how to validate the not acppting null values for aspx design page

<script language="javascript" type="text/javascript">

        function ValidateForm()

        {

            if (document.getElementById("<%=txtSuppName.ClientID%>").value == null)

            {

                alert("Supplier Name can not be Empty!");

                document.getElementById("<%=txtSuppName.ClientID%>").focus();

                return false;

            }

      return true;

     }

</script>

OnClientClick="return ValidateForm();"

John Bhatt
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