Code coverage measures how much of your code is executed by tests. It is a useful indicator and a dangerous target. The difference between the two determines whether coverage improves your software or just your numbers.
An honest assessment of test-driven development - the contexts where writing tests first genuinely improves code design, and the ones where it adds friction without benefit.
What each type of test double actually is, when to use each one, and how to avoid the trap of over-mocking tests into uselessness.
What separates a unit test that catches real regressions from one that only verifies your test setup - and how to write tests that stay useful as code changes.
What unit, integration, and end-to-end tests are actually good for, and how to find the right mix so your test suite catches real bugs without being a maintenance burden.
