Maybe you return string instead of number?

If your comment is about my solution, then answer is no. It doesn't modify the initial list passed, as assigning new value to list variable doesn't modify the list itself, but only changes the reference.

It looks like there is no way to specify different kata description for JS and Ruby. (Maybe I just cannot find it though).

Is there a way to see the sample test cases when actually doing the kata though? I couldn't find one without clicking over to the discussion and risking spoilers.

You should have sample test cases preloaded in the box which is there for your own test cases.

It should say that "3" should come out to 3, "3 4" should come out to 4

This should be obvious from the sample test cases.

"2 -" should come out to -2

This isn't tested for.

Hmm, it is present in sample test cases.

Maybe you try to print the resulting list and it is printed recursively?

I guess you use recursion and it results in too deep stack level. You likely need to rewrite your code in non-recursive manner as JS doesn't have tail-call optimization.

Added it

It isn't tested.

What language do you use? May you try submitting your solution instead of just testing it (it should allow me to see it).

Yes, it is intended.

Given that expression is in postfix notation I don't think eval() makes big difference.

Added, thanks for suggestion

If you are logging original/resulting list and it is huge, it may easily be a problem (console.log would attempt to convert it to string recursively).