Fix undefined check

[Seb33300]

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined#description

A variable that has not been assigned a value is of type undefined.

Fix error in #63

[SimJoSt]

While this doesn't fix the error in it entirety, I do prefer proper error handling. Thanks for the PR.

[SimJoSt]

@antonmedv it would be great if you could provide a comment on why you closed this PR. It would help to create better contributions :)
Was the code quality or approach not up to snuff?
Is this approach superseded by another change? Maybe you want to focus on #65?

[antonmedv]

Now sure what to say here.

[SimJoSt]

@antonmedv I would love to give back a little, try to provide PRs, report bug reports, and testing.
If a PR is closed without a comment, I don't know what to make of it. Was it wrong? Broken? Unnecessary?
Any hint, why it was closed would help. It could be, that the reason is obvious, and I am just blind. I can't figure it out without some help :)

[Seb33300]

I think I forgot the typeof

[SimJoSt]

Ah, true. Could be.
Well, that would be an easy fix.

[antonmedv]

It will be actually cool to refracture all this part more robust and include all the corner cases just type of could help but I’m not sure