Recursion return undefined instead true

I am wonder why this function return undefined? Any one may explain it for me?

function isEven(i) {
  if (i === 0 ) {
    return true;
  } else if (i == 1) {
    return false;
  } else {
    console.log(i);
    isEven(i - 2);
  }
}
console.log(isEven(6));