auto group option in optgroup by attribute

jquery.autogroup.js

$.fn.autoGroup = function (selector) {
    var elem = $(this);
    let a = [];
    selector = selector || "data-group";

    elem.find("option").appendTo(elem); //remove options dentro de optgroups e coloca no select
    elem.find("optgroup").remove(); // remove optgroups

    elem.find("[" + selector + "]").each(function (index) {
        a.push($(this).attr(selector));
    });

    var unique = a.filter(function (itm, i, a) {
        return i == a.indexOf(itm);
    });

    for (var i = 0; i < unique.length; i++) {
        if (unique[i].length > 0) {
            elem.find("[" + selector + "='" + unique[i] + "']")
                .wrapAll("<optgroup label='" + unique[i] + "'>");
        }
    }

    elem.val(elem.find("[selected]").attr("value"));
    elem.trigger("change");


};