nextjs

---
name: "nextjs"
description: "Guide for implementing Next.js - a React framework for production with server-side rendering, static generation, and modern web features. Use when building Next.js applications, implementing App Router, working with server components, data fetching, routing, or optimizing performance."
license: "MIT"
---

# Next.js Skill

Next.js is a React framework for building full-stack web applications with server-side rendering, static generation, and powerful optimization features built-in.

## Reference

https://nextjs.org/docs/llms.txt

## When to Use This Skill

Use this skill when:

- Building new Next.js applications (v15+)
- Implementing App Router architecture
- Working with Server Components and Client Components
- Setting up routing, layouts, and navigation
- Implementing data fetching patterns
- Optimizing images, fonts, and performance
- Configuring metadata and SEO
- Setting up API routes and route handlers
- Migrating from Pages Router to App Router
- Deploying Next.js applications

## Core Concepts

### App Router vs Pages Router

**App Router (Recommended for v13+):**

- Modern architecture with React Server Components
- File-system based routing in `app/` directory
- Layouts, loading states, and error boundaries
- Streaming and Suspense support
- Nested routing with layouts

**Pages Router (Legacy):**

- Traditional page-based routing in `pages/` directory
- Uses `getStaticProps`, `getServerSideProps`, `getInitialProps`
- Still supported for existing projects

### Key Architectural Principles

1. **Server Components by Default**: Components in `app/` are Server Components unless marked with `'use client'`
2. **File-based Routing**: File system defines application routes
3. **Nested Layouts**: Share UI across routes with layouts
4. **Progressive Enhancement**: Works without JavaScript when possible
5. **Automatic Optimization**: Images, fonts, scripts auto-optimized

## Installation & Setup

### Create New Project

```bash
npx create-next-app@latest my-app
# or
yarn create next-app my-app
# or
pnpm create next-app my-app
# or
bun create next-app my-app
```

**Interactive Setup Prompts:**

- TypeScript? (Yes recommended)
- ESLint? (Yes recommended)
- Tailwind CSS? (Optional)
- `src/` directory? (Optional)
- App Router? (Yes for new projects)
- Import alias? (Default: @/\*)

### Manual Setup

```bash
npm install next@latest react@latest react-dom@latest
```

**package.json scripts:**

```json
{
	"scripts": {
		"dev": "next dev",
		"build": "next build",
		"start": "next start",
		"lint": "next lint"
	}
}
```

### Project Structure

```
my-app/
├── app/                    # App Router (v13+)
│   ├── layout.tsx         # Root layout
│   ├── page.tsx           # Home page
│   ├── loading.tsx        # Loading UI
│   ├── error.tsx          # Error UI
│   ├── not-found.tsx      # 404 page
│   ├── global.css         # Global styles
│   └── [folder]/          # Route segments
├── public/                # Static assets
├── components/            # React components
├── lib/                   # Utility functions
├── next.config.js         # Next.js configuration
├── package.json
└── tsconfig.json
```

## Routing

### File Conventions

- `page.tsx` - Page UI for route
- `layout.tsx` - Shared UI for segment and children
- `loading.tsx` - Loading UI (wraps page in Suspense)
- `error.tsx` - Error UI (wraps page in Error Boundary)
- `not-found.tsx` - 404 UI
- `route.ts` - API endpoint (Route Handler)
- `template.tsx` - Re-rendered layout UI
- `default.tsx` - Parallel route fallback

### Basic Routing

**Static Route:**

```
app/
├── page.tsx              → /
├── about/
│   └── page.tsx         → /about
└── blog/
    └── page.tsx         → /blog
```

**Dynamic Route:**

```tsx
// app/blog/[slug]/page.tsx
export default function BlogPost({ params }: { params: { slug: string } }) {
	return <h1>Post: {params.slug}</h1>;
}
```

**Catch-all Route:**

```tsx
// app/shop/[...slug]/page.tsx
export default function Shop({ params }: { params: { slug: string[] } }) {
	return <h1>Category: {params.slug.join("/")}</h1>;
}
```

**Optional Catch-all:**

```tsx
// app/docs/[[...slug]]/page.tsx
// Matches /docs, /docs/a, /docs/a/b, etc.
```

### Route Groups

Organize routes without affecting URL:

```
app/
├── (marketing)/          # Group without URL segment
│   ├── about/page.tsx   → /about
│   └── blog/page.tsx    → /blog
└── (shop)/
    ├── products/page.tsx → /products
    └── cart/page.tsx     → /cart
```

### Parallel Routes

Render multiple pages in same layout:

```
app/
├── @team/               # Slot
│   └── page.tsx
├── @analytics/          # Slot
│   └── page.tsx
└── layout.tsx           # Uses both slots
```

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

### Intercepting Routes

Intercept routes to show in modal:

```
app/
├── feed/
│   └── page.tsx
├── photo/
│   └── [id]/
│       └── page.tsx
└── (..)photo/           # Intercepts /photo/[id]
    └── [id]/
        └── page.tsx
```

## Layouts

### Root Layout (Required)

```tsx
// app/layout.tsx
export default function RootLayout({
	children,
}: {
	children: React.ReactNode;
}) {
	return (
		<html lang="en">
			<body>{children}</body>
		</html>
	);
}
```

### Nested Layouts

```tsx
// app/dashboard/layout.tsx
export default function DashboardLayout({
	children,
}: {
	children: React.ReactNode;
}) {
	return (
		<section>
			<nav>Dashboard Nav</nav>
			{children}
		</section>
	);
}
```

**Layouts are:**

- Shared across multiple pages
- Preserve state on navigation
- Do not re-render on navigation
- Can fetch data

## Server and Client Components

### Server Components (Default)

Components in `app/` are Server Components by default:

```tsx
// app/page.tsx (Server Component)
async function getData() {
	const res = await fetch("https://api.example.com/data");
	return res.json();
}

export default async function Page() {
	const data = await getData();
	return <div>{data.title}</div>;
}
```

**Benefits:**

- Fetch data on server
- Access backend resources directly
- Keep sensitive data on server
- Reduce client-side JavaScript
- Improve initial page load

**Limitations:**

- Cannot use hooks (useState, useEffect)
- Cannot use browser APIs
- Cannot add event listeners

### Client Components

Mark components with `'use client'` directive:

```tsx
// components/counter.tsx
"use client";

import { useState } from "react";

export function Counter() {
	const [count, setCount] = useState(0);

	return <button onClick={() => setCount(count + 1)}>Count: {count}</button>;
}
```

**Use Client Components for:**

- Interactive UI (onClick, onChange)
- State management (useState, useReducer)
- Effects (useEffect, useLayoutEffect)
- Browser APIs (localStorage, navigator)
- Custom hooks
- React class components

### Composition Pattern

```tsx
// app/page.tsx (Server Component)
import { ClientComponent } from "./client-component";

export default function Page() {
	return (
		<div>
			<h1>Server-rendered content</h1>
			<ClientComponent />
		</div>
	);
}
```

## Data Fetching

### Server Component Data Fetching

```tsx
// app/posts/page.tsx
async function getPosts() {
	const res = await fetch("https://api.example.com/posts", {
		next: { revalidate: 3600 }, // Revalidate every hour
	});

	if (!res.ok) {
		throw new Error("Failed to fetch");
	}

	return res.json();
}

export default async function PostsPage() {
	const posts = await getPosts();

	return (
		<ul>
			{posts.map((post) => (
				<li key={post.id}>{post.title}</li>
			))}
		</ul>
	);
}
```

### Caching Strategies

**Force Cache (Default):**

```tsx
fetch("https://api.example.com/data", { cache: "force-cache" });
```

**No Store (Dynamic):**

```tsx
fetch("https://api.example.com/data", { cache: "no-store" });
```

**Revalidate:**

```tsx
fetch("https://api.example.com/data", {
	next: { revalidate: 3600 }, // Seconds
});
```

**Tag-based Revalidation:**

```tsx
fetch("https://api.example.com/data", {
	next: { tags: ["posts"] },
});

// Revalidate elsewhere:
import { revalidateTag } from "next/cache";
revalidateTag("posts");
```

### Parallel Data Fetching

```tsx
async function getData() {
	const [posts, users] = await Promise.all([
		fetch("https://api.example.com/posts").then((r) => r.json()),
		fetch("https://api.example.com/users").then((r) => r.json()),
	]);

	return { posts, users };
}
```

### Sequential Data Fetching

```tsx
async function getData() {
	const post = await fetch(`https://api.example.com/posts/${id}`).then((r) =>
		r.json(),
	);
	const author = await fetch(
		`https://api.example.com/users/${post.authorId}`,
	).then((r) => r.json());

	return { post, author };
}
```

## Route Handlers (API Routes)

### Basic Route Handler

```tsx
// app/api/hello/route.ts
export async function GET(request: Request) {
	return Response.json({ message: "Hello" });
}

export async function POST(request: Request) {
	const body = await request.json();
	return Response.json({ received: body });
}
```

### Dynamic Route Handler

```tsx
// app/api/posts/[id]/route.ts
export async function GET(
	request: Request,
	{ params }: { params: { id: string } },
) {
	const post = await getPost(params.id);
	return Response.json(post);
}

export async function DELETE(
	request: Request,
	{ params }: { params: { id: string } },
) {
	await deletePost(params.id);
	return new Response(null, { status: 204 });
}
```

### Request Helpers

```tsx
export async function GET(request: Request) {
	const { searchParams } = new URL(request.url);
	const id = searchParams.get("id");

	const cookies = request.headers.get("cookie");

	return Response.json({ id });
}
```

### Response Types

```tsx
// JSON
return Response.json({ data: "value" });

// Text
return new Response("Hello", { headers: { "Content-Type": "text/plain" } });

// Redirect
return Response.redirect("https://example.com");

// Status codes
return new Response("Not Found", { status: 404 });
```

## Navigation

### Link Component

```tsx
import Link from "next/link";

export default function Page() {
	return (
		<>
			<Link href="/about">About</Link>
			<Link href="/blog/post-1">Post 1</Link>
			<Link href={{ pathname: "/blog/[slug]", query: { slug: "post-1" } }}>
				Post 1 (alternative)
			</Link>
		</>
	);
}
```

### useRouter Hook (Client)

```tsx
"use client";

import { useRouter } from "next/navigation";

export function NavigateButton() {
	const router = useRouter();

	return <button onClick={() => router.push("/dashboard")}>Dashboard</button>;
}
```

**Router Methods:**

- `router.push(href)` - Navigate to route
- `router.replace(href)` - Replace current history
- `router.refresh()` - Refresh current route
- `router.back()` - Navigate back
- `router.forward()` - Navigate forward
- `router.prefetch(href)` - Prefetch route

### Programmatic Navigation (Server)

```tsx
import { redirect } from "next/navigation";

export default async function Page() {
	const session = await getSession();

	if (!session) {
		redirect("/login");
	}

	return <div>Protected content</div>;
}
```

## Metadata & SEO

### Static Metadata

```tsx
// app/page.tsx
import { Metadata } from "next";

export const metadata: Metadata = {
	title: "My Page",
	description: "Page description",
	keywords: ["nextjs", "react"],
	openGraph: {
		title: "My Page",
		description: "Page description",
		images: ["/og-image.jpg"],
	},
	twitter: {
		card: "summary_large_image",
		title: "My Page",
		description: "Page description",
		images: ["/twitter-image.jpg"],
	},
};

export default function Page() {
	return <div>Content</div>;
}
```

### Dynamic Metadata

```tsx
// app/blog/[slug]/page.tsx
export async function generateMetadata({ params }): Promise<Metadata> {
	const post = await getPost(params.slug);

	return {
		title: post.title,
		description: post.excerpt,
		openGraph: {
			title: post.title,
			description: post.excerpt,
			images: [post.coverImage],
		},
	};
}
```

### Metadata Files

- `favicon.ico`, `icon.png`, `apple-icon.png` - Favicons
- `opengraph-image.png`, `twitter-image.png` - Social images
- `robots.txt` - Robots file
- `sitemap.xml` - Sitemap

## Image Optimization

### Image Component

```tsx
import Image from "next/image";

export default function Page() {
	return (
		<>
			{/* Local image */}
			<Image src="/profile.png" alt="Profile" width={500} height={500} />

			{/* Remote image */}
			<Image
				src="https://example.com/image.jpg"
				alt="Remote"
				width={500}
				height={500}
			/>

			{/* Responsive fill */}
			<div style={{ position: "relative", width: "100%", height: "400px" }}>
				<Image src="/hero.jpg" alt="Hero" fill style={{ objectFit: "cover" }} />
			</div>

			{/* Priority loading */}
			<Image src="/hero.jpg" alt="Hero" width={1200} height={600} priority />
		</>
	);
}
```

**Image Props:**

- `src` - Image path (local or URL)
- `alt` - Alt text (required)
- `width`, `height` - Dimensions (required unless fill)
- `fill` - Fill parent container
- `sizes` - Responsive sizes
- `quality` - 1-100 (default 75)
- `priority` - Preload image
- `placeholder` - 'blur-sm' | 'empty'
- `blurDataURL` - Data URL for blur

### Remote Image Configuration

```js
// next.config.js
module.exports = {
	images: {
		remotePatterns: [
			{
				protocol: "https",
				hostname: "example.com",
				pathname: "/images/**",
			},
		],
	},
};
```

## Font Optimization

### Google Fonts

```tsx
// app/layout.tsx
import { Inter, Roboto_Mono } from "next/font/google";

const inter = Inter({
	subsets: ["latin"],
	display: "swap",
});

const robotoMono = Roboto_Mono({
	subsets: ["latin"],
	display: "swap",
	variable: "--font-roboto-mono",
});

export default function RootLayout({ children }) {
	return (
		<html lang="en" className={`${inter.className} ${robotoMono.variable}`}>
			<body>{children}</body>
		</html>
	);
}
```

### Local Fonts

```tsx
import localFont from "next/font/local";

const myFont = localFont({
	src: "./fonts/my-font.woff2",
	display: "swap",
	variable: "--font-my-font",
});
```

## Loading States

### Loading File

```tsx
// app/dashboard/loading.tsx
export default function Loading() {
	return <div>Loading dashboard...</div>;
}
```

### Streaming with Suspense

```tsx
// app/page.tsx
import { Suspense } from "react";

async function Posts() {
	const posts = await getPosts();
	return (
		<ul>
			{posts.map((p) => (
				<li key={p.id}>{p.title}</li>
			))}
		</ul>
	);
}

export default function Page() {
	return (
		<div>
			<h1>My Posts</h1>
			<Suspense fallback={<div>Loading posts...</div>}>
				<Posts />
			</Suspense>
		</div>
	);
}
```

## Error Handling

### Error File

```tsx
// app/error.tsx
"use client";

export default function Error({
	error,
	reset,
}: {
	error: Error & { digest?: string };
	reset: () => void;
}) {
	return (
		<div>
			<h2>Something went wrong!</h2>
			<p>{error.message}</p>
			<button onClick={() => reset()}>Try again</button>
		</div>
	);
}
```

### Global Error

```tsx
// app/global-error.tsx
"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>
	);
}
```

### Not Found

```tsx
// app/not-found.tsx
export default function NotFound() {
	return (
		<div>
			<h2>404 - Not Found</h2>
			<p>Could not find requested resource</p>
		</div>
	);
}

// Trigger programmatically
import { notFound } from "next/navigation";

export default async function Page({ params }) {
	const post = await getPost(params.id);

	if (!post) {
		notFound();
	}

	return <div>{post.title}</div>;
}
```

## Middleware

```tsx
// middleware.ts
import { NextResponse } from "next/server";
import type { NextRequest } from "next/server";

export function middleware(request: NextRequest) {
	// Authentication check
	const token = request.cookies.get("token");

	if (!token) {
		return NextResponse.redirect(new URL("/login", request.url));
	}

	// Add custom header
	const response = NextResponse.next();
	response.headers.set("x-custom-header", "value");

	return response;
}

export const config = {
	matcher: ["/dashboard/:path*", "/api/:path*"],
};
```

## Environment Variables

```env
# .env.local
DATABASE_URL=postgresql://...
NEXT_PUBLIC_API_URL=https://api.example.com
```

```tsx
// Server-side only
const dbUrl = process.env.DATABASE_URL;

// Client and server (NEXT_PUBLIC_ prefix)
const apiUrl = process.env.NEXT_PUBLIC_API_URL;
```

## Configuration

### next.config.js

```js
/** @type {import('next').NextConfig} */
const nextConfig = {
	// React strict mode
	reactStrictMode: true,

	// Image domains
	images: {
		remotePatterns: [{ protocol: "https", hostname: "example.com" }],
	},

	// Redirects
	async redirects() {
		return [
			{
				source: "/old-page",
				destination: "/new-page",
				permanent: true,
			},
		];
	},

	// Rewrites
	async rewrites() {
		return [
			{
				source: "/api/:path*",
				destination: "https://api.example.com/:path*",
			},
		];
	},

	// Headers
	async headers() {
		return [
			{
				source: "/(.*)",
				headers: [{ key: "X-Frame-Options", value: "DENY" }],
			},
		];
	},

	// Environment variables
	env: {
		CUSTOM_KEY: "value",
	},
};

module.exports = nextConfig;
```

## Best Practices

1. **Use Server Components**: Default to Server Components, use Client Components only when needed
2. **Optimize Images**: Always use `next/image` for automatic optimization
3. **Metadata**: Set proper metadata for SEO
4. **Loading States**: Provide loading UI with Suspense
5. **Error Handling**: Implement error boundaries
6. **Route Handlers**: Use for API endpoints instead of separate backend
7. **Caching**: Leverage built-in caching strategies
8. **Layouts**: Use nested layouts to share UI
9. **TypeScript**: Enable TypeScript for type safety
10. **Performance**: Use `priority` for above-fold images, lazy load below-fold

## Common Patterns

### Protected Routes

```tsx
// app/dashboard/layout.tsx
import { redirect } from "next/navigation";
import { getSession } from "@/lib/auth";

export default async function DashboardLayout({ children }) {
	const session = await getSession();

	if (!session) {
		redirect("/login");
	}

	return <>{children}</>;
}
```

### Data Mutations (Server Actions)

```tsx
// app/actions.ts
"use server";

import { revalidatePath } from "next/cache";

export async function createPost(formData: FormData) {
	const title = formData.get("title");

	await db.post.create({ data: { title } });

	revalidatePath("/posts");
}

// app/posts/new/page.tsx
import { createPost } from "@/app/actions";

export default function NewPost() {
	return (
		<form action={createPost}>
			<input name="title" type="text" required />
			<button type="submit">Create</button>
		</form>
	);
}
```

### Static Generation

```tsx
// Generate static params for dynamic routes
export async function generateStaticParams() {
	const posts = await getPosts();

	return posts.map((post) => ({
		slug: post.slug,
	}));
}

export default async function Post({ params }) {
	const post = await getPost(params.slug);
	return <article>{post.content}</article>;
}
```

## Deployment

### Vercel (Recommended)

```bash
# Install Vercel CLI
npm i -g vercel

# Deploy
vercel
```

### Self-Hosting

```bash
# Build
npm run build

# Start production server
npm start
```

**Requirements:**

- Node.js 18.17 or later
- `output: 'standalone'` in next.config.js (optional, reduces size)

### Docker

```dockerfile
FROM node:18-alpine AS base

FROM base AS deps
WORKDIR /app
COPY package*.json ./
RUN npm ci

FROM base AS builder
WORKDIR /app
COPY --from=deps /app/node_modules ./node_modules
COPY . .
RUN npm run build

FROM base AS runner
WORKDIR /app
ENV NODE_ENV production
COPY --from=builder /app/public ./public
COPY --from=builder /app/.next/standalone ./
COPY --from=builder /app/.next/static ./.next/static

EXPOSE 3000
CMD ["node", "server.js"]
```

## Troubleshooting

### Common Issues

1. **Hydration errors**
   - Ensure server and client render same content
   - Check for browser-only code in Server Components
   - Verify no conditional rendering based on browser APIs

2. **Images not loading**
   - Add remote domains to `next.config.js`
   - Check image paths (use leading `/` for public)
   - Verify width/height provided

3. **API route 404**
   - Check file is named `route.ts/js` not `index.ts`
   - Verify export named GET/POST not default export
   - Ensure in `app/api/` directory

4. **"use client" errors**
   - Add `'use client'` to components using hooks
   - Import Client Components in Server Components, not vice versa
   - Check event handlers have `'use client'`

5. **Metadata not updating**
   - Clear browser cache
   - Check metadata export is named correctly
   - Verify async generateMetadata returns Promise<Metadata>

## Resources

- Documentation: https://nextjs.org/docs
- Learn Course: https://nextjs.org/learn
- Examples: https://github.com/vercel/next.js/tree/canary/examples
- Blog: https://nextjs.org/blog
- GitHub: https://github.com/vercel/next.js

## Implementation Checklist

When building with Next.js:

- [ ] Create project with `create-next-app`
- [ ] Configure TypeScript and ESLint
- [ ] Set up root layout with metadata
- [ ] Implement routing structure
- [ ] Add loading and error states
- [ ] Configure image optimization
- [ ] Set up font optimization
- [ ] Implement data fetching patterns
- [ ] Add API routes as needed
- [ ] Configure environment variables
- [ ] Set up middleware if needed
- [ ] Optimize for production build
- [ ] Test in production mode
- [ ] Configure deployment platform
- [ ] Set up monitoring and analytics