要不要重新认识一下递归与迭代？

// recursive process
function fact(n) {
  return n === 0
         ? 1
         : n * fact(n - 1)
}

fact(4)
4 * fact(3)
4 * 3 * fact(2)
4 * 3 * 2 * fact(1)
4 * 3 * 2 * 1 * fact(0)
4 * 3 * 2 * 1 * 1
4 * 3 * 2 * 1
4 * 3 * 2
4 * 6
24

// iterative process
function fact(n) {
  function fact_iter(a, n) {
    return n === 0
           ? a
           : fact_iter(a * n, n - 1)
  }
  return fact_iter(1, n)
}

fact(4)
fact_iter(1, 4)
fact_iter(4, 3)
fact_iter(12, 2)
fact_iter(24, 1)
fact_iter(24, 0)
24

// recursive process
function fast_expt(b, n) {
  return n === 0
          ? 1
          : is_even(n)
            ? square(fast_expt(b, n / 2))
            : b * fast_expt(b, n - 1);
}
// iterative process
function fast_expt(b, n) {
  function iter(a, b, count) {
    return count === 0
            ? a
            : is_even(count)
              ? iter(a, square(b), count / 2)
              : iter(a * b, b, count - 1);
  }
  return iter(1, b, n);
}
function is_even(x) {
  return x % 2 === 0;
}
function square (x) {
  return x * x;
}