Recursive power function: Why does this work if there's no initial return value?

Question

because power(base, exponent) has no return value unless exponent is 0, initially, shouldn't power(base, exponent -1) return 'undefined', and therefore be unmultipliable, initially? So, I am having trouble following the logic of this code. Why/how does it work?

function power(base, exponent) {
  if (exponent == 0)
    return 1;
  else
    return base * power(base, exponent - 1);
}

Answer 1

It could be more concise:

function power(base, exponent) {
  return exponent == 0? 1 : base * power(base, --exponent);
}

Howerver an iterative solution is very much faster:

function powerNR(base, exp) {
  var result = 1;
  while(exp--) {
    result *= base;
  }
  return result;
}