用JS獲取DropDownList選中得值

HTML：

<asp:DropDownList ID="DropdownList1" runat="server" AutoPostBack="true" />

JS：

<script type="text/JavaScript">
function getvalue(){

                var select1 = document.all.<%= DropdownList1.ClientID %>;
                var selectvalue = select1.options[select1.selectedIndex].value;
                var selectText= select1.options[select1.selectedIndex].text;
               alert(selectvalue+"  "+selectText);
}
</script>