i wonder why this very simple solution doesn't get nearly as much "best practice" than the other one above there >^.^<

Nice

https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/from
Array.from({length: n+1}) is like creating array as usual by new Array(length) (but this is bad approach as it is not recommended to use constructor for creating array). The bad thing with that is that we can't use map to define each of it's element because all of them are undefined. So this what is the feature of the Array.from I think, that the second parameter is a function which allows us to define each element of the array like you can do it with map method.

Define "productive" and "sophisticated", considering that it is competitive coding and you are meant to show off ;)

It looks not productive and too sophisticated

It does not look like the same to me and every single array item would be negative on that check.

if replace "0123456789".indexOf(a) on isNaN(a) will be shorter and probably faster is not it ?

I broke my brain )))

I completely agree!