How to cache on a +server.(js/ts)?

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I am trying to cache a +server.ts file , and I tried using setHeaders, but it doesn't appear to work...

Ex:

import { json } from "@sveltejs/kit";

export async function GET({ setHeaders }) {
  setHeaders({
    "Cache-Control": "max-age=60"
  });
  return json({
    number: Math.round(Math.random() * 30)
  });
}

The random number is different after each reload within that 60 seconds.