using cache.addAll in DOM event handler

cache-add-all.js

/**
purpose:
example is for a music player app, where we want to store
selective playlists for offline usage

on user click to `Save Offline` button, we want to add all 
songs in playlist to the cache storage
*/
document.getElementBy('saveOfflineBtn').addEventListener('click', function(event) {
  event.preventDefault();
  const id = this.currentPlaylist.id;
  caches.open('songs-cache').then(function(cache) {
    fetch(`/get-all-songs-url-from-playlist?id=${id}`).then(function(response) {
      // response is a JSON-encoded array of
      // songs URL that a given playlist has
      return response.json();
    }).then(function(urls) {
      cache.addAll(urls);
    });
  });
});