Expected: 5, instead got: undefined
Expected: 8, instead got: undefined
Expected: 9, instead got: undefined

The error messages are completely useless.

Double the satisfaction. Once with some good coding practice, and a second time seeing the creative ways used to cheat 😋

Yes, my solution follows the task literally. But since title is to implement the List and there are descriptions of what methods should do still I think solution should work as if no Array existed at all. We can not remove it totally, but we can develop test criterias which will check if this requirement is fulfilled. For example, remove all methods from Array prototype and as you said check return types of methods.

But maybe I'm just not satisfied with Kata's complexity and digging to deep. :) Then again if we follow the rules literally, .filter() should work exactly as Array's one does, thus return an Array.

Pfff... who knows? ^^

You're returning an object with a length property, values on numerical keys, and method getters pointing to Array methods.

It's as close to an array as you'll come, but it's an object which is not an array. It's legal. It's not a cheat.

It may feel like a cheat, but because after all everything is an object, how are you going to distinguish a too simple one from a sufficiently complicated one, and where do you draw the line?

I have no better solution.

Edit: Hmm .. .filter() doesn't return a List; it returns an Array. (More solutions are guilty of this.) Would that be a fair requirement? (It's not explicitly specified.)

Why the downvote? What did I do wrong? If I don't know, I can't fix it.

(Aimed at downvoter, not necessarily at OP.)

Why the downvote? What did I do wrong? If I don't know, I can't fix it.

(Aimed at downvoter, not necessarily at OP.)

Why the downvote? What did I do wrong? If I don't know, I can't fix it.

(Aimed at downvoter, not necessarily at OP.)

Why the downvote? What did I do wrong? If I don't know, I can't fix it.

(Aimed at downvoter, not necessarily at OP.)

Still can cheat. :)

The most blatant workarounds have been invalidated by a test for a List not actually being an Array.

Actually completely disabling Array is rather tricky.

Improved. Won't say it's fixed, but it's better than it was.

A test for a List not actually being an Array has been added.

Deleting Array is rather tricky.

Fixed.

Should be fixed.