Test Driven Development

Test-Driven Development — Red, Green, Refactor

Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST.

If you're about to write implementation code, STOP. Write the test first.

The Cycle

1. RED — Write a Failing Test

Write a test that describes the behavior you want:

describe('GET /api/resource', () => {
  it('returns items filtered by scope', async () => {
    const response = await fetch('/api/resource?scope=test')
    const data = await response.json()
    expect(data.success).toBe(true)
    expect(data.items).toBeDefined()
  })
})

def test_calculate_score_returns_weighted_average():
    result = calculate_score(grades=[90, 80, 70], weights=[0.5, 0.3, 0.2])
    assert result == 83.0

2. RUN — Verify It Fails

Run the test and confirm it fails for the RIGHT reason:

npm test -- --grep "returns items filtered"
# or
pytest tests/test_scoring.py::test_calculate_score -v

Expected: FAIL with a clear error (function not found, endpoint 404, assertion error, etc.)

If it passes without implementation — your test is wrong. Fix the test.

3. GREEN — Write Minimal Code

Write the MINIMUM code to make the test pass. No extra features, no "while I'm here" additions.

4. RUN — Verify It Passes

Run the same test again. Expected: PASS

5. REFACTOR — Clean Up (If Needed)

Only if the code is messy. Don't refactor for the sake of it.

6. COMMIT

git add -A
git commit -m "feat: [what the test proves works]"

Anti-Patterns

• "Too simple to test" — Simple code breaks too. Write the test.
• "I'll write tests after" — You won't. And if you do, they'll test implementation, not behavior.
• "Tests slow me down" — Tests prevent 3-hour debugging sessions. They speed you up.
• "I'll just manually test it" — Manual tests don't run in CI. Write automated tests.
• "Mocking everything" — Only mock external services. Integration tests use real connections.

What Makes a Good Test

• Tests behavior, not implementation — "returns filtered results" not "calls findMany with where clause"
• One assertion per concept — Each test proves one thing
• Clear name — it('returns 401 when no auth token') not it('works')
• No test-only code in production — Never add methods just for testing