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

Quality	Good	Bad
Minimal	One thing. "and" in name? Split it.	test('validates email and domain and whitespace')
Clear	Name describes behavior	test('test1')
Shows intent	Demonstrates desired API	Obscures what code should do
Real code	Tests actual implementation	Tests mock behavior

Common Rationalizations (And Why They're Wrong)

Excuse	Reality
"Too simple to test"	Simple code breaks. Test takes 30 seconds.
"I'll test after"	Tests passing immediately prove nothing.
"Tests after achieve same goals"	Tests-after = "what does this do?" Tests-first = "what should this do?"
"Already manually tested"	Ad-hoc is not systematic. No record, can't re-run.
"Deleting X hours is wasteful"	Sunk cost fallacy. Keeping unverified code is technical debt.
"Keep as reference, write tests first"	You'll adapt it. That's testing after. Delete means delete.
"Need to explore first"	Fine. Throw away exploration, start with TDD.
"Test hard = design unclear"	Listen to the test. Hard to test = hard to use.
"TDD will slow me down"	TDD is faster than debugging. Pragmatic = test-first.
"Manual test is faster"	Manual doesn't prove edge cases. You'll re-test every change.
"Existing code has no tests"	You're improving it. Add tests for existing code you touch.
"TDD is dogmatic"	TDD IS pragmatic: finds bugs before commit, prevents regressions.

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

Problem	Solution
Don't know how to test	Write wished-for API. Write assertion first. Ask your human partner.
Test too complicated	Design too complicated. Simplify interface.
Must mock everything	Code too coupled. Use dependency injection.
Test setup huge	Extract helpers. Still complex? Simplify design.

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
• systematic-debugging - TDD approach to bug fixes
• subagent-driven-development - Sub-agents follow TDD