All 5 Route Handlers (API Route.js) questions
More in Route Handlers (API Route.js)
How do you read query parameters and the request body inside a route handler? 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)

What is a route handler in the Next.js App Router, and how do you create one?

Beginner

Answer

A route handler is a file named route.js placed inside a folder within your app directory, exporting functions named after HTTP methods like GET or POST, letting you build backend API endpoints directly within the App Router, similar to what API routes did in the Pages Router.
// app/api/hello/route.js
export async function GET() {
  return Response.json({ message: 'Hello from a route handler' });
}
Real-world example A weather application builds a simple route handler that returns current weather data as JSON, accessible at the api/hello path within its App Router based project.

Common follow-ups: Can a single route.js file export functions for multiple HTTP methods?;What is the difference between a route handler and a server action?

API Routes;Server Actions & Mutations
All 5 Route Handlers (API Route.js) questions