Test-Driven Development

Write the test first. Watch it fail. Write minimal code to pass.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

If you didn't watch the test fail, you don't know if it tests the right thing.

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

When to Use

Always:

• New features
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (ask first):

• Throwaway prototypes
• Generated code
• Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

Red-Green-Refactor

RED - Write Failing Test

Write one minimal test showing what should happen.

// Good: Clear name, tests real behavior, one thing
test('rejects empty email', async () => {
  const result = await submitForm({ email: '' });
  expect(result.error).toBe('Email required');
});

// Bad: Vague name, tests mock not code
test('form works', async () => {
  const mock = jest.fn().mockResolvedValue('success');
  await submitForm(mock);
  expect(mock).toHaveBeenCalled();
});

Requirements:

• One behavior per test
• Clear descriptive 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 the test.

GREEN - Minimal Code

Write the simplest code to pass the test.

// Good: Just enough to pass
function validateEmail(email: string): string | null {
  if (!email?.trim()) return 'Email required';
  return null;
}

// Bad: Over-engineered
function validateEmail(
  email: string,
  options?: {
    allowEmpty?: boolean;
    customRegex?: RegExp;
    onValidate?: (email: string) => void;
  }
): ValidationResult {
  // YAGNI - don't add features not tested
}

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 clean (no errors, warnings)

Test fails? Fix code, not test.

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.

Red Flags - STOP and Start Over

• Code before test
• Test after implementation
• Test passes immediately (didn't watch it fail)
• Can't explain why test failed
• "I'll test after"
• "Too simple to test"
• "Just this once"
• "Already manually tested it"
• "Keep as reference" (delete means delete)

All of these mean: Delete code. Start over with TDD.

Common Rationalizations

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
• You never saw it catch the bug

Test-first forces you to see the test fail, proving it actually tests something.

Quick Reference

Test Types

What to Test

Always test:

• Happy path (normal usage)
• Edge cases (empty, null, boundaries)
• Error handling (invalid input, failures)

Skip testing:

• Framework code (React, Express)
• Third-party libraries
• Simple getters/setters

Verification Checklist

Before marking complete:

• [ ] Every new function has a test
• [ ] Watched each test fail before implementing
• [ ] Each test failed for expected reason
• [ ] Wrote minimal code to pass each test
• [ ] All tests pass
• [ ] Tests use real code (mocks only if unavoidable)

Can't check all boxes? You skipped TDD. Start over.

Example: Bug Fix with TDD

Bug: Empty email accepted

RED

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

Verify RED

$ npm test
FAIL: expected 'Email required', got undefined

GREEN

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

Verify GREEN

$ npm test
PASS

REFACTOR - Extract validation if needed.

When Stuck

See Also

• frameworks.md - Framework-specific patterns (Jest, Vitest, Pytest)
• patterns.md - Common test patterns and examples
• anti-patterns.md - Testing mistakes to avoid