sw.js

var cacheName = 'app-cache';
// Not a lot to cache in here...
var cacheUrls = [
  '/',
];

self.addEventListener('install', function(event) {
  event.waitUntil(
    caches.open(cacheName).then(function(cache) {
      return cache.addAll(cacheUrls);
    }
  ));
});

self.addEventListener('fetch', function(event) {
  event.respondWith(
    caches.match(event.request)
      .then(function(response) {
        if (response) {
          return response;
        }
        return fetch(event.request).then(function(response) {
          // Check if we received a valid response
          if(!response || response.status !== 200 || response.type !== 'basic') {
            return response;
          }

          // IMPORTANT: Clone the response. A response is a stream
          // and because we want the browser to consume the response
          // as well as the cache consuming the response, we need
          // to clone it so we have two streams.
          var responseToCache = response.clone();

          caches.open(cacheName)
            .then(function(cache) {
              cache.put(event.request, responseToCache);
            });

          return response;
        });
      }
    )
  );
});