Calculate Digit Sum of a String

export function digitSum(s: string, k: number): string {
  if (s.length <= k) {
    return s;
  }
  let result = '';
  for (let i = 0; i < s.length; i += k) {
    const substr = s.substring(i, i + k);
    const sum = [...substr].reduce((acc, c) => acc + parseInt(c), 0);
    result += sum.toString();
  }
  return digitSum(result, k);
}