Requiring the test before writing the code also ensures you dont forget to write a test to match the scenario.
So what is gained by test after... is that it is almost as good?
I still dont get it.
This order also helps verify I didn't typo something in the test itself and end up TDD-ing myself into broken code.
You don't need something to work with to write it. You can, by definition, write an e2e test against an app that doesnt exist.
This test isnt special as far as TDD is concerned - red-green-refactor works the same way.
Im sensing a pattern in the answers to my question though. I keep getting "well, if you assume TDD is only done with low level unit tests..."
Completely wrong.
Even with your example, there's an initial exploratory stage where you're still figuring out the interface that the tests would use. I, personally, am not capable of using something that doesn't exist. I have to make that initial version first before I can use it in a test.
Quick edit aside: This is also why I rarely work top-down or bottom-up, I work mostly throughline - following the data flow and jumping up and down the abstraction stack as needed.
What happens when you then show it to stakeholders (e.g. other teams consuming your API, customers or UX people) or and they tell you to change it again?
Rewrite everything again?
Thats gonna be reaaaaaaaalllly labor intensive and could damage your code base too.
Im equally perplexed about why people dont try to build top down. It's one of those few things in programming that always makes sense regardless of circumstance.
> Rewrite everything again?
> Thats gonna be reaaaaaaaalllly labor intensive and could damage your code base too.
Why would I do that? Only the thing they have issue with would need to be changed, it wouldn't take any longer than another way of doing it.
You seem to have forgotten what I said, something needs to exist for me to work with. Well, in this "stakeholders want something changed", something exists. It's not a rewrite from scratch.
If you change the spec (e.g. changing the contract on a REST API), you will probably need to consult to make sure it aligns with everybody's expectations. Does the team calling it even have the customer ID you've just decided to require on, say, this new endpoint?
>You seem to have forgotten what I said, something needs to exist for me to work with.
No. I'm assuming here that a code base exists and that you are mostly (if not 100%) familiar with it.
So you end up writing the unit's boilerplate, that doesn't yet do anything but needs to compile/run for a suite to test it throwing a adhoc error of the notImplemented sort or whatever, then break flow to set the test suite and check the reds (that aren't telling you anything useful because of course it's red), then actually write the code.
I find it flow-breaking and cumbersome for little to no gain.
For additions to existing code I find TDD more useful, but even then it's not unusual to decide to move logic around until deciding a final structure, so the units you end up with might be solidified later in time. Writing tests for scraped units is a waste of time then.
Right. In those situations I TDD with an e2e or integration test.
I dont get why youd restrict yourself to doing TDD with just with low level unit tests.
The type of test is, in my mind, a completely different topic to red-green-refactor and for the decision about which one to write I follow a set of rules which is also unconnected.
TDD is just red-green-refactor. It works with any test.
I only use that technique for pieces of code that really fit that well - usually functions that have a very strong relationship between their input and output - so I'll write tests first for those, but not for most of my other stuff.
Almost every user story I follow in production code follows the form of given/when/then scenario which can always be transformed into a test of some kind (e2e, integration, sometimes even unit).
Where it's something like "do x, y and z and then a graph appears" I find TDD with a snapshot test with, say, playwright works best.
If you're using snapshot tests (a technique I really like) surely you can't write the tests before the implementation, because you need the implementation in order to generate the snapshot?
(This is what I hate about the term TDD: sometimes it means test-first, sometimes it doesn't - which leads to frustrating conversations where people are talking past each other.)
I agree that "unit test"/"integration test" as a definition sucks horribly and leads to people talking past each other, but I think with TDD the main issue is that lots of people have developed a fixed and narrow idea of the kind of test you are "supposed" to write with it which makes the process miserable if the type of code doesnt fit that type of test.
The whole idea of a unit test being "the" kind of "default" test and being "tests a class/method as a unit" definitely needs to die.
No, it provides live feedback about whether your code is passing your tests
If you have written your tests poorly then set out to make the tests pass, then your tests become the target rather than the correct behavior
If you are continuously updating your tests while your code evolves because you missed test cases or your understanding of the behavior has improved, then writing the tests first didn't actually give you any value. In fact it just wasted a lot of your time
Write the code Manually test to verify correctness and to identify the test cases you have to write THEN write tests to protect against regressions