ServiceWorker.js

var CACHE_NAME = "$(ProjectName)";
var CACHED_URLS = [
  "$(ProjectHTML)",
  (FileList)
]

self.addEventListener("install", function(event) {
  event.waitUntil(
    caches.open(CACHE_NAME).then(function(cache) {
      return cache.addAll(CACHED_URLS)
    })
  )
})

self.addEventListener("fetch", function(event) {
  event.respondWith(
    fetch(event.request).catch(function() {
      return caches.match(event.request).then (
        function(response) {
          if (response) {
            return response
          }
          else {
            if (event.request.headers.get("accept").includes("text/html")) {
              return caches.match("$(ProjectHTML)")
            }
            else {
              return false
            }
          }
        }
      )
    })
  )
})