Jon Chery
49 lines
2.2 KiB
Markdown
49 lines
2.2 KiB
Markdown
# Test Fixtures — Derived Rules
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> Derives from `domains/testing/first-principles.md` P7 (Realism), P2 (Independence), P3 (Determinism).
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## Fixtures are Real Data (P7 Realism)
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- A fixture resembles production data in shape, distribution, and edge cases.
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- A fixture with `name: "test"` and `email: "a@b.c"` hides bugs that real data surfaces.
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- Use realistic names, realistic emails, realistic dates. `"Jane Doe", "jane.doe@example.com", "2026-03-15"`.
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## Factory Over Fixture (P2 Independence, P3 Determinism)
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- A fixture file is shared state. A factory is fresh state per test.
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- Prefer factories (e.g., `factory.User()` returning a new instance) over shared fixture files.
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- A shared fixture is mutated by one test, breaks another. Independence is violated.
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## Builders for Complex Data
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- A builder (`UserBuilder().withEmail().withAdmin().build()`) composes only the fields the test needs.
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- A builder with defaults: every field has a sensible default; tests override only what they test.
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- A builder is the test's API to data. Stable, composable, readable.
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## Setup and Teardown (P2 Independence)
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- Every test cleans up after itself. No test leaves state for the next.
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- `setUp`/`tearDown` (or `beforeEach`/`afterEach`) restore the world.
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- A test that depends on the order of execution is not independent.
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## Determinism (P3)
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- No `Date.now()`, no `Math.random()` in fixtures. Inject the clock, inject the RNG.
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- A fixture that uses "now" is non-deterministic. It passes today and fails tomorrow.
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- Fix timestamps: `createdAt: new Date("2026-01-01T00:00:00Z")`.
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## Edge Case Fixtures (P9 Edge Case Coverage)
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- A fixture set includes: the empty case, the single-item case, the max-size case, the unicode case.
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- A fixture set includes invalid data: malformed email, negative age, future date.
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- Edge case fixtures are first-class, not "extra credit."
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## What Violates Fixture Discipline
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| Violation | Principle |
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|-----------|-----------|
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| `name: "test"` fixture | P7 Realism |
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| Shared fixture file mutated across tests | P2 Independence |
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| `createdAt: new Date()` (now) in fixture | P3 Determinism |
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| No edge-case fixtures | P9 Edge Case Coverage |
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| A 500-line fixture file | P3 (complexity) | |