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.

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.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over.

No exceptions:

• Don't keep it as "reference"
• Don't "adapt" it while writing tests
• Don't look at it
• Delete means delete

Implement fresh from tests. Period.

Test Process Discipline (CRITICAL)

Problem: Test runners (Vitest, Jest) default to watch mode, leaving processes hanging indefinitely.

Mandatory Rules:

1. Always use run mode — Never invoke watch mode:
   • Vitest: npx vitest run (NOT npx vitest)
   • Jest: CI=true npx jest or npx jest --watchAll=false
   • npm scripts: CI=true npm test or npm test -- --run
2. Prefer CI=true prefix for all test commands: CI=true npm test
3. After TDD cycle complete, verify no orphaned processes: pgrep -f "vitest|jest" || echo "Clean"
4. Kill if found: pkill -f "vitest" 2>/dev/null || true

Red-Green-Refactor

    ┌─────────┐       ┌─────────┐       ┌───────────┐
    │   RED   │──────>│  GREEN  │──────>│ REFACTOR  │
    │ (Fail)  │       │ (Pass)  │       │ (Clean)   │
    └─────────┘       └─────────┘       └───────────┘
         ^                                    │
         │                                    │
         └────────────────────────────────────┘
                    Next Feature

RED - Write Failing Test

Write one minimal test showing what should happen.

Good:

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:

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 not code

Requirements:

• One behavior
• Clear name
• Real code (no mocks unless unavoidable)

Verify RED - Watch It Fail

MANDATORY. Never skip.

CI=true npm test path/to/test.test.ts

Confirm:

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

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

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

GREEN - Minimal Code

Write simplest code to pass the test.

Good:

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:

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, refactor other code, or "improve" beyond the test. Don't hard-code test values - implement general logic that works for ALL inputs.

Verify GREEN - Watch It Pass

MANDATORY.

CI=true 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.

Repeat

Next failing test for next feature.

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

Factory Pattern for Tests (Reference Pattern)

Create getMockX(overrides?: Partial<X>) functions for reusable test data:

interface User {
  id: string;
  name: string;
  email: string;
  role: 'admin' | 'user';
}

const getMockUser = (overrides?: Partial<User>): User => ({
  id: '123',
  name: 'John Doe',
  email: 'john@example.com',
  role: 'user',
  ...overrides,
});

// Usage - override only what matters for the test
it('shows admin badge for admin users', () => {
  const user = getMockUser({ role: 'admin' });
  render(<UserCard user={user} />);
  expect(screen.getByText('Admin')).toBeTruthy();
});

Benefits:

• Sensible defaults - less boilerplate per test
• Override specific properties - focus on what test cares about
• Type-safe - catches missing properties
• DRY - change mock in one place

Mocking External Dependencies (When Unavoidable)

Rule: Prefer real code. Mock only when:

• External API (network calls)
• Database (test isolation)
• Time-dependent logic
• Third-party services

Common Mock Patterns

Supabase:

jest.mock('@/lib/supabase', () => ({
  supabase: {
    from: jest.fn(() => ({
      select: jest.fn(() => ({
        eq: jest.fn(() => Promise.resolve({ data: mockData, error: null }))
      }))
    }))
  }
}))

Fetch/API:

global.fetch = jest.fn(() =>
  Promise.resolve({ ok: true, json: () => Promise.resolve(mockResponse) })
) as jest.Mock

Redis:

jest.mock('@/lib/redis', () => ({
  get: jest.fn(() => Promise.resolve(cachedValue)),
  set: jest.fn(() => Promise.resolve('OK'))
}))

Environment Variables:

beforeEach(() => {
  process.env.API_KEY = 'test-key'
})
afterEach(() => {
  delete process.env.API_KEY
})

Time:

jest.useFakeTimers()
// In test:
jest.advanceTimersByTime(1000)

Mock quality check: If mock setup > test code, reconsider design.

Why Order Matters

"I'll write tests after to verify it works"

Tests written after code pass immediately. Passing immediately proves nothing:

• Might test wrong thing
• Might test implementation, not behavior
• Might miss edge cases you forgot
• You never saw it catch the bug

Test-first forces you to see the test fail, proving it actually tests something.

"I already manually tested all the edge cases"

Manual testing is ad-hoc. You think you tested everything but:

• No record of what you tested
• Can't re-run when code changes
• Easy to forget cases under pressure
• "It worked when I tried it" ≠ comprehensive

Automated tests are systematic. They run the same way every time.

"Deleting X hours of work is wasteful"

Sunk cost fallacy. The time is already gone. Your choice now:

• Delete and rewrite with TDD (X more hours, high confidence)
• Keep it and add tests after (30 min, low confidence, likely bugs)

The "waste" is keeping code you can't trust. Working code without real tests is technical debt.

Red Flags - STOP and Start Over

If you catch yourself:

• Code before test
• Test after implementation
• Test passes immediately
• Can't explain why test failed
• Tests added "later"
• Rationalizing "just this once"
• "I already manually tested it"
• "Tests after achieve the same purpose"
• "It's about spirit not ritual"
• "Keep as reference" or "adapt existing code"
• "Already spent X hours, deleting is wasteful"
• "TDD is dogmatic, I'm being pragmatic"
• "This is different because..."

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

Rationalization Prevention

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 ≠ 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 test. Hard to test = hard to use.
"TDD will slow me down"	TDD faster than debugging. Pragmatic = test-first.
"Manual test 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.

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' };
  }
  // ...
}

Verify GREEN

$ npm test
PASS

REFACTOR Extract validation for multiple fields if needed.

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
• No hanging test processes (pgrep -f "vitest|jest" returns empty)

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

Coverage Threshold (Project Default)

Target: 80%+ code coverage across:

• Branches: 80%
• Functions: 80%
• Lines: 80%
• Statements: 80%

Verify with: npm run test:coverage or equivalent.

Below threshold? Add missing tests before claiming completion.

Test Smells (Anti-Patterns)

Smell	Bad Example	Why It's Bad	Fix
Testing implementation	expect(component.state.count).toBe(5)	Breaks when internals change	Test user-visible behavior
Dependent tests	Test B relies on Test A's state	Flaky, order-dependent	Each test sets up own data
Mocking everything	Every dependency mocked	Tests mock, not code	Use real code where feasible
Giant setup	50 lines of setup per test	Hard to understand	Extract factories
Magic numbers	expect(result).toBe(42)	Meaning unclear	Use named constants
Test name lies	test('works') passes but doesn't test 'works'	Misleading	Name describes actual behavior
No assertions	test('loads', () => { loadData() })	Tests nothing	Always assert outcomes
Commented tests	// test('edge case'...	Dead code, skipped coverage	Delete or uncomment

If you spot these in your tests: Fix before claiming TDD cycle complete.

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.

Output Format

## TDD Cycle

### Requirements
[What functionality is being built]

### RED Phase
- Test: [test name]
- Command: `npm test -- --grep "test name"`
- Result: exit 1 (FAIL as expected)
- Failure reason: [function not defined / expected X got Y]

### GREEN Phase
- Implementation: [summary]
- File: [path:line]
- Command: `npm test -- --grep "test name"`
- Result: exit 0 (PASS)

### REFACTOR Phase
- Changes: [what was improved]
- Command: `npm test`
- Result: exit 0 (all tests pass)

Final Rule

Production code → test exists and failed first
Otherwise → not TDD

No exceptions without your human partner's permission.