js-neo

let swap = function (array, pos1, pos2) {
    var temp = array[pos1];
    array[pos1] = array[pos2];
    array[pos2] = temp;
};
let combinations = (array, output, n) => {
    n = n || array.length;
    if (n === 1) {
        output(array);
    } else {
        for (var i = 1; i <= n; i += 1) {
        combinations(array, output, n - 1);
        if (n % 2) {
            var j = 1;
        } else {
            var j = i;
        }
        swap(array, j - 1, n - 1); 
    }
}
};
combinations(['a', 'b', 'c', ], (result)=>console.log(result));