A simple implementation of a merge sort algorithm.

mergeSort.ts

function mergeSort(arr: number[]): number[] {
  const length = arr.length;

  if (length === 1) return arr;

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

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

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

function merge(left: number[], right: number[]): number[] {
  const sorted: number[] = [];

  while (left.length > 0 && right.length > 0) {
    if (left[0] < right[0]) {
      sorted.push(left[0]);
      left.shift();
    } else {
      sorted.push(right[0]);
      right.shift();
    }
  }

  return sorted.concat(left);
}