Can I ask why you were expecting undefined as the return value?

I think right now, when providing undefined as a key-path, it's interpreted as the empty key path. I don't have a strong opinion about this being The Right Way, so I'm open to a discussion about it.

Another approach may be to consider undefined an illegal input and throw an exception instead. What do you think of that?

I think undefined should be considered an illegal input since it's neither an Array nor an Iterable.

@leebyron I conceived a bad analogy between get(undefined) and getIn(undefined): indeed undefined isn't a valid input for getIn.

Ok, I hear the consensus for this being illegal input and I agree.

It seems that hasIn is still affected.