function* countdown(num) {
  while (true) {
    yield num--;
  }
}

function setCountDown(n) {
  n = n || 5;
  var cd = countdown(n);
  for(var i = 0; i <= n; i++) {
   setTimeout(function() {
      console.log(cd.next().value);
    }, 1000 * i);
  }
}