recursive fatorial using y-combinator

recursive factorial through y-combinator.js

var factorial = (f => {
  return y => {
    return f(y, f);
  };
})((num, f) => {
  return (num => {
    if (num < 0) {
      return -1;
    } else if (num === 0) {
      return 1;
    } else {
      return (num * f(num - 1, f));
    }
  })(num);
});

console.log(factorial(8));