Merge and sort 1 or more arrays

mergeSort.js

const array = [0, 1, 3, 4, 31, 2, 4, 6, 7, 10, 23, 30];

/**
 * Merge Sort is a divide and conquer algorithm. It divides input array in two halves,
 * calls itself for the two halves and then merges the two sorted halves.
 * time complexity: O(nLogn)
 * space complexity: O(n)
 * @param arr | Array<number>
 * @return Array<number>
 */
function mergeSort(arr) {
  if (!Array.isArray(arr)) {
    throw new Error('input must be an array');
  }

  if (arr.length > 1) {
    let mid = arr.length / 2;
    let leftArr = arr.slice(0, mid);
    let rightArr = arr.slice(mid);

    mergeSort(leftArr);
    mergeSort(rightArr);

    let i = j = k = 0;

    while (i < leftArr.length && j < rightArr.length) {
      if (leftArr[i] < rightArr[j]) {
        arr[k] = leftArr[i];
        i++;
      }
      else {
        arr[k] = rightArr[j];
        j++;
      }
      k++;
    }

    while (i < leftArr.length) {
      arr[k] = leftArr[i];
      i++;
      k++;
    }

    while (j < rightArr.length) {
      arr[k] = rightArr[j];
      j++;
      k++;
    }

    return arr;
  }
}

mergeSort(array); // result: [ 0, 1, 2, 3, 4, 4, 6, 7, 10, 23, 30, 31 ]