It is a bit late but maybe it helps.

I use the Mozilla- Documentation(link below) for Javascript
https://developer.mozilla.org/de/docs/Web/JavaScript

and to really get in to the details i use a Stackoverflow search.
https://stackoverflow.com/

hope i could help you :)

It does make sense since the empty string ε is a substring, prefix and suffix of every string.

If you want to learn more about it you can start here, and more in depth here :)

Yup, doesn't make sense to me too. I am getting similar error with second string being empty. But still, have made the adjustment as per the error.

Please read the description.
Each line is seperated with \n