simpleEvenNumber.js

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);
}

console.log(isEven(50));
// → true
console.log(isEven(75));
// → false
console.log(isEven(-1));
// → ?? runs out of stack space -> false