You are misreading the sample tests. Look how tribonacci(sig, len) is called in the sample tests. User solution is called with two arguments, not one.

The tests are being ran with Node18. at() method should not be a problem/missing function.

Please explain why there are two arguments then for this function, when in the test samples you are feeding a single array, since I cannot de-structure. The sequence being an array in the [0] index, the 'n' value being the [1] index.

That's because you messed up the function signature. The corrrect one, present initially in the solution setup, is function tribonacci(signature,n). You changed it in a way which turns the signature argument into not a sequence.

It's not a kata issue, you need to fix your solution. Also make sure you run testss with version of Node which supports at.