function mergeSort(arr) {
  if (arr.length === 1) {
    return arr
  }

  center = Math.floor(arr.length / 2)
  left = arr.slice(0, center)
  right = arr.slice(center)

  return merge(mergeSort(left), mergeSort(right))
}

function merge(left, right) {
  const results = []

  while (left.length && right.length) {
    if (left[0] < right[0]) {
      results.push(left.shift())
    } else {
      results.push(right.shift())
    }
  }

  return [...results, ...left, ...right]
}