Nextjs 15

App Router File Conventions

app/
├── layout.tsx          # Root layout (required)
├── page.tsx            # Home page (/)
├── loading.tsx         # Loading UI (Suspense)
├── error.tsx           # Error boundary
├── not-found.tsx       # 404 page
├── (auth)/             # Route group (no URL impact)
│   ├── login/page.tsx  # /login
│   └── signup/page.tsx # /signup
├── api/
│   └── route.ts        # API handler
└── _components/        # Private folder (not routed)

Server Components (Default)

// No directive needed - async by default
export default async function Page() {
  const data = await db.query();
  return <Component data={data} />;
}

Server Actions

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

import { revalidatePath } from "next/cache";
import { redirect } from "next/navigation";

export async function createUser(formData: FormData) {
  const name = formData.get("name") as string;

  await db.users.create({ data: { name } });

  revalidatePath("/users");
  redirect("/users");
}

// Usage
<form action={createUser}>
  <input name="name" required />
  <button type="submit">Create</button>
</form>

Data Fetching

// Parallel
async function Page() {
  const [users, posts] = await Promise.all([
    getUsers(),
    getPosts(),
  ]);
  return <Dashboard users={users} posts={posts} />;
}

// Streaming with Suspense
<Suspense fallback={<Loading />}>
  <SlowComponent />
</Suspense>

Route Handlers (API)

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

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

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

Middleware

// middleware.ts (root level)
import { NextResponse } from 'next/server';
import type { NextRequest } from 'next/server';

export function middleware(request: NextRequest) {
  const token = request.cookies.get('token');

  if (!token && request.nextUrl.pathname.startsWith('/dashboard')) {
    return NextResponse.redirect(new URL('/login', request.url));
  }

  return NextResponse.next();
}

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

Metadata

// Static
export const metadata = {
  title: 'My App',
  description: 'Description',
};

// Dynamic
export async function generateMetadata({ params }) {
  const product = await getProduct(params.id);
  return { title: product.name };
}

server-only Package

import 'server-only';

// This will error if imported in client component
export async function getSecretData() {
  return db.secrets.findMany();
}