Test Driven Development

What This Skill Does

This skill guides developers through the Test-Driven Development (TDD) process, emphasizing writing tests before code. It ensures that code is written to meet specific requirements and that tests are reliable. This skill is best suited for new features, bug fixes, refactoring, and behavior changes.

When to Use

Developing new features with clear requirements. Fixing bugs by writing tests that reproduce the issue. Refactoring existing code while maintaining functionality. Implementing behavior changes with confidence. Ensuring code meets specific requirements through testing. Following the Red-Green-Refactor cycle for development.

Key Features

Enforces writing failing tests before production code. Follows the Red-Green-Refactor cycle. Ensures tests fail correctly before implementing code. Promotes writing minimal code to pass tests. Encourages refactoring to clean up code while staying green. Provides a structured approach to test-driven development.

Manual Installation

Manual installation

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;

}

RED - Write Failing Test

Write one minimal test showing what should happen.

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

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

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

Verify RED - Watch It Fail

MANDATORY. Never skip.

npm test path/to/test.test.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.

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

async function retryOperation<T>( fn: () => Promise<T>, options?: { maxRetries?: number; backoff?: 'linear' | 'exponential'; onRetry?: (attempt: number) => void; } ): Promise<T> { // YAGNI } Over-engineered

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

Verify GREEN - Watch It Pass

MANDATORY.

npm test path/to/test.test.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

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

Red Flags - STOP and Star