sw.js

  var CACHE_NAME = 'static-cache';

  var urlsToCache = [
    '/',
    '/index.html',
    '/assets/main.css',
    '/assets/materialize.css',
    '/images/Digital.jpg',
    '//maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css',
    '//fonts.googleapis.com/icon?family=Material+Icons'
  ];

  self.addEventListener('install', function(event) {
    event.waitUntil(
      caches.open(CACHE_NAME)
      .then(function(cache) {
        return cache.addAll(urlsToCache);
      })
    );
  });
  self.addEventListener('fetch', function(event) {
    event.respondWith(
      caches.match(event.request)
      .then(function(response) {
        return response || fetchAndCache(event.request);
      })
    );
  });

  function fetchAndCache(url) {
    return fetch(url)
    .then(function(response) {
      // Check if we received a valid response
      if (!response.ok) {
        throw Error(response.statusText);
      }
      return caches.open(CACHE_NAME)
      .then(function(cache) {
        cache.put(url, response.clone());
        return response;
      });
    })
    .catch(function(error) {
      console.log('Request failed:', error);
      // You could return a custom offline 404 page here
    });
  }