Specialty Testing

For unit / E2E / component testing see Testing. This page covers the specialized branches.

Visual regression

• ★ Chromatic — Storybook-native; every PR gets visual diffs. Generous free tier; paid above.
• ★ Argos CI — Playwright-native; cheaper than Chromatic if you don't use Storybook; generous free tier.
• Percy (BrowserStack) — pioneer; full-page diffs.
• Loki (Storybook plugin) — open source self-host visual regression.
• BackstopJS — older, self-host, configuration-driven.
• Playwright toHaveScreenshot() — built-in; pair with a CI cache for diffs.
• reg-cli / reg-suit — minimal screenshot diff tool.

Mutation testing

• ★ Stryker Mutator — JS / TS / .NET / Scala. Mutate your code, run your tests, see what should fail but doesn't. The default for measuring test quality (vs. test coverage).
• PIT (Java) — non-JS but worth knowing.

Contract testing

• ★ Pact — consumer-driven contracts; the standard. JS / Python / Go / Java / .NET. Hosted Pact Broker or self-host.
• Spring Cloud Contract — Java-flavored.
• Schemathesis — property-based, OpenAPI-driven (see OpenAPI Tooling).
• Optic — captures live API traffic and diffs against spec.

Coverage

• ★ c8 — uses native V8 coverage; what Vitest uses by default. The fastest.
• Istanbul (@vitest/coverage-istanbul, nyc) — instrumentation-based; produces the same reports.
• @vitest/coverage-v8 — Vitest's c8-flavored.
• Codecov / Coveralls — hosted reporting; generous free tiers; PR comments with coverage diffs.

Property-based / fuzz testing

• ★ fast-check — TS-first property-based testing; integrates with Vitest / Jest. The default.
• @hapi/hoek — runtime contract checks.
• jsfuzz — JS fuzzer.
• Atheris (Python), AFL++, libFuzzer — non-JS.
• OSS-Fuzz — Google's continuous fuzzing for OSS projects.

Snapshot testing patterns

• Vitest's toMatchSnapshot() / toMatchInlineSnapshot().
• Jest's same APIs.
• Use sparingly — easy to commit broken snapshots without noticing.

API / load testing

• See Load Testing — k6, Artillery, Autocannon.

Accessibility testing

• See Accessibility Testing — axe, Pa11y, jsx-a11y.

End-to-end with auth / state

• Playwright auth state — storageState; reuse logged-in session across tests.
• @playwright/test projects — split tests by browser / device.
• Vitest browser mode — run unit tests in a real browser.
• Faker.js + factories — see Faker & Seed Data.

Patterns to know

• Visual regression on Storybook stories — much faster than full-page snapshots; isolates components.
• Mutation testing as a periodic CI job, not per-PR — too slow for inline gating.
• Contract tests in monorepos — Pact for cross-service contracts; tRPC / oRPC types for same-repo.
• Coverage thresholds — set realistic ones (70–85% line is healthy); track trend, not absolute.
• Don't chase 100% coverage — diminishing returns; quality > quantity.

Pick this if…

• Default visual regression with Storybook: Chromatic.
• Default visual regression with Playwright: Argos.
• Self-host visual regression: Loki.
• Measure test quality (not just coverage): Stryker.
• Contract testing across services: Pact.
• Coverage reports + PR comments: Codecov.
• Property-based / fuzz: fast-check.