it doesn't matter how well your app is designed, your UX designer is not going to tell you you need a function that does X. you just build something that looks like the thing they want, and then write some tests to make sure someone doesn't break that thing, and if you have to write a dozen functions to create it and they're testable then you test them but you don't say oh no i can't write a 13th function now because that wasn't part of the preordained plan