respond with static files from cache only.

cache-only.js

//no network call is made, only local store is looked into
self.addEventListener('fetch', function (event) {
  const requestUrl = new URL(event.request.url);
  if(myItemsToPrefetchRequestArray.contains(requestUrl){
    event.respondWith(
       caches.open(staticCacheName).then(
        cache => cache.match(event.request)
      )
    )
     return;
   }
});