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What is a route handler in the Next.js App Router, and how do you create one? How do you access dynamic route parameters inside a route handler defined with a dynamic segment? How would you implement rate limiting inside a route handler to prevent abuse of a public API endpoint? How do you set custom response headers, such as CORS headers, in a route handler to allow requests from a different domain?
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Next.js › Route Handlers (API Route.js)

How do you read query parameters and the request body inside a route handler?

Intermediate

Answer

You access query parameters through the request URL's searchParams property, and read the request body by calling a method like json on the request object, which returns a promise that resolves to the parsed body data sent by the client.
export async function POST(request) {
  const { searchParams } = new URL(request.url);
  const category = searchParams.get('category');
  const body = await request.json();
  return Response.json({ category, received: body });
}
Real-world example A search endpoint reads a category filter from the query string while also accepting a JSON body containing search preferences, combining both sources of input inside a single route handler.

Common follow-ups: How is reading the request body different between GET and POST route handlers?;What happens if the request body is not valid JSON?

API Routes;Form Handling & Validation in Next.js
All 5 Route Handlers (API Route.js) questions