I still don't understand why mutating or not the input would be relevant to this.
My first answer mutate the input, returned the right answer and I was getting “Value is not what was expected”. That's fine, but if I hardcode the return to the right answer it works, and the input is being mutated all the same.
I even console.log a test to see if the result coming from the algorithm and the hardcoded version are the same, and they are, but only the hardcoded version is accepted as correct. Would anyone be bothered to explain why?

Don't mutate the input.

Why is there no report button to report hateful users that can't solve a puzzle nor even stay calm?

I had this problem, but it was apparently because I was modifying the input. If you solve this kata without modifying the input, then you probabaly won't have this problem.

if (answer) return answer;