Javascript to not allow number in textbox

Call javascript function as shown below in content or header section:-

 <script type="text/javascript">
function isNumberKey(evt, element) {

            var charCode = (evt.which) ? evt.which : event.keyCode;
            if (charCode != 46 && charCode > 31 && (charCode < 48 || charCode > 57))
                return true;
            return false;
        }
 </script>

Call javascript function from inside the Textbox :-

  <asp:TextBox ID="textbox1" MaxLength="30" runat="server" onkeypress="return isNumberKey(event,this)"    ></asp:TextBox>