implement-with-tdd

Test-Driven Development (TDD)

Overview

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

Core principle: Watch the test fail. Only then is it proven to test 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 (skip TDD only for these; otherwise follow the full TDD cycle):

• Throwaway spikes that will never be merged (branch must be deleted before implementation begins; any code carried forward requires full TDD)
• Generated code (scaffolded, not behavior-bearing)
• Configuration-only changes

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.

Choosing Test Type

Default to unit tests. Escalate only when the thing you're testing can't be isolated:

• Unit — pure logic, validation, transformations, calculations. No I/O, no wiring. Fast (<100ms).
• Integration — components wired together: database queries, middleware chains, API clients, config reaching runtime. If the bug would only manifest when components connect, it needs an integration test.
• E2E — critical user flows only (login, checkout, onboarding). Expensive, slow, flaky-prone. Don't use for logic that can be tested at a lower level.

When in doubt: can you test it with a function call and an assertion? Unit test. Does it require spinning up real infrastructure or connecting real components? Integration test.

From Spec to Tests

Each scenario in the spec becomes one test. Write the test name and assertion from the scenario before writing any implementation.

Spec scenario:

WHEN a user with an expired session requests a protected resource THEN they receive a 401 and a redirect to login

Test:

test('expired session returns 401 with login redirect', async () => {
  const session = createExpiredSession();
  const response = await requestProtectedResource(session);
  expect(response.status).toBe(401);
  expect(response.headers.location).toBe('/login');
});

One test per scenario, one scenario per behavior. If a scenario needs multiple assertions, that's fine — but if it needs multiple setups, split it. If the spec numbers its criteria (AC-1, R-001, etc.), reference the ID in the test name or comment for traceability. (Note: the spec skill does not generate IDs by default — IDs are an optional custom extension you may add to your spec for traceability.)

Red-Green-Refactor

digraph tdd_cycle {
    rankdir=LR;
    red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"];
    verify_red [label="Verify fails\ncorrectly", shape=diamond];
    green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"];
    verify_green [label="Verify passes\nAll green", shape=diamond];
    refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"];
    next [label="Next", shape=ellipse];

    red -> verify_red;
    verify_red -> green [label="yes"];
    verify_red -> red [label="wrong\nfailure"];
    green -> verify_green;
    verify_green -> refactor [label="yes"];
    verify_green -> green [label="no"];
    refactor -> verify_green [label="stay\ngreen"];
    verify_green -> next;
    next -> red;
}

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.

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
}

Over-engineered

Don't add features, refactor other code, or "improve" beyond the test. Never add test-only methods to production classes — put test helpers in test utilities.

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.

Repeat

Next failing test for next feature.

Why Order Matters (Common Rationalizations)

"I'll write tests after" — Tests written after pass immediately. That proves nothing. Tests-after are biased by your implementation: you test what you built, not what's required. Test-first forces you to see failure, proving the test catches something real.

"Already manually tested" — Ad-hoc, no record, can't re-run, can't prove edge cases. Automated tests are systematic and repeatable.

"Deleting X hours of work is wasteful" — Sunk cost fallacy. Keeping unverified code is the real waste. Delete and rewrite with TDD gives high confidence; bolting tests onto existing code gives false confidence.

"TDD is dogmatic / too slow" — TDD is faster than debugging in production. It finds bugs before commit, prevents regressions, documents behavior, and enables safe refactoring.

"Keep as reference, write tests first" — You'll adapt it. That's testing after with extra steps. Delete means delete.

"Hard to test" — Listen to the test. Hard to test = hard to use. Simplify the design.

If you catch yourself rationalizing, delete code and 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' };
  }
  // ...
}

Verify GREEN

$ npm test
PASS

REFACTOR Extract validation for multiple fields if needed.

Verification Checklist

Before marking work complete:

• Every spec scenario has a test
• 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.

When Stuck

Problem	Solution
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.
Mock setup > test logic	Understand real method's side effects before mocking. Mock at the lowest level necessary. Consider integration tests instead.

Final Rule

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