ItemAdded プロパティ

// The client event 'ItemAdded' takes two parameters sender and e
// sender  is the object which is raising the event
// e is the DropDownControlEventArgs

//Fires only when items are added programmatically on the client-side
function WebDropDown_ItemAdded(sender, e) {

    //Gets the drop-down item that has mouse on it
    var item = e.get_value();

    //Gets the text of the drop-down item
   var text = item.get_text();

    //Gets the index of the drop-down item
    var index = item.get_index();

   alert("New Item '"+text+"'added at index " + index); 
          
}