用JS获取DropDownList选中得值

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>