Test-Driven Development (TDD)

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.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over.

Red-Green-Refactor

RED — Write Failing Test

Write one minimal test showing what should happen.

test('rejects empty email', async () => {
  const result = await submitForm({ email: '' });
  expect(result.error).toBe('Email required');
});

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

GREEN — Minimal Code

Write simplest code to pass the test.

function submitForm(data: FormData) {
  if (!data.email?.trim()) {
    return { error: 'Email required' };
  }
  // ...
}

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

REFACTOR — Clean Up

After green only:

• •Remove duplication
• •Improve names
• •Extract helpers

Keep tests green. Don't add behavior.

Common Rationalizations

Red Flags — STOP and Start Over

• •Code before test
• •Test passes immediately
• •Can't explain why test failed
• •"Just this once"

Verification Checklist

Before marking work complete:

• • Every new function has a test
• • Watched each test fail before implementing
• • Wrote minimal code to pass each test
• • All tests pass
• • Output pristine (no errors, warnings)