Set service-worker max-age to 0

[nolanlawson]

As of Chrome 68 and Firefox 57 (and Edge and Safari), service workers are always revalidated regardless of the cache-control directive. So it's recommended to use cache-control: max-age=0, since it aligns older browsers with the newer behavior and also makes it clear to developers what's going on.

For the service worker sourcemap file I don't think it really matters what we do, since it's a request that is only made when DevTools are open, but I'd say align it with service-worker.js just in case the two get out of sync somehow.

Sources:

• https://developers.google.com/web/updates/2018/06/fresher-sw
• https://stackoverflow.com/a/38854905

[lukeed]

They should be private, no-cache, no?

[nolanlawson]

I mean, max-age=0 should be the same as no-cache, right? Not sure about private; my intuition is actually to set public for everything, but I don't know if that's a sensible default since I'm not very familiar with how proxies handle that stuff.

[lukeed]

Well, service-workers should completely bypass any cache so that it doesn't ever risk being stale.

While it's true that max-age=0 is effectively the same thing, you're still adding an entry into the browser and CDN cache, only to have them immediately expire. It's like doing this:

let obj = {};

function onRequest()
  obj.foo = 'bar';
  if (obj.foo) {
    delete obj.foo;
  }
}

onRequest();
onRequest();
// etc

The combination of private and no-cache completely skips the cache assignment(s), avoiding useless work. It's as equally supported as the public directive.

[Rich-Harris]

From that MDN page:

Preventing caching

To turn off caching, you can send the following response header. In addition, see also the Expires and Pragma headers.

Cache-Control: no-cache, no-store, must-revalidate

[Rich-Harris]

I went with the MDN recommendation, we can always change it later if we think it's wrong. Thanks