Test-driven development (XP-style)

Tests define the specification. Design them from requirements before any implementation. The RED-GREEN-REFACTOR cycle is the heartbeat: write a failing test, make it pass with minimal code, then clean up while green.

Modern insight (2025): TDD + property-based testing pairing is the standard -- example tests prevent regressions, property tests discover edge cases. TDD also serves AI-assisted development: structural integrity keeps code understandable for both human and AI collaborators (Kent Beck, "Augmented Coding"). Mutation testing validates test quality beyond coverage metrics (TDD+Mutation: 63.3% vs TDD-alone: 39.4% mutation coverage).

See frameworks for language-specific test runners, property testing, coverage, and mutation tools. See examples for brief TDD cycle patterns per language.

When to Apply

• New features with clear requirements (both inside-out and outside-in approaches valid)
• Bug fixes -- write a failing test that proves the bug before fixing
• Refactoring -- ensure coverage exists before restructuring
• API contract enforcement -- test the interface, not internals
• Property-based invariants -- complement example tests with PBT
• Legacy code -- add characterization tests before modifying (Michael Feathers pattern)