# Test Driven Development

> Use London Test Driven Development (TDD) when implementing any feature or bugfix, before writing implementation code

- Skill: `jimweller/test-driven-development` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add jimweller/test-driven-development`
- Raw SKILL.md: https://api.skillmd.com/api/skills/jimweller/test-driven-development/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: jimweller (https://skillmd.com/u/jimweller)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/jimweller/test-driven-development

---


<!-- markdownlint-disable-file MD041 -->

STARTER_CHARACTER = 🧪

# Test-Driven Development (TDD) - London School

## Overview

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

**School:** London (mock-first). Mock collaborators to isolate the unit under test. Let test doubles drive interface discovery.

**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 the user):**

- Throwaway prototypes
- Generated code
- Configuration files

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

## The Iron Law

`````text
NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST
```text

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.

## London School Mocking

Mock collaborators by default. The unit under test is real. Everything it talks to is a test double.

**Why:** Mocks drive interface design. When you mock a collaborator, you define the contract the unit expects. This discovers interfaces before implementing them.

**What to mock:**

- Collaborator objects (services, repositories, clients)
- External systems (APIs, databases, filesystems)
- Anything the unit delegates to

**What NOT to mock:**

- The unit under test itself
- Value objects and data structures
- Language/stdlib primitives

**Mock quality matters:** Mocks must reflect real collaborator contracts. Incomplete or invented mock behavior is worse than no mock. See @testing-anti-patterns.md.

## Red-Green-Refactor

- RED: Write failing test. Verify it fails for the expected reason. If the failure is wrong (import error, typo, wrong assertion), fix the test and re-verify before proceeding.
- GREEN: Write minimal code to pass. Run ALL tests, not just the new one. If anything breaks, fix before moving on.
- REFACTOR: Clean up. Re-run tests. If anything breaks, revert the refactor and try a different approach.

### RED - Write Failing Test

Write one minimal test showing what should happen. Mock collaborators to isolate the unit.

<Good>
```typescript
test('retries failed operations 3 times', async () => {
  const mockClient = { send: vi.fn() };
  mockClient.send
    .mockRejectedValueOnce(new Error('fail'))
    .mockRejectedValueOnce(new Error('fail'))
    .mockResolvedValueOnce('success');

const result = await retryOperation(mockClient.send);

expect(result).toBe('success');
expect(mockClient.send).toHaveBeenCalledTimes(3);
});

````text
Clear name, mocks the collaborator, tests unit behavior
</Good>

<Bad>
```typescript
test('retry works', async () => {
  await retryOperation(realHttpClient.send);
  // Depends on network, slow, flaky
});
````text

Vague name, no isolation, tests infrastructure
</Bad>

**Requirements:**

- One behavior
- Clear name
- Mock collaborators, test the unit

### Verify RED - Watch It Fail

**MANDATORY. Never skip.**

Run the project's test command targeting the specific test file.

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');
}
```text
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
}
```text
Over-engineered
</Bad>

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

### Verify GREEN - Watch It Pass

**MANDATORY.**

Run the project's test command targeting the specific test file.

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                        |
| **Isolated**     | Mocks collaborators, tests unit     | Tests entire call chain                             |

## 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" does not equal 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 does not equal 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 does not equal 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

**Bug:** Empty email accepted

**RED**

```typescript
test("rejects empty email", async () => {
  const mockRepo = { save: vi.fn() };
  const result = await submitForm({ email: "" }, mockRepo);
  expect(result.error).toBe("Email required");
  expect(mockRepo.save).not.toHaveBeenCalled();
});
```text

**Verify RED** - Run tests, confirm failure: expected 'Email required', got undefined.

**GREEN**

```typescript
function submitForm(data: FormData, repo: UserRepo) {
  if (!data.email?.trim()) {
    return { error: "Email required" };
  }
  // ...
}
```text

**Verify GREEN** - Run tests, confirm 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)
- [ ] Collaborators mocked, unit tested in isolation
- [ ] 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 the user. |
| Test too complicated   | Design too complicated. Simplify interface.                |
| Too many mocks         | 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.

## 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

## Final Rule

```text
Production code -> test exists and failed first
Otherwise -> not TDD
```text

No exceptions without the user's permission.
`````