I think this is just missing a few newlines to be perfectly readable production-quality code.

I never decalre them unless they're needed.

You should, but it's not required. This way a function pollutes the global scope, and that's bad.

wow! Don't you have to declare const or let before the function expression name?

good job you are amazing

I also wrote the solution with ternary operators and was really skeptical whether this is the best way and so was seeing the arguments against it. And ya, I think you are right. Clean code looks more elegant than complex, hard-to-read, seemingly-smart-looking-code. I think simple if-else wins here over ternary operator.

In my opinion the test cases should include partial numbers such as "12.test". And/or the test details amended to indicate whether these values should be accepted or return null (I would suggest null).