# App Router Architecture
## File-Based Routing
```
app/
├── layout.tsx # Root layout (required)
├── page.tsx # Home page (/)
├── loading.tsx # Loading UI
├── error.tsx # Error boundary
├── not-found.tsx # 404 page
├── template.tsx # Re-mounted layout
│
├── (marketing)/ # Route group (no URL segment)
│ ├── layout.tsx
│ ├── about/
│ │ └── page.tsx # /about
│ └── contact/
│ └── page.tsx # /contact
│
├── dashboard/
│ ├── layout.tsx # Shared dashboard layout
│ ├── page.tsx # /dashboard
│ ├── settings/
│ │ └── page.tsx # /dashboard/settings
│ └── @analytics/ # Parallel route (slot)
│ └── page.tsx
│
├── blog/
│ ├── [slug]/
│ │ └── page.tsx # /blog/my-post (dynamic)
│ └── [...slug]/
│ └── page.tsx # /blog/a/b/c (catch-all)
│
└── api/
└── users/
└── route.ts # API route handler
```
## Root Layout (Required)
```tsx
// app/layout.tsx
import type { Metadata } from 'next'
import { Inter } from 'next/font/google'
import './globals.css'
const inter = Inter({ subsets: ['latin'] })
export const metadata: Metadata = {
title: {
default: 'My App',
template: '%s | My App'
},
description: 'Next.js 14 application',
}
export default function RootLayout({
children,
}: {
children: React.ReactNode
}) {
return (
{children}
)
}
```
## Nested Layouts
```tsx
// app/dashboard/layout.tsx
import { Sidebar } from '@/components/sidebar'
import { auth } from '@/lib/auth'
import { redirect } from 'next/navigation'
export default async function DashboardLayout({
children,
}: {
children: React.ReactNode
}) {
const session = await auth()
if (!session) {
redirect('/login')
}
return (
{children}
)
}
```
## Templates (Re-mount on Navigation)
```tsx
// app/template.tsx
'use client'
import { useEffect } from 'react'
export default function Template({ children }: { children: React.ReactNode }) {
useEffect(() => {
// Runs on every navigation
console.log('Template mounted')
}, [])
return {children}
}
```
## Loading States
```tsx
// app/dashboard/loading.tsx
export default function Loading() {
return (
)
}
```
## Error Boundaries
```tsx
// app/error.tsx
'use client'
export default function Error({
error,
reset,
}: {
error: Error & { digest?: string }
reset: () => void
}) {
return (
Something went wrong!
reset()}>Try again
)
}
```
## Route Groups
```tsx
// (marketing) and (shop) share the same URL level
app/
├── (marketing)/
│ ├── layout.tsx # Marketing layout
│ └── about/
│ └── page.tsx # /about
└── (shop)/
├── layout.tsx # Shop layout
└── products/
└── page.tsx # /products
```
## Parallel Routes
```tsx
// app/dashboard/layout.tsx
export default function Layout({
children,
analytics,
team,
}: {
children: React.ReactNode
analytics: React.ReactNode
team: React.ReactNode
}) {
return (
<>
{children}
{analytics}
{team}
)
}
// app/dashboard/@analytics/page.tsx
export default function Analytics() {
return Analytics Dashboard
}
```
## Intercepting Routes
```tsx
// Show modal when navigating from same app
// but show full page on direct navigation
// app/photos/[id]/page.tsx (full page)
export default function PhotoPage({ params }: { params: { id: string } }) {
return Photo {params.id} - Full Page
}
// app/@modal/(.)photos/[id]/page.tsx (modal)
export default function PhotoModal({ params }: { params: { id: string } }) {
return Photo {params.id} - Modal
}
```
## Dynamic Routes
```tsx
// app/blog/[slug]/page.tsx
export default function BlogPost({ params }: { params: { slug: string } }) {
return Post: {params.slug}
}
// Generate static params at build time
export async function generateStaticParams() {
const posts = await fetch('https://api.example.com/posts').then(res => res.json())
return posts.map((post: { slug: string }) => ({
slug: post.slug,
}))
}
// Opt out of static generation
export const dynamic = 'force-dynamic'
// Revalidate every 60 seconds
export const revalidate = 60
```
## Catch-All Routes
```tsx
// app/docs/[...slug]/page.tsx
// Matches: /docs/a, /docs/a/b, /docs/a/b/c
export default function Docs({ params }: { params: { slug: string[] } }) {
return Docs: {params.slug.join('/')}
}
// Optional catch-all: [[...slug]]
// Also matches: /docs
```
## Route Handlers (API Routes)
```tsx
// app/api/users/route.ts
import { NextRequest, NextResponse } from 'next/server'
export async function GET(request: NextRequest) {
const users = await db.user.findMany()
return NextResponse.json(users)
}
export async function POST(request: NextRequest) {
const body = await request.json()
const user = await db.user.create({ data: body })
return NextResponse.json(user, { status: 201 })
}
// Dynamic routes: app/api/users/[id]/route.ts
export async function GET(
request: NextRequest,
{ params }: { params: { id: string } }
) {
const user = await db.user.findUnique({ where: { id: params.id } })
return NextResponse.json(user)
}
```
## Metadata API
```tsx
// app/blog/[slug]/page.tsx
import type { Metadata } from 'next'
export async function generateMetadata(
{ params }: { params: { slug: string } }
): Promise {
const post = await fetchPost(params.slug)
return {
title: post.title,
description: post.excerpt,
openGraph: {
title: post.title,
description: post.excerpt,
images: [{ url: post.coverImage }],
},
}
}
```
## Quick Reference
| File | Purpose | Use Case |
|------|---------|----------|
| `layout.tsx` | Persistent UI across routes | Shared navigation, auth wrapper |
| `page.tsx` | Route UI | Actual page content |
| `loading.tsx` | Loading fallback | Automatic Suspense boundary |
| `error.tsx` | Error boundary | Handle errors gracefully |
| `template.tsx` | Re-mounted layout | Analytics, animations |
| `not-found.tsx` | 404 page | Custom not found UI |
| `route.ts` | API handler | Backend API endpoints |