Nothing undocumented going on here. Read the MDN docs again and look for "offset". It is of course the second parameter in this case since there's no groups in the regex.

Had the same issue a few times now. Maybe I didn't look in the correct places but I've never seen a specification for the function parameters of the replace method. At least now I learned something :)

This is how I wanted to implement my solution, but I couldn't find the documentation for how to get the index position of the current item in replace(). Anyone else have the same problem?

I know the answer is somehwere in the "Specifying a function as a parameter" section of MDN, but when I read over that section, nothing there stands out as a method of returning an index value. (Great solution, BTW)