sw.js

self.addEventListener("install", function(event) {
    event.waitUntil(preLoad());
  });
  
  var preLoad = function(){
    //console.log("Installing web app");
  };
  
  self.addEventListener("fetch", function(event) {
    event.respondWith(checkResponse(event.request).catch(function() {
      return;
    }));
    event.waitUntil(addToCache(event.request));
  });
  
  var checkResponse = function(request){
    return new Promise(function(fulfill, reject) {
      fetch(request).then(function(response){
        if(response.status !== 404) {
          fulfill(response);
        } else {
          reject();
        }
      }, reject);
    });
  };


var addToCache = function(request){
  return caches.open("offline").then(function (cache) {
    return fetch(request).then(function (response) {
      //console.log(response.url + " was cached");
      return cache.put(request, response);
    });
  });
};

var returnFromCache = function(request){
  return caches.open("offline").then(function (cache) {
    return cache.match(request).then(function (matching) {
     if(!matching || matching.status == 404) {
       return cache.match("index.html");
     } else {
       return matching;
     }
    });
  });
};