DropDownOpening プロパティ

// The client event 'DropDownOpening' takes two parameters sender and e
// sender  is the object which is raising the event
// e is the DropDownContainerEventArgs
function WebDropDown_DropDownOpening(sender, e) {

    if (!confirm("Are you sure you want to open the drop-down container?"))
    //Cancels the DropDownOpening event 
        e.set_cancel(true);
}