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.

Prefer E2E with Playwright

•Unit tests: Pure logic, utilities, isolated behavior (no UI).
•E2E tests: Flows a user can do (login, submit form, navigate). Use Playwright.

To explore and generate e2e tests, use the playwright-cli skill: open the app, take snapshots, perform actions—CLI output gives you Playwright code to paste into test files.

The Iron Law

code

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.

Red-Green-Refactor

dot

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.

E2E (Playwright) – preferred for user flows:

<Good> ```typescript import { test, expect } from '@playwright/test';

test('login shows dashboard', async ({ page }) => { await page.goto('/login'); await page.getByRole('textbox', { name: 'Email' }).fill('user@example.com'); await page.getByRole('textbox', { name: 'Password' }).fill('password123'); await page.getByRole('button', { name: 'Sign In' }).click();

await expect(page).toHaveURL(/.*dashboard/); await expect(page.getByRole('heading', { name: 'Dashboard' })).toBeVisible(); });

code

One flow, semantic locators, clear assertions, tests in real browsers.
</Good>

**Unit – for pure logic:**

<Good>
```typescript
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 </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>

Requirements:

•One behavior
•Clear name
•Real code (no mocks unless unavoidable)
•E2E: Use semantic locators (getByRole, getByLabel); avoid fragile CSS. Add assertions playwright-cli generates actions, not expectations.

Verify RED - Watch It Fail

MANDATORY. Never skip.

bash

# Unit (from app or monorepo root)
bun test path/to/test.test.ts

# E2E (from app, e.g. apps/web or apps/spa)
bun run test path/to/e2e.spec.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> ```typescript 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 </Good> <Bad> ```typescript async function retryOperation<T>( fn: () => Promise<T>, options?: { maxRetries?: number; backoff?: 'linear' | 'exponential'; onRetry?: (attempt: number) => void; } ): Promise<T> { // YAGNI } ``` Over-engineered </Bad>

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

Verify GREEN - Watch It Pass

MANDATORY.

bash

bun test path/to/test.test.ts
# or for e2e: bun run test path/to/e2e.spec.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

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.

"TDD is dogmatic, being pragmatic means adapting"

TDD IS pragmatic:

•Finds bugs before commit (faster than debugging after)
•Prevents regressions (tests catch breaks immediately)
•Documents behavior (tests show how to use code)
•Enables refactoring (change freely, tests catch breaks)

"Pragmatic" shortcuts = debugging in production = slower.

"Tests after achieve the same goals - it's spirit not ritual"

No. Tests-after answer "What does this do?" Tests-first answer "What should this do?"

Tests-after are biased by your implementation. You test what you built, not what's required. You verify remembered edge cases, not discovered ones.

Tests-first force edge case discovery before implementing. Tests-after verify you remembered everything (you didn't).

30 minutes of tests after ≠ TDD. You get coverage, lose proof tests work.

Common Rationalizations

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.

Red Flags - STOP and Start Over

•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.

Example: Bug Fix (E2E)

Bug: Empty email accepted on login form

RED

typescript

import { test, expect } from '@playwright/test';

test('shows error when email is empty', async ({ page }) => {
  await page.goto('/login');
  await page.getByRole('textbox', { name: 'Email' }).fill('');
  await page.getByRole('button', { name: 'Sign In' }).click();

  await expect(page.getByText('Email required')).toBeVisible();
  await expect(page).toHaveURL(/\/login$/);
});

Verify RED – run bun run test; test fails (no validation yet).

GREEN – add validation in form handler so error is shown and submit is blocked.

Verify GREEN – test passes.

REFACTOR – extract validation for multiple fields if needed.

Example: Bug Fix (Unit)

Bug: Empty email accepted (API/utility)

RED

typescript

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

Verify RED → GREEN (add if (!data.email?.trim()) return { error: 'Email required' };) → REFACTOR.

Verification Checklist

Before marking work complete:

• Every new function/flow has a test (e2e for user flows, unit for pure logic)
• 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 (including bun run test for e2e in the app)
• Output pristine (no errors, warnings)
• Tests use real code (mocks only if unavoidable)
• E2E: semantic locators and explicit assertions
• Edge cases and errors covered

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

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 failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

E2E with Playwright – Best Practices

(From playwright-cli skill and references/test-generation.md.)

•Semantic locators – Prefer getByRole('button', { name: 'Submit' }), getByLabel, getByText over CSS like locator('#submit-btn').
•Explore then record – Use playwright-cli open <url>, playwright-cli snapshot, then perform actions; paste generated code into spec files.
•Add assertions – Generated code is actions only. Always add expect(...) for visible text, URL, or state.
•Auth/setup – Use storage state (e.g. state-save / state-load in playwright-cli) or project setup in playwright.config to reuse login; avoid logging in in every test.
•Mock sparingly – Prefer real backend in e2e when feasible; use page.route() only when necessary (flaky APIs, payments). See playwright-cli references/request-mocking.md.

Testing Anti-Patterns

When adding mocks or test utilities, read @testing-anti-patterns.md to avoid common pitfalls:

•Testing mock behavior instead of real behavior
•Adding test-only methods to production classes
•Mocking without understanding dependencies

When considering heavy mocks for UI flows, prefer an e2e test with Playwright instead.

Final Rule

code

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

No exceptions without your human partner's permission.