Maybe it's easier to imagine x = [];, then x.slice(). Or even ([]).slice. In all cases, it looks on the instance of the array for the slice method and, failing to find one, walks the prototype chain until it gets there. Hypothetically less performant but it's a constant time optimization and I think some or most compilers optimize for the autoboxing this involves.

I tried that, but the test says

TypeError: Cannot call method 'toArray' of null
at Test.it
at Test.describe

If I use Cons(null, null) I get this error:

TypeError: Cannot call method 'toArray' of undefined
at Test.it
at Test.describe

Based on the link to the algebraic data type documentation, I think Cons.fromArray([]) should return null, but I wasn't positive.

What should Cons.fromArray([]) return?

null

or

new Cons(null, null)

or something else?