This one works:

```js
function fact(n) {
  return n > 1 ? n * fact(n - 1) : n;
}

console.log(fact(4)) // Returns 24
```

This returns `NaN`:

```js
function fact(n) {
  return n > 1 ? n * fact(n - 1) : `Factorial is : ${n}`;
}

console.log(fact(4)) // Returns NaN
```

Why the second function returns `NaN`?