[mcv]

End-to-end tests are more overhead than unit tests. End-to-end tests are also not great for testing the many edge cases that you are likely to mess up if you refactor carelessly.

I also don't see how a robust type really helps there. It might even be part of the thing that needs to be refactored. Besides, many languages don't have a very robust type system.

End-to-end tests and type systems certainly have their uses, but for refactoring messy code, I don't think there's a good substitute for thorough unit tests.

Although there are different kinda of refactoring of course. The case I'm referring to was about one very messy module. This makes it very easy to unit test. If instead it's the entire architecture of your application that needs to be refactored, then you're looking at a very different case, and end-to-end tests become more important than edge cases.

[agloeregrets]

Unit tests are too. What the comment you are replying to is referencing is Integration tests, the type that instead of working on a “this class returns this” instead works on “component A calls Component B to do something, are the two still speaking the same language and expectations?”

[mcv]

There's three levels of testing: unit, integration, and end-to-end.

End-to-end (e2e) tests the entire stack: deploy the site with database and all, run a script that visits a page, clicks and stuff, and checks if the right things become visible.

Unit tests are the other extreme: you take a single unit of code and test whether it does what it's supposed to, while mocking all communication with other units of code.

Integration tests are in between those two extremes, and probably the least understood as a result (at least by me). They look like unit tests, and don't generally test the UI, but they don't mock the other components, and ideally also set up a database to test against.

I think there's a bit of overlap between unit and integration tests; a sloppy unit test where you don't mock (some) other components but treat them as part of the unit you're testing, start to look like integration tests at some point. If you want a clear demarcation, I think you might consider a database connection essential to count as integration test.