JS判斷下拉框asp:DropDownList的那個選項被選中了

 function myClick() {
            var objDrp = document.getElementById('<% =ddlSP_TDEPT1.ClientID%>');

            var ID = objDrp.options[objDrp.selectedIndex].value;
            if (ID == "") {
                alert("請選擇承辦單位！");
                return false;
            }
            
        }

<asp:DropDownList ID="ddlSP_TDEPT1" DataTextField="Name" DataValueField="code" runat="server"> </asp:DropDownList>