How do you create API endpoints in Next.js?

Next.js provides built-in serverless backend capabilities for creating REST or GraphQL endpoints:

### 1. In App Router (Route Handlers - Modern):
Create a route.ts (or route.js) inside any folder in the app directory. Export named HTTP verb functions (GET, POST, PUT, DELETE, PATCH):

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

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

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

### 2. In Pages Router (API Routes - Legacy):
Create files inside pages/api/ exporting a default handler:

// pages/api/users.js
export default async function handler(req, res) {
  if (req.method === 'GET') {
    res.status(200).json({ message: 'Success' });
  } else {
    res.status(405).end();
  }
}