Next.js Best Practices (App Router, v15+)

Server Components (default)

• All components are Server Components by default — no 'use client' needed
• Only add 'use client' when you need: hooks, event handlers, browser APIs, or Context
• Prefer server components for data fetching — no useEffect, no loading states
• Pass server data to client components as props (serializable only)

Route Conventions

• page.tsx — route page
• layout.tsx — shared layout (wraps children, persists across navigations)
• loading.tsx — streaming fallback (instant loading UI)
• error.tsx — error boundary ('use client' required)
• not-found.tsx — 404 page
• proxy.ts — request proxy/middleware (NOT middleware.ts in v16+)
• route.ts — API route handler

Data Fetching

• Fetch in server components directly (no getServerSideProps)
• Use fetch() with Next.js caching extensions
• Deduplicate requests — React auto-deduplicates identical fetches per render
• For mutations: server actions ('use server')

Caching Strategy

// Static (build time)
export const dynamic = 'force-static';

// Dynamic (every request)
export const dynamic = 'force-dynamic';

// ISR (revalidate every N seconds)
export const revalidate = 3600;

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

Partial Prerendering (PPR)

• Enable in next.config.ts: experimental: { ppr: true }
• Static shell renders instantly, dynamic parts stream in
• Use <Suspense> boundaries to mark dynamic regions
• No code changes needed — just wrap dynamic content in Suspense

Metadata

// Static
export const metadata: Metadata = {
  title: 'Page Title',
  description: 'Page description',
};

// Dynamic
export async function generateMetadata({ params }: Props): Promise<Metadata> {
  const data = await fetchData(params.id);
  return { title: data.title };
}

Server Actions

'use server';

export async function createItem(formData: FormData) {
  const name = formData.get('name') as string;
  // Validate, write to DB, revalidate
  revalidatePath('/items');
}

• Always validate input (use Zod)
• Always revalidate affected paths after mutations
• Return errors as values, not thrown exceptions

Image Optimization

import Image from 'next/image';

<Image
  src="/hero.jpg"
  alt="Description"
  width={1200}
  height={630}
  priority // above the fold
  placeholder="blur"
/>

Common Mistakes to Avoid

1. Don't use 'use client' on pages that don't need interactivity
2. Don't fetch data in client components when server components work
3. Don't use useEffect for data fetching — use server components or server actions
4. Don't put middleware.ts at the root — use proxy.ts in Next.js 16+
5. Don't use router.push() for mutations — use server actions + revalidatePath()
6. Don't wrap entire pages in Suspense — wrap individual dynamic sections

React 19 Conventions

• No useCallback, useMemo, or React.memo — React Compiler handles memoization
• Use use() instead of useContext() for reading context
• Use ternary {condition ? <A/> : null} instead of {condition && <A/>}
• ref is a regular prop now — no forwardRef needed

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