function primeGenerator(num, showPrimes) {
  var i,
      arr = [];

  function isPrime(num) {
    // try primes <= 16
    if (num <= 16) return (
      num == 2 || num == 3 || num == 5 || num == 7 || num == 11 || num == 13
    );
    // cull multiples of 2, 3, 5 or 7
    if (num % 2 == 0 || num % 3 == 0 || num % 5 == 0 || num % 7 == 0)
      return false;
    // cull square numbers ending in 1, 3, 7 or 9
    for (var i = 10; i * i <= num; i += 10) {
      if (num % (i + 1) == 0) return false;
      if (num % (i + 3) == 0) return false;
      if (num % (i + 7) == 0) return false;
      if (num % (i + 9) == 0) return false;
    }
    return true;
  }

  if (typeof num == "number") {
    for (i = 0; arr.length < num; i++) if (isPrime(i)) arr.push(i);
    // first x primes
    if (showPrimes) return arr;
    // xth prime
    else return arr.pop();
  }

  if (Array.isArray(num)) {
    for (i = num[0]; i <= num[1]; i++) if (isPrime(i)) arr.push(i);
    // primes between x .. y
    if (showPrimes) return arr;
    // number of primes between x .. y
    else return arr.length;
  }
  // throw a default error if nothing returned yet
  // (surrogate for a quite long and detailed try-catch-block anywhere before)
  throw("Invalid arguments for primeGenerator()");
}