TDD

Test-drive browser JavaScript behavior with DOM fixtures, test runners, smoke tests, onload timing, selector refactors, and integration tradeoffs with Selenium or end-to-end tests. Use when adding client-side behavior, debugging DOM tests, choosing between JS unit tests and browser functional tests, or making JavaScript testable without overusing Selenium.

Use Python mocks and fakes as a design tool without losing behavioral confidence. Use when testing external dependencies, choosing monkeypatch versus unittest.mock.patch, isolating slow boundaries, avoiding mock-heavy tests, interpreting mock call assertions, or refactoring toward clearer dependency seams.

Design a sustainable test architecture using test desiderata, test pyramid tradeoffs, fast and slow test separation, integration boundaries, CI feedback loops, and architecture choices that make code testable. Use when a test suite is too slow, too brittle, too mock-heavy, unclear about unit vs integration coverage, or needs a testing strategy before major growth.