Get only new items from Firebase

1_query_timestamp.js

// assumes you add a timestamp field to each record (see Firebase.ServerValue.TIMESTAMP)
// pros: fast and done server-side (less bandwidth, faster response), simple
// cons: a few bytes on each record for the timestamp
var ref = new Firebase(...);
ref.orderByChild('timestamp').startAt(Date.now()).on('child_added', function(snapshot) {
  console.log('new record', snap.key());
});

2_once_value.js

// pros: works?
// cons: fetches entire record set, requires discarding the first record
// credits: http://stackoverflow.com/questions/12850789/is-there-a-way-to-know-in-what-context-child-added-is-called-particularly-page/12851236#12851236
var ref = new Firebase(...);
ref.once("value", function(snap) {
  //TODO: display initial state...
  // Object.keys not supported in IE 8, but has a polyfill: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys
  var keys = Object.keys(snap.val()||{});
  var lastIdInSnapshot = keys[keys.length-1]

  ref.orderByKey().startAt(lastIdInSnapshot).on("child_added", function(newMessSnapshot) {
    if( snap.key() === lastIdInSnapshot ) { return; }
    console.log('new record', snap.key());
  }
}

3_limit_endat.js

// pros: does not fetch entire record set
// cons: triggers child_removed events when last item changes, grabs the last record which needs to be ignored
var first = true;
var ref = new Firebase(...);
ref.limitToLast(1).on("child_added", function(snap) {
   if( first ) {
       first = false; 
   } 
   else {
       console.log('new record', snap.key());
   }
});

dont forget to add an index for the timestamp to make the filtration quicker https://www.firebase.com/docs/security/guide/indexing-data.html

This is also one solution,
ref.startAt(null,ref.push().key).on('child-added,...'
works only when keys are generated by push()

This is also one solution,
ref.startAt(null,ref.push().key).on('child-added,...'
works only when keys are generated by push()

This is interesting, do you know why this works? Are keys generated in order by RTDB?

On 2), ref.orderByKey().startAt(lastIdInSnapshot).on("child_added"..., I found out that instead of throwing away the first value, you can add a '-' to the lastIdSnapshot. It's just an character with a low ASCII value, so any push'ed item will go after the previous one + '-'.

This is interesting, do you know why this works?

if we were to use ref.push().key to put a new entry on ref, it would be stored as the last child.So, using second parameter of startAt function, we retrive childs with key greater than or equal to ref.push().key
https://firebase.google.com/docs/reference/js/firebase.database.Query#startat

Are keys generated in order by RTDB?

yes, https://firebase.google.com/docs/database/admin/save-data#push-vs-transaction