nextjs-app-router

What this skill does

# Next.js App Router (Next.js 16+)

Build modern React applications using Next.js 16+ with App Router architecture.

## Overview

This skill provides patterns for Server Components (default) and Client Components ("use client"), Server Actions for mutations and form handling, Route Handlers for API endpoints, explicit caching with "use cache" directive, parallel and intercepting routes, and Next.js 16 async APIs and proxy.ts.

## When to Use

Activate when user requests involve:
- "Create a Next.js 16 project", "Set up App Router"
- "Server Component", "Client Component", "use client"
- "Server Action", "form submission", "mutation"
- "Route Handler", "API endpoint", "route.ts"
- "use cache", "cacheLife", "cacheTag", "revalidation"
- "parallel routes", "`@`slot", "intercepting routes"
- "proxy.ts", "migrate from middleware.ts"
- "layout.tsx", "page.tsx", "loading.tsx", "error.tsx", "not-found.tsx"
- "generateMetadata", "next/image", "next/font"

## Quick Reference

| File | Purpose | Directive | Purpose |
|------|---------|-----------|---------|
| `page.tsx` | Route page | `"use server"` | Server Action function |
| `layout.tsx` | Shared layout | `"use client"` | Client Component boundary |
| `loading.tsx` | Suspense loading | `"use cache"` | Explicit caching (Next.js 16) |
| `error.tsx` | Error boundary | | |
| `not-found.tsx` | 404 page | | |
| `route.ts` | API Route Handler | | |
| `proxy.ts` | Routing boundary | | |

## Instructions

### Create New Project

```bash
npx create-next-app@latest my-app --typescript --tailwind --app --turbopack
```

### Server Component

Server Components are the default in App Router. They run on the server and can use async/await.

```tsx
// app/users/page.tsx
async function getUsers() {
  const apiUrl = process.env.API_URL;
  const res = await fetch(`${apiUrl}/users`);
  return res.json();
}

export default async function UsersPage() {
  const users = await getUsers();
  return <main>{users.map(user => <UserCard key={user.id} user={user} />)}</main>;
}
```

### Client Component

Add `"use client"` when using hooks, browser APIs, or event handlers.

```tsx
"use client";

import { useState } from "react";

export default function Counter() {
  const [count, setCount] = useState(0);
  return <button onClick={() => setCount(c => c + 1)}>Count: {count}</button>;
}
```

### Server Action

Define actions in separate files with `"use server"` directive.

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

import { revalidatePath } from "next/cache";

export async function createUser(formData: FormData) {
  const name = formData.get("name") as string;
  const email = formData.get("email") as string;
  await db.user.create({ data: { name, email } });
  revalidatePath("/users");
}
```

Use with forms in Client Components:

```tsx
"use client";

import { useActionState } from "react";
import { createUser } from "./actions";

export default function UserForm() {
  const [state, formAction, pending] = useActionState(createUser, {});
  return (
    <form action={formAction}>
      <input name="name" />
      <input name="email" type="email" />
      <button type="submit" disabled={pending}>{pending ? "Creating..." : "Create"}</button>
    </form>
  );
}
```

See [references/server-actions.md](references/server-actions.md) for Zod validation, optimistic updates, and advanced patterns.

### Configure Caching

Use `"use cache"` directive for explicit caching (Next.js 16+).

```tsx
"use cache";

import { cacheLife, cacheTag } from "next/cache";

export default async function ProductPage({ params }: { params: Promise<{ id: string }> }) {
  const { id } = await params;
  cacheTag(`product-${id}`);
  cacheLife("hours");
  const product = await fetchProduct(id);
  return <ProductDetail product={product} />;
}
```

See [references/caching-strategies.md](references/caching-strategies.md) for cache profiles, on-demand revalidation, and advanced patterns.

### Route Handler

```ts
// app/api/users/route.ts
import { NextRequest, NextResponse } from "next/server";

export async function GET(request: NextRequest) {
  return NextResponse.json(await db.user.findMany());
}

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

Dynamic segments use `[param]`:

```ts
// app/api/users/[id]/route.ts
export async function GET(request: NextRequest, { params }: { params: Promise<{ id: string }> }) {
  const { id } = await params;
  const user = await db.user.findUnique({ where: { id } });
  if (!user) return NextResponse.json({ error: "Not found" }, { status: 404 });
  return NextResponse.json(user);
}
```

### Next.js 16 Async APIs

All Next.js APIs are async in version 16.

```tsx
import { cookies, headers } from "next/headers";

export default async function Page() {
  const cookieStore = await cookies();
  const headersList = await headers();
  const session = cookieStore.get("session")?.value;
  const userAgent = headersList.get("user-agent");
  return <div>...</div>;
}
```

Params and searchParams are also Promise-based:

```tsx
export default async function Page({
  params,
  searchParams,
}: {
  params: Promise<{ slug: string }>;
  searchParams: Promise<{ sort?: string }>;
}) {
  const { slug } = await params;
  const { sort } = await searchParams;
  // ...
}
```

See [references/nextjs16-migration.md](references/nextjs16-migration.md) for migration guide and proxy.ts configuration.

### Parallel Routes

Use `@folder` convention for parallel route slots.

```tsx
// app/dashboard/layout.tsx
export default function DashboardLayout({ children, team, analytics }: Record<string, React.ReactNode>) {
  return (
    <div>
      {children}
      <div className="grid grid-cols-2">{team}{analytics}</div>
    </div>
  );
}
```

```tsx
// app/dashboard/@team/page.tsx
export default function TeamPage() { return <div>Team Section</div>; }

// app/dashboard/@analytics/page.tsx
export default function AnalyticsPage() { return <div>Analytics Section</div>; }
```

See [references/routing-patterns.md](references/routing-patterns.md) for intercepting routes, route groups, and dynamic routes.

## Best Practices

**Server vs Client Decision:**
- Start with Server Component (default)
- Use Client Component only for: React hooks (useState, useEffect), browser APIs (window, document), event handlers (onClick, onSubmit), or client-only libraries

**Data Fetching:**
- Fetch in Server Components when possible
- Use React's `cache()` for deduplication
- Parallelize independent fetches
- Add Suspense boundaries with `loading.tsx`

**Performance Checklist:**
- Use `loading.tsx` for Suspense boundaries
- Use `next/image` for optimized images
- Use `next/font` for font optimization
- Add `error.tsx` and `not-found.tsx` for error handling

## Examples

### Example 1: Blog Post Form with Server Action

**Input:** Create a form to submit blog posts with Zod validation

**Output:**
```tsx
// app/blog/actions.ts
"use server";

import { z } from "zod";
import { revalidatePath } from "next/cache";

const schema = z.object({ title: z.string().min(5), content: z.string().min(10) });

export async function createPost(formData: FormData) {
  const parsed = schema.safeParse({ title: formData.get("title"), content: formData.get("content") });
  if (!parsed.success) return { errors: parsed.error.flatten().fieldErrors };
  await db.post.create({ data: parsed.data });
  revalidatePath("/blog");
  return { success: true };
}
```

```tsx
// app/blog/new/page.tsx
"use client";

import { useActionState } from "react";
import { createPost } from "../actions";

export default function NewPostPage() {
  const [state, formAction, pending] = useActionState(createPost, {});
  return (
    <form action={formAction}>
      <input name="title" placeholder="Title" />
      {state.errors?.title && <span>{state.errors.title[0]}</span>}
      <textarea name="content" placeholder="Content" />
      {