เอา Code ASP.Net มาฝาก button get dropdown value ด้วย javascript

<form id="form1" runat="server">
    <div>
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
        <asp:Button ID="Button1" runat="server" Text="Button"  OnClientClick="test()"/>
        <asp:DropDownList ID="DropDownList1" runat="server">
            <asp:ListItem Value="sdasd">asdsada</asp:ListItem>
            <asp:ListItem Value="dasdas">asdas</asp:ListItem>
            <asp:ListItem Value="dasdasd">asdsa</asp:ListItem>
            <asp:ListItem Value="dasdasd">adas</asp:ListItem>
            <asp:ListItem Value="asda">adasd</asp:ListItem>
            <asp:ListItem></asp:ListItem>
        </asp:DropDownList>
    </div>
    <script>
        function test() {            
            var e = document.getElementById("<%=DropDownList1.ClientID%>");
            //var strUser = e.options[e.selectedIndex].value;
            alert(e.value);

            mypopup = window.open('Default.aspx?carid=' + e.value, 'mypopup', 'nenuber=no,toorlbar=no,location=no,scrollbars=no, status=no,resizable=no,width=180,height=180,top=220,left=650 ');
            mypopup.focus();
        }
    </script>
    </form>