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fast-check
What it is
fast-check is a free, open-source property-based testing library for JavaScript and TypeScript. It generates inputs and checks expected behavior across many cases, then reduces a failure to a smaller counterexample to help diagnose it. Tests are assembled from arbitraries that generate values, properties that define behavior, runners that execute checks, and plugins that extend runners. The library works with Jest, Mocha, Vitest, Ava, Bun, Deno, and the Node.js test runner. Runs can be configured with seeds, generated-value sizes, time limits, and reporting options, globally or for an individual assertion. It supports stateful testing and reproducible failures, and the project describes fuzzing and generative testing for issues such as race conditions and prototype poisoning. Official integrations include AVA, Jest, and Vitest; the ecosystem also lists connectors for Zod and io-ts. Version 5.x requires Node.js 22.12.0 or later and ECMAScript 2025; TypeScript 5.8 or later is optional. The project is released under the MIT license.
For and against
For
- Shrinks failures to smaller counterexamples.
- Supports stateful testing and reproducible failures.
- Runs with Jest, Mocha, Vitest, and other test runners.
- MIT-licensed and free.
Against
- Version 5.x requires Node.js 22.12.0 or later.
- Version 5.x requires ECMAScript 2025.
- No specific stability guarantees are listed for non-official packages.
The verdict
fast-check provides input generation, failure reduction, and configuration options for property-based testing. Check its version 5.x runtime requirements and the maintenance details available for integrations before adopting it.
Who it suits
fast-check suits JavaScript and TypeScript developers who want to exercise expected behavior across generated inputs. It can fit projects using a range of test runners.
Plans and prices
fast-check
MIT-licensed JavaScript/TypeScript testing library
Read 30 Sep 2026 · github.com
On the record
- Additional testing
- The project describes fuzzing and generative testing for finding issues including race conditions and prototype poisoning.fast-check.dev · read 30 Sep 2026
- API surface
- The API reference documents classes such as Arbitrary, Random, Stream and Value, plus enumerations, interfaces, variables and functions.fast-check.dev · read 15 Oct 2026
- Arbitraries
- Arbitraries generate values and include shrinkers that reduce failures to readable counterexamples.fast-check.dev · read 15 Oct 2026
- Bug discovery
- The project says generated testing can find edge cases, complex input combinations, race conditions, prototype poisoning and zero-days.fast-check.dev · read 15 Oct 2026
- Community activity
- Snyk describes the project as actively maintained, with a README, contributing guide, code of conduct, funding and 80 contributors.security.snyk.io · read 15 Oct 2026
- Compatibility
- Version 4 requires Node.js 12.17.0 or newer, ES2020 support and TypeScript 5.0 or newer when TypeScript is used.github.com · read 15 Oct 2026
- Configuration
- Users can configure runs, seeds, generated value sizes, time limits, and result reporting globally or per assertion.fast-check.dev · read 30 Sep 2026
- Creator
- The Developers' Bakery identifies Nicolas Dubien as fast-check's creator.thebakery.dev · read 15 Oct 2026
- Dependency footprint
- The same index reports one direct dependency, pure-rand, and an unpacked npm size of about 1.5 MB.javascripts.com · read 15 Oct 2026
- Documentation depth
- Documentation includes runnable tutorials, quick start material, reference pages, configuration, advanced topics, migration guides and API reference.fast-check.dev · read 15 Oct 2026
- Ecosystem caveat
- The maintainers state that stability and maintenance are guaranteed for official packages but not for non-official ecosystem packages.fast-check.dev · read 15 Oct 2026
- Ecosystem limitations
- The ecosystem page says it can provide stability and maintenance details for official packages, but no specific guarantees for non-official packages.fast-check.dev · read 30 Sep 2026
- Failure diagnosis
- When a property fails, fast-check shrinks the input to a smaller counterexample and reports information that helps diagnose the failure.github.com · read 30 Sep 2026
- Independent assessment
- DEV.co rates maintenance, documentation, licence clarity, deployment complexity and overall fit positively, with high confidence.dev.co · read 15 Oct 2026
- Integration connectors
- Official connectors are provided for AVA, Jest and Vitest, with additional third-party bridges listed for other tools.fast-check.dev · read 15 Oct 2026
- Integrations
- The ecosystem lists official integrations for AVA, Jest, and Vitest, along with connectors for libraries such as Zod and io-ts.fast-check.dev · read 30 Sep 2026
- Latest notable release
- GitHub's release page lists v4.10.2 with an interrupt-plugin fix and v4.10.0 with a new plugin API, global plugin configuration and timeout plugin.github.com · read 15 Oct 2026
- Licence
- fast-check is released under the MIT licence.github.com · read 15 Oct 2026