title: Get key of an element from Firebase database snapshot
body: <p>I have the next query for a Firebase database</p>
  
  <pre><code>firebase.database().ref('users/'+userId+ubication)
      .orderByChild('timestamp')
      .equalTo(parseInt(timeStamp))
      .once('value')
      .then(snapshot=&gt;{
          console.log(snapshot);
      }
  );
  </code></pre>
  
  <p>This query return my only one element, I need remove this element from the database but I don't know how I can get the key of it ! </p>