How to validate field in asp.net using Javascript

Write in header section of page

<script type="text/javascript">

        function ValidateFiled() {

if (document.getElementById("<%= txt_LeaveNOD.ClientID  %>").value == "") {

                document.getElementById("<%= lbl_Msg.ClientID  %>").innerHTML = "Please Enter Number of Days...!";

                return false;

            }

}

    </script>

Write in body section of page

<asp:Button ID="btn_Save" runat="server" Text="Save" Width="15%" OnClientClick="return ValidateFiled();"/>