sw.js

      // after a service worker is installed and the user navigates to a different page or refreshes,the service worker will begin to receive fetch events
          
      self.addEventListener('fetch', function(event) {
        event.respondWith(caches.open('cache').then(function(cache) {
          return cache.match(event.request).then(function(response) {
            console.log("cache request: " + event.request.url);
              var fetchPromise = fetch(event.request).then(function(networkResponse) {           
  // if we got a response from the cache, update the cache                   
                console.log("fetch completed: " + event.request.url, networkResponse);
                  if (networkResponse) {
                    console.debug("updated cached page: " + event.request.url, networkResponse);
                      cache.put(event.request, networkResponse.clone());}
                      return networkResponse;
                      }, function (e) {   
  // rejected promise - just ignore it, we're offline!   
                      console.log("Error in fetch()", e);
                      e.waitUntil(
                      caches.open('cache').then(function(cache) { // our cache here is named *cache* in the caches.open()
                      return cache.addAll
                      ([            
  //cache.addAll(), takes a list of URLs, then fetches them from the server and adds the response to the cache.           
  // add your entire site to the cache- as in the code below; for offline access
  // If you have some build process for your site, perhaps that could generate the list of possible URLs that a user might load.               
                      '/', // do not remove this
                      '/index.html', //default
                      '/index.html?homescreen=1', //default
                      '/?homescreen=1', //default
                      '/assets/main.css', // configure as by your site ; just an example
                      '/icons/pwafire128white.png', // add more icons path below as in this line. Check out the icons path in the icons folder.
                      
  // Do not replace/delete/edit the sw.js/ and manifest.js paths below
                      '/sw.js/',
                      '/manifest.js',
  //These are links to the extenal social media buttons that should be cached; we have used twitter's as an example
                      'https://platform.twitter.com/widgets.js',       
                      ]);
                      })
                      );
                      });
  // respond from the cache, or the network
                      return response || fetchPromise;
                  });
                  }));
                  });