function mystery(arr, depth = 0) {
  if (arr.length <= 1) return arr;
  
  const mid = Math.floor(arr.length / 2);
  const left = mystery(arr.slice(0, mid), depth + 1);
  const right = mystery(arr.slice(mid), depth + 1);
  
  const result = [];
  let i = 0, j = 0;
  
  while (i < left.length && j < right.length) {
    result.push(left[i] <= right[j] ? left[i++] : right[j++]);
  }
  
  return result.concat(left.slice(i)).concat(right.slice(j));
}

const arr = [3, 1, 4, 1, 5];
console.log(mystery(arr));