sort.js

function merge(left, right) {
  let arr = [], leftIndex = 0, rightIndex = 0;

  while(left.length > leftIndex && right.length > rightIndex) {
    if (left[leftIndex] < right[rightIndex]) {
      arr.push(left[leftIndex]);
      leftIndex++;
    } else {
      arr.push(right[rightIndex]);
      rightIndex++;
    }
  }

  return arr.concat(
    left.slice(leftIndex).concat(
      right.slice(rightIndex)
    )
  );
}

function sort(arr) {
  if (arr.length <= 1) {
    return arr;
  }

  const middle = Math.floor(arr.length / 2);

  const left = arr.slice(0, middle);
  const right = arr.slice(middle, arr.length);

  return merge(sort(left), sort(right));
}