function isEven(x) {
  if (x < 0 ) {
        return false
  }
   else if (x % 2 == 0) {
        return true
  } else {
        return isEven(x-2)
  }
}

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));
// → false

else if (n < 0)
    return isEven(-n);

else
    return isEven(n - 2);

function isEven(n) {
  // make sure we have a number
  if(isNAN(n)) {
    console.log('Non-integer passed to isEven()');
    return false;
  }
  // parse it as int to weed out floats
  if(n !== parseInt(n)) {
    console.log('Non-integer passed to isEven()');
    return false;
  }
  if (n === 0)
    return true;
  else if (n === 1)
    return false;
  else if (n < 0)
    return isEven(-n);
  else
    return isEven(n - 2);
}