self.addEventListener('activate',
function(event) {
  event.waitUntil(
    caches.keys().then(function(cacheNames)
{
      return Promise.all(

 cacheNames.filter(function(cacheName) {
          // Return true if you want to remove this cache,
          // but remember that caches are shared across
          // the whole origin
        }).map(function(cacheName) {
          return caches.delete(cacheName);
        })
      );
    })
  );
 });