Fixed titles not working when any of the nodes has id `0`.

HISTORY.md

@@ -9,6 +9,10 @@ http://visjs.org
 - Fixed #761: Timeline and Graph2d throwing an error when locale is not found.
   Gives a warning message instead.
 
+### Network
+
+- Fixed titles not working when any of the nodes has id `0`.
+
 
 ## 2015-04-07, version 3.12.0
 

lib/network/Network.js

@@ -1449,7 +1449,7 @@ Network.prototype._checkShowPopup = function (pointer) {
   };
 
   var id;
-  var previousPopupObjId = this.popupObj === undefined ? "" : this.popupObj.id;
+  var previousPopupObjId = this.popupObj === undefined ? undefined : this.popupObj.id;
   var nodeUnderCursor = false;
   var popupType = "node";