merge2.js

function merge(arr1, arr2) {
  let result = [];
  while (arr1.length && arr2.length) {
    if (arr1[0] < arr2[0]) {
      result.push(arr1.shift());
    } else {
      result.push(arr2.shift());
    }
  }
  return [...result, ...arr1, ...arr2];
}
function mergeSort(arr) {
  if (arr.length <= 1) return arr;
  const mid = Math.floor(arr.length / 2);
  const left = mergeSort(arr.slice(0, mid));
  const right = mergeSort(arr.slice(mid));
  return merge(left, right);
}