获得列表第一个

limit_endat.js

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

once_value.js

// pros: does not return last record
// cons: fetches entire record set
// 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.startAt(null, lastIdInSnapshot).on("child_added", function(newMessSnapshot) {
    //TODO: render new child and flash title bar.
  }
}