Test-Driven Development: Red-Green-Refactor

Core Principle

Tests verify behavior through public interfaces, not implementation details. A test that breaks when you refactor internals -- but behavior is unchanged -- is testing implementation, not behavior. Good tests survive refactors.

When fixing a bug: prove it exists with a failing test before touching production code. The test is evidence. The fix is the response to that evidence.

Workflow Overview

RED   ->  Write a test that fails (proves the bug or defines missing behavior)
GREEN ->  Write the minimum code to make the test pass
REFACTOR -> Improve structure, naming, duplication -- tests stay green