text: 
  const FILES_TO_CACHE =[
      "/",
      "/index.html",
      "index.js",
      "/manifest.webmanifest",
      "/icons/icon-192x192.png",
      "/icons/icon-512x512.png",
      "/styles.css"
  ];
  
  const CACHE_NAME = "static-cache-v2";
  const DATA_CACHE_NAME = "data-cache-v1";
  
  //install
  self.addEventListener("install", function(evt) {
      evt.waitUntil(
          caches.open(CACHE_NAME).then(cache => {
              console.log("Your files were pre-cached successfully!");
              return cache.addAll(FILES_TO_CACHE);
          })
      );
      self.skipWaiting();
  });
  
  self.addEventListener("activate", function(evt) {
      evt.waitUntil(
          caches.keys().then(keyList => {
              return Promise.all(
                  keyList.map(key => {
                      if (key !== CACHE_NAME && key !== DATA_CACHE_NAME) {
                          console.log("Removing old cache data", key);
                          return caches.delete(key);
                      }
                  })
              )
          })
      );
      self.clients.claim();
  });
  
  // fetch
  self.addEventListener("fetch", function(evt) {
      // cache successful requests to the API
      if (evt.request.url.includes("/api/")) {
        evt.respondWith(
          caches.open(DATA_CACHE_NAME).then(cache => {
            return fetch(evt.request)
              .then(response => {
                // If the response was good, clone it and store it in the cache.
                if (response.status === 200) {
                  cache.put(evt.request.url, response.clone());
                }
    
                return response;
              })
              .catch(err => {
                // Network request failed, try to get it from the cache.
                return cache.match(evt.request);
              });
          }).catch(err => console.log(err))
        );
    
        return;
      }
    
      // if the request is not for the API, serve static assets using "offline-first" approach.
      // see https://developers.google.com/web/fundamentals/instant-and-offline/offline-cookbook#cache-falling-back-to-network
      evt.respondWith(
        caches.match(evt.request).then(function(response) {
          return response || fetch(evt.request);
        })
      );
    });