Javascript to validate a text filed to add only number

<asp:TextBox ID="txtboxForDays" runat="server" OnKeyPress="return AllowNumericOnly(this);"></asp:TextBox>

<script type="text/javascript">
                function AllowNumericOnly(e) {
                    var keycode;
                    if (window.event)
                        keycode = window.event.keyCode;
                    else if (event)
                        keycode = event.keyCode;
                    else if (e)
                        keycode = e.which;
                    else return true;
                    if ((keycode > 47 && keycode <= 57)) { return true; }
                    else { return false; } return true;
                }
    </script>