Firebase push() collections item key, when is it generated?

Firebase push() collections item key, is it generated immediately on client side or after sync done?

eg this code:

var messageListRef = new Firebase('http://ift.tt/1ltO9mZ');
messageListRef.push({ 'user_id': 'fred', 'text': 'Yabba Dabba Doo!' });

And if immediately generated on client side, how can i acquire it immediately? without using callback