I have the same question and also diag1Sym, where is it from ?

No problem, it's not very intuitive either, but every post you make is public unless you mark it as having spoiler content (even when commenting a solution).

Chrono79, oh, I don't know yet how exactly spoiler flags work, but next time i will use one :D. Sorry!

Use spoiler flag next time, please.

{'0':'O','5':'S','1':'I'} is an object, let's call it symbolObj.
let symbolObj = {'0':'O','5':'S','1':'I'};
As you see, the map method will return symbolObj[a] or a.
So if a is 0, 5 or 1 it will be true and will return the correspondent symbol from the object. Otherwise, it will be false and the returned value will be a.

I'm beginner, so i don't know if i explained it very well but i hope i helped you.

Read this
I see where your confusion comes from, in sample tests assertDeepEquals is used and in random tests assertSimilar is used instead.
Changed it so all tests use assertDeepEquals.

AFAIK Javascript tests are ok and aren't vulnerable to input mutation, so the problem must be in your code. Post it and mark your code as having spoiler content.

Each positive becomes negatives, and the negatives become positives.

Just in case, 0 is neither positive nor negative.