SelectionChanging プロパティ (DropDownClientEvents)

// 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);       
      
}