selection of one item from ASP.Net ListBox

     <asp:UpdatePanel ID="UpdatePanel2" runat="server" >
                <ContentTemplate>

<div runat="server" style="position: absolute; z-index: 122; " id="divcity">

 
                            <asp:ListBox ID="drpcity" runat="server" Height="252px"  SelectionMode="Multiple"
                                Width="141px"></asp:ListBox></div>

 </ContentTemplate>
                <Triggers>
                    <asp:AsyncPostBackTrigger ControlID="btnpb" />
                </Triggers>
            </asp:UpdatePanel>



 var list = document.getElementById('drpcity');
            debugger;
            for (var i = 0; i <=list.options.length-1; ++i)
            
             {
             
                if (list.options[i].selected)
                 {
                      
var lstval ;               
                      lstval =  list.options[i].value +","+  lstval  ;
                
                }
            }