function startListeners() {
  firebase.database().ref('/posts').on('child_added', function(postSnapshot) {
    const postReference = postSnapshot.ref;
    const uid = postSnapshot.val().uid;
    const postId = postSnapshot.key;
    // Update the star count.
    // [START post_value_event_listener]
    postReference.child('stars').on('value', function(dataSnapshot) {
      updateStarCount(postReference);
      // [START_EXCLUDE]
      updateStarCount(firebase.database().ref('user-posts/' + uid + '/' + postId));
      // [END_EXCLUDE]
    }, function(error) {
      console.log('Failed to add "value" listener at /posts/' + postId + '/stars node:', error);
    });
    // [END post_value_event_listener]
    // Send email to author when a new star is received.
    // [START child_event_listener_recycler]
    postReference.child('stars').on('child_added', function(dataSnapshot) {
      sendNotificationToUser(uid, postId);
    }, function(error) {
      console.log('Failed to add "child_added" listener at /posts/' + postId + '/stars node:', error);
    });
    // [END child_event_listener_recycler]
  });
  console.log('New star notifier started...');
  console.log('Likes count updater started...');
}
Keep the likes count updated and send email notifications for new likes.