nextjs-app-router

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';

<Image
  src="/hero.jpg"
  alt="Hero"
  width={1200}
  height={600}
  priority // For LCP images
  placeholder="blur"
  blurDataURL={blurHash}
/>

// 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