Required Field By Code

<script type="text/javascript">
     function fnCheck() {
         if ((document.getElementById("<%=txtname.ClientID%>").value).length == 0) {
             alert("Name should not be empty!");
             return false;
         }
         else if ((document.getElementById("<%=txtEmail.ClientID%>").value).length == 0) {
             alert("Email should not be empty!");
             return false;
         }
         else if ((document.getElementById("<%=txtfeedback.ClientID%>").value).length == 0) {
             alert("Feedback should not be empty!");
             return false;
         }
         else {
             return true;
         }
     }
    </script>

<asp:Button ID="btnsubmit" runat="server" Text="SUBMIT" OnClientClick="return fnCheck();" />