Extending prototypes, even if not for the faint of heart, on Array ( and, yes, even on Object ! ), can be done correctly and safely. But that isn't specified or tested for here.

This deserves to be marked as an Issue ( which will need a new comment ).

#Taking about JavaScript

It's an extremely bad practice to declare properties/methods on Builtin Objects such as Array.
The author of this Kata should correct this detestable part.