c# - deselect item in multiitem listbox with javascript -

i trying deselect item (once selected) when user click on item in listbox. have script deselecting item on user click happening when item selected first time well. have: js:

 function deselect()           {               var list = document.getelementbyid('<%=ddlcity.clientid%>');               var listlength = list.options.length;                (var = 0; < listlength; i++) {                   if (list.options[i].selected) {                       list.selectedindex = -1;                   }               }           } 

and html:

  <asp:listbox id="ddlcity" cssclass="formfield" runat="server" datasourceid="sqldatasourcecity"                 datatextfield="description" datavaluefield="cityid" selectionmode="multiple" onclick="deselect();"                 ondatabound="dropdownlist3_databound"></asp:listbox>   

any idea how can edit js deselect value once value selected, when user selects value first time value selected. thanks, laziale