TDD Overview

When to Use

You're starting a new feature, fixing a bug, or working on complex business logic where you want to catch errors early and design better code through tests.

What is TDD?

Test-Driven Development (TDD) is a software development practice where you write a failing test before you write any production code. This inverts the traditional approach of writing code first and testing later.

The core cycle is simple: 1. Red - Write a test that fails (because the feature doesn't exist yet) 2. Green - Write the minimum code to make the test pass 3. Refactor - Clean up the code while keeping tests green

Why TDD Matters

Early Bug Detection: Writing tests first means bugs are caught during development, not in production. Teams using TDD report 40-90% reduction in defect density compared to traditional approaches.

Better Design: Tests force you to think about how your code will be used before you write it. This leads to more modular, loosely coupled designs because tightly coupled code is hard to test.

Living Documentation: Tests describe what the code does in executable form. Unlike comments, tests never lie - if they pass, the behavior is correct.

Refactoring Confidence: Comprehensive test coverage means you can refactor fearlessly. If tests stay green, behavior is preserved.

Faster Feedback: Running tests takes seconds compared to manual testing cycles that take hours or days.

When TDD Shines

Complex business logic with many edge cases
Long-lived projects where maintenance matters
Team environments where code changes hands
Regulated industries requiring audit trails and quality evidence
Refactoring legacy code where you need safety nets

When to Skip TDD

Quick prototypes or throwaway spikes to explore ideas
UI exploration where visual design is still fluid
Tight deadlines with simple, well-understood problems (though this is debatable)
Trivial code like getters/setters with no logic

Common Mistakes

Writing tests after code - Defeats the design benefits; tests become an afterthought
Skipping refactor step - Accumulates technical debt, code becomes messy despite test coverage
Testing implementation instead of behavior - Tests break on every refactor, defeating the purpose
Aiming for 100% coverage - Wastes time on trivial tests, creates false confidence (see Test Coverage Strategy)