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:

typescript

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

✅ CORRECT:

typescript

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

Common Next.js Type Patterns

typescript

// 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:

typescript

// 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:

typescript

// 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>
    </>
  );
}

typescript

// 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:

typescript

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

typescript

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

Step 4: Update Navigation

Replace anchor tags with Next.js Link:

typescript

// 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

typescript

// 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

typescript

// 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

typescript

// 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

typescript

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

Catch-All Routes

typescript

// 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

typescript

// 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:

typescript

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

Correct:

typescript

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

Pitfall 2: Using

next/head

in App Router

Wrong:

typescript

import Head from 'next/head';

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

Correct:

typescript

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:

typescript

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

Correct:

typescript

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:

typescript

// 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

typescript

// 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

typescript

// 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

typescript

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

typescript

// 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

typescript

// 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

typescript

// 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

typescript

// 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

typescript

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

Pattern 2: Fetch from API

typescript

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

typescript

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

Migration from Pages Router

Before (Pages Router):

typescript

// 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):

typescript

// 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'

typescript

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

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

❌ Wrong: Using Pages Router pattern

typescript

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

❌ Wrong: Missing export keyword

typescript

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

✅ Correct: Clean Server Component

typescript

// 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

bash

# 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.

nextjs-app-router-fundamentals

NPX Install

Tags

SKILL.md Content

Next.js App Router Fundamentals

Overview

TypeScript: NEVER Use any Type

Common Next.js Type Patterns

When to Use This Skill

Core Concepts

App Router vs Pages Router

File Conventions

Migration Guide: Pages Router to App Router

Step 1: Understand the Current Structure

Step 2: Create Root Layout

Step 3: Migrate Pages to Routes

Step 4: Update Navigation

Step 5: Clean Up Pages Directory

Metadata Handling

Static Metadata

Dynamic Metadata

Layouts and Nesting

Creating Nested Layouts

Routing Patterns

Dynamic Routes

Catch-All Routes

Optional Catch-All

Route Groups

Common Migration Pitfalls

Pitfall 1: Forgetting Root Layout HTML Tags

Pitfall 2: Using next/head in App Router

Pitfall 3: Not Removing Pages Directory

Pitfall 4: Missing page.tsx Files

Pitfall 5: Incorrect Link Usage

Server Components vs Client Components

Default: Server Components

Client Components

Data Fetching Patterns

Server Component Data Fetching

Parallel Data Fetching

Static Site Generation with generateStaticParams

Overview

Basic Usage

Fetching Data for Static Params

Multiple Dynamic Segments

Dynamic Behavior Configuration

Common Patterns

Migration from Pages Router

Common Mistakes to Avoid

Testing and Validation

Quick Reference

File Structure Mapping

Common Commands

Additional Resources

TypeScript: NEVER Use
`any`
Type

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

Pitfall 4: Missing
`page.tsx`
Files