Test-Driven Development (TDD)

Overview

Write the test first. Watch it fail. Write minimal code to pass.

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

The Iron Law: NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Violating the letter of the rules IS violating the spirit of the rules.

When to Use

Always:

• New features
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (ask your human partner):

• Throwaway prototypes
• Generated code
• Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

Red-Green-Refactor Cycle

RED: Write failing test
  |
  v
VERIFY RED: Confirm fails correctly (not errors, not typos)
  |
  v
GREEN: Write minimal code to pass
  |
  v
VERIFY GREEN: Confirm all tests pass
  |
  v
REFACTOR: Clean up (stay green)
  |
  v
REPEAT: Next failing test

RED - Write Failing Test

Write one minimal test showing what should happen.

Good Example:

test('retries failed operations 3 times', async () => {
  let attempts = 0;
  const operation = () => {
    attempts++;
    if (attempts < 3) throw new Error('fail');
    return 'success';
  };

  const result = await retryOperation(operation);

  expect(result).toBe('success');
  expect(attempts).toBe(3);
});

Clear name, tests real behavior, one thing.

Bad Example:

test('retry works', async () => {
  const mock = jest.fn()
    .mockRejectedValueOnce(new Error())
    .mockRejectedValueOnce(new Error())
    .mockResolvedValueOnce('success');
  await retryOperation(mock);
  expect(mock).toHaveBeenCalledTimes(3);
});

Vague name, tests mock behavior not real code.

Requirements:

• One behavior per test
• Clear, descriptive name
• Real code (no mocks unless unavoidable)

VERIFY RED - Watch It Fail

MANDATORY. Never skip.

npm test path/to/test.test.ts
# or: pytest test_file.py::test_name
# or: go test -run TestName

Confirm:

• Test FAILS (not errors)
• Failure message is expected
• Fails because feature is missing (not typos)

Test passes? You're testing existing behavior. Fix the test.

Test errors? Fix error, re-run until it fails correctly.

GREEN - Write Minimal Code

Write the simplest code to pass the test. Nothing more.

Good Example:

async function retryOperation<T>(fn: () => Promise<T>): Promise<T> {
  for (let i = 0; i < 3; i++) {
    try {
      return await fn();
    } catch (e) {
      if (i === 2) throw e;
    }
  }
  throw new Error('unreachable');
}

Just enough to pass.

Bad Example:

async function retryOperation<T>(
  fn: () => Promise<T>,
  options?: {
    maxRetries?: number;
    backoff?: 'linear' | 'exponential';
    onRetry?: (attempt: number) => void;
  }
): Promise<T> {
  // YAGNI - You Ain't Gonna Need It
}

Over-engineered. Don't add features not required by tests.

VERIFY GREEN - Watch It Pass

MANDATORY.

npm test path/to/test.test.ts

Confirm:

• Test passes
• Other tests still pass
• Output pristine (no errors, warnings)

Test fails? Fix code, not test.

Other tests fail? Fix now.

REFACTOR - Clean Up

After green only:

• Remove duplication
• Improve names
• Extract helpers

Keep tests green. Don't add behavior.

Good Tests

Common Rationalizations (And Why They're Wrong)

Red Flags - STOP and Start Over

If you find yourself:

• Writing code before test
• Writing test after implementation
• Test passes immediately (no red phase)
• Can't explain why test failed
• Adding tests "later"
• Rationalizing "just this once"
• Saying "I already manually tested it"
• Saying "Tests after achieve the same purpose"
• Saying "It's about spirit not ritual"
• Keeping code as "reference" to adapt
• Saying "Already spent X hours, deleting is wasteful"
• Saying "TDD is dogmatic, I'm being pragmatic"
• Saying "This is different because..."

All of these mean: Delete the code. Start over with TDD.

Example: Bug Fix

Bug: Empty email accepted

RED

test('rejects empty email', async () => {
  const result = await submitForm({ email: '' });
  expect(result.error).toBe('Email required');
});

Verify RED

$ npm test
FAIL: expected 'Email required', got undefined

GREEN

function submitForm(data: FormData) {
  if (!data.email?.trim()) {
    return { error: 'Email required' };
  }
  // ...existing code
}

Verify GREEN

$ npm test
PASS

REFACTOR Extract validation for multiple fields if needed.

When Stuck

Debugging Integration

Bug found? Write a failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

Testing Anti-Patterns

Avoid these common mistakes:

1. Testing mock behavior - Test real components, not mock existence
2. Test-only methods in production - Put cleanup methods in test utilities
3. Mocking without understanding - Know side effects before mocking
4. Incomplete mocks - Mirror real API structure completely
5. Over-complex mocks - Consider integration tests instead

The Iron Laws:

• NEVER test mock behavior
• NEVER add test-only methods to production classes
• NEVER mock without understanding dependencies

Verification Checklist

Before marking work complete:

• Every new function/method has a test
• Watched each test fail before implementing
• Each test failed for expected reason (feature missing, not typo)
• Wrote minimal code to pass each test
• All tests pass
• Output pristine (no errors, warnings)
• Tests use real code (mocks only if unavoidable)
• Edge cases and errors covered

Can't check all boxes? You skipped TDD. Start over.

The Bottom Line

Production code -> test exists and failed first
Otherwise -> not TDD

No exceptions without your human partner's explicit permission.

Integration

Always loaded: This is a Process skill, loaded at session start.

Pairs with:

• verification - Verify tests pass before claiming complete
• debug - TDD approach to bug fixes
• subagent-driven-development - Sub-agents follow TDD