Test-Driven Development

Write test first → watch fail → minimal code → pass → refactor.

Iron Law: No production code without failing test first. Violate → delete code, start over.

Red-Green-Refactor Cycle

RED - Write Failing Test

One minimal test showing desired behavior.

typescript

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

Requirements: one behavior, clear name, real code (mocks only if unavoidable).

Verify RED

MANDATORY. Never skip.

bash

npm test path/to/test.test.ts

Confirm:

•Test fails (not errors)
•Failure = feature missing (not typo)

Test passes? Testing existing behavior. Fix test.

GREEN - Minimal Code

Simplest code to pass. No extra features, no refactoring yet.

Verify GREEN

MANDATORY.

•Test passes
•Other tests still pass
•Output pristine (no warnings)

REFACTOR

After green only: remove duplication, improve names, extract helpers. Keep tests green.

Repeat

Next failing test → next feature.

When to Use

Always: new features, bug fixes, refactoring, behavior changes.

Exceptions (ask human): throwaway prototypes, generated code, config files.

Common Rationalizations

Excuse	Reality
"Too simple to test"	Simple code breaks. Test takes 30s.
"I'll test after"	Tests passing immediately prove nothing.
"Already manually tested"	Ad-hoc != systematic. Can't re-run.
"Deleting X hours is wasteful"	Sunk cost. Unverified code = tech debt.
"Keep as reference"	You'll adapt it = testing after. Delete means delete.
"Need to explore first"	Fine. Throw away exploration, TDD from scratch.
"Test hard = skip TDD"	Hard to test = hard to use. Simplify design.
"TDD slows me down"	TDD faster than debugging production.
"Existing code has no tests"	Improving it? Add tests for touched code.

Verification Checklist

Before marking complete:

• Every new function has test
• Watched each test fail before implementing
• Each test failed for expected reason
• Wrote minimal code to pass
• All tests pass
• Output pristine
• Real code tested (minimal mocks)
• Edge cases + errors covered

Can't check all? Skipped TDD. Start over.

Red Flags - STOP and Restart

•Code before test
•Test passes immediately
•Can't explain why test failed
•"Just this once"
•"Keep as reference"
•"Already spent X hours"
•"This is different because..."

All mean: delete code, start over with TDD.

When Stuck

Problem	Solution
Don't know how to test	Write wished-for API. Ask human.
Test too complicated	Design too complicated. Simplify.
Must mock everything	Code too coupled. Use DI.
Test setup huge	Extract helpers or simplify design.

Bug Fixes

Bug found → write failing test reproducing it → TDD cycle → test proves fix + prevents regression.

Never fix bugs without test.

tdd