Merge sort

merge_sort.js

const mergeSort = (xs) => {
  if (xs.length <= 1) return xs;
  const midIndex = Math.trunc(xs.length / 2);
  return merge(
    mergeSort(xs.slice(0, midIndex)),
    mergeSort(xs.slice(midIndex)),
  );
}

const merge = (xs, ys) => {
  if (xs.length <= 0) return ys;
  if (ys.length <= 0) return xs;
  
  return xs[0] < ys[0]
    ? [xs[0], ...merge(xs.slice(1), ys)]
    : [ys[0], ...merge(xs, ys.slice(1))];
};

merge_sort.ts

const mergeSort = (xs: number[]): number[] => {
  if (xs.length <= 1) return xs;
  const midIndex: number = Math.trunc(xs.length / 2);
  return merge(
    mergeSort(xs.slice(0, midIndex)),
    mergeSort(xs.slice(midIndex))
  );
}

const merge = (xs: number[], ys: number[]): number[] => {
  if (xs.length <= 0) return ys;
  if (ys.length <= 0) return xs;
  
  return xs[0] < ys[0]
    ? [xs[0], ...merge(xs.slice(1), ys)]
    : [ys[0], ...merge(xs, ys.slice(1))];
};