Fired before the default event delete action. You can cancel the default action (which is to remove the card from the control when cardDeleteHandling is set to "Update") using args.preventDefault();
DayPilot.Kanban.onCardDelete(args);
<div id="dp"></div>
<script type="text/javascript">
var dp = new DayPilot.Kanban("dp");
// ...
dp.cardDeleteHandling = "Update";
dp.onCardDelete = function(args) {
if (!confirm("Do you really want to delete this card?")) {
args.preventDefault();
}
};
dp.init();
</script>