Next.js App Router Fundamentals

Overview

Provide comprehensive guidance for Next.js App Router (Next.js 13+), covering migration from Pages Router, file-based routing conventions, layouts, metadata handling, and modern Next.js patterns.

TypeScript: NEVER Use `any` Type

CRITICAL RULE: This codebase has @typescript-eslint/no-explicit-any enabled. Using any will cause build failures.

❌ WRONG:

function handleSubmit(e: any) { ... }
const data: any[] = [];

✅ CORRECT:

function handleSubmit(e: React.FormEvent<HTMLFormElement>) { ... }
const data: string[] = [];

Common Next.js Type Patterns

// Page props
function Page({ params }: { params: { slug: string } }) { ... }
function Page({ searchParams }: { searchParams: { [key: string]: string | string[] | undefined } }) { ... }

// Form events
const handleSubmit = (e: React.FormEvent<HTMLFormElement>) => { ... }
const handleChange = (e: React.ChangeEvent<HTMLInputElement>) => { ... }

// Server actions
async function myAction(formData: FormData) { ... }

When to Use This Skill

Use this skill when:

Migrating from Pages Router (pages/ directory) to App Router (app/ directory)
Creating Next.js 13+ applications from scratch
Working with layouts, templates, and nested routing
Implementing metadata and SEO optimizations
Building with App Router routing conventions
Handling route groups, parallel routes, or intercepting routes basics

Core Concepts

App Router vs Pages Router

Pages Router (Legacy - Next.js 12 and earlier):

pages/
├── index.tsx              # Route: /
├── about.tsx              # Route: /about
├── _app.tsx               # Custom App component
├── _document.tsx          # Custom Document component
└── api/                   # API routes
    └── hello.ts           # API endpoint: /api/hello

App Router (Modern - Next.js 13+):

app/
├── layout.tsx             # Root layout (required)
├── page.tsx               # Route: /
├── about/                 # Route: /about
│   └── page.tsx
├── blog/
│   ├── layout.tsx         # Nested layout
│   └── [slug]/
│       └── page.tsx       # Dynamic route: /blog/:slug
└── api/                   # Route handlers
    └── hello/
        └── route.ts       # API endpoint: /api/hello

File Conventions

Special Files in App Router:

layout.tsx - Shared UI for a segment and its children (preserves state, doesn't re-render)
page.tsx - Unique UI for a route, makes route publicly accessible
loading.tsx - Loading UI with React Suspense
error.tsx - Error UI with Error Boundaries
not-found.tsx - 404 UI
template.tsx - Similar to layout but re-renders on navigation
route.ts - API endpoints (Route Handlers)

Colocation:

Components, tests, and other files can be colocated in app/
Only page.tsx and route.ts files create public routes
Other files (components, utils, tests) are NOT routable

Migration Guide: Pages Router to App Router

Step 1: Understand the Current Structure

Examine existing Pages Router setup:

Read pages/ directory structure
Identify _app.tsx - handles global state, layouts, providers
Identify _document.tsx - customizes HTML structure
Note metadata usage (next/head, <Head> component)
List all routes and dynamic segments

Step 2: Create Root Layout

Create app/layout.tsx - REQUIRED for all App Router applications:

// app/layout.tsx
export const metadata = {
  title: 'My App',
  description: 'App description',
};

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

Migration Notes:

Move _document.tsx HTML structure to layout.tsx
Move _app.tsx global providers/wrappers to layout.tsx
Convert <Head> metadata to metadata export
The root layout MUST include <html> and <body> tags

Step 3: Migrate Pages to Routes

Simple Page Migration:

// Before: pages/index.tsx
import Head from 'next/head';

export default function Home() {
  return (
    <>
      <Head>
        <title>Home Page</title>
      </Head>
      <main>
        <h1>Welcome</h1>
      </main>
    </>
  );
}

// After: app/page.tsx
export default function Home() {
  return (
    <main>
      <h1>Welcome</h1>
    </main>
  );
}

// Metadata moved to layout.tsx or exported here
export const metadata = {
  title: 'Home Page',
};

Nested Route Migration:

// Before: pages/blog/[slug].tsx
export default function BlogPost() { ... }

// After: app/blog/[slug]/page.tsx
export default function BlogPost() { ... }

Step 4: Update Navigation

Replace anchor tags with Next.js Link:

// Before (incorrect in App Router)
<a href="/about">About</a>

// After (correct)
import Link from 'next/link';
<Link href="/about">About</Link>

Step 5: Clean Up Pages Directory

After migration:

Remove all page files from pages/ directory
Keep pages/api/ if you're not migrating API routes yet
Remove _app.tsx and _document.tsx (functionality moved to layout)
Optionally delete empty pages/ directory

Metadata Handling

Static Metadata

// app/page.tsx or app/layout.tsx
import type { 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'],
  },
};

Dynamic Metadata

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

  return {
    title: post.title,
    description: post.excerpt,
  };
}

Layouts and Nesting

Creating Nested Layouts

// app/layout.tsx - Root layout
export default function RootLayout({ children }) {
  return (
    <html>
      <body>
        <Header />
        {children}
        <Footer />
      </body>
    </html>
  );
}

// app/blog/layout.tsx - Blog layout
export default function BlogLayout({ children }) {
  return (
    <div>
      <BlogSidebar />
      <main>{children}</main>
    </div>
  );
}

Layout Behavior:

Layouts preserve state across navigation
Layouts don't re-render on route changes
Parent layouts wrap child layouts
Root layout is required and wraps entire app

Routing Patterns

Dynamic Routes

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

Catch-All Routes

// app/shop/[...slug]/page.tsx - Matches /shop/a, /shop/a/b, etc.
export default function Shop({
  params
}: {
  params: { slug: string[] }
}) {
  return <div>Path: {params.slug.join('/')}</div>;
}

Optional Catch-All

// app/shop/[[...slug]]/page.tsx - Matches /shop AND /shop/a, /shop/a/b

Route Groups

Group routes without affecting URL:

app/
├── (marketing)/
│   ├── about/
│   │   └── page.tsx      # /about
│   └── contact/
│       └── page.tsx      # /contact
└── (shop)/
    └── products/
        └── page.tsx      # /products

Common Migration Pitfalls

Pitfall 1: Forgetting Root Layout HTML Tags

Wrong:

export default function RootLayout({ children }) {
  return <div>{children}</div>; // Missing <html> and <body>
}

Correct:

export default function RootLayout({ children }) {
  return (
    <html lang="en">
      <body>{children}</body>
    </html>
  );
}

Pitfall 2: Using `next/head` in App Router

Wrong:

import Head from 'next/head';

export default function Page() {
  return (
    <>
      <Head><title>Title</title></Head>
      <main>Content</main>
    </>
  );
}

Correct:

export const metadata = { title: 'Title' };

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

Pitfall 3: Not Removing Pages Directory

After migrating routes, remove the old pages/ directory files to avoid confusion. The build will fail if you have conflicting routes.

Pitfall 4: Missing `page.tsx` Files

Routes are NOT accessible without a page.tsx file. Layouts alone don't create routes.

app/
├── blog/
│   ├── layout.tsx   # NOT a route
│   └── page.tsx     # This makes /blog accessible

Pitfall 5: Incorrect Link Usage

Wrong:

<a href="/about">About</a>  // Works but causes full page reload

Correct:

import Link from 'next/link';
<Link href="/about">About</Link>  // Client-side navigation

Server Components vs Client Components

Default: Server Components

All components in app/ are Server Components by default:

// app/page.tsx - Server Component (default)
export default async function Page() {
  const data = await fetch('https://api.example.com/data');
  const json = await data.json();

  return <div>{json.title}</div>;
}

Benefits:

Can use async/await directly
Direct database/API access
Zero client-side JavaScript
Automatic code splitting

Client Components

Use 'use client' directive when you need:

Interactive elements (onClick, onChange, etc.)
React hooks (useState, useEffect, useContext, etc.)
Browser APIs (window, localStorage, etc.)
Event listeners

// app/components/Counter.tsx
'use client';

import { useState } from 'react';

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

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

Data Fetching Patterns

Server Component Data Fetching

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

Parallel Data Fetching

export default async function Page() {
  // Fetch in parallel
  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 (/* render */);
}

Static Site Generation with generateStaticParams

Overview

generateStaticParams is the App Router equivalent of getStaticPaths from the Pages Router. It generates static pages at build time for dynamic routes.

Basic Usage

// app/blog/[id]/page.tsx
export async function generateStaticParams() {
  // Return array of params to pre-render
  return [
    { id: '1' },
    { id: '2' },
    { id: '3' },
  ];
}

export default function BlogPost({
  params
}: {
  params: { id: string }
}) {
  return <article>Blog post {params.id}</article>;
}

Key Points:

Returns an array of objects with route parameter keys
Each object represents one page to pre-render at build time
Function must be exported and named generateStaticParams
Works ONLY in Server Components (no 'use client' directive)
Replaces Pages Router's getStaticPaths

Fetching Data for Static Params

// app/blog/[slug]/page.tsx
export async function generateStaticParams() {
  const posts = await fetch('https://api.example.com/posts').then(r => r.json());

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

export default async function BlogPost({
  params
}: {
  params: { slug: string }
}) {
  const post = await fetch(`https://api.example.com/posts/${params.slug}`).then(r => r.json());

  return (
    <article>
      <h1>{post.title}</h1>
      <p>{post.content}</p>
    </article>
  );
}

Multiple Dynamic Segments

// app/products/[category]/[id]/page.tsx
export async function generateStaticParams() {
  const categories = await getCategories();

  const params = [];
  for (const category of categories) {
    const products = await getProducts(category.slug);
    for (const product of products) {
      params.push({
        category: category.slug,
        id: product.id,
      });
    }
  }

  return params;
}

export default function ProductPage({
  params
}: {
  params: { category: string; id: string }
}) {
  return <div>Category: {params.category}, Product: {params.id}</div>;
}

Dynamic Behavior Configuration

// app/blog/[id]/page.tsx
export async function generateStaticParams() {
  return [{ id: '1' }, { id: '2' }];
}

// Control behavior for non-pre-rendered paths
export const dynamicParams = true; // default - allows runtime generation
// export const dynamicParams = false; // returns 404 for non-pre-rendered paths

export default function BlogPost({
  params
}: {
  params: { id: string }
}) {
  return <article>Post {params.id}</article>;
}

Options:

dynamicParams = true (default): Non-pre-rendered paths generated on-demand
dynamicParams = false: Non-pre-rendered paths return 404

Common Patterns

Pattern 1: Simple ID-based routes

export async function generateStaticParams() {
  return [
    { id: '1' },
    { id: '2' },
    { id: '3' },
  ];
}

Pattern 2: Fetch from API

export async function generateStaticParams() {
  const items = await fetch('https://api.example.com/items').then(r => r.json());
  return items.map(item => ({ id: item.id }));
}

Pattern 3: Database query

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

Migration from Pages Router

Before (Pages Router):

// pages/blog/[id].tsx
export async function getStaticPaths() {
  return {
    paths: [
      { params: { id: '1' } },
      { params: { id: '2' } },
    ],
    fallback: false,
  };
}

export async function getStaticProps({ params }) {
  return { props: { id: params.id } };
}

After (App Router):

// app/blog/[id]/page.tsx
export async function generateStaticParams() {
  return [
    { id: '1' },
    { id: '2' },
  ];
}

export const dynamicParams = false; // equivalent to fallback: false

export default function BlogPost({ params }: { params: { id: string } }) {
  return <div>Post {params.id}</div>;
}

Common Mistakes to Avoid

❌ Wrong: Using 'use client'

'use client'; // ERROR! generateStaticParams only works in Server Components

export async function generateStaticParams() {
  return [{ id: '1' }];
}

❌ Wrong: Using Pages Router pattern

export async function getStaticPaths() { // Wrong API!
  return { paths: [...], fallback: false };
}

❌ Wrong: Missing export keyword

async function generateStaticParams() { // Must be exported!
  return [{ id: '1' }];
}

✅ Correct: Clean Server Component

// app/blog/[id]/page.tsx
// No 'use client' directive

export async function generateStaticParams() {
  return [{ id: '1' }, { id: '2' }];
}

export default function Page({ params }: { params: { id: string } }) {
  return <div>Post {params.id}</div>;
}

CRITICAL IMPLEMENTATION NOTE:

When asked to "write" or "implement" generateStaticParams:

DO use the Edit or Write tool to modify the actual file
DO add the function to the existing page.tsx file
DO remove any TODO comments about generateStaticParams
DON'T just output code in markdown - actually implement it
DON'T show code without writing it to the file

Testing and Validation

When migrating or building with App Router, verify:

Structure:
- app/ directory exists
- Root layout.tsx exists with <html> and <body>
- Each route has a page.tsx file
Metadata:
- No next/head imports in App Router
- Metadata exported from pages or layouts
- Metadata properly typed with Metadata type
Navigation:
- Using Link component from next/link
- Not using plain <a> tags for internal navigation
Cleanup:
- No remaining page files in pages/ directory
- _app.tsx and _document.tsx removed
- Old metadata patterns removed

Quick Reference

File Structure Mapping

Pages Router	App Router	Purpose
`pages/index.tsx`	`app/page.tsx`	Home route
`pages/about.tsx`	`app/about/page.tsx`	About route
`pages/[id].tsx`	`app/[id]/page.tsx`	Dynamic route
`pages/_app.tsx`	`app/layout.tsx`	Global layout
`pages/_document.tsx`	`app/layout.tsx`	HTML structure
`pages/api/hello.ts`	`app/api/hello/route.ts`	API route

Common Commands

# Create new Next.js app with App Router
npx create-next-app@latest my-app

# Run development server
npm run dev

# Build for production
npm run build

# Start production server
npm start

Additional Resources

For more advanced routing patterns (parallel routes, intercepting routes, route handlers), refer to the nextjs-advanced-routing skill.

For Server vs Client component best practices and anti-patterns, refer to the nextjs-server-client-components and nextjs-anti-patterns skills.