Next.jseasy

Route Handlers in the App Router: how do you define them and what are they used for?

Answer

You create a `route.ts`/`route.js` file and export HTTP methods like `GET`, `POST`. Handlers use the Web `Request`/`Response` APIs. They’re used for custom APIs, webhooks, or backend logic without a separate server.

export async function GET() {
  return Response.json({ ok: true })
}

Advanced answer

Deep dive

Expanding on the short answer — what usually matters in practice:

Context (tags): nextjs, route-handlers, api, app-router
Lifecycle: what happens at runtime (render/build, request/response, background jobs).
Caching: where cache lives, cache keys, how to invalidate without chaos.
Security: authn/authz, secrets, attack surface (SSRF/CSRF).
Explain the "why", not just the "what" (intuition + consequences).
Trade-offs: what you gain/lose (time, memory, complexity, risk).
Edge cases: empty inputs, large inputs, invalid inputs, concurrency.

Examples

Here’s an additional example (building on the short answer):

export async function GET() {
  return Response.json({ ok: true })
}

Common pitfalls

Too generic: no concrete trade-offs or examples.
Mixing average-case and worst-case (e.g., complexity).
Ignoring constraints: memory, concurrency, network/disk costs.

Interview follow-ups

When would you choose an alternative and why?
What production issues show up and how do you diagnose them?
How would you test edge cases?