Yeah, I found the same issue when I tried to solve it in Codewars. So I had to do the same. Looking through other people's solutions, there's one that I find interesting that uses split() instead of a regular expression. Something like this:
const xo = str => str.toLowerCase().split('x').length == str.toLowerCase().split('y').length
The split approach is interesting. I was playing with different options, but not with this. Also, I found that using toLowerCase() made the code considerably slower, it works considerably faster if you use a regular expression in the split:
const xo6 = str => str.split(/x/i).length == str.split(/o/i).length;
I think this is my favourite solution. I hadn't checked its speed, but I thought that toLowerCase() probably didn't make it any smoother than returning an empty array if there are no matches.
I checked different solutions with jsperf and this was the second fastest by a little (but results may vary.)
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