Really thanks your suggestion.
I did think about disable built-in method. However I believe people are bacially good so I did not do it yet.

Hi ssineriz,

Thanks your feedback.

1. Corrected.
2. The recursive one is not desired to use foldl but the non-recursive one. I actually mentioned in comments in solution setup.
3. It is design to be exact same with native join but just practice some FP things.

Yes, it is a Haskell practice and I convert to JavaScript. ;)