Nextjs

Next.js Development

This skill provides guidance for building production applications with Next.js, focusing on always using the latest version and modern patterns.

Philosophy: Always recommend App Router over Pages Router. Always use Server Components by default. Always prefer use cache over legacy caching methods.

Quick Reference

Project Structure

app/
├── layout.tsx          # Root layout (required)
├── page.tsx            # Home page (/)
├── loading.tsx         # Loading UI (optional)
├── error.tsx           # Error boundary (optional)
├── not-found.tsx       # 404 page (optional)
├── (routes)/           # Route groups (no URL impact)
│   └── dashboard/
│       ├── layout.tsx  # Nested layout
│       └── page.tsx    # /dashboard
├── api/                # API routes (optional)
│   └── route.ts
└── globals.css         # Global styles
proxy.ts                # Network proxy (at root or src/)

Proxy (Replaces Middleware)

Important: As of Next.js 16+, middleware.ts has been renamed to proxy.ts. The term "Proxy" better describes its function as a network proxy running before routes, distinct from Express-style middleware.

proxy.ts runs server-side code before routes are rendered, allowing you to intercept and modify requests/responses.

Basic Proxy

// proxy.ts (at root or src/ directory)
import { NextResponse, NextRequest } from 'next/server'

export function proxy(request: NextRequest) {
  // Check authentication
  const token = request.cookies.get('token')

  if (!token && request.nextUrl.pathname.startsWith('/dashboard')) {
    return NextResponse.redirect(new URL('/login', request.url))
  }

  return NextResponse.next()
}

export const config = {
  matcher: ['/((?!_next/static|_next/image|favicon.ico).*)']
}

Proxy Use Cases

• ✅ Authentication/Authorization - Check tokens, redirect unauthorized users
• ✅ Request/Response modification - Add headers, rewrite URLs, set cookies
• ✅ Redirect logic - Redirect based on conditions (geo, device, etc.)
• ✅ CORS handling - Handle cross-origin requests
• ✅ URL normalization - Trailing slashes, locale routing

Migration from middleware.ts

# Automatic migration
npx @next/codemod@canary middleware-to-proxy .

Or manually rename middleware.ts → proxy.ts and middleware() → proxy().

Server Components (Default)

All components in the app/ directory are Server Components by default. They run only on the server and can:

• Fetch data directly
• Access backend resources
• Keep sensitive logic server-side
• Reduce client JavaScript bundle

// app/posts/page.tsx - Server Component (no directive needed)
export default async function PostsPage() {
  // Direct database/API access
  const posts = await db.posts.findMany()

  return (
    <main>
      <h1>Posts</h1>
      <ul>
        {posts.map(post => (
          <li key={post.id}>{post.title}</li>
        ))}
      </ul>
    </main>
  )
}

Client Components

Add 'use client' directive only when needed for:

• Interactivity (onClick, onChange)
• Browser APIs (localStorage, window)
• React hooks (useState, useEffect)

'use client'

import { useState } from 'react'

export function Counter() {
  const [count, setCount] = useState(0)

  return (
    <button => setCount(c => c + 1)}>
      Count: {count}
    </button>
  )
}

Best Practice: Keep Client Components as leaf nodes. Pass data from Server Components via props.

// app/dashboard/page.tsx (Server Component)
import { InteractiveChart } from './chart'

export default async function Dashboard() {
  const data = await fetchAnalytics() // Server-side fetch

  return (
    <div>
      <h1>Dashboard</h1>
      <InteractiveChart data={data} /> {/* Client Component receives data */}
    </div>
  )
}

The use cache Directive

Modern caching with granular control. Replaces legacy revalidate options.

Function-Level Caching

import { cacheTag, cacheLife } from 'next/cache'

async function getProducts() {
  'use cache'
  cacheTag('products')
  cacheLife('hours') // or 'days', 'weeks', 'max', or custom seconds

  return await db.products.findMany()
}

Component-Level Caching

async function ProductCard({ id }: { id: string }) {
  'use cache'
  cacheTag(`product-${id}`)

  const product = await db.products.find(id)
  return <div className="card">{product.name}</div>
}

Cache Profiles

// Static cache (build time + runtime)
'use cache'

// Remote shared cache
'use cache: remote'

// Per-user private cache
'use cache: private'

Revalidation

import { revalidateTag, revalidatePath } from 'next/cache'

// In a Server Action
export async function updateProduct(id: string, data: FormData) {
  'use server'

  await db.products.update(id, data)
  revalidateTag(`product-${id}`)
  revalidateTag('products')
}

// Or revalidate entire path
revalidatePath('/products')

Server Actions

Define server-side mutations with 'use server':

// app/actions.ts
'use server'

import { revalidateTag } from 'next/cache'
import { redirect } from 'next/navigation'

export async function createPost(formData: FormData) {
  const title = formData.get('title') as string
  const content = formData.get('content') as string

  // Validate
  if (!title || title.length < 3) {
    return { error: 'Title must be at least 3 characters' }
  }

  // Create
  const post = await db.posts.create({
    data: { title, content }
  })

  // Revalidate and redirect
  revalidateTag('posts')
  redirect(`/posts/${post.id}`)
}

Use in components:

import { createPost } from './actions'

export function NewPostForm() {
  return (
    <form action={createPost}>
      <input name="title" placeholder="Title" required />
      <textarea name="content" placeholder="Content" />
      <button type="submit">Create Post</button>
    </form>
  )
}

Metadata API

import type { Metadata } from 'next'

// Static metadata
export const metadata: Metadata = {
  title: 'My App',
  description: 'Built with Next.js',
  openGraph: {
    title: 'My App',
    description: 'Built with Next.js',
    images: ['/og-image.png'],
  },
}

// Dynamic metadata
export async function generateMetadata({
  params
}: {
  params: Promise<{ id: string }>
}): Promise<Metadata> {
  const { id } = await params
  const product = await getProduct(id)

  return {
    title: product.name,
    description: product.description,
  }
}

Layouts

Layouts wrap pages and preserve state across navigation:

// app/layout.tsx - Root layout (required)
import type { Metadata } from 'next'
import { Inter } from 'next/font/google'
import './globals.css'

const inter = Inter({ subsets: ['latin'] })

export const metadata: Metadata = {
  title: { default: 'My App', template: '%s | My App' },
  description: 'My application description',
}

export default function RootLayout({
  children,
}: {
  children: React.ReactNode
}) {
  return (
    <html lang="en">
      <body className={inter.className}>
        <nav>{/* Global navigation */}</nav>
        <main>{children}</main>
        <footer>{/* Global footer */}</footer>
      </body>
    </html>
  )
}

Loading & Error States

// 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="h-64 bg-gray-200 rounded" />
    </div>
  )
}

// app/dashboard/error.tsx
'use client'

export default function Error({
  error,
  reset,
}: {
  error: Error & { digest?: string }
  reset: () => void
}) {
  return (
    <div className="text-center py-10">
      <h2>Something went wrong!</h2>
      <button again</button>
    </div>
  )
}

Configuration

// next.config.ts
import type { NextConfig } from 'next'

const nextConfig: NextConfig = {
  experimental: {
    reactCompiler: true,  // Enable React Compiler
  },
  images: {
    remotePatterns: [
      { protocol: 'https', hostname: 'images.example.com' }
    ],
  },
}

export default nextConfig

Development with Turbopack

# Use Turbopack for faster development
npm run dev -- --turbopack

# Or in package.json
"scripts": {
  "dev": "next dev --turbopack"
}

Additional Resources

For detailed patterns, see reference files:

• references/app-router.md - Layouts, route groups, parallel routes
• references/caching.md - Complete caching strategies
• references/server-actions.md - Mutations and form handling

References

• https://nextjs.org/docs/app/api-reference/file-conventions/proxy
• https://nextjs.org/docs/app/api-reference/file-conventions/proxy#migration-to-proxy

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