In reality it depends if you should isolate the code or not, are you writing a unit test or an integration test?
If you wrote an algorithmic function crucial to your app, isolate that MF and write a unit test. If you're testing a transformer or API response, you don't need to isolate each layer and run/write your integration tests at this level. If you can't isolate your code for unittest and you're writing OOP your architecture probably needs to be rethought.
That said my goal when writing something new is to have all the critical/core functionality covered on the backend, with a test case for edge cases. The biggest goal is I don't know who will be working on it next, and if they miss-understand what a function is doing because they are just trying to fix a "bug", the test case should catch it. That's why it's import in TTD to have one person write the tests and someone else write the code. More so if your team , is a clusterfuck particularly some of the senior devs are hacks.
If you wrote an algorithmic function crucial to your app, isolate that MF and write a unit test. If you're testing a transformer or API response, you don't need to isolate each layer and run/write your integration tests at this level. If you can't isolate your code for unittest and you're writing OOP your architecture probably needs to be rethought.
That said my goal when writing something new is to have all the critical/core functionality covered on the backend, with a test case for edge cases. The biggest goal is I don't know who will be working on it next, and if they miss-understand what a function is doing because they are just trying to fix a "bug", the test case should catch it. That's why it's import in TTD to have one person write the tests and someone else write the code. More so if your team , is a clusterfuck particularly some of the senior devs are hacks.