[TimJYoung]

Yes, I understand how JS works. My company markets and sells a web development IDE that includes an Object Pascal->JS compiler.

Re: your definition of "internal": I suspected that this was what you meant. I also disagree with this particular version of "correctness". If the developer intended that a particular function accept an integer, then accepting anything else is essentially a bug, and the fact that it works at all is down to pure coincidence.

As for writing tests in OP: you write tests in the same manner that you do with Java, C++, C#, etc.