Overview
Test-Driven Development (TDD) is a software development practice where you write tests before writing the production code that makes them pass. This test-first approach fundamentally changes how you design systems and ensures high test coverage from the start.
Core Concepts
Red-Green-Refactor Cycle - Write a failing test (Red), make it pass with minimal code (Green), then improve the design while keeping tests green (Refactor). This rhythm becomes natural with practice.
Test as Specification - Tests document what the code should do before the code exists. This forces clarity about requirements and edge cases upfront.
Design Through Tests - Writing tests first reveals design problems immediately. Hard-to-test code indicates coupling or complexity that needs addressing.
Refactoring Safety Net - Comprehensive test coverage enables aggressive refactoring with confidence. Green tests mean behavior is preserved.
When to Use TDD
Greenfield Projects - When starting new codebases where you control the architecture and can establish test-first habits.
Critical Business Logic - When bugs would be costly or dangerous. Financial calculations, medical systems, and security-critical code benefit enormously.
Refactoring Legacy Code - When modernizing existing systems, write characterization tests first to understand current behavior before changing it.
Learning New Technologies - When exploring unfamiliar frameworks, writing tests first helps you understand the API and catch misunderstandings early.
Industry Impact
Kent Beck formalized TDD in the late 1990s, and it has become standard practice at organizations like Google, Amazon, and Microsoft. Studies show TDD reduces defect density by 40-90% while improving design quality.
Learning Path
- By Example - 75-85 annotated examples covering the complete TDD workflow from basics to enterprise patterns
- Beginner (0-40%) - Red-Green-Refactor, basic assertions, test fixtures
- Intermediate (40-75%) - Mocking, asynchronous testing, test architecture
- Advanced (75-95%) - Legacy code, microservices, performance-sensitive TDD
Start with the By Example tutorial to learn through hands-on practice with working code.