排列组合

    var data = ['a', 'b', 'c', 'd'];
    function getGroup(data, index = 0, group = []) {
        var newArr = [];
        newArr.push(data[index]);
        for (var i = 0; i < group.length; i++) {
            newArr.push(group[i] + ',' + data[index]);
        }
        group.push(...newArr);
        //  group.push.apply(group, newArr);
        if (index + 1 >= data.length) return group;
        else return getGroup(data, index + 1, group);
    }
    console.log(getGroup(data));