merge sort js toy problem code example

Example: implement a merge sort algorithm JS

function merge(left, right) {
  const resultArray = []
  let leftIndex = 0
  let rightIndex = 0

  while (leftIndex < left.length && rightIndex < right.length) {
    if (left[leftIndex] < right[rightIndex]) {
      resultArray.push(left[leftIndex])
      leftIndex+=1
    } else {
      resultArray.push(right[rightIndex])
      rightIndex+=1
    }
  }
  return resultArray
    .concat(left.slice(leftIndex))
    .concat(right.slice(rightIndex))
}
function mergeSort(unsortedArray) {
  if (unsortedArray.length <= 1) {
    return unsortedArray;
  }
  const middle = Math.floor(unsortedArray.length / 2);
  const left = unsortedArray.slice(0, middle);
  const right = unsortedArray.slice(middle);
  return merge(
    mergeSort(left), mergeSort(right)
  );
}
mergeSort([10, 9, 8, 7, 6, 5, 4, 3, 2, 1])