// The client event 'SelectionChanging' takes two parameters sender and e
// sender is the object which is raising the event
// e is the DropDownSelectionEventArgs
function WebDropDown_SelectionChanging(sender, e) {
//Gets the new collection of selected drop-down items
//Only one item will be in the collection when single selection is enabled
var newSelection = e.getNewSelection();
//Gets the text of the first drop-down item in the collection
var text = newSelection[0].get_text();
//Gets the index of the first drop-down item in the collection
var index = newSelection[0].get_index();
if (!confirm("Are you sure you want to select the item '" + text + "' at index " + index + "?"))
//Cancels the SelectionChanging client event
e.set_cancel(true);
}