// The client event 'SelectionChanging' takes two parameters sender and e
// sender is the object which is raising the event
// e is the DataTreeSelectionEventArgs
function WebDataTree_SelectionChanging(sender, e) {
//Get the newly selected tree nodes array
var newSelectedNodes = e.getNewSelectedNodes();
//Gets the text of the node at index 0 in the array
var nodeText = newSelectedNodes[0].get_text();
if (!confirm("Are you sure you want to select the node '" + nodeText + "'?"))
//Cancels the SelectionChanging event
e.set_cancel(true);
}