merge-sort.js

function merge(left, right) {
  const result = [];

  while (left.length > 0 && right.length > 0) {
    if (left[0] <= right[0]) {
      result.push(left[0]);
      left = left.slice(1);
    } else {
      result.push(right[0]);
      right = right.slice(1);
    }
  }
  while (left.length > 0) {
    result.push(left[0]);
    left = left.slice(1);
  }
  while (right.length > 0) {
    result.push(right[0]);
    right = right.slice(1);
  }
  return result;
}

function mergeSort(array) {
  if (array.length <= 1) {
    return array;
  }

  let left = [];
  let right = [];

  for (let i = 0; i < array.length; i += 1) {
    if (i < array.length / 2) {
      left.push(array[i]);
    } else {
      right.push(array[i]);
    }
  }
  left = mergeSort(left);
  right = mergeSort(right);

  return merge(left, right);
};