function quick_sort(arr) {

    if (arr.length <= 1) {
        return arr;
    }

    let pivot = arr[0];

    let left = [];
    let right = [];

    for (let i = 1; i < arr.length; i++) {
        if (arr[i] < pivot) {
            left.push(arr[i]);
        } else {
            right.push(arr[i]);
        }
    }

    return quick_sort(left).concat([pivot], quick_sort(right));
}