Unit testing

<objective> Write unit tests that fail when the code is wrong and pass when it is right — nothing weaker. A test that mocks every collaborator stays green while the integration is broken; the doubles taxonomy below stops that. A `coverageThreshold` typo (or the plural `coverageThresholds`, which Jest silently ignores) lets 40%-covered code ship on a green pipeline; the config and Verification sections below make the gate actually fire. This skill covers Jest, Vitest, and pytest: doubles, coverage gating, snapshots, fake timers, and mutation testing as a behavior check on top of coverage. </objective>

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Discovery Questions

Check .agents/qa-project-context.md first — if it exists, use it and skip anything answered there.

1. Framework: Jest, Vitest, or pytest? Check package.json or pyproject.toml. The runner decides config keys and mock APIs.
2. Coverage tooling: Already configured? Look for jest.config.*, vitest.config.*, .nycrc, [tool.coverage]. Determines whether you add the gate or just tune it.
3. Mocking strategy: Manual mocks, auto-mocking, or dependency injection? Check for __mocks__/ dirs or DI containers — this sets which doubles you reach for.
4. Conventions: Co-location (*.test.ts next to source) or a __tests__/tests/ tree? Match what exists; don't introduce a third location.

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Core Principles

1. Test behavior, not implementation. Verify what code does, not how. Refactoring internals should not break tests.

// Bad — implementation detail        // Good — observable behavior
expect(svc._cache.size).toBe(3);      expect(svc.getUser("abc")).toEqual({ id: "abc", name: "Alice" });

2. Fast, isolated, deterministic. No network/disk/DB. No shared mutable state. No uncontrolled Date.now() or Math.random() — freeze them with fake timers and seeded values.

3. Arrange-Act-Assert. One clear shape per test.

it("should apply discount for orders over $100", () => {
  // Arrange
  const order = createOrder({ subtotal: 150 });
  const svc = new DiscountService(0.1);
  // Act
  const result = svc.apply(order);
  // Assert
  expect(result.total).toBe(135);
});

4. One assertion concept per test. Multiple expect calls are fine when they verify the same concept.

5. Descriptive names. "should [behavior] when [condition]", not "test calculateTotal".

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Framework-Specific Patterns

The full setup/teardown, mocking, spying, timer, in-source, and monorepo examples for each runner live in references/patterns.md. Below is what is current and what to reach for; copy the code from the reference.

Jest

Current is Jest 30.x (30.4.2, May 2026). Jest 30 added --collect-tests, jest.config.mts support, Temporal-aware fake timers, and clearMocksOnScope. If the code under test uses the Temporal API or time-zone logic, Jest 30's Temporal-aware fake timers remove a class of brittle setup.

Reach for: jest.mock() for module boundaries (jest.requireActual for partial mocks), jest.spyOn() to wrap a real method, jest.Mocked<T> for typed mocks, and jest.useFakeTimers() for time. See references/patterns.md § Jest.

Vitest

Same API as Jest, Vite-native. Stable: Vitest 4.1.x (June 2026); 5.0.0-beta is out (beta.3, May 2026). Vitest 4 added coverage.changed (changed-files-only coverage), mockThrow/mockThrowOnce, and a stable browser mode. Vitest 5 beta removes the sequential option and requires Node 22 / Vite 6.4 — wait for stable before adopting. Mock with vi.mock/vi.spyOn; the standout features are in-source testing (import.meta.vitest) and browser mode for component rendering. See references/patterns.md § Vitest.

pytest

Use fixtures + conftest.py (with yield for teardown), @pytest.mark.parametrize for data-driven cases, and monkeypatch for env/attr substitution. Prefer fixtures over setUp/tearDown methods — fixtures compose and isolate per test. See references/patterns.md § pytest.

Bun / Deno

bun test (Jest-compatible, no extra config) and deno test (native TS, permission flags) are reasonable defaults when your runtime is already Bun or Deno. Prefer Vitest/Jest for Node projects with deeper plugin ecosystems.

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Mocking Taxonomy

Pick the simplest double that does the job. Most of the time that is a stub.

Double	What it does	When to use
Stub	Returns canned data, no verification	Control a dependency's return value
Spy	Wraps real impl, records calls	Verify calls without changing behavior
Mock	Replaces impl + records calls	Control return AND verify interaction
Fake	Simplified working impl (in-memory DB)	Complex stateful dependencies

Rule of thumb: prefer stubs over mocks; reserve fakes for stateful dependencies; never call a real external API in a unit test. Only mock the external boundary (network, filesystem, DB, time) — let fast, deterministic internal collaborators run for real, or you get a suite that is green while the integration is broken. The four doubles in code: references/patterns.md § Test doubles.

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Coverage

Configuration

Jest — the threshold key is coverageThreshold (singular). The plural coverageThresholds is not a Jest key: Jest ignores it silently, the gate never enforces, and CI stays green at 30% coverage. This is the single most common config bug.

// jest.config.js
module.exports = {
  coverageProvider: "v8",
  collectCoverageFrom: ["src/**/*.ts", "!src/**/*.{d,test,stories}.ts", "!src/**/index.ts"],
  coverageThreshold: { global: { branches: 80, functions: 80, lines: 80, statements: 80 } },
};

Vitest — set test.coverage.thresholds in vitest.config.ts with provider: "v8" (see references/patterns.md § Vitest for the full block).

pytest:

# pyproject.toml
[tool.coverage.run]
source = ["src"]
omit = ["src/**/test_*.py", "src/**/conftest.py"]
[tool.coverage.report]
fail_under = 80
show_missing = true
exclude_lines = ["pragma: no cover", "if TYPE_CHECKING:"]

Coverage types and what to gate on

Type	Measures	Blind spots
Branch	Every if/else path taken?	Misses value combinations
Line	Each line executed?	Misses untested branches in one line
Statement	Each statement executed?	Similar to line
Function	Each function called?	Nothing about correctness

Priority: Branch > Line > Statement > Function. Use 80% line as the baseline gate, not a vanity target, and weight branch coverage higher. Focus coverage on business logic, transformations, error paths, and edge cases; skip generated code, type definitions, barrel exports, trivial getters, and framework boilerplate.

For interpreting which uncovered lines matter and doing gap analysis, that's coverage-analysis, not this skill.

CI gate

Jest and Vitest exit non-zero when thresholds fail — that exit code IS the gate. pytest needs the flag explicitly:

- run: pytest --cov=src --cov-fail-under=80

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Mutation Testing

Coverage tells you what code ran. Mutation testing tells you whether the tests would catch a bug. It makes small source changes (> → >=, true → false) and reruns the suite against each mutant. If the suite still passes, the mutant survived — your tests executed that logic but did not assert on it.

Stryker (JS/TS)

npm i -D @stryker-mutator/core @stryker-mutator/jest-runner  # or vitest-runner

// stryker.config.json  (Stryker's documented default; .mjs/.mts also load)
{
  "testRunner": "jest",
  "coverageAnalysis": "perTest",
  "mutate": ["src/**/*.ts", "!src/**/*.test.ts"],
  "thresholds": { "high": 80, "low": 60, "break": 50 },
  "reporters": ["html", "clear-text", "progress"]
}

Stryker's own defaults are { high: 80, low: 60, break: null } — break: null means no failing exit. Set break (e.g. 50) to make a low score fail CI. Run: npx stryker run.

mutmut (Python) — mutmut 3.x

mutmut 3 dropped the old CLI surface. Configure paths in a [mutmut] block, run, then review survivors in the TUI:

# setup.cfg  (or a [tool.mutmut] table in pyproject.toml)
[mutmut]
paths_to_mutate=src/

pip install mutmut          # 3.5.x
mutmut run                  # paths come from config, not a flag
mutmut browse               # interactive TUI: inspect and retest survivors
mutmut apply <mutant_id>    # write a survivor to disk to see what it changed

Avoid: mutmut run --paths-to-mutate=src/, mutmut results, and mutmut show 42 — that was the mutmut <3 surface. The --paths-to-mutate flag is gone (paths move to the [mutmut] config block) and results/show are replaced by browse/apply (mutmut 3.5.x, verified June 2026). Following the old commands errors out on a current install.

Interpreting scores

Score	Meaning
90%+	Strong — catching most logic changes
70–89%	Decent — review survivors in critical paths
<70%	Tests execute code but do not verify behavior

Run mutation testing on critical business logic, not the whole codebase (it is slow). Ignore equivalent mutants — logically identical code where no test could ever tell the difference.

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Snapshot Testing

Use for: UI component render output, serialized data structures, CLI formatting — output where exact structure matters and is tedious to assert field-by-field.

Do not use for: frequently changing output (snapshot fatigue → rubber-stamp reviews), large snapshots (unreviewable), implementation details (CSS classes, internal IDs), or as a substitute for a targeted assertion when one specific value is what matters.

Prefer inline snapshots for small output (<20 lines) and property matchers (expect.any(String)) for dynamic fields like ids and timestamps. Always run CI with --ci so an unknown snapshot fails instead of being silently written and committed. Code: references/patterns.md § Snapshot testing.

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Anti-Patterns

Testing private methods — Test through the public API. If a private method really needs its own tests, extract it to its own module with a public surface.

Mocking everything — Only mock external boundaries (network, filesystem, DB, time). A suite where every collaborator is mocked passes while the wiring between them is broken.

The plural coverageThresholds — Jest ignores it; the gate never fires; CI is green at any coverage. The key is coverageThreshold (singular). See Coverage above.

Faking all timers blindly — jest.useFakeTimers() / vi.useFakeTimers() with no allowlist can deadlock code awaiting a real microtask. Fake only what the test needs (doNotFake / toFake). See references/patterns.md § Jest timers.

Async test without await — a forgotten await makes the assertion never run and the test passes vacuously. Add expect.assertions(n) / expect.hasAssertions() to async tests so a missing assertion fails them.

Snapshot overuse — Use expect(x).toBe("active") for a specific value; reserve snapshots for structured output you can't assert field-by-field.

Non-descriptive names — Replace "works" with "should return empty array when no items match the filter".

Shared mutable state — Initialize in beforeEach, not at module scope:

// Bad: shared mutation               // Good: fresh per test
const items = [];                     let items: string[];
it("A", () => items.push("a"));       beforeEach(() => { items = []; });
it("B", () => {                       it("A", () => { items.push("a"); expect(items).toHaveLength(1); });
  items.push("b");                    it("B", () => { items.push("b"); expect(items).toHaveLength(1); });
  expect(items).toHaveLength(1); // FAILS
});

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Verification

Prove the suite runs and the gate actually fails on under-coverage — the exact thing the coverageThreshold typo silently disables.

1. Tests run and pass: npx jest (or vitest run, pytest -q) exits 0.
2. The gate bites. Run coverage and confirm a non-zero exit when below threshold:

   npx jest --coverage --ci          # Jest/Vitest exit !=0 below coverageThreshold
   vitest run --coverage             # same for Vitest
   pytest --cov=src --cov-fail-under=80   # pytest exits !=0 below the floor
   

   Temporarily set a threshold above current coverage (e.g. 99) and confirm the command fails. If it exits 0, your threshold key is wrong (likely the plural coverageThresholds).
3. Snapshots are safe in CI: the run uses --ci, so an unknown snapshot fails rather than being written. git status shows no new *.snap after a CI-mode run.

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Done When

• Coverage thresholds configured in jest.config.* (key coverageThreshold, singular), vitest.config.* (coverage.thresholds), or pyproject.toml (fail_under) AND verified to exit non-zero below threshold (Verification step 2)
• Test files all live in the project's single chosen location (co-located OR __tests__/tests/) — git ls-files shows no ad-hoc test paths
• External boundaries (HTTP, DB, time) are mocked and internal collaborators are not — grep finds no real network/DB clients constructed in test files
• No test reaches outside the process boundary — suite passes with the network disabled and no test DB running
• CI runs the test command with --ci (Jest/Vitest) so an unknown snapshot fails the build instead of being auto-written

Reference Files (in references/)

• patterns.md — full runnable examples per framework: Jest setup/teardown, module/spy/timer mocks, async guards; Vitest config, in-source tests, concurrency, browser mode; pytest fixtures/parametrize/monkeypatch; Bun/Deno; the four test doubles; snapshot file/inline/property matchers.

Related Skills

• coverage-analysis — interpreting coverage reports, finding meaningful gaps, mutation score as a first-class signal. Go there to read coverage; stay here to configure and gate it.
• ci-cd-integration — test stages in pipelines, parallelization, caching, deployment gating.
• ai-test-generation — when an AI writes the test code from a spec/PRD; this skill is for writing and structuring tests by hand.
• ai-qa-review — auditing existing tests for hallucinated APIs, fabricated imports, and closed-loop tests.
• shift-left-testing — pre-commit hooks, IDE integration, and TDD workflow around these tests.