Test

Tests — write & review

Per-test rules that operationalize one principle: test the contract / responsibility, not the implementation. This is the how-to for a single it, and the lens for reviewing one.

Two modes, one rule set

• Write mode — authoring a test. Apply the rules below to produce it.
• Review mode — auditing an existing test or test diff. Apply the same rules as a checklist; report each violation with file:line, the rule it breaks, and the fix. See "Review mode" near the end.

The rules are identical in both modes — only the posture changes (produce vs. audit).

Test contract, not implementation

• Drive the system through its public control plane and assert on observable effects (returned values, persisted state, emitted events, injected messages), never on source details.
• Resolve collaborators through their contract — the interface plus its identifier — not the module that binds a concrete implementation.
• Do not reach into private fields or add backdoors "for testing". If you feel the need, the seam is wrong — fix the design, not the test.

One behavior per it

Each it covers exactly one responsibility / scenario. If the name needs "and", split it.

it('returns 401 when the caller is unauthorized', ...);
it('does not double-fire when the same tick repeats', ...);

Name and structure

• describe('<slice> (<responsibilities>)' — name the responsibility, not the class.
• An it(...) reads as a sentence, but it must still encode three things — the behavior / method, the state or condition, and the expected outcome: it('<behavior> when <condition>, <outcome>'). A name like does X when Y with no result is too vague to fail usefully.
  • Use spaces, not the Java-style method_state_outcome underscores — that convention exists only because Java test methods cannot contain spaces. A string-named test reads fine as a sentence.
  • Good: it('returns 401 when the caller is unauthorized') · it('advances the cursor and does not double-fire on a repeat tick')
  • Bad: it('works') · it('handles auth correctly') — no condition, no outcome
• Arrange / Act / Assert. A short // Given // When // Then is fine when it aids reading; do not paste it mechanically on trivial tests.

Build a small rig

When several tests share setup, write a factory (rig(), createHost(), whatever fits the codebase) that returns the smallest surface the test needs. Tests reach into the rig; they do not rebuild the world each time. Keep the rig dumb: wiring only, no assertions.

Stub only the real external boundary

Default to real collaborators wired the way production wires them. Stub the minimum seam that is genuinely external:

• A remote / model / service boundary — spy on the contract method (the interface), and capture what the system sends across it. Do not stand up the real external thing.
• Network / other-process boundaries — stub at the boundary, not the internals.
• Time, timers, jitter — use the documented control knobs the system exposes (env, an injected clock, a manual tick). Do not use fake timers or real setTimeout to drive time.
• Env / config knobs are usually snapshotted at bootstrap — set them before building the system under test, and restore them in afterEach.

Keep tests DAMP and keep cause next to effect

• DAMP over DRY: use literal expected values in assertions; do not compute the expectation with the same logic as the code under test.
• Keep the key preconditions inside the it (or its rig), where the reader can see cause next to effect. Reserve beforeEach for cross-cutting plumbing (env snapshot, cleanup), not for hiding the scenario's setup.

// Good — the expected value is a literal the reader can check.
expect(discount).toBe(15);
// Bad — re-derives the expectation; mirrors the implementation.
expect(discount).toBe(price * rate);

Assert only what is relevant

Assert the effect that proves the contract. Use matchers / partial-object matching to ignore incidental fields. Do not assert internal counters, call orders, or shapes the user cannot rely on.

Isolate and clean up (no flakes)

Every test must be hermetic and order-independent. In afterEach:

• restore every mock / spy
• restore every env var you touched (snapshot in beforeEach)
• dispose the host / container and reset its reference

No dependence on wall-clock time, run order, or leftover on-disk state — give each scenario its own isolated identity / workspace when state persists.

Quality bar: CCCR

Before finishing, check each test against:

• Clarity — a stranger can tell what broke from the failure message alone.
• Completeness — covers the responsibility's success, error, and boundary paths.
• Conciseness — no duplicate or speculative cases; one scenario per it.
• Resilience — survives an internal refactor with no test change (because it asserts contract, not implementation).

Per-file scenario header

Start each test file with a short header comment: the scenario, the responsibilities asserted, the wiring (which collaborators are real vs. the single stubbed boundary), and how to run it.

Review mode — auditing existing tests

Apply the rules above as a checklist against each test in scope (a file, a diff, or a named it). For every hit, report file:line + the rule it breaks + the fix; do not rewrite unless asked. Lead with the contract question: what observable behavior does this test prove, and would it survive a refactor?

Check, in order:

1. Contract, not implementation — asserts observable effects, not private fields, call order, or internal shapes the user cannot rely on.
2. One behavior per it — the name carries behavior + condition + outcome; "and" in the name means a split is owed.
3. Boundary discipline — only the true external seam is stubbed; time is driven by documented knobs, not fake timers / real setTimeout.
4. DAMP expectations — expected values are literals, not re-derived by the code under test's logic.
5. Isolation — mocks / spies / env / host restored in afterEach; no wall-clock, run-order, or leftover on-disk dependence.
6. CCCR read-through — Clarity, Completeness (success / error / boundary), Conciseness, Resilience.

Report findings as evidence + fix, e.g. "foo.test.ts:42 asserts on service.internalMap (contract) — assert the returned value instead." If a test passes the lens, say so briefly; silence on a rule means it held.

Quick checklist (write & review)

• Resolved through the contract; no concrete-impl import
• One behavior per it; name carries behavior + condition + outcome; AAA
• Stubbed only the true external seam; time via knobs, not fake timers
• Literal expectations; relevant assertions only
• Mocks / env / host restored in afterEach; hermetic, no flakes
• CCCR read-through done