Unit Testing Foundations

Description

Write effective unit tests in any language - testing behaviours rather than functions, Arrange-Act-Assert, the four test doubles (dummy, stub, mock, fake) and when each fits, the testing pyramid, FIRST principles, test names as documentation, error paths and systematic edge cases (zero, one, many, boundaries, invalid), common test smells, and using coverage to find blind spots rather than as a target. Use whenever the user writes their first tests, asks how or what to test, asks mock vs stub vs fake, has flaky, slow or brittle tests, wonders whether a test is any good, or is chasing a coverage number.