SOURCE

function quickSort(arr) {
    if (arr.length <= 1) {
        return arr;
    }
    const num = arr[0];
    const left = [], right = [];
    for(let i = 1; i < arr.length; i ++) {
        if (arr[i] <= num) {
            left.push(arr[i]);
        } else {
            right.push(arr[i]);
        }
    }
    return quickSort(left).concat([num], quickSort(right));
}

console.log(quickSort([1,3,9,3,5,6]))