the error is in your code, at this line:

EmptyList.prototype.push = function(x) {return new ListNode(x, new EmptyList());};

You create a brand new EmptyList() for no reason there, you can reuse the current one instead

That's your opinion and it's fine, but I think it's no more than that: your opinion.

Personally I do prefer to cover all possible cases that I can think of, even if they aren't necessary to pass the tests, regardless if the kata is actually meant to be like that or if the creator didn't take such cases into account by mistake.

In the real world you will often need to cover all possible cases in order to prevent errors, so this way it helps me develop that kind of skills, if you want to call it like that.

done

upgraded to chai.assert.strictEqual

Raised as issue

invalidated

Too late to change...

Approved some time ago

I agree. And I would add that I see no point in using a map instead of a single string as argument.

Reraised as issue

fixed! i just couldn't bear to see it :D

added some time ago

Approved some time ago

Not so easy. Imagine this: https://www.codewars.com/kata/reviews/55661000acc711e892000009/groups/5fab78001696050001928386
Really, this digs up a builtin.