Test-Driven Development

Overview

Write a failing test first, then implement the minimum code to pass, then refactor. Tests are proof that the system behaves as intended. This is the canonical Build-phase testing skill.

When to Use

• Implementing new logic or features
• Fixing bugs (write a failing test that reproduces the bug)
• Refactoring existing behavior
• Changing any code that has correctness requirements

Test Pyramid

• 80% Unit tests: fast, isolated, test individual functions and components
• 15% Integration tests: test interactions between modules
• 5% End-to-end tests: test critical user flows through the full stack