jimweller 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
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.