Question 1

Jest vs Vitest, when do I pick which?

Accepted Answer

Jest: most jobs, React ecosystem, 10+ years maturity, great documentation, slower startup (but parallel execution is fast). Vitest: Vite-native, ESM-first, instant startup, faster cold runs, smaller bundles, 2026 trend is Vitest. If you're Vite + TypeScript: Vitest. If you're CRA or Jest-heavy legacy: Jest. Both understand mocks identically, skill transfers. Learn Jest first for employability.

Question 2

Mocking strategies, jest.mock vs jest.spyOn vs Manual Mocks?

Accepted Answer

jest.mock('/path'): hoist to top, replaces entire module, best for external deps. jest.spyOn: partial replacement, you can call the real function, best for tracking calls. Manual Mocks: (/__mocks__/) for complex stubs, persistence across tests. Rule of thumb: spy on internal, mock external. Always useRestoreMocks: true in jest.config to auto-clean after each test.

Question 3

Snapshot tests, when are they useful vs a code smell?

Accepted Answer

Snapshots are GOLD for UI regressions: Jest render component → snapshot changes if JSX breaks. NOT useful for logic tests (brittle). Snapshot best practices: Review snapshots in PR, commit them, use -u sparingly, keep snapshots < 100 lines else split them. Bad: snapshot of entire page. Good: snapshot of single Card component. Trend: visual regression tools (Percy/Chromatic) are replacing snapshots for visual work.

Question 4

Coverage thresholds, how high should they go?

Accepted Answer

Industry median: 70-80% coverage. High-integrity (fintech/healthcare): 85%+. Startups: 60%. Jest coverage = line + branch + function + statement. High coverage ≠ high quality (can mock everything). Better metric: test-to-code ratio (1:1 is healthy). Set thresholds in jest.config.js to fail CI if you drop below target. Red flags: 100% coverage with no branch coverage, or high coverage but low integration tests.

Question 5

Integration vs Unit tests in Jest, how to split them?

Accepted Answer

Unit: single function/component in isolation, fast (< 1ms), mocked deps. Integration: multiple modules working together, slower (10-100ms), real DB or MSW mocks. Jest can do both: unit tests run parallel in <2s, integration tests in <10s. Use testMatch: ['**/__tests__/unit/**', '**/__tests__/integration/**'] to split folders. CI: run unit tests on every commit, integration tests on PR/merge only (they're slower).

Question 6

ESM support in Jest 30, how does it differ from CommonJS?

Accepted Answer

Jest 30+ has native ESM: set "type": "module" in package.json, use --experimental-vm-modules flag. ESM advantages: top-level await, tree-shaking, dynamic imports. Trade-off: slower startup than CommonJS. ts-jest handles TypeScript → ESM transparently. If stuck on Jest 29: use ts-jest as transpiler, it handles ESM-CommonJS bridge. Vitest is ESM-native, no flag needed.

Question 7

Parallel test performance, why are tests slow and how do I fix it?

Accepted Answer

Jest runs tests in parallel by default (maxWorkers = num CPUs). Slowness causes: (1) shared state not cleaned up, (2) heavy setup (DB connections, file I/O), (3) missing beforeEach/afterEach, (4) too many tests in one file (split into groups). Fixes: mock heavy deps, use beforeAll for expensive setup, run --detectOpenHandles to find leaks. For 1000+ tests: --maxWorkers=4 (avoid over-parallelizing), consider splitting into suites or using testPathPattern. Typical: 500 tests in 3-5s.

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