mergesort.js

let mergeSort = (arr) => {

  if (arr.length < 2) return arr;

  let middle = parseInt(arr.length / 2),
  left = arr.slice(0, middle),
  right = arr.slice(middle);

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

let merge = (left, right) => {
  let result = [];

  while (left.length && right.length) {
    left[0] <= right[0] ?
    result.push(left.shift()) :
    result.push(right.shift());
  }

  while (left.length) result.push(left.shift());
  while (right.length) result.push(right.shift());

  return result;
}