So, uh. Why did you write it like that?

MergeSort.js

const MergeSort = ({ unsortedArray }) => {
  const { length } = unsortedArray
  if (length <= 1) {
    return unsortedArray
  } else {
    const middle = Math.floor(length / 2)
    const unsortedLeft = unsortedArray.slice(0, middle)
    const unsortedRight = unsortedArray.slice(middle)
    const { result: left } = MergeSort({ unsortedArray: unsortedLeft })
    const { result: right } = MergeSort({ unsortedArray: unsortedRight })
    const { result } = Merge({ left, right })
    return { result }
  }
}

const Merge = ({ left, right }) => {
  const resultArray = []
  let leftIndex = 0
  let rightIndex = 0
  while (leftIndex < left.length && rightIndex < right.length) {
    if (left[leftIndex] < rightIndex[rightIndex]) {
      resultArray.push(left[leftIndex])
      leftIndex++
    } else {
      resultArray.push(right[rightIndex])
      rightIndex++
    }
  }
  const result = resultArray.concat(left.slice(leftIndex)).concat(right.slice(rightIndex))
  return { result }
}