code: function onActivate(event) {
    // Delete old caches to save space.
    const dropOldCaches = async () => {
      const cacheNames = await caches.keys();
  
      // Return true on all the caches we want to clean up.
      // Note that caches are shared across the origin, so only remove
      // caches we are sure we created.
      const cleanTheseUp = cacheNames.filter((cacheName) =>
          cacheName.startsWith(CACHE_NAME_PREFIX) && cacheName != CACHE_NAME);
  
      const cleanUpPromises =
          cleanTheseUp.map((cacheName) => caches.delete(cacheName));
  
      await Promise.all(cleanUpPromises);
    };
  
    event.waitUntil(dropOldCaches());
  }
docs: This event fires when the service worker is activated.
  This can be after installation or upgrade.
  
  @param {!ExtendableEvent} event