Test Driven Development

Test-Driven Development

The Iron Law of TDD

Write code before test? Delete it. Start over.

This is not negotiable. This is not optional. You cannot rationalize your way out of this.

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

If you catch yourself thinking ANY of these, STOP:

• "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)

Don't write code you don't need yet:

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

DRY (Don't Repeat Yourself)

But only during REFACTOR phase:

• 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