mergesort.js

function merge(leftArray, rightArray) {
  const sorted = [];
  while (letArray.length > 0 && rightArray.length > 0) {
    const leftItem = leftArray[0];
    const rightItem = rightArray[0];
    if (leftItem > rightItem) {
      sorted.push(rightItem);
      rightArray.shift();
    } else {
      sorted.push(leftItem);
      leftArray.shift();
    }
  }
  while (leftArray.length !== 0) {
    const value = leftArray.shift();
    sorted.push(value);
  }
  while (rightArray.length !== 0) {
    const value = rightArray.shift();
    sorted.push(value);
  }
  return sorted;
}
function mergeSort(array) {
  const length = array.length;
  if (length === 1) {
    return array;
  }
  const middleIndex = Math.ceil(length / 2);
  const leftArray = array.slice(0, middleIndex);
  const rightArray = array.slice(middleIndex, length);
  leftArray = mergeSort(leftArray);
  rightArray = mergeSort(rightArray);
  return merge(leftArray, rightArray);
}