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nextjs

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SKILL.md

Next.js Development Patterns

App Router vs Pages Router

The App Router (Next.js 13.4+) is recommended for new projects. The Pages Router remains supported.

Use App Router when: starting a new project, you need Server Components/streaming/Server Actions, nested layouts, or parallel routes. Use Pages Router when: maintaining an existing codebase, or depending on libraries without App Router support.

Both routers can coexist. Files in app/ use App Router; files in pages/ use Pages Router. Do not define the same route in both.

File-Based Routing (App Router)

app/
  layout.tsx        # Root layout (required, wraps all pages, must have <html> and <body>)
  page.tsx          # Home route (/)
  loading.tsx       # Suspense fallback (shown while page streams)
  error.tsx         # Error boundary (must be a Client Component)
  not-found.tsx     # 404 UI (triggered by notFound())
  global-error.tsx  # Error boundary for the root layout
  template.tsx      # Like layout but re-mounts on navigation
  dashboard/
    layout.tsx      # Nested layout for /dashboard/*
    page.tsx        # /dashboard
  • layout.tsx persists across navigations. page.tsx makes a route publicly accessible.
  • loading.tsx auto-wraps the page in <Suspense>.
  • error.tsx requires "use client" and receives error and reset props.

Server Components vs Client Components

All components are Server Components by default in the App Router.

Server Components run only on the server, can access databases/secrets directly, cannot use hooks or event handlers, and reduce client bundle size.

Client Components require "use client" as the first line. Needed for interactivity, hooks, browser APIs. They still SSR on initial load, then hydrate.

"use client";
import { useState } from "react";
export function Counter() {
  const [count, setCount] = useState(0);
  return <button onClick={() => setCount(count + 1)}>Count: {count}</button>;
}

Composition pattern: Keep Server Components as outer wrappers, pass Client Components as children. Only add "use client" when genuinely needed.

Data Fetching

App Router (Server Components)

// Cached indefinitely (like getStaticProps)
const res = await fetch("https://api.example.com/data");
// Revalidate every 60s (ISR)
const res = await fetch(url, { next: { revalidate: 60 } });
// No caching (like getServerSideProps)
const res = await fetch(url, { cache: "no-store" });

Segment-level config for non-fetch sources:

export const revalidate = 60;
export const dynamic = "force-dynamic";

Pages Router

  • getStaticProps -- build time (or ISR revalidation).
  • getServerSideProps -- every request.
  • getStaticPaths -- defines dynamic paths for static generation.

Server Actions

Server-side functions callable from forms and Client Components.

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

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

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

Can also be called with startTransition in Client Components. Always validate input server-side.

Dynamic Routes

app/blog/[slug]/page.tsx          -- /blog/hello-world
app/docs/[...catchAll]/page.tsx   -- /docs/a, /docs/a/b/c
app/shop/[[...optional]]/page.tsx -- /shop, /shop/a, /shop/a/b

export default async function BlogPost({ params }: { params: Promise<{ slug: string }> }) {
  const { slug } = await params;
  const post = await getPost(slug);
  return <article>{post.content}</article>;
}

export async function generateStaticParams() {
  const posts = await getAllPosts();
  return posts.map((post) => ({ slug: post.slug }));
}

Route Groups and Parallel Routes

Route groups use parentheses to organize without affecting URLs:

app/(marketing)/about/page.tsx  --> /about
app/(shop)/cart/page.tsx        --> /cart

Each group can have its own layout.tsx.

Parallel routes use @-prefixed slots rendered simultaneously in a layout:

app/layout.tsx     # Props: children, analytics, team
app/@analytics/page.tsx
app/@team/page.tsx

export default function Layout({ children, analytics, team }: {
  children: React.ReactNode; analytics: React.ReactNode; team: React.ReactNode;
}) {
  return <div>{children}{analytics}{team}</div>;
}

Middleware

Define middleware.ts at the project root (or src/). Runs before requests complete, on the Edge Runtime.

import { NextResponse } from "next/server";
import type { NextRequest } from "next/server";

export function middleware(request: NextRequest) {
  const token = request.cookies.get("session")?.value;
  if (!token && request.nextUrl.pathname.startsWith("/dashboard")) {
    return NextResponse.redirect(new URL("/login", request.url));
  }
  const country = request.geo?.country;
  if (country === "DE") {
    return NextResponse.rewrite(new URL("/de" + request.nextUrl.pathname, request.url));
  }
  const response = NextResponse.next();
  response.headers.set("x-request-id", crypto.randomUUID());
  return response;
}

export const config = {
  matcher: ["/((?!api|_next/static|_next/image|favicon.ico).*)"],
};

Keep middleware lightweight. Avoid heavy computation or large dependencies.

API Routes

App Router (Route Handlers)

// app/api/posts/route.ts
import { NextResponse } from "next/server";

export async function GET() {
  const posts = await db.post.findMany();
  return NextResponse.json(posts);
}

export async function POST(request: Request) {
  const body = await request.json();
  const post = await db.post.create({ data: body });
  return NextResponse.json(post, { status: 201 });
}

Supports GET, POST, PUT, PATCH, DELETE, HEAD, OPTIONS. GET-only route files are cached by default.

Pages Router

// pages/api/posts.ts
export default function handler(req: NextApiRequest, res: NextApiResponse) {
  if (req.method === "POST") { /* handle */ }
  res.status(200).json({ posts: [] });
}

Image Optimization

import Image from "next/image";
import heroImage from "@/public/hero.jpg";

<Image src={heroImage} alt="Hero" priority />
<Image src="https://cdn.example.com/photo.jpg" alt="Photo"
  width={800} height={600} sizes="(max-width: 768px) 100vw, 800px" />
  • priority on above-the-fold images (disables lazy loading).
  • sizes helps the browser pick the correct srcset image.
  • Configure remotePatterns in next.config.js for external domains.
  • fill prop with a positioned parent for responsive container-filling images.

Metadata and SEO

// Static
export const metadata = {
  title: "My App",
  description: "A description",
  openGraph: { title: "My App", images: ["/og.png"] },
};

// Dynamic
export async function generateMetadata({ params }: { params: Promise<{ slug: string }> }) {
  const { slug } = await params;
  const post = await getPost(slug);
  return { title: post.title, description: post.summary };
}

Sitemap and robots:

// app/sitemap.ts
export default async function sitemap() {
  const posts = await getAllPosts();
  return [
    { url: "https://example.com", lastModified: new Date() },
    ...posts.map((p) => ({ url: `https://example.com/blog/${p.slug}`, lastModified: p.updatedAt })),
  ];
}

// app/robots.ts
export default function robots() {
  return {
    rules: { userAgent: "*", allow: "/", disallow: "/private/" },
    sitemap: "https://example.com/sitemap.xml",
  };
}

Environment Variables

  • NEXT_PUBLIC_ prefix: inlined into the client bundle at build time, visible to the browser.
  • All other variables: server-only (Server Components, Route Handlers, Middleware).
  • Use .env.local for local overrides. Commit .env.example with placeholders.
  • Consider @t3-oss/env-nextjs or zod for startup validation.
DATABASE_URL=postgres://localhost:5432/mydb        # server only
NEXT_PUBLIC_API_URL=https://api.example.com         # available in browser

Caching

Four layers:

  1. Request Memoization -- Duplicate fetch calls with same URL/options are deduplicated within a single render.
  2. Data Cache -- fetch responses persisted. Invalidate with revalidatePath, revalidateTag, or time-based revalidation.
  3. Full Route Cache -- Static routes cached as HTML + RSC payload at build time. Invalidated when Data Cache invalidates.
  4. Router Cache -- RSC payloads cached in the browser per session. Configurable staleness.

Invalidation:

import { revalidatePath, revalidateTag } from "next/cache";

const res = await fetch(url, { next: { tags: ["posts"] } });
revalidateTag("posts");     // tag-based
revalidatePath("/blog");    // path-based
export const dynamic = "force-dynamic"; // opt out entirely

Tune Router Cache in next.config.js:

module.exports = { experimental: { staleTimes: { dynamic: 0, static: 180 } } };

Deployment

Vercel: Push to a connected Git repo. Zero config. Supports Edge Functions, ISR, image optimization.

Self-hosted:

next build && next start -p 3000

Docker:

FROM node:20-alpine AS base
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production
FROM base AS build
RUN npm ci
COPY . .
RUN npm run build
FROM base AS runner
COPY --from=build /app/.next/standalone ./
COPY --from=build /app/.next/static ./.next/static
COPY --from=build /app/public ./public
EXPOSE 3000
CMD ["node", "server.js"]

Requires output: "standalone" in next.config.js.

Static export: Set output: "export" in next.config.js. Outputs static HTML/CSS/JS to out/. No ISR, middleware, non-GET route handlers, or image optimization.

Common Patterns

Auth Middleware

const publicPaths = ["/login", "/register", "/api/auth"];
export function middleware(request: NextRequest) {
  const isPublic = publicPaths.some((p) => request.nextUrl.pathname.startsWith(p));
  if (isPublic) return NextResponse.next();
  const session = request.cookies.get("session");
  if (!session) return NextResponse.redirect(new URL("/login", request.url));
  return NextResponse.next();
}

Internationalization (i18n)

const locales = ["en", "fr", "de"];
const defaultLocale = "en";
export function middleware(request: NextRequest) {
  const { pathname } = request.nextUrl;
  const hasLocale = locales.some((l) => pathname.startsWith(`/${l}/`) || pathname === `/${l}`);
  if (hasLocale) return NextResponse.next();
  const preferred = request.headers.get("accept-language")?.split(",")[0].split("-")[0];
  const locale = locales.includes(preferred ?? "") ? preferred : defaultLocale;
  return NextResponse.rewrite(new URL(`/${locale}${pathname}`, request.url));
}

Dynamic OG Images

// app/api/og/route.tsx
import { ImageResponse } from "next/og";
export async function GET(request: Request) {
  const title = new URL(request.url).searchParams.get("title") ?? "Default";
  return new ImageResponse(
    (<div style={{ display: "flex", fontSize: 60, background: "white",
      width: "100%", height: "100%", alignItems: "center", justifyContent: "center" }}>
      {title}
    </div>),
    { width: 1200, height: 630 }
  );
}

Performance Tips

  • Streaming with Suspense: Wrap slow components in <Suspense> with a fallback. The shell renders immediately.
  • Partial Prerendering (experimental): Static shell with dynamic holes that stream in.
  • Parallel data fetching: Use Promise.all for independent fetches instead of sequential awaits.
  • Minimize "use client": Push interactivity to leaf components to reduce client JS.
  • Lazy-load heavy components: 
    import dynamic from "next/dynamic";
const Chart = dynamic(() => import("./chart"), { ssr: false });
  • Optimize fonts: Use next/font for self-hosted fonts with zero layout shift.
  • Analyze bundles: ANALYZE=true next build with @next/bundle-analyzer.
  • Prefer Server Components for data display -- zero client bundle cost.
  • Use React.cache for request-level deduplication of non-fetch functions.
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