function isPrime(number) {
  if (number < 2) { return false; }
  const max = Math.sqrt(number);
  for (let divisor = 2; divisor <= max; divisor++) {
    if (number % divisor === 0) {
      return false;
    }
  }
  return true;
}
Prime number tester
Runtime: O(n ^ 0.5)
@param {integer} number
@returns {boolean} true if number is prime, otherwise false.