Nextjs

# Next.js App Router - Production Patterns

**Version**: Next.js 16.0.3
**React Version**: 19.2.0
**Node.js**: 20.9+
**Last Verified**: 2025-11-21

## Table of Contents
1. [When to Use This Skill](#when-to-use-this-skill)
2. [When NOT to Use This Skill](#when-not-to-use-this-skill)
3. [Next.js 16 Breaking Changes](#nextjs-16-breaking-changes)
4. [Cache Components & Caching APIs](#cache-components--caching-apis)
5. [Server Components](#server-components)
6. [Server Actions](#server-actions)
7. [Route Handlers](#route-handlers)
8. [React 19.2 Features](#react-192-features)
9. [Metadata API](#metadata-api)
10. [Image & Font Optimization](#image--font-optimization)
11. [Top 5 Critical Errors](#top-5-critical-errors)
12. [Performance Patterns](#performance-patterns)
13. [TypeScript Configuration](#typescript-configuration)

---

## When to Use This Skill

Use this skill when you need:

- **Next.js 16 App Router patterns** (layouts, loading, error boundaries, routing)
- **Server Components** best practices (data fetching, composition, streaming)
- **Server Actions** patterns (forms, mutations, revalidation, error handling)
- **Cache Components** with `"use cache"` directive (NEW in Next.js 16)
- **New caching APIs**: `revalidateTag()`, `updateTag()`, `refresh()` (Updated in Next.js 16)
- **Migration from Next.js 15 to 16** (async params, proxy.ts, parallel routes)
- **Route Handlers** (API endpoints, webhooks, streaming responses)
- **Proxy patterns** (`proxy.ts` replaces `middleware.ts` in Next.js 16)
- **Async route params** (`params`, `searchParams`, `cookies()`, `headers()` now async)
- **Parallel routes with default.js** (breaking change in Next.js 16)
- **React 19.2 features** (View Transitions, `useEffectEvent()`, React Compiler)
- **Metadata API** (SEO, Open Graph, Twitter Cards, sitemaps)
- **Image optimization** (`next/image` with updated defaults in Next.js 16)
- **Performance optimization** (lazy loading, code splitting, PPR, ISR)

## When NOT to Use This Skill

Do NOT use this skill for:

- **Cloudflare Workers deployment** → Use `cloudflare-nextjs` skill instead
- **Pages Router patterns** → This skill covers App Router ONLY (Pages Router is legacy)
- **Authentication libraries** → Use `clerk-auth`, `auth-js`, or other auth-specific skills
- **Database integration** → Use `cloudflare-d1`, `drizzle-orm-d1`, or database-specific skills
- **UI component libraries** → Use `tailwind-v4-shadcn` skill for Tailwind + shadcn/ui
- **State management** → Use `zustand-state-management`, `tanstack-query` skills
- **Form libraries** → Use `react-hook-form-zod` skill

---

## Next.js 16 Breaking Changes

**CRITICAL**: Next.js 16 has multiple breaking changes. For detailed migration steps, see `references/next-16-migration-guide.md`.

| Breaking Change | Before | After |
|-----------------|--------|-------|
| **Async params** | `params.slug` | `const { slug } = await params` |
| **Async headers** | `cookies()` sync | `await cookies()` |
| **Middleware** | `middleware.ts` | `proxy.ts` (renamed) |
| **Parallel routes** | `default.js` optional | `default.js` **required** |
| **Caching** | Auto-cached fetch | Opt-in with `"use cache"` |
| **revalidateTag()** | 1 argument | 2 arguments (tag + cacheLife) |
| **Node.js** | 18.x+ | **20.9+** required |
| **React** | 18.x | **19.2+** required |

**Quick Fix for Async Params**:
```typescript
// ✅ Next.js 16 pattern
export default async function Page({ params }: { params: Promise<{ id: string }> }) {
  const { id } = await params
  return <div>{id}</div>
}
```

**Codemod**: `bunx @next/codemod@canary upgrade latest`

**See**: `references/next-16-migration-guide.md` for complete migration guide with examples.

---

## Cache Components & Caching APIs

### "use cache" Directive (NEW in Next.js 16)

```typescript
'use cache'

export async function BlogPosts() {
  const posts = await db.posts.findMany()
  return posts.map(post => <article key={post.id}>{post.title}</article>)
}
```

### Caching APIs Summary

| API | Purpose | Example |
|-----|---------|---------|
| `"use cache"` | Opt-in component/function caching | `'use cache'` at top |
| `revalidateTag()` | Invalidate by tag | `revalidateTag('posts', 'max')` |
| `updateTag()` | Update cache without revalidation | `updateTag('posts', newData)` |
| `refresh()` | Refresh current page | `refresh()` |
| `revalidatePath()` | Invalidate by path | `revalidatePath('/posts')` |

### PPR (Partial Prerendering)

```typescript
// next.config.ts
const config = { experimental: { ppr: true } }

// page.tsx
export const experimental_ppr = true

export default function Page() {
  return (
    <>
      <StaticHeader />
      <Suspense fallback={<Skeleton />}>
        <DynamicContent />
      </Suspense>
    </>
  )
}
```

**See**: `references/caching-apis.md` for complete caching API reference with ISR, tag-based revalidation, and advanced patterns.

---

## Server Components

Server Components are the **default** in App Router. They run on the server and can fetch data, access databases, and keep logic server-side.

```typescript
// app/posts/page.tsx (Server Component by default)
export default async function PostsPage() {
  const posts = await db.posts.findMany()
  return <div>{posts.map(p => <article key={p.id}>{p.title}</article>)}</div>
}
```

### Streaming with Suspense

```typescript
import { Suspense } from 'react'

export default function Page() {
  return (
    <div>
      <Header />
      <Suspense fallback={<Skeleton />}>
        <Posts />
      </Suspense>
    </div>
  )
}
```

### Server vs Client Components

| Server Components | Client Components |
|-------------------|-------------------|
| Data fetching, DB access | Interactivity (onClick) |
| Sensitive logic | React hooks (useState) |
| Large dependencies | Browser APIs |
| Static content | Real-time updates |

**Client Component** (requires `'use client'`):
```typescript
'use client'
import { useState } from 'react'
export function Counter() {
  const [count, setCount] = useState(0)
  return <button onClick={() => setCount(count + 1)}>{count}</button>
}
```

---

## Server Actions

Server Actions are async functions that run on the server, callable from Client or Server Components.

### Basic Server Action

```typescript
// app/actions.ts
'use server'

import { revalidatePath } from 'next/cache'

export async function createPost(formData: FormData) {
  const title = formData.get('title') as string
  await db.posts.create({ data: { title } })
  revalidatePath('/posts')
}
```

### Form Handling

```typescript
// Server Component Form (simplest)
import { createPost } from './actions'

export default function NewPostPage() {
  return (
    <form action={createPost}>
      <input name="title" required />
      <button type="submit">Create</button>
    </form>
  )
}
```

### Available Patterns

| Pattern | Use Case | Reference |
|---------|----------|-----------|
| Client Form with Loading | useFormState + useFormStatus | `templates/server-action-form.tsx` |
| Error Handling | Return { error } or { success } | `references/server-actions-patterns.md` |
| Optimistic Updates | useOptimistic hook | `references/server-actions-patterns.md` |
| File Upload | FormData + blob storage | `references/server-actions-patterns.md` |
| Redirect After Action | redirect() function | `references/server-actions-patterns.md` |

**See**: `references/server-actions-patterns.md` for error handling, optimistic updates, file uploads, and advanced patterns.

---

## Route Handlers

Route Handlers are the App Router equivalent of API Routes.

```typescript
// app/api/posts/route.ts
export async function GET(request: Request) {
  const posts = await db.posts.findMany()
  return Response.json({ posts })
}

export async function POST(request: Request) {
  const body = await request.json()
  const post = await db.posts.create({ data: body })
  return Response.json({ post }, { status: 201 })
}
```

**Dynamic Routes** (with async params):
```typescript
// app/api/posts/[id]/route.ts
export async function GET(
  request: Request,
  { params }: { params: Promise<{ id: string }> }
) {
  const { id } = await params // Await in Next.js 16
  const post = await db.posts.findUnique({ where: { id } })
  return post ? Response.json({ post }) : Response.json({ error: 'Not found' }, { status: 404 })
}
```

**See**: `templates/route-handler-api.ts` for search params, webhooks, and streaming patterns.

---

## React 19.2 Features

| Feature | Usage |
|---------|-------|
| **React Compiler** | `experimental: { reactCompiler: true }` - Auto-memoization |
| **View Transitions** | `useTransition()` + `router.push()` |
| **useEffectEvent** | Stable event handlers without deps |

---

## Metadata API

```typescript
// Static metadata
export const metadata: Metadata = {
  title: 'My Blog',
  description: 'A blog about Next.js',
  openGraph: { title: 'My Blog', images: ['/og-image.jpg'] },
}

// Dynamic metadata (await params in Next.js 16)
export async function generateMetadata({ params }: { params: Promise<{ id: string }> }) {
  const { id } = await params
  const post = await db.posts.findUnique({ where: { id } })
  return { title: post.title, description: post.excerpt }
}
```

---

## Image & Font Optimization

```typescript
// next/image
import Image from 'next/image'
<Image src="/profile.jpg" alt="Profile" width={500} height={500} priority />

// next/font
import { Inter } from 'next/font/google'
const inter = Inter({ subsets: ['latin'], variable: '--font-inter' })
<html className={inter.variable}>
```

**Remote images**: Configure `images.remotePatterns` in `next.config.ts`.

---

## Top 5 Critical Errors

### Error 1: `params` is a Promise

**Error**: `Type 'Promise<{ id: string }>' is not assignable to type '{ id: string }'`

**Solution**: Await params in Next.js 16:
```typescript
export default async function Page({ params }: { params: Promise<{ id: string }> }) {
  const { id } = await params
}
```

---

### Error 2: `middleware.ts` is deprecated

**Warning**: `middleware.ts is deprecated. Use proxy.ts instead.`

**Solution**: Rename file and function:
```typescript
// Rename: middleware.ts → proxy.ts
// Rename function: middleware → proxy
export function proxy(request: NextRequest) {
  // Same logic
}
```

---

### Error 3: Parallel route missing `default.js`

**Error**: `Parallel route @modal was matched but no default.js was found`

**Solution**: Add default.tsx:
```typescript
// app/@modal/default.tsx
export default function ModalDefault() {
  return null
}
```

---

### Error 4: Cannot use React hooks in Server Component

**Error**: `You're importing a component that needs useState. It only works in a Client Component`

**Solution**: Add `'use client'`:
```typescript
'use client'

import { useState } from 'react'

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

---

### Error 5: `fetch()` not caching

**Cause**: Next.js 16 uses opt-in caching with `"use cache"`.

**Solution**: Add `"use cache"` directive:
```typescript
'use cache'

export async function getPosts() {
  const response = await fetch('/api/posts')
  return response.json()
}
```

---

**See All 18 Errors**: `references/error-catalog.md`

---

## Performance Patterns

| Pattern | Usage |
|---------|-------|
| **Lazy Loading** | `const HeavyComp = dynamic(() => import('./Heavy'), { ssr: false })` |
| **Code Splitting** | Automatic per route - each `page.tsx` gets own bundle |
| **Turbopack** | Default in Next.js 16, opt out with `--webpack` flag |
| **PPR** | `experimental: { ppr: true }` + `<Suspense>` boundaries |

---

## TypeScript Configuration

```json
{
  "compilerOptions": {
    "strict": true,
    "baseUrl": ".",
    "paths": { "@/*": ["./*"] }
  }
}
```

---

## When to Load References

| Reference | Load When... |
|-----------|--------------|
| `next-16-migration-guide.md` | Migrating from Next.js 15, async params errors, proxy.ts setup |
| `server-actions-patterns.md` | Error handling, optimistic updates, file uploads, advanced forms |
| `caching-apis.md` | ISR, tag-based revalidation, updateTag(), refresh(), PPR details |
| `error-catalog.md` | Debugging any Next.js error, comprehensive error solutions |
| `top-errors.md` | Quick fixes for the 5 most common Next.js errors |

---

## Bundled Resources

| Type | Files |
|------|-------|
| **References** | `error-catalog.md`, `top-errors.md`, `next-16-migration-guide.md`, `server-actions-patterns.md`, `caching-apis.md` |
| **Templates** | `async-params-page.tsx`, `server-action-form.tsx`, `route-handler-api.ts`, `cache-component-use-cache.tsx`, `parallel-routes-with-default.tsx`, `proxy-migration.ts` |

---

## Related Skills

| Skill | Purpose |
|-------|---------|
| `cloudflare-nextjs` | Deploy to Cloudflare Workers |
| `tailwind-v4-shadcn` | Styling |
| `clerk-auth` | Authentication |
| `drizzle-orm-d1` | Database |
| `react-hook-form-zod` | Forms |
| `zustand-state-management` | Client state |

**Official Docs**: https://nextjs.org/docs | **App Router**: https://nextjs.org/docs/app

---

**Version**: Next.js 16.0.0 | React 19.2.0 | Node.js 20.9+ | TypeScript 5.3+
**Production Tested**: E-commerce, SaaS, content sites | **Token Savings**: 65-70%