Firebase child_added only get child added

To track things added since some checkpoint without fetching previous records, you can use endAt() and limit() to grab the last record:

// retrieve the last record from `ref`
ref.endAt().limitToLast(1).on('child_added', function(snapshot) {

   // all records after the last continue to invoke this function
   console.log(snapshot.name(), snapshot.val());

});

limit() method is deprecated. limitToLast() and limitToFirst() methods replace it.

// retrieve the last record from `ref`
ref.limitToLast(1).on('child_added', function(snapshot) {

   // all records after the last continue to invoke this function
   console.log(snapshot.name(), snapshot.val());
   // get the last inserted key
   console.log(snapshot.key());

});

Since calling the ref.push() method without data generates path keys based on time, this is what I did:

// Get your base reference
const messagesRef = firebase.database().ref().child("messages");

// Get a firebase generated key, based on current time
const startKey = messagesRef.push().key;

// 'startAt' this key, equivalent to 'start from the present second'
messagesRef.orderByKey().startAt(startKey)
.on("child_added", 
    (snapshot)=>{ /*Do something with future children*/}
);

Note that nothing is actually written to the reference(or 'key') that ref.push() returned, so there's no need to catch empty data.