module.exports = function thirteen(number) {
  if (number !== undefined && typeof number !== "number") {
    // Can number be cast to a number?
    if (!isNaN(+number.toString())) {
      return +number.toString() * 13;
    } else if (!isNaN(+number.valueOf())) {
      return +number.valueOf() * 13;
    } else {
      return new Error("I can only deal with numbers");
    }
  } else {
    return number * 13;
  }
};

module.exports = function thirteen(number) {
  if (number !== undefined && typeof number !== "number") {
    // Can number be cast to a number?
    if (!isNaN(+number.toString())) {
      return +number.toString() * 13;
    } else if (!isNaN(+number.valueOf())) {
      return +number.valueOf() * 13;
    } else {
      return new Error("I can only deal with numbers");
    }
  } else {
    // Respond with great energy
    return number * 13  + "!!!";
  }
};

Enhance user experience with more energetic response
Enhance user experience with more energetic response

Previous standard output of the thirteen() function did not sufficiently convey how revolutionary the project is to arithmetic and indeed mathematics as we understand it. Adding exclamation points helps communicate the proper element of surprise. This does not cause the overall 'thirteen()' function to break.