Test-Driven Development (TDD)

Overview

Test first. Watch it fail. Write minimal code to pass.

Core principle: Didn't watch the test fail? You don't know it tests the right thing.

Break the letter of the rules = break the spirit.

When to Use

Always: new features, bug fixes, refactors, behavior changes.

Exceptions (ask your human partner): throwaway prototypes, generated code, config files.

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

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Code before the test? Delete it. Start over. Don't keep it as "reference", don't "adapt" it, don't look at it. Delete means delete. Implement fresh from tests.

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

One minimal test. Shows what should happen.

const result = await retryOperation(operation);

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

Clear name, real behavior, one thing
</Good>

<Bad>
```typescript
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 </Bad>

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

Verify RED — watch it fail

MANDATORY. Never skip. Run via bash:

npm test path/to/test.test.ts

Confirm: fails (not errors), failure message is the expected one, fails because feature missing (not a typo).

Passes? Testing existing behavior — fix the test. Errors? Fix, rerun till it fails right.

GREEN — minimal code

Simplest code that passes.

Don't add features, refactor other code, or "improve" past the test.

Verify GREEN — watch it pass

MANDATORY.

npm test path/to/test.test.ts

Confirm: passes, other tests still pass, output pristine (no errors/warnings).

Fails? Fix code, not test. Other tests fail? Fix now.

"Other tests" means the project's full suite. A passing focused test does not establish that the change is done. Before reporting completion, run the repository's test command (npm test, pytest, cargo test, or its equivalent) even when the task names one file. Report every failure by name, including a failure you did not cause; a scoped deliverable does not narrow verification.

REFACTOR — clean up

After green only: kill duplication, improve names, extract helpers. Stay green. Don't add behavior.

Repeat

Next failing test, next feature.

Good Tests

Common Rationalizations

Red Flags — STOP and Start Over

• Code before test
• Test passes immediately, or you can't explain why it failed
• "I'll test after" / "tests after achieve the same purpose"
• "Keep as reference" / "adapt existing code"
• "Already spent X hours, deleting is wasteful"
• "TDD is dogmatic, I'm being pragmatic" / "this is different because…"

All mean: delete 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' };
  }
  // ...
}

Verify GREEN

$ npm test
PASS

REFACTOR — extract validation for more fields if needed.

Verification Checklist

• Every new function/method has a test
• Watched each test fail before implementing
• Each test failed for the expected reason (feature missing, not a typo)
• Wrote minimal code to pass
• All tests pass, output pristine
• Tests use real code (mocks only if unavoidable)
• Edge cases and errors covered

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

When Stuck

Debugging Integration

Bug? Write a failing test reproducing it. Follow the cycle — the test proves the fix and blocks regression. Never fix bugs without a test. (See superpowers-systematic-debugging.)

Testing Anti-Patterns

Adding mocks/test utils? Consult the Testing Anti-Patterns reference (auto-included below): testing mock not real behavior; test-only methods in prod classes; mocking without understanding deps.

Final Rule

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

No exceptions without your human partner's permission.