I may be fundamentally misunderstanding something here but it seems to me that if I want to fetch something I can call

let r = await fetch("someurl");

and the browser will use a cached version if it has one. Or I could call

let r = await fetch("someurl", {
  cache: "reload"
});

and I would always get a new version.

What I cannot find is how to tell it that I only want the cached version if it is younger than X minutes.

I could do

let r = await fetch("someurl", {
  headers: {
    "Cache-control": "max-age=180"
  }
});

but afaiu that would just get sent to the server and not actually impact what the browser is doing about values in its own cache.