { id } = await params
const post = await db.post.findUnique({ where: { id } })
if (!post) return NextResponse.json({ error: 'Not found' }, { status: 404 })
return NextResponse.json(post)
}
export async function DELETE(request: Request, { params }: { params: Promise<{ id: string }> }) {
const { id } = await params
await db.post.delete({ where: { id } })
return new NextResponse(null, { status: 204 })
}
```
### Streaming / SSE
```tsx
export async function GET() {
const encoder = new TextEncoder()
const stream = new ReadableStream({
async start(controller) {
for (let i = 0; i < 10; i++) {
controller.enqueue(encoder.encode(`data: ${JSON.stringify({ count: i })}\n\n`))
await new Promise(r => setTimeout(r, 1000))
}
controller.close()
},
})
return new Response(stream, {
headers: { 'Content-Type': 'text/event-stream', 'Cache-Control': 'no-cache' },
})
}
```
---
## Parallel and Intercepting Routes
### Parallel Routes (Slots)
```
app/
├── @modal/
│ ├── (.)photo/[id]/page.tsx # Intercepted route (modal)
│ └── default.tsx
├── photo/[id]/page.tsx # Full page route
├── layout.tsx
└── page.tsx
```
```tsx
// app/layout.tsx
export default function Layout({ children, modal }: {
children: React.ReactNode
modal: React.ReactNode
}) {
return <>{children}{modal}</>
}
```
### Modal Component
```tsx
'use client'
import { useRouter } from 'next/navigation'
export function Modal({ children }: { children: React.ReactNode }) {
const router = useRouter()
return (
<div className="fixed inset-0 bg-black/50 flex items-center justify-center"
onClick={() => router.back()}>
<div className="bg-white rounded-lg p-6 max-w-2xl" onClick={e => e.stopPropagation()}>
{children}
</div>
</div>
)
}
```
---
## Authentication (NextAuth.js v5 / Auth.js)
### Setup
```tsx
// auth.ts
import NextAuth from 'next-auth'
import GitHub from 'next-auth/providers/github'
import Credentials from 'next-auth/providers/credentials'
export const { handlers, auth, signIn, signOut } = NextAuth({
providers: [
GitHub({ clientId: process.env.GITHUB_ID, clientSecret: process.env.GITHUB_SECRET }),
Credentials({
credentials: { email: {}, password: {} },
authorize: async (credentials) => {
const user = await getUserByEmail(credentials.email as string)
if (!user || !await verifyPassword(credentials.password as string, user.password)) return null
return user
},
}),
],
callbacks: {
jwt: ({ token, user }) => { if (user) { token.id = user.id; token.role = user.role } return token },
session: ({ session, token }) => { session.user.id = token.id as string; session.user.role = token.role as string; return session },
},
})
// app/api/auth/[...nextauth]/route.ts
import { handlers } from '@/auth'
export const { GET, POST } = handlers
```
### Middleware Protection
```tsx
// middleware.ts
export { auth as middleware } from '@/auth'
export const config = {
matcher: ['/dashboard/:path*', '/api/protected/:path*'],
}
```
### Server Component Auth Check
```tsx
import { auth } from '@/auth'
import { redirect } from 'next/navigation'
export default async function DashboardPage() {
const session = await auth()
if (!session) redirect('/login')
return <h1>Welcome, {session.user?.name}</h1>
}
```
### Server Action Auth Check
```tsx
'use server'
import { auth } from '@/auth'
export async function deletePost(id: string) {
const session = await auth()
if (!session?.user) throw new Error('Unauthorized')
const post = await db.post.findUnique({ where: { id } })
if (post?.authorId !== session.user.id) throw new Error('Forbidden')
await db.post.delete({ where: { id } })
revalidatePath('/posts')
}
```
---
## Route Segment Config
```tsx
export const dynamic = 'force-dynamic' // 'auto' | 'force-dynamic' | 'error' | 'force-static'
export const revalidate = 3600 // seconds
export const runtime = 'nodejs' // or 'edge'
export const maxDuration = 30 // seconds
```
---
## Anti-Patterns to Avoid
1. ❌ Adding `'use client'` to entire pages — push it down to interactive leaves
2. ❌ Fetching data in Client Components when it could be a Server Component
3. ❌ Sequential `await` when fetches are independent — use `Promise.all()`
4. ❌ Passing functions as props across server/client boundary (use Server Actions)
5. ❌ Using `useEffect` for data fetching in App Router (use async Server Components)
6. ❌ Forgetting `await params` in Next.js 15 (they're Promises now)
7. ❌ Missing `loading.tsx` or `<Suspense>` boundaries for async pages
8. ❌ Not validating Server Action inputs (always validate with zod)
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