Next.js 15 Best Practices

Core Principles

1. Default to Server Components — only add "use client" when you need interactivity or browser APIs
2. Colocate by feature — keep components, hooks, and utils near the routes that use them
3. Type everything — leverage TypeScript with strict mode enabled
4. Cache deliberately — understand the four caching layers and opt in/out explicitly

Project Structure

app/
  (marketing)/        # route group: no URL segment
    page.tsx
  (dashboard)/
    layout.tsx
    [id]/
      page.tsx
  api/
    route.ts
components/
  ui/                 # shared, "dumb" UI components
  features/           # feature-specific components
lib/
  db.ts               # database client (singleton)
  auth.ts             # auth helpers
  utils.ts

Server vs. Client Components

// ✅ Server Component (default) — runs on server, no JS sent to client
export default async function ProductList() {
  const products = await db.product.findMany()
  return <ul>{products.map(p => <li key={p.id}>{p.name}</li>)}</ul>
}

// ✅ Client Component — only when needed
"use client"
import { useState } from "react"
export function Counter() {
  const [n, setN] = useState(0)
  return <button onClick={() => setN(n + 1)}>{n}</button>
}

Data Fetching Patterns

// Parallel fetching (avoid sequential waterfalls)
export default async function Dashboard() {
  const [user, stats] = await Promise.all([
    fetchUser(),
    fetchStats(),
  ])
  return <View user={user} stats={stats} />
}

// fetch with cache control
const data = await fetch("https://api.example.com/data", {
  next: { revalidate: 60 },   // ISR: revalidate every 60s
  // cache: "no-store"         // always fresh
  // cache: "force-cache"      // static, until manual revalidation
})

Server Actions

// app/actions.ts
"use server"
import { revalidatePath } from "next/cache"

export async function createPost(formData: FormData) {
  const title = formData.get("title") as string
  await db.post.create({ data: { title } })
  revalidatePath("/posts")
}

// app/posts/new/page.tsx
import { createPost } from "../actions"
export default function NewPost() {
  return (
    <form action={createPost}>
      <input name="title" />
      <button type="submit">Create</button>
    </form>
  )
}

Metadata & SEO

// Static metadata
export const metadata = {
  title: "My App",
  description: "...",
}

// Dynamic metadata
export async function generateMetadata({ params }) {
  const post = await fetchPost(params.slug)
  return { title: post.title, description: post.excerpt }
}

Error & Loading States

// app/posts/loading.tsx  — automatic Suspense boundary
export default function Loading() {
  return <Skeleton />
}

// app/posts/error.tsx  — automatic error boundary
"use client"
export default function Error({ error, reset }) {
  return <button onClick={reset}>Retry: {error.message}</button>
}

Performance Checklist

• [ ] Images use next/image with explicit width/height
• [ ] Fonts use next/font (zero layout shift)
• [ ] Dynamic imports for heavy client components: dynamic(() => import(...))
• [ ] generateStaticParams for known dynamic routes
• [ ] Bundle analyzer run: ANALYZE=true next build
• [ ] Partial Prerendering (PPR) considered for mixed static/dynamic pages

References

See references/ folder for routing patterns, caching deep-dive, and migration guide.