Next.js App Router Core

Full Reference: See advanced.md for WebSocket integration, Socket.IO, Server-Sent Events, TanStack Query real-time sync, and Vercel/Pusher patterns.

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: nextjs for comprehensive documentation.

File Conventions

File Purpose

page.tsx

Route UI (required for route)

layout.tsx

Shared layout, preserves state

loading.tsx

Loading UI (Suspense)

error.tsx

Error boundary

not-found.tsx

404 page

route.ts

API endpoint

Component Types

Server Components (Default)

// No 'use client' - runs on server async function Page() { const data = await db.query(...); // Direct DB access return <div>{data.name}</div>; }

Client Components

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

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

Data Fetching

// Server Component async function Page() { const data = await fetch('https://api.example.com/data', { cache: 'force-cache', // Static (default) // cache: 'no-store', // Dynamic // next: { revalidate: 60 } // ISR }); return <Component data={await data.json()} />; }

Server Actions

'use server' export async function createItem(formData: FormData) { await db.items.create({ name: formData.get('name') }); revalidatePath('/items'); }

Decision Rules

Scenario Use

Interactive UI 'use client'

Data fetching Server Component

Form mutations Server Actions

Shared state Client Component

Production Readiness

Security Configuration

// next.config.js - Security headers const securityHeaders = [ { key: 'X-DNS-Prefetch-Control', value: 'on' }, { key: 'Strict-Transport-Security', value: 'max-age=63072000; includeSubDomains; preload' }, { key: 'X-Content-Type-Options', value: 'nosniff' }, { key: 'X-Frame-Options', value: 'DENY' }, { key: 'X-XSS-Protection', value: '1; mode=block' }, { key: 'Referrer-Policy', value: 'origin-when-cross-origin' }, { key: 'Content-Security-Policy', value: default-src 'self'; script-src 'self' 'unsafe-eval' 'unsafe-inline'; style-src 'self' 'unsafe-inline';, }, ];

module.exports = { async headers() { return [{ source: '/:path*', headers: securityHeaders }]; }, };

// Server-side secrets (never exposed to client) // Use server components or API routes async function SecureComponent() { const apiKey = process.env.API_SECRET_KEY; // Server only const data = await fetch(url, { headers: { Authorization: apiKey } }); return <div>{/* render data */}</div>; }

// Environment variables // NEXT_PUBLIC_* = exposed to client (careful!) // Other vars = server-side only

Error Handling

// app/error.tsx - Error boundary 'use client'

export default function Error({ error, reset, }: { error: Error & { digest?: string } reset: () => void }) { useEffect(() => { // Log to error reporting service captureException(error); }, [error]);

return ( <div> <h2>Something went wrong!</h2> <button onClick={reset}>Try again</button> </div> ); }

// app/global-error.tsx - Root error boundary 'use client'

export default function GlobalError({ error, reset, }: { error: Error & { digest?: string } reset: () => void }) { return ( <html> <body> <h2>Something went wrong!</h2> <button onClick={reset}>Try again</button> </body> </html> ); }

Performance Optimization

// Dynamic imports for client components import dynamic from 'next/dynamic';

const HeavyChart = dynamic(() => import('@/components/Chart'), { loading: () => <Skeleton />, ssr: false, // Client-only if needed });

// Image optimization import Image from 'next/image';

// Font optimization import { Inter } from 'next/font/google'; const inter = Inter({ subsets: ['latin'], display: 'swap' });

Caching Strategy

// Static data (cached indefinitely) async function StaticPage() { const data = await fetch(url, { cache: 'force-cache' }); }

// Dynamic data (never cached) async function DynamicPage() { const data = await fetch(url, { cache: 'no-store' }); }

// ISR (revalidate every 60 seconds) async function ISRPage() { const data = await fetch(url, { next: { revalidate: 60 } }); }

// On-demand revalidation import { revalidatePath, revalidateTag } from 'next/cache';

async function updateData() { 'use server'; await db.update(...); revalidatePath('/posts'); // Revalidate specific path revalidateTag('posts'); // Revalidate by tag }

Middleware Security

// middleware.ts import { NextResponse } from 'next/server'; import type { NextRequest } from 'next/server';

export function middleware(request: NextRequest) { // Rate limiting header for upstream proxy const response = NextResponse.next();

// Auth check const token = request.cookies.get('session'); if (!token && request.nextUrl.pathname.startsWith('/dashboard')) { return NextResponse.redirect(new URL('/login', request.url)); }

// CORS for API routes if (request.nextUrl.pathname.startsWith('/api')) { response.headers.set('Access-Control-Allow-Origin', process.env.ALLOWED_ORIGIN!); }

return response; }

export const config = { matcher: ['/dashboard/:path*', '/api/:path*'], };

Monitoring Metrics

Metric Alert Threshold

Largest Contentful Paint (LCP)

2.5s

First Input Delay (FID)

100ms

Cumulative Layout Shift (CLS)

0.1

Time to First Byte (TTFB)

800ms

Server Component render time

200ms

Build size increase

10%

Build & Deployment

// next.config.js - Production optimizations module.exports = { output: 'standalone', // For Docker deployments images: { remotePatterns: [ { protocol: 'https', hostname: 'cdn.example.com' }, ], }, experimental: { // Enable if using PPR ppr: true, }, };

Production build with analysis

ANALYZE=true npm run build

Check bundle size

npx @next/bundle-analyzer

Checklist

Security headers configured
CSP policy defined
No secrets in NEXT_PUBLIC_* vars
Error boundaries at route level
Global error boundary
Image optimization with next/image
Font optimization with next/font
Dynamic imports for heavy components
Caching strategy per route
Middleware for auth/rate limiting
Core Web Vitals monitored
Standalone output for containerization

When NOT to Use This Skill

This skill is for Next.js App Router (v13+). DO NOT use for:

React without Next.js: Use frontend-react skill instead
Next.js Pages Router (v12 and below): Consult KB for migration guidance
Nuxt.js (Vue meta-framework): Use nuxt3 skill instead
Remix (React meta-framework): Use remix skill instead
SvelteKit: Use sveltekit skill instead
Astro: Use astro skill instead
API-only backend: Consider nestjs or fastapi instead

Anti-Patterns

Anti-Pattern Why It's Wrong Correct Approach

Using 'use client' everywhere Defeats Server Component benefits, increases bundle size Only use 'use client' for interactive components

Fetching in Client Components Waterfalls, no SSR, poor SEO Fetch in Server Components or use Server Actions

Not using loading.tsx Poor UX during data fetching Create loading.tsx for route-level loading states

Secrets in NEXT_PUBLIC_* Exposed to client, security risk Use server-only env vars or Server Components

Ignoring caching strategy Poor performance or stale data Set explicit cache: 'force-cache', 'no-store', or revalidate

No error boundaries Uncaught errors crash entire app Add error.tsx at route and root level

fetch() without cache option Unpredictable caching behavior Always specify cache or revalidate strategy

Using useEffect for data Client-side only, no SSR Use Server Components with async/await

Quick Troubleshooting

Issue Possible Cause Solution

"Cannot use useState in Server Component" Missing 'use client' directive Add 'use client' at top of file

Data not updating after mutation Cache not revalidated Use revalidatePath() or revalidateTag() in Server Action

Hydration mismatch error Server/client render differently Ensure consistent data, check Date/random values

"Cannot access cookies" in component Cookies only in Server Components/Actions Move logic to Server Component or API route

Build fails with "Dynamic server usage" Using dynamic APIs in static route Add export const dynamic = 'force-dynamic'

Images not optimized Not using next/image Replace with from next/image

Slow page load Large client bundle Use dynamic imports, check bundle analyzer

CORS errors with API routes Missing headers in route.ts Add CORS headers in route handler

Reference Documentation

File Conventions
Data Fetching Patterns
Deep: Caching
Deep: Server Components

nextjs-app-router

Safety Notice

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Production build with analysis

Check bundle size

Source Transparency

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