you're missing a unit test case for your division function -- the case of 0. That test will reveal the function's domain is non-zero numbers. And you know thi

you're missing a unit test case for your division function -- the case of 0. That test will reveal the function's domain is non-zero numbers. And you know this because you got the fast feedback from your unit test. No integration test using 0 necessary unless you want to test the failure scenario. BTW, nobody is saying not to use integration tests. But unit tests are faster, more stable, and easier to maintain and execute.