service-worker.js

 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) {           
              return networkResponse;
                }, function (event) {   
                // rejected promise - just ignore it, we're offline!   
                console.log("Error in fetch()", event);
                event.waitUntil(
                caches.open('cache').then(function(cache) { 
                // our cache is named *cache* in the caches.open() above
                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
                '/quran.html', //default
                '/assets/js/*', //default
                '/assets/css/*',// configure as by your site ; just an example
                '/assets/images/*',// choose images to keep offline; just an example
                // Do not replace/delete/edit the manifest.js paths below
                //These are links to the extenal social media buttons that should be cached;
                // we have used twitter's as an example
              ]);
            })
          );
        });
              // respond from the cache, or the network
              return response || fetchPromise;
            });
          }));
        });
        
        self.addEventListener('install', function(event) {
          // The promise that skipWaiting() returns can be safely ignored.
          self.skipWaiting();
          console.log("Latest version installed!");
        });