asp.net中怎样用Javascript控制RequiredFieldValidator控件什么时候启用，什么时候不启用验证？

Enable/Disable RequiredFieldValidator with Javascript
 
document.getElementById("requiredfieldvalidatorid").enabled = false;
 
 <asp:DropDownList ID="ddlServiceName" runat="server" onchange='var DateValidator = document.all ? document.all["DateValidator "] : document.getElementById("DateValidator ");DateValidator .enabled = false;'></asp:DropDownList>