fast-check

fast-check is a free property-based testing library for JavaScript and TypeScript. It is best suited to developers who want generated-input checks within an existing test runner; version 5.x’s runtime requirements may rule it out for older environments.

Overview

Instead of relying only on hand-picked examples, fast-check generates inputs and evaluates properties that express expected behavior. When a property fails, it shrinks the failing input to a smaller counterexample and reports information to help diagnose it. That makes it useful for probing behavior across cases a developer may not think to write individually. It is a complement to conventional example-based tests, not necessarily a replacement for them.

Key features

Tests combine arbitraries that generate values, properties that specify expected behavior, runners that execute those properties, and plugins that extend runners. fast-check is test-runner agnostic and supports workflows with Jest, Mocha, Vitest, Ava, Bun, Deno, and the Node.js test runner. Official integrations include AVA, Jest, and Vitest; connectors for Zod and io-ts extend its reach to those libraries.

Runs can be configured globally or per assertion, including seeds, generated-value sizes, time limits, and result reporting. Seeds allow a failure to be reproduced, while shrinking makes the counterexample more focused. Stateful testing is also supported. These controls are useful for tuning generated checks, though teams need to decide how those checks fit alongside their existing tests.

The project also describes fuzzing and generative testing for issues such as race conditions and prototype poisoning. Its official @fast-check/poisoning package checks whether specially crafted inputs cause global prototype modifications. This is a targeted security-testing aid rather than a claim that the library covers every security concern.

For version 5.x, fast-check requires Node.js 22.12.0 or later and ECMAScript 2025; TypeScript 5.8 or later is optional. That is a meaningful adoption constraint for projects that cannot move to those runtime requirements.

Pricing

fast-check is open source under the MIT license, with a free plan at no cost. There is no free trial because no paid plan is required. The project welcomes code and non-code contributions, community help, and financial support through GitHub Sponsors or OpenCollective. Its free price is compelling for teams comfortable using a community-maintained library; there is no paid tier described for buyers seeking a commercial support plan.

Platforms

fast-check is available for API, Linux, and web environments. It supports JavaScript and TypeScript, with test-runner support spanning Jest, Mocha, Vitest, Ava, Bun, Deno, and the Node.js test runner.

Who it's for

Choose fast-check if you write JavaScript or TypeScript and want to explore behavior across generated cases without changing test runners. It is particularly relevant when hard-to-anticipate inputs, reproducible failures, or stateful behavior matter. Teams on older Node.js versions should check compatibility before adopting 5.x.

Pros and cons

• Pro: Shrinking turns a failing generated input into a smaller counterexample, helping focus debugging.
• Pro: Configurable seeds, sizes, time limits, and reporting give teams control over runs, including per-assertion settings.
• Pro: Runner agnosticism and integrations for common JavaScript test runners make it possible to add generated checks to an existing workflow.
• Pro: The MIT-licensed library is free, and the ecosystem includes a specific package for checking prototype-poisoning behavior.
• Con: Version 5.x requires Node.js 22.12.0 or later and ECMAScript 2025, which can block use in older environments.
• Con: Property-based testing requires expressing expected behavior as properties, a different approach from writing only individual examples.
• Con: The ecosystem describes stability and maintenance details for official packages, but makes no specific guarantees for non-official packages.

Alternatives

For Go, Gopter is a free MIT-licensed property-testing package for Linux and self-hosted use. Rapid is another free, open-source Go library and has no dependencies outside the Go standard library. Pick either instead when Go is the language you need to test.

Scala Hedgehog is a free open-source Scala testing library for Linux, making it the language-specific option for Scala projects. CsCheck is free forever under Apache 2 and is self-hosted; it is an alternative when that license and deployment model suit your needs. QuickCheck is a free Haskell library for random testing of program properties, so Haskell teams should consider it instead.

Hypothesis is a free alternative for Linux, macOS, and Windows. RapidCheck is free and supports Linux, self-hosted, and Windows environments. Proptest is a free alternative available on the web.

Browse Property-Based Testing Tools for more options.

Verdict

JavaScript and TypeScript developers who want configurable generated-input tests, reproducible failures, and smaller counterexamples should choose fast-check, especially if they can use an existing supported runner. Its strongest reason to adopt is that it brings those checks into familiar test workflows at no cost. Look elsewhere if version 5.x’s runtime requirements do not fit your project or you need a language-specific alternative.