sw-vs-ss.js

// Caching fallback from ServiceWorker to sessionStorage
function setCacheItem(){}
function getCacheItem(){ return undefined; }
if ( ! ( 'serviceWorker' in window.navigator ) )
{
  var cache = window.sessionStorage;
  setCacheItem = function( key, text ) {
    return cache.setItem( key, text );
  };
  getCacheItem = function( key ) {
    return cache.getItem( key );
  };
}