Cool!

this is dark magic

nice

Iterate 2 times unnecessarily causes the script to be more inefficient...
Using less lines of code does not mean writting good code...

It would iterate over string two times. It's an easy solution, but strange that it's rated as best.

out of the box type of thinking, my admiration!

nah bro

see my solution

On another note, has anyone solved this kata using the regex exec function instead of match and could link to their solution? Due to their similarity (with the lastIndex difference), I'm curious to see a successful implementation (for some reason I couldn't get it to work). Thanks! :)

why the x&& and o&&? Is there something i'm not aware of or should x.length and o.length be sufficient?

because null === null evaluates to true
if I'm understanding your question correctly

It just ignore the "Case sensitive".
So, if its x or X it counts anyway

I think replace is better anyways. Don't need to worry about getting null returned, then have to check for it. null.length doesn't brake your code, because you where not turned a string or array.

you provide the series