Test-Driven Development

The Iron Law of TDD

The RED-GREEN-REFACTOR Cycle

1. RED - Write a Failing Test First

• Write the smallest test that fails for the right reason
• The test should fail because the feature doesn't exist yet
• Run the test and verify it fails

2. GREEN - Make It Pass (Minimum Code)

• Write the minimum code to make the test pass
• Don't write more than necessary
• Don't optimize yet
• Don't refactor yet

3. REFACTOR - Improve the Code

• Now you can clean up
• Remove duplication
• Improve naming
• Extract methods/functions
• All tests must still pass after refactoring

Forbidden Rationalizations

• "This is just a simple function" → WRONG. Simple functions need tests.
• "I'll write tests after" → WRONG. That's not TDD.
• "Let me just get it working first" → WRONG. Tests first.
• "This doesn't need a test" → WRONG. Everything needs tests.
• "I'll test it manually" → WRONG. Write automated tests.
• "The test is obvious" → WRONG. Write it anyway.
• "I'm just exploring" → WRONG. Explore with tests.

Test Quality Guidelines

Good Tests Are:

• Fast - Tests should run in milliseconds
• Isolated - No dependencies between tests
• Repeatable - Same result every time
• Self-validating - Pass or fail, no interpretation
• Timely - Written before the code

Test Structure (Arrange-Act-Assert):

// Arrange - Set up the test conditions
const input = createTestInput();

// Act - Execute the code under test
const result = functionUnderTest(input);

// Assert - Verify the expected outcome
expect(result).toBe(expectedValue);

When to Use This Skill

• Starting any new feature
• Fixing any bug (write a test that reproduces the bug first)
• Refactoring existing code (ensure tests exist first)
• Any code change that could break existing functionality

YAGNI (You Aren't Gonna Need It)

• No speculative features
• No "just in case" abstractions
• No premature optimization
• Build what's needed now, nothing more

DRY (Don't Repeat Yourself)

• First make it work (GREEN)
• Then make it right (REFACTOR)
• Duplication is okay temporarily
• Remove it during refactor

Commit Strategy

• Commit after each GREEN
• Commit after each REFACTOR
• Small, frequent commits
• Each commit should pass all tests