tdd

Test-Driven Development

Iron Law

No prod code without failing test first. No exceptions.

Anti-Pattern: Horizontal Slices

DO NOT write all tests first, then all impl. Bulk tests test imagined behavior, not actual.

Correct: Vertical slices -- one test -> one impl -> repeat.

WRONG:  RED: test1,test2,test3  ->  GREEN: impl1,impl2,impl3
RIGHT:  RED->GREEN: test1->impl1  ->  RED->GREEN: test2->impl2

State Machine

Full state diagram: REFERENCE.md#state-machine.

Workflow

0. PLAN -- Coverage gap analysis

• Use project's domain glossary for test/interface names; respect ADRs in the area
• Run vitest run --coverage.enabled --coverage.reporter=text
• Find uncovered lines/branches/functions in changed files -> test targets
• Confirm behaviors w/ user (prioritize gaps over covered)
• Find deep module chance (small interface, deep impl)
• Design interfaces for testability; test behaviour through public interface, not impl (tests.md)

1. RED -- Failing test (tracer bullet)

• ONE test, ONE behavior, clear name
• Real code, no mocks (unless unavoidable -- see mocking.md)
• Verify fails for RIGHT reason

2. GREEN -- Minimal code to pass

• Only enough to pass | no premature optimization
• Run test | see green

3. REFACTOR -- Clean up while green

• Kill duplication | fix naming | deepen modules
• Tests after every change -- stay green
• Never refactor while RED. Get GREEN first.
• Flag unit tests >500ms, integration >2s
• Avoid per-keystroke sim (slow, flaky) -> bulk input
• Commit when clean

Reactive TDD with Monitor

Monitor: vitest --watch -- stream pass/fail as edit. Edit->fail->fix->pass->refactor->repeat.

4. REPEAT -- Next behavior

RED->GREEN->REFACTOR per behavior. One at a time.

Per-Cycle Checklist

• Test describe behavior, not impl
• Test use public interface only
• Test survive internal refactor
• Code minimal for this test
• No speculative features

Test Classification

Suffix	Purpose	DOM?
.test.ts	Unit -- pure logic	No
.test.tsx / .integration.tsx	Integration -- render components	Yes
e2e/*.spec.ts	E2E -- Playwright browser	Browser

Visual Regression Tests (Route Files)

New TanStack Router routes need *.browser.test.tsx sibling -- only if project use vitest browser mode (existing *.browser.test.* files or @vitest/browser dep). Skip layout/redirect-only routes. See REFERENCE.md.

When Done

• All pass (vitest run)
• Zero warnings -- green run w/ DeprecationWarning / React act() / unhandled rejection / @ts-ignore is NOT done. test-warning-check hook surfaces; fix at source. Escape: // allow: test-warning w/ reason.
• No async leaks (vitest run --detectAsyncLeaks) -- Stop hook run auto
• No setTimeout hacks -- condition-based wait
• Coverage gaps closed -- re-run coverage, verify changed files
• Selector priority: getByRole > getByText > getByTestId > querySelector
• Portal tests: defaultOpen for content tests | waitFor for close assertions
• Tests verify behavior, not impl | expect.soft() for multi-assertion state tests
• CI green AND ci-warning-audit clean -- gh run view <id> --log | grep -E 'Warning|Deprecation' returns nothing

See REFERENCE.md for element selectors, portal testing, mock patterns, diagnostics, Vitest config.