Why is parity minus one not defined?

Question

JavaScript

habrahabr.ru/post/240349 - in this article on Habré there is a task of writing a recursive function that returns the parity of a number ..
The code actually from their example is this

function isEven(n) {
  if (n == 0)
    return true;
  else if (n == 1)
    return false;
  else if (n < 0)
    return isEven(-n);
  else
    return isEven(n - 2);
}

if you pass -1 as a parameter, then it returns undefined... I was able to fix it - just a separate conditional case, but I did not understand the causes of the problem ..

Answer 1

console.log(-1 % 2); // => -1
console.log(-3 % 2); // => -1

Answer 2

The code looks correct. How was the work checked?

function isEven(n) {
  if (n == 0)
    return true;
  else if (n == 1)
    return false;
  else if (n < 0)
    return isEven(-n);
  else
    return isEven(n - 2);
}

alert( isEven('-1') )ж

Prints false as it should.

Answer 3

cycling