Why sweet alert in window.onbeforeunload just skips return values

Question

When running a sweet alert on window.onbeforeunload it ignores the return values and just leaves the page straight away without you even getting to choose an option. What am I missing.

window.onbeforeunload = ConfirmExit;

    function ConfirmExit() {
        swal({
            title: "Are you sure?",
            text: "If you leave this page without submitting you will lose your progress.",
            type: "warning",
            showCancelButton: true,
            confirmButtonColor: "#5cb85c",
            confirmButtonText: "✔ Continue",
            cancelButtonText: "✖ Cancel",
            closeOnConfirm: true,
            closeOnCancel: true,
            animation: false
        },
            function (isConfirm) {
                if (isConfirm) {
                    return true;
                } else {
                    return false;
                }
            });
    }