title: Child_added get last item added javascript
body: <p>For my Firebase cloud functions, i'am trying to get the last child item added, in the aim to proceed a task for this item.
  My structure is like this: </p>
  
  <pre><code>  -Tips
        -TipsId
               -Safe
                    -SafeId
                    -SafeId
                     etc
  </code></pre>
  
  <p>My goal is to get the last child SafeId item added.
  i found this snippet, but the problem is that it is also trigger when item SafeId is remove.</p>
  
  <pre><code>ref.orderByChild('timestamp').startAt(Date.now()).on('child_added',function(snapshot) {
    console.log('new record', snapshot.val());
  });
  </code></pre>