service-worker.js

var log = console.log.bind(console); //bind our console to a variable
var version = "0.0.1";
var cacheName = "swETH";
var cache = cacheName + "-" + version;
var filesToCache = [
  "vendor/bootstrap/css/bootstrap.min.css",
  "vendor/font-awesome/css/font-awesome.min.css",
  "vendor/jquery/jquery.min.js",
  "vendor/bootstrap/js/bootstrap.bundle.min.js",
  "img/bg.mov",
  "img/*.svg" //Note that this is different from below
];

//Add event listener for install
self.addEventListener("install", function(event) {
  log("[ServiceWorker] Installing....");
  event.waitUntil(
    caches
      .open(cache) //open this cache from caches and it will return a Promise
      .then(function(cache) {
        //catch that promise
        log("[ServiceWorker] Caching files");
        cache.addAll(filesToCache); //add all required files to cache it also returns a Promise
      })
  );
});

//Add event listener for fetch
self.addEventListener("fetch", function(event) {
  //note that event.request.url gives URL of the request so you could also intercept the request and send a response based on your URL
  //e.g. you make want to send gif if anything in jpeg form is requested.
  event.respondWith(
    //it either takes a Response object as a parameter or a promise that resolves to a Response object
    caches
      .match(event.request) //If there is a match in the cache of this request object
      .then(function(response) {
        if (response) {
          log("Fulfilling " + event.request.url + " from cache.");
          //returning response object
          return response;
        } else {
          log(event.request.url + " not found in cache fetching from network.");
          //return promise that resolves to Response object
          return fetch(event.request);
        }
      })
  );
});

self.addEventListener("activate", function(event) {
  log("[ServiceWorker] Activate");
  event.waitUntil(
    caches
      .keys() //it will return all the keys in the cache as an array
      .then(function(keyList) {
        //run everything in parallel using Promise.all()
        Promise.all(
          keyList.map(function(key) {
            if (key !== cacheName) {
              log("[ServiceWorker] Removing old cache ", key);
              //if key doesn`t matches with present key
              return caches.delete(key);
            }
          })
        );
      })
  );
});