| name | tdd |
| description | Test-driven development — write the test first, watch it fail, write minimal code to pass. Use when implementing features or fixing bugs. |
Test-Driven Development
The Iron Law
NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST
Write code before the test? Delete it. Start over. No exceptions.
Red-Green-Refactor
RED — Write Failing Test
Write ONE minimal test showing what should happen.
Requirements:
- One behavior per test
- Clear name describing the scenario and expected outcome
- Real code, not mocks (unless external dependency)
Verify RED — Watch It Fail
MANDATORY. Never skip.
Run the test. Confirm:
- Test FAILS (not errors from typos)
- Failure message matches what you expect
- Fails because the feature is missing
Test passes? You're testing existing behavior. Fix the test.
GREEN — Minimal Code
Write the SIMPLEST code to pass the test. Nothing more.
Don't add features. Don't refactor. Don't "improve" beyond the test.
Verify GREEN — Watch It Pass
MANDATORY.
Run the test. Confirm:
- Test passes
- ALL other tests still pass
- No warnings or errors in output
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 behavior.
Common Rationalizations
| Excuse | Reality |
|---|
| "Too simple to test" | Simple code breaks. Test takes 30 seconds. |
| "I'll test after" | Tests written after pass immediately — proves nothing. |
| "Need to explore first" | Fine. Throw away exploration, start with TDD. |
| "Test is hard to write" | Listen to the test. Hard to test = hard to use. Fix the design. |
| "TDD will slow me down" | TDD is faster than debugging. Every time. |
| "Keep as reference" | You'll adapt it. That's testing-after. Delete means delete. |
Bug Fix Example
Bug: empty email accepted.
RED: test('rejects empty email', () => { expect(validate('')).toBe(false); })
Verify RED: FAIL — expected false, got undefined
GREEN: function validate(email) { return email.trim().length > 0; }
Verify GREEN: PASS
Red Flags — STOP and Start Over
- Code written before test
- Test passes immediately (you're testing existing behavior)
- Can't explain why the test failed
- Rationalizing "just this once"
- Multiple behaviors in one test
All of these mean: delete code, start over with TDD.
