This is the only solution I can find with clean, understandable code. Great work

This is very cool.

Nice solution!
You can replace Math.floor(x) with ~~(x) if you're looking to save even more space.

The function in the initial code is Japanese(), but the function in the test cases is japanese(), leading to errors unless corrected.

Or you could just use a.localeCompare(b) instead of trying to find the index. Misread that.

I'd say the best say is

[...'abcdefghijklmnopqrstuvwxyz']

This is masterful. Well done!

This is what I started thinking about too, but you save time with just [...'abcdefghijklmnopqrstuvwxyz'] compared to your Array map function, and you don't need to call the spread operator on the Includes string.