Test-Driven Development

Philosophy

Core principle: Tests verify behavior through public interfaces, not implementation details. Code can change entirely; tests should not.

Good tests are integration-style: they exercise real code paths through public APIs. They describe what the system does, not how. A good test reads like a specification. These tests survive refactors because they don't depend on internal structure.

Bad tests are coupled to implementation. They mock internal collaborators, test private methods, or verify through external means. Warning sign: your test breaks when you refactor, but behavior hasn't changed.

See reference/tests.md for examples and reference/mocking.md for mocking guidelines.

Framework Agnostic

This skill works with any test framework (Vitest, Jest, Playwright, Cypress, pytest, JUnit, etc.) and any stack (Vue.js, React, Node.js, backend services). Adapt examples to the project's language and framework.

Anti-Pattern: Horizontal Slices

DO NOT write all tests first, then all implementation. This is "horizontal slicing" — treating RED as "write all tests" and GREEN as "write all code."