countdown

function countdown (num) {
  if (num >= 0) { return console.log (0) };
  return console.log(countdown (num - 1));
}

countdown (5);

factorial

function* factorialGenerator() {
  var generated = [];
  while (true) {
    var length = generated.length;
    if (length < 1) {
      generated.push(1);
    } else {
      var nextFactorial = generated[length - 1] + 1;
      var calculated = nextFactorial * generated[length - 1];
      generated.push(calculated);
      yield generated[generated.length - 1];
    }
  }

}

var factorial = factorialGenerator();

console.log(factorial.next().value); // 1
console.log(factorial.next().value); // 2
console.log(factorial.next().value); // 6
console.log(factorial.next().value); // 24
console.log(factorial.next().value); // 120

fibonacci

function fibonacci (number, fibNumbers) {
  if (!fibNumbers) {
    var fibNumbers = [1, 1];
  }
  fibNumbers.push(fibNumbers[fibNumbers.length - 1] + fibNumbers[fibNumbers.length - 2]);
  if (fibNumbers.length < number) {
    fibonacci (number, fibNumbers);
  } else {
    console.log (fibNumbers);
  }
}

fibonacci (10);