next-js

Server vs Client Components

• Default is Server Component in App Router — no useState, useEffect, browser APIs
• 'use client' at top of file for client — marks component and descendants as client
• Can't import Server Component into Client — only pass as children or props
• Client components can't be async — only Server Components can await directly

Caching Gotchas

• fetch cached by default in Server Components — add cache: 'no-store' for dynamic
• revalidate in seconds — next: { revalidate: 60 } for ISR
• Route Handlers not cached by default — except GET with no dynamic data
• revalidatePath('/path') or revalidateTag('tag') for on-demand — in Server Actions

Data Fetching

• Fetch in Server Components, not useEffect — no waterfalls, better performance
• Parallel fetches with Promise.all — not sequential awaits
• loading.tsx for Suspense boundary — automatic streaming
• Error boundaries with error.tsx — catches errors in segment

Environment Variables

• NEXT_PUBLIC_ prefix for client-side — otherwise only available on server
• Server Components access all env vars — no prefix needed
• .env.local for secrets — .env checked into repo, .env.local gitignored
• Runtime env with process.env — build-time with env in next.config.js

Route Handlers (API Routes)

• route.ts in App Router — pages/api is Pages Router
• Export named functions: GET, POST, etc. — not default export
• NextRequest and NextResponse — typed request/response
• Dynamic by default — export const dynamic = 'force-static' to cache

Server Actions

• 'use server' at top of function or file — marks as server action
• Can be called from Client Components — automatic RPC
• Form action={serverAction} — progressive enhancement, works without JS
• Revalidate cache after mutation — revalidatePath or revalidateTag

Middleware

• Runs on Edge, not Node — limited APIs, no fs, limited npm packages
• matcher config to limit routes — don't run on static assets
• Can't throw errors — return redirect or next()
• Cookies and headers access — but can't call database directly

Dynamic Routes

• [slug] folder name for params — page.tsx receives params.slug
• [...slug] for catch-all — array of segments
• generateStaticParams for SSG — return array of param objects
• dynamicParams = false to 404 unknown — otherwise renders on demand

Common Mistakes

• Using router.push in Server Component — only works in Client, use redirect()
• <Link> prefetches in viewport — can cause excessive requests, prefetch={false} to disable
• next/image without width/height — required unless fill prop with positioned parent
• Metadata in Client Component — generateMetadata only in Server Components
• cookies() makes route dynamic — can't be statically generated