Test Driven Development

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.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over. Do not keep it as "reference" — delete means delete.

Red-Green-Refactor

1. RED — Write a Failing Test

Write one minimal test for the specific behavior you're implementing.

• One behavior per test
• Name describes the behavior
• Use real code, not mocks (unless unavoidable)

2. Verify RED — Watch It Fail

MANDATORY. Never skip.

Run the test and confirm:

• It fails (not errors out)
• The failure message is expected
• It fails because the feature is missing, not due to a typo

Test passes immediately? You're testing existing behavior. Fix the test.

3. GREEN — Write Minimal Code

Write the simplest code that makes the test pass. No extra features. No "nice-to-haves." Don't refactor other code.

4. Verify GREEN — Watch It Pass

MANDATORY. Never skip.

Run the test and confirm:

• Your test passes
• All other tests still pass
• No new errors or warnings

Test still fails? Fix the code, not the test.

5. REFACTOR — Clean Up

After green only: remove duplication, improve names, extract helpers. Keep tests green. Do not add behavior.

6. Repeat

Move to the next failing test.

Good Tests

Bug Fix Example

Bug: Empty email accepted

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

// Verify RED: FAIL — expected 'Email required', got undefined

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

// Verify GREEN: PASS

Verification Checklist

Before marking work complete:

• Every new function/method has a test written before the implementation
• Watched each test fail before implementing
• Each test failed for the expected reason (feature missing, not typo)
• Wrote minimal code to pass each test
• All tests pass with clean output
• Edge cases covered

Red Flags — Stop and Start Over

Any of these? Delete code. Start over with TDD.

When Stuck

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