This snippet shows how to serve a cached file instead of having to load it from the network

serving-cached-assets.js

// Offline Web Applications course on Udacity https://classroom.udacity.com/courses/ud899

//Following on from previous example here: https://gist.github.com/luke-denton/ef791e5150470814a7a155cd85b1bf80

self.addEventListener('fetch', function(event) {
  event.respondWith(
    //Look for a match to the current request in all caches, resolves to a Promise
    caches.match(event.request).then(function(response) {
      //If the repsonse is truthy, meaning something was found in the cache, then return it
      if (response) return response;
      
      //If no match was found in the cache, then make a network call to download the asset, and return that
      return fetch(event.request);
    })
  );
});