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testing-anti-patterns.md 8.1 KB name ferrox-test-driven-development description Use when implementing any feature or bugfix, before writing implementation code
Adapted from Superpowers by Obra (MIT) — see NOTICE.
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.
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.
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;
}
```typescript
test('retries failed operations 3 times', async () => {
let attempts = 0;
const operation = () => {
attempts++;
if (attempts < 3) throw new Error('fail');
return 'success';
};
```typescript
async function retryOperation(fn: () => Promise): Promise {
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
```typescript
async function retryOperation(
fn: () => Promise,
options?: {
maxRetries?: number;
backoff?: 'linear' | 'exponential';
onRetry?: (attempt: number) => void;
}
): Promise {
// YAGNI
}
```
Over-engineered
RED - Write Failing Test Write one minimal test showing what should happen.
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
One behavior
Clear name
Real code (no mocks unless unavoidable)
Verify RED - Watch It Fail npm test path/to/test.test.ts
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.
Don't add features, refactor other code, or "improve" beyond the test.
Verify GREEN - Watch It Pass npm test path/to/test.test.ts
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
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"
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 Bug: Empty email accepted
test ('rejects empty email' , async () => {
const result = await submitForm ({ email : '' });
expect (result.error ).toBe ('Email required' );
});
$ npm test
FAIL: expected 'Email required' , got undefined
function submitForm (data : FormData if (!data.email ?.trim ()) {
return { error : 'Email required' };
}
}
REFACTOR
Extract validation for multiple fields if needed.
Verification Checklist Before marking work complete:
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.
Testing Anti-Patterns
Testing mock behavior instead of real behavior
Adding test-only methods to production classes
Mocking without understanding dependencies
Final Rule Production code → test exists and failed first
Otherwise → not TDD
No exceptions without your human partner's permission.
