test-driven-development

Write a test that reproduces the bug first and confirm it fails, then implement the fix and confirm the test passes. Finally run the full test suite to verify no regressions were introduced.

Red means writing a test that fails because the behavior doesn't exist yet. Green means writing the minimal code to make it pass. Refactor means cleaning up the implementation while keeping all tests green.

Prefer real implementations first, then fakes, stubs, and mocks last. Mock only when the real dependency is too slow, non-deterministic, or has uncontrollable side effects like external APIs or email sending.

Skip tests for pure configuration changes, documentation updates, or static content changes with no behavioral impact. Any change that could alter or break existing behavior should have a corresponding test.

Tests that assert on internal method calls or implementation details break during refactoring even when behavior is unchanged. Assert on inputs and outputs (state-based testing) instead of interaction sequences.

Check the build files like package.json, pom.xml, pyproject.toml, go.mod, or Cargo.toml, and prefer checked-in wrappers like ./gradlew or make test. README, CONTRIBUTING, and CI workflows show the commands that gate merges.