title: Map items of collection snapshot in Firebase Firestore
body: <p><a href="https://firebase.google.com/docs/firestore/query-data/get-data?authuser=0#get_all_documents_in_a_collection" rel="noreferrer">Firebase Firestore Guides</a> show how to iterate documents in a collection snapshot with <code>forEach</code>:</p>
  
  <pre><code>db.collection("cities").get().then(function(querySnapshot) {
      querySnapshot.forEach(function(doc) {
          console.log(doc.id, " =&gt; ", doc.data());
      });
  });
  </code></pre>
  
  <p>I imagined it would support <code>map</code> as well, but it doesn't. How can I map the snapshot?</p>