TDD Workflow

Overview

This skill guides the implementation of features using Test-Driven Development (TDD) principles. TDD is a disciplined, framework-agnostic approach where tests are written before implementation code, ensuring comprehensive test coverage and driving clean, testable design.

Core Principle

Never write implementation code before a failing test exists for that behavior.

Red-Green-Refactor Cycle

1. Red Phase: Write a Failing Test

Goal: Express the expected behavior through a test that fails.

Process:

• Select one small, specific behavior from the requirements
• Write a descriptive test that clearly expresses the expected behavior
• Run the test to confirm it fails (red)
• The failure should be for the right reason (not a syntax error or missing dependency)

Why it must fail first: A test that passes before any implementation is written might be testing the wrong thing or not actually exercising the code.

2. Green Phase: Minimal Implementation

Goal: Make the test pass with the simplest possible implementation.

Process:

• Implement only the minimal code necessary to pass the failing test
• Resist the urge to add extra features or handle edge cases not yet covered by tests
• Focus on making it work, not making it perfect
• Run the test to confirm it passes (green)

Key principle: Write the simplest code that could possibly work. Even hardcoding values is acceptable if it makes the current test pass – subsequent tests will force generalization.

3. Refactor Phase: Improve Quality

Goal: Improve code quality while maintaining passing tests.

Process:

• Review both implementation and test code for improvements
• Remove duplication (DRY principle)
• Improve naming and clarity
• Extract methods or classes if responsibilities are growing
• Apply language-specific idioms and patterns
• Ensure tests remain readable and maintainable
• Run tests after each refactoring step to ensure they still pass

Safety net: Tests provide confidence that refactoring doesn’t break functionality.

Workflow Decision Tree

Starting a new feature or bug fix:

1. Understand requirements and identify testable behaviors
2. Break down into small, independent behaviors
3. Prioritize behaviors from simple to complex
4. Enter Red-Green-Refactor cycle for each behavior

For each behavior:

1. Red: Write failing test → Confirm failure
2. Green: Minimal implementation → Confirm pass
3. Refactor: Improve code → Confirm still passing
4. Repeat for next behavior

When all behaviors are complete:

1. Run full test suite
2. Verify all requirements are met
3. Review code organization and naming
4. Clean up any debugging code or commented-out code
5. Commit in small, meaningful commits

Test Sequencing Strategy

Order tests from simple to complex:

1. Happy path – The core behavior with valid inputs

   • Start here to establish the foundation
   • Example: “User can successfully log in with valid credentials”

2. Validation tests – Required fields, format constraints

   • Example: “Email must be present”, “Password must be at least 8 characters”

3. Edge cases – Boundary conditions, empty values, unusual inputs

   • Example: “Handles empty string”, “Handles maximum length input”

4. Error handling – Invalid inputs, failure scenarios

   • Example: “Returns error for invalid email format”, “Handles network timeout”

5. Integration points – Interactions with other components

   • Example: “Sends welcome email after registration”, “Updates user profile in database”

Test Quality Guidelines

Each test should:

• Cover exactly one behavior – If a test name includes “and”, it might be testing multiple things
• Be isolated – Tests should not depend on each other or share state
• Have a clear, descriptive name – Should explain what behavior is being tested
• Be easy to read – Test code should be even clearer than production code
• Fail for only one reason – When a test fails, it should be obvious why

Avoid:

• Testing implementation details instead of behavior
• Writing tests after the code (not TDD)
• Making tests too broad or too narrow
• Sharing test setup that creates hidden dependencies
• Skipping the refactor phase

When to Use TDD

Ideal for:

• New feature development
• Bug fixes (write test that reproduces the bug first)
• Refactoring existing code (write characterization tests first)
• Complex business logic
• Code that will be maintained long-term

May skip for:

• Quick prototypes or proof-of-concepts
• Simple glue code
• UI layout and styling (consider other testing approaches)
• Well-understood, trivial implementations

Benefits of TDD

• Comprehensive test coverage – Every line of code has a corresponding test
• Better design – Writing tests first forces consideration of interfaces and dependencies
• Documentation – Tests serve as executable specifications
• Confidence to refactor – Tests provide safety net for improvements
• Fewer bugs – Issues are caught during development, not production
• Faster debugging – When tests fail, the problem is in recently written code

Framework-Specific Implementation

This skill provides the framework-agnostic TDD workflow. For framework-specific implementation details, use the appropriate testing skill for your project:

• RSpec (Ruby): Use the rspec-coder skill for RSpec syntax and patterns
• Minitest (Ruby): Use the minitest-coder skill for Minitest syntax and patterns
• Other frameworks: Consult framework-specific documentation or skills

Example Reference

A Ruby on Rails example implementation is available in references/rails-tdd-workflow.md to illustrate how these principles apply in a specific framework, but the TDD workflow itself remains the same across all languages and frameworks.