Moment of undefined vs moment of null

Question

If wonder why

> moment(undefined).isBefore()
true

but

> moment(null).isBefore()
false

Is there any rational explanation for this behavior?

Answer 1

moment(undefined) is equivalent to moment(), which assumes the initial state to be the current date/time.

moment(null), on the other hand, is not a thing. It is not valid (at least not in the version I'm playing with), and has undocumented results.

Of course, you can read the source code, and find that isBefore doesn't check for undefined either. In other words, momentjs is not expecting itself to be used this way, twice.