Test-Driven Development (TDD)

Core Rule: Write tests BEFORE implementation code.

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

The Red-Green-Refactor Cycle

1. RED: Write Failing Test First

// Write this FIRST
it('should validate email format', () => {
  const result = validateEmail('invalid');
  expect(result.valid).toBe(false);
  expect(result.error).toBe('Invalid email format');
});

Run test → MUST FAIL (for the right reason: missing feature, not typo)

2. GREEN: Implement Minimum Code

// Write this SECOND - simplest code to pass
function validateEmail(email: string) {
  if (!email.includes('@')) {
    return { valid: false, error: 'Invalid email format' };
  }
  return { valid: true };
}

Run test → MUST PASS

3. REFACTOR: Clean Up

Improve code quality while keeping tests green.

Mandatory Verification Points

Point	What to Check
After writing test	Test fails for correct reason (missing feature)
After implementing	Test passes, no other tests break

Non-Negotiable Rules

• ❌ Delete any production code written before tests
• ❌ Never keep pre-written code "as reference"
• ❌ No exceptions - TDD applies to ALL features and bug fixes
• ❌ No unnecessary mocking - tests use real code paths

For Bug Fixes (Critical!)

1. Write test that reproduces the bug (must fail)
2. Verify test fails for the bug reason
3. Implement fix
4. Verify test passes
5. Verify no regression in other tests

// Bug: User with spaces in name causes crash
it('should handle names with spaces', () => {
  // This MUST fail before fix
  expect(() => createUser('John Doe')).not.toThrow();
});

Rationalizations to REJECT

Excuse	Reality
"Manual testing is enough"	Manual testing doesn't catch regressions
"TDD slows me down"	Debugging untested code takes 10x longer
"This is too simple"	Simple code still needs regression protection
"I'll add tests later"	You won't. And you'll introduce bugs.

Output Format

When implementing:

**TDD Cycle:**
1. ✅ Test written: `validateEmail returns error for invalid format`
2. ✅ Test fails: "validateEmail is not defined" (correct reason)
3. ✅ Implementation: Added validateEmail function
4. ✅ Test passes: 1/1 assertions passed
5. ✅ No regressions: All 47 tests still pass