Nextjs Expert

---
name: nextjs-expert
version: 1.0.0
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 boundaries, dynamic routes, parallel routes, intercepting routes, or any Next.js architecture question.
triggers:
  - Next.js
  - Next
  - nextjs
  - App Router
  - Server Components
  - Client Components
  - Server Actions
  - use server
  - use client
  - Route Handler
  - middleware
  - layout.tsx
  - page.tsx
  - loading.tsx
  - error.tsx
  - revalidatePath
  - revalidateTag
  - NextAuth
  - Auth.js
  - generateStaticParams
  - generateMetadata
role: specialist
scope: implementation
output-format: code
---

# 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 d