Enter Numbers only in TextBox Using Javascript

function isNumber(evt) {

            evt = (evt) ? evt : window.event;

            var charCode = (evt.which) ? evt.which : evt.keyCode;

            if (charCode > 31 && (charCode < 48 || charCode > 57)) {

                return false;

            }

            return true;

        }

<asp:TextBox ID="txt_No" runat="server" onkeypress="return isNumber(event)"></asp:TextBox>