This snippet shows how to update files that have been cached

updating-cached-files.js

// Offline Web Applications course on Udacity https://classroom.udacity.com/courses/ud899

//To trigger a new service worker to be created, the service worker code needs to be updated. This could be as simple
//as adding a comment to the file or, as we're doing here, increasing the cache version from 1 to 2, meaning there
//are likely new elements that need to be cached
var staticCacheName = "my-cache-v2";

//Fires when a service worker becomes active. Means its install has completed and it is now controlling the page
//Cache cleanup has to happen in the activate event because other service workers could still be using an old cache
//whilst the new service worker is spinning up. If the old cache is removed, that old service worker will no longer be
//able to work effectively, and therefore the user will be without any service worker benefits until the new service worker
//is activated and takes over controlling the page
self.addEventListener('activate', function(event) {
  event.waitUntil(
    //Get all the cache keys
    caches.keys().then(function(cacheNames) {
      return Promise.all( //Wait for completion of all Promises
        //Filter the cache keys to only ones starting with 'my-cache', this will prevent other caches from being found and deleted
        cachesNames.filter(function(cacheName) {
          return cacheName.startsWith('my-cache') && cacheName != staticCacheName;
        }).map(function(cacheName) {
          //For each of the found caches that aren't the current cache name and match the required string, delete
          return cache.delete(cacheName);
        });
      );
    });
  );
})