const shuffleArray = arr => {
  const len = arr.length
  for (let i = 0; i < len - 1; i += 1) {
    let r = Math.floor(Math.random() * (len - i))
    r && ([arr[i], arr[i + r]] = [arr[i + r], arr[i]])
  }
  return arr
}

const chaoticArray = shuffleArray(Array(20).fill(0).map((_, i) => i + 1))

const selectionSort = array => {
  for (let i = 0; i < array.length - 1; i += 1) {
    let min = i;
    for (let j = i + 1; j < array.length; j += 1) {
      if (array[min] > array[j]) min = j;
    }
    [array[i], array[min]] = [array[min], array[i]];
  }

  return array
}

const mergeSort = array => {
  const pivot = (start, end) => Math.floor((start + end) / 2);  
  const len = array.length
  let tempA = [...array]
  let tempB = tempA.splice(pivot(0, len), len)
  
  tempA = selectionSort(tempA)
  tempB = selectionSort(tempB)
  
  let i = 0
  let j = 0
  let k = 0
  const result = []
  
  while (k < len) {
    if (tempA[i] < tempB[j]) {
      result.push(tempA[i])
      i += 1
    } else {
      result.push(tempB[j])
      j += 1
    }
    k += 1
  }
  return result;
}

console.log(mergeSort(chaoticArray))