sw.offline.js

var cacheName = "app-cache-v1";
var offlineUrl = "./offline.html";

self.addEventListener("install", event => {
  event.waitUntil(
    caches
    .open(cacheName)
    .then(cache =>
      cache.addAll([
        "./dist/vendor.css",
        "./css/site.min.css",
        "./scripts/lib.js",
        "./images/logo.png",
         offlineUrl
      ])
    ).then(() => self.skipWaiting())
  );
});

// Cache any new resources as they are fetched
self.addEventListener("fetch", function (event) {
  //ignore these request
  if (event.request.method !== "GET"
  || /api/.test(event.request.url)
  || event.request.destination === "document") {
    return;
  }

  event.respondWith(
    caches
    .match(event.request, {
      ignoreSearch: true
    })
    .then(function (response) {
      if (response) {
        return response;
      }
      var fetchRequest = event.request.clone();

      return fetch(fetchRequest)
        .then(function (res) {
          if (!res || res.status !== 200) {
            return res;
          }

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

          return res;
        }).catch(error => {
          // Check if the user is offline first and is trying to navigate to a web page
          if (event.request.method === 'GET' && event.request.headers.get('accept').includes('text/html')) {
          // Return the offline page
          return caches.match(offlineUrl);
        }
        });
      })
    );

});