# Test Fixtures — Derived Rules

> Derives from `domains/testing/first-principles.md` P7 (Realism), P2 (Independence), P3 (Determinism).

## Fixtures are Real Data (P7 Realism)

- A fixture resembles production data in shape, distribution, and edge cases.
- A fixture with `name: "test"` and `email: "a@b.c"` hides bugs that real data surfaces.
- Use realistic names, realistic emails, realistic dates. `"Jane Doe", "jane.doe@example.com", "2026-03-15"`.

## Factory Over Fixture (P2 Independence, P3 Determinism)

- A fixture file is shared state. A factory is fresh state per test.
- Prefer factories (e.g., `factory.User()` returning a new instance) over shared fixture files.
- A shared fixture is mutated by one test, breaks another. Independence is violated.

## Builders for Complex Data

- A builder (`UserBuilder().withEmail().withAdmin().build()`) composes only the fields the test needs.
- A builder with defaults: every field has a sensible default; tests override only what they test.
- A builder is the test's API to data. Stable, composable, readable.

## Setup and Teardown (P2 Independence)

- Every test cleans up after itself. No test leaves state for the next.
- `setUp`/`tearDown` (or `beforeEach`/`afterEach`) restore the world.
- A test that depends on the order of execution is not independent.

## Determinism (P3)

- No `Date.now()`, no `Math.random()` in fixtures. Inject the clock, inject the RNG.
- A fixture that uses "now" is non-deterministic. It passes today and fails tomorrow.
- Fix timestamps: `createdAt: new Date("2026-01-01T00:00:00Z")`.

## Edge Case Fixtures (P9 Edge Case Coverage)

- A fixture set includes: the empty case, the single-item case, the max-size case, the unicode case.
- A fixture set includes invalid data: malformed email, negative age, future date.
- Edge case fixtures are first-class, not "extra credit."

## What Violates Fixture Discipline

| Violation | Principle |
|-----------|-----------|
| `name: "test"` fixture | P7 Realism |
| Shared fixture file mutated across tests | P2 Independence |
| `createdAt: new Date()` (now) in fixture | P3 Determinism |
| No edge-case fixtures | P9 Edge Case Coverage |
| A 500-line fixture file | P3 (complexity) |