Yes, perhaps I wasn’t clear — I’m advocating for just the inlined version, not the nested function in the general case.
The nested function is, I think, fine when it’s so tiny it’s not worth unit testing (like, it-could-have-been-a-lambda small) or when bar is returned by foo (ie foo is higher order), in which case you can test the return value. Apart from that… restraint!
When you write the initial code to figure out bar, just throw that code in a unit test so you can run it any time. Stop throwing tests away!