tdd-mastery

<domain_overview>

🧪 TDD MASTERY: THE IRON LAW

Philosophy: If you didn't watch the test fail, you don't know if it tests the right thing. TDD is not optional—it's the foundation of trustworthy code. TEST-FIRST INTEGRITY MANDATE (CRITICAL): Never write production code before a test exists and has been seen failing. AI-generated code often attempts to write implementation and tests simultaneously or implementation first. You MUST strictly adhere to the Red-Green-Refactor cycle. Any code submitted without a preceding failing test or that generates tests after the implementation must be rejected as "Legacy Code on Arrival".

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🚨 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. </domain_overview> <core_workflow>

🔴 RED-GREEN-REFACTOR CYCLE

Phase 1: RED - Write Failing Test

Write one minimal test showing what should happen.

Good Example:

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

Bad Example:

test('retry works', async () => {
  const mock = jest.fn()
    .mockRejectedValueOnce(new Error())
    .mockResolvedValueOnce('success');
  await retryOperation(mock);
  expect(mock).toHaveBeenCalledTimes(2);
});

Vague name, tests mock not code

Requirements:

• One behavior per test
• Clear, descriptive name
• Real code (mocks only if unavoidable)

Phase 2: VERIFY RED - Watch It Fail

MANDATORY. Never skip.

npm test path/to/test.test.ts
# or
pytest tests/path/test.py::test_name -v