title: Promise.all problem in making pwa for website
body: <p>i am trying to make a pwa of my website for the first time but i have a problem
  <br>
  its my code</p>
  <pre><code>let expectedCacheNames = Object.values(CURRENT_CACHE);
      event.waitUntil(
          caches.keys().then((cacheNames) =&gt; {
              return Promise.all(
                  cacheNames.forEach(cacheName =&gt; {
                      if(!expectedCacheNames.indexOf(cacheName) &gt; -1){
                          return caches.delete(cacheName)
                      }
                  })
              )
          })
      )
  </code></pre>
  <p>and i get this error</p>
  <blockquote>
  <p>Uncaught (in promise) TypeError: undefined is not iterable (cannot read property Symbol(Symbol.iterator))</p>
  </blockquote>