Test-Driven Development (TDD)

Overview

Test-Driven Development (TDD) is a software development process where you write a test before writing the actual code. The cycle is: Red-Green-Refactor.

1. Red: Write a failing test.
2. Green: Write the minimal code to pass the test.
3. Refactor: Improve the code while keeping the test green.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST.

• If you write code before the test, delete it and start over.
• Exceptions: Throwaway prototypes, generated code, configuration files (ask for permission).

The TDD Cycle

1. RED - Write Failing Test

• Write one minimal test that defines the expected behavior.
• Ensure the test name is clear and describes the behavior, not the implementation.
• Example: test('rejects empty email', ...)

2. Verify RED - Watch It Fail

• Run the test.
• Confirm it fails.
• Confirm it fails for the expected reason (e.g., "Email required" vs "undefined").
• If it passes or errors out (compilation error, etc.), fix the test/setup until it fails correctly.

3. GREEN - Minimal Code

• Write the simplest code to make the test pass.
• Do not add extra features or future-proofing.
• Do not refactor yet.

4. Verify GREEN - Watch It Pass

• Run the test.
• Confirm it passes.
• Ensure all other tests also pass.

5. REFACTOR - Clean Up

• Remove duplication.
• Improve variable/function names.
• Extract helper functions.
• Keep tests green throughout this process.

Common Rationalizations to Avoid

• "It's too simple to test": Simple code breaks too.
• "I'll write tests later": You won't, or they will be biased by the implementation.
• "I already manually tested it": Manual testing is ad-hoc and not repeatable.
• "Deleting code is wasteful": Keeping unverified code is technical debt.

Verification Checklist

Before marking work as complete:

• Every new function/method has a corresponding test.
• You watched each test fail before implementing the code.
• Each test failed for the expected reason.
• You wrote only the minimal code to pass the test.
• All tests pass (green).
• Code was refactored after passing tests.
• Edge cases are covered.