Dot and zero as first character in textbox using javascript ?

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

        String.prototype.startsWith = function (str)

        {

            return (this.indexOf(str) === 0);

        }

        function ValidationofPaymentAmount{

            var txtNo = document.getElementById("txtNumber").value;

            if (txtNo .startsWith(".") || txtNo.startsWith("0")) // true

            {

                document.getElementById("txtNumber").value = "";

                alert("you can not insert dot and zero as first character");

            }

        }

    < /script>

In code Behind: 

<asp:TextBox ID="txtNumber" runat="server" onkeyup="ValidationofPaymentAmount();"></asp:TextBox>