So please forgive me if I'm missing something basic and this is a moronic question,

but it's a key part of this not to mutate the input?

Tests pass, but I get "Cannot read property 'reduce' of undefined
at arr.filter" on attempt. Can anyone explain what this means? I'm not using reduce or filter in my code. Is this a bug or a user error?

Can someone explain to me how the characters are ending up in the 'count' variable in this solution? I understand that it works, I'm not quite getting how.