Javascript Confirmation message on DropDownList Selected Index Changed Event

 Suppose a  user entering records in fields and before save, changed the selected DropDownList suddenly there should be a message . Otherwise the values will be lose those are loaded under   DropDownList selected index change event.I took a another textbox to hold the previous selected index value of  DropDownList .(Also we can take HiddenField)

     protected void DDlPo_SelectedIndexChanged(object sender, EventArgs e)

        {

            DDlPo.Attributes.Add("onchange", "return OnSelectedIndexChange();");

            LoadMaterialBatchDetails();//this is load method

         }

   <script language="javascript" type="text/javascript">

            function OnSelectedIndexChange() {

                  var IndexValue = document.getElementById('DDlPo');               

                  var PreviousIndexValue = document.getElementById('Txt PreviousIndexValue ').value;

                  if (IndexValue.selectedIndex != PreviousIndexValue) {

                      if (confirm("Are you sure to select another Order Number, Data will be loss for this Order Number?") == true) {

                          document.getElementById('BtnClose').click();

                          IndexValue.selectedIndex = IndexValue.selectedIndex;

                      }

                      else {

                          IndexValue.selectedIndex = PreviousIndexValue;

                      }

                }

               

            }

        </script>