How to sum the digits of a number

function digitSum(n) {
  if (!(typeof n === 'number' || n instanceof Number)) {
    return 0;
  }

  if (n <= 0) {
    return 0;
  } else if (n < 10) {
    return n;
  } else if (n === Infinity) {
    return Infinity;
  }

  return (n % 10) + digitSum((n / 10)>>0);
}

console.log(digitSum(1234)); // 10
console.log(digitSum(3890)); 20
console.log(digitSum(Infinity)); // Infinity

console.log(digitSum(-1234)); // 0
console.log(digitSum(-3890)); // 0
console.log(digitSum(-Infinity)); // 0