This is a very curious and fun way to put it:
https://stackoverflow.com/a/57249968
In essence, `undefined` is non-existence. And `null` is existence with a non- (or "invalid", or "unknown") value.
Although using them like that depends on the needs of the application. If "invalid" or "unknown" was an expected user choice, I would still rather encode it as an enum better than a nullable boolean, as mentioned in my comment above.
