Javascript to enter only digits in asp.net

<script type="text/javascript">
    function AllowNumsOnly(evt)
{
    var charCode = (evt.which) ? evt.which : event.keyCode
    if (charCode > 31 && (charCode < 48 || charCode > 57))
        return false;
 else
        return true;
}
    </script>



<asp:TextBox ID="txt_Qty" runat="server" onkeypress="return AllowNumsOnly(event);"></asp:TextBox>