var count = 0;

function quicksort(arr) {

  let length = arr.length,
    pos = Math.floor(Math.random() * length); //pick a random index position

  if (length <= 1) return arr;
  //if (++count > 10) return console.log('error');

  /*
  console.log({
    arr,
    pos,
    posv: arr[pos]
  })
  */


  let left = arr.filter((v) => v < arr[pos]),
    center = arr.filter((v) => v === arr[pos]), //center is all equal already
    right = arr.filter((v) => v > arr[pos]);

  return [...quicksort(left), ...center, ...quicksort(right)]
}

console.log(quicksort([]))
console.log(quicksort([3]))
console.log(quicksort([7, 5]))
console.log(quicksort([4, 7, 5]))
console.log(quicksort([4, 7, 5, 6]))

var test = n => { //test n times
  let error = false;
  for (let i = 0; i < n && !error; i++) {
    let length = Math.floor(Math.random() * n);
    let arr = [];
    for (let j = 0; j < length; j++) {
      arr.push(Math.floor(Math.random() * n))
    }
    //console.log(arr)
    const arr_sort = quicksort(arr);

    //check whether arr_sort is sorted
    for (let j = 1; j < arr_sort.length; j++) {
      if (arr_sort[j] < arr_sort[j - 1]) {
        console.log('error');
        error = true;
        break;
      }
    }
    //console.log(arr_sort)

  }

  if (!error) console.log('all pass');
}

test(1E2)