how to validate at client side

Question

Hi to all master of asp.net and c#
how to do the client side validation using c# and with out using required field validation control i wrote the code of javascript in head tab and i write in page_load method
protected void Page_Load(object sender, EventArgs e)
    {
        btnAdd.Attributes.Add("onclick", "return validate()");
    }
but its not checking the fields and javascript is
<script language="javascript" type="text/javascript">
        function Validate()
        {
            if(document.getElementById("<%=txtRefNo.ClientID%>").value=="")
            {
                alert("Please Enter Your Reference Number");
                document.getElementById("<%=txtRefNo.ClientID%>").focus();
                return false;
            }
               
        }
    </script>
where i am doing mistake i don't know