function mergeSort(arr) {
  let l = arr.length
  if (l === 1) return arr
  let m = Math.round(l / 2)
  let left = arr.slice(0, m)
  let right = arr.slice(m)
  return merge(mergeSort(left), mergeSort(right))
}

function merge(arr1, arr2) {
  let l1 = arr1.length
  let l2 = arr2.length
  let i1 = 0
  let i2 = 0
  let r = []
  while (i1 < l1 && i2 < l2) {
    let item1 = arr1[i1]
    let item2 = arr2[i2]
    if (item1 < item2) {
      r.push(item1)
      i1++
    } else {
      r.push(item2)
      i2++
    }
  }

  while (i1 < l1) {
    r.push(arr1[i1++])
  }

  while (i2 < l2) {
    r.push(arr2[i2++])
  }

  return r
}

console.log(mergeSort([1, 4, 2, 3, 5, 2, 7, 3, 17, 25, 19]))