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fast-check

fast-check.dev

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Spec score 8.9 #1 of 28 in Property-Based Testing Tools

What it is

fast-check is a 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 investigate it. Tests combine arbitraries that create values, properties that describe behavior, runners that execute checks, and plugins that extend runners. It works with Jest, Mocha, Vitest, Ava, Bun, Deno, the Node.js test runner, and other test runners. Runs can be configured with seeds, generated-value sizes, time limits, and result reporting globally or per assertion. fast-check supports stateful testing and reproducible failures; its project also describes fuzzing and generative testing for issues such as race conditions and prototype poisoning. The project is open source under the MIT license, and the listed plan is free. Version 5.x requires Node.js 22.12.0 or later and ECMAScript 2025; TypeScript 5.8 or later is optional.

Spec sheet

What it does
fast-check is a property-based testing framework for JavaScript and TypeScript.Read 15 Oct 2026 · fast-check.dev
Purpose
fast-check is a property-based testing library for JavaScript and TypeScript that generates inputs, runs assertions over many cases, and shrinks failures to smaller counterexamples.Read 30 Sep 2026 · fast-check.dev
Integrations
The ecosystem lists official integrations for AVA, Jest, and Vitest, along with connectors for libraries such as Zod and io-ts.Read 30 Sep 2026 · fast-check.dev
Support
The project invites code and non-code contributions, community help, and financial support through GitHub Sponsors or OpenCollective.Read 30 Sep 2026 · fast-check.dev
Additional testing
The project describes fuzzing and generative testing for finding issues including race conditions and prototype poisoning.Read 30 Sep 2026 · fast-check.dev
API surface
The API reference documents classes such as Arbitrary, Random, Stream and Value, plus enumerations, interfaces, variables and functions.Read 15 Oct 2026 · fast-check.dev
Arbitraries
Arbitraries generate values and include shrinkers that reduce failures to readable counterexamples.Read 15 Oct 2026 · fast-check.dev
Bug discovery
The project says generated testing can find edge cases, complex input combinations, race conditions, prototype poisoning and zero-days.Read 15 Oct 2026 · fast-check.dev
Community activity
Snyk describes the project as actively maintained, with a README, contributing guide, code of conduct, funding and 80 contributors.Read 15 Oct 2026 · security.snyk.io
Compatibility
Version 4 requires Node.js 12.17.0 or newer, ES2020 support and TypeScript 5.0 or newer when TypeScript is used.Read 15 Oct 2026 · github.com
Configuration
Users can configure runs, seeds, generated value sizes, time limits, and result reporting globally or per assertion.Read 30 Sep 2026 · fast-check.dev
Creator
The Developers' Bakery identifies Nicolas Dubien as fast-check's creator.Read 15 Oct 2026 · thebakery.dev
Dependency footprint
The same index reports one direct dependency, pure-rand, and an unpacked npm size of about 1.5 MB.Read 15 Oct 2026 · javascripts.com
Documentation depth
Documentation includes runnable tutorials, quick start material, reference pages, configuration, advanced topics, migration guides and API reference.Read 15 Oct 2026 · fast-check.dev
Ecosystem caveat
The maintainers state that stability and maintenance are guaranteed for official packages but not for non-official ecosystem packages.Read 15 Oct 2026 · fast-check.dev
Ecosystem limitations
The ecosystem page says it can provide stability and maintenance details for official packages, but no specific guarantees for non-official packages.Read 30 Sep 2026 · fast-check.dev
Failure diagnosis
When a property fails, fast-check shrinks the input to a smaller counterexample and reports information that helps diagnose the failure.Read 30 Sep 2026 · github.com
Independent assessment
DEV.co rates maintenance, documentation, licence clarity, deployment complexity and overall fit positively, with high confidence.Read 15 Oct 2026 · dev.co
Integration connectors
Official connectors are provided for AVA, Jest and Vitest, with additional third-party bridges listed for other tools.Read 15 Oct 2026 · fast-check.dev
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.Read 15 Oct 2026 · github.com
Licence
fast-check is released under the MIT licence.Read 15 Oct 2026 · github.com
Limit: language scope
DEV.co notes that fast-check targets JavaScript and TypeScript, so Python, Go, Java and Rust projects need other libraries.Read 15 Oct 2026 · dev.co
Limit: learning curve
DEV.co warns that projects with minimal testing needs may not justify the learning curve and setup overhead of property-based testing.Read 15 Oct 2026 · dev.co
Limit: security claims
DEV.co says fast-check makes no security guarantees and recommends checking generated-input distributions for security-critical fuzzing.Read 15 Oct 2026 · dev.co
Maker location
The creator's GitHub profile lists the location as Remote, France.Read 15 Oct 2026 · github.com
Model-based testing
The API includes modelRun, asyncModelRun and scheduledModelRun functions for model and real-system testing.Read 15 Oct 2026 · fast-check.dev
Module formats
An independent package index reports ESM and CommonJS support with bundled TypeScript declarations.Read 15 Oct 2026 · javascripts.com
Node runner support
The documentation shows installation through npm, Yarn, pnpm or Bun for Node's built-in test runner.Read 15 Oct 2026 · fast-check.dev
Open source
The GitHub repository identifies fast-check as a public project released under the MIT license.Read 30 Sep 2026 · github.com
Plugins
Plugins extend runners and provide additional lifecycle or execution behavior.Read 15 Oct 2026 · fast-check.dev
Popularity
JavaScripts.com reports about 30 million weekly npm downloads, 5.2 thousand GitHub stars and 212 forks in its September 2026 snapshot.Read 15 Oct 2026 · javascripts.com
Praise: coverage
The independent Developers' Bakery interview describes fast-check as helping developers write property tests that provide better code coverage.Read 15 Oct 2026 · thebakery.dev
Praise: documentation
DEV.co characterizes the documentation as strong and the deployment complexity as low.Read 15 Oct 2026 · dev.co
Properties
Properties express predicates that must remain true for values produced by one or more arbitraries.Read 15 Oct 2026 · fast-check.dev
Release activity
JavaScripts.com reports 12 releases in the prior year and version 4.10.2 published on September 19, 2026.Read 15 Oct 2026 · javascripts.com
Runner support
The library is test-runner agnostic and works with Jest, Mocha, Vitest and other runners.Read 15 Oct 2026 · fast-check.dev
Runners
Runners control execution count, seeds, failure reporting and whether results are thrown or returned.Read 15 Oct 2026 · fast-check.dev
Runtime requirements
The repository lists fast-check 5.x as requiring Node.js 22.12.0 or later, ECMAScript 2025, and optionally TypeScript 5.8 or later.Read 30 Sep 2026 · github.com
Schema integrations
The ecosystem lists adapters for Effect Schema, Zod, io-ts, GraphQL schemas, JSON Schema and TypeSpec.Read 15 Oct 2026 · fast-check.dev
Security status
Snyk reports no known security issues and no direct vulnerabilities for version 4.10.2, while noting dependency vulnerabilities are separate.Read 15 Oct 2026 · security.snyk.io
Security testing
The ecosystem documents an official @fast-check/poisoning package for checking whether specially crafted inputs cause global prototype modifications.Read 30 Sep 2026 · fast-check.dev
Support funding
OpenCollective offers a Backer contribution starting at $1 USD per month, a Sponsor contribution starting at $100 USD per month and custom one-time or recurring donations.Read 15 Oct 2026 · opencollective.com
Test building blocks
Tests use arbitraries to generate values, properties to specify expected behavior, runners to execute properties, and plugins to extend runners.Read 30 Sep 2026 · fast-check.dev
Testing workflow
The library is test-runner agnostic and works with Jest, Mocha, Vitest, and other test runners.Read 30 Sep 2026 · fast-check.dev
Vitest connector
@fast-check/vitest enriches Vitest test and it functions with property syntax and handles timeouts, lifecycle hooks and configuration.Read 15 Oct 2026 · fast-check.dev

Plans and prices

fast-check Free MIT-licensed JavaScript/TypeScript testing library Read 30 Sep 2026 · github.com

Every fast-check plan and price →

Strong points and limits

Strong points

  • Shrinks failing inputs to smaller counterexamples
  • Works with Jest, Mocha, Vitest, and others
  • Supports stateful testing and reproducible failures
  • Configurable seeds, sizes, time limits, and reporting
  • Free under the MIT license

Limits

  • Version 5.x requires Node.js 22.12.0 or later
  • Version 5.x requires ECMAScript 2025
  • TypeScript 5.8 or later is optional, not required

Who it suits

It suits JavaScript and TypeScript developers who want to check behavior across generated inputs, including teams using different test runners. Note the Node.js and ECMAScript requirements for version 5.x.

Verdict

fast-check offers configurable property-based testing across several JavaScript and TypeScript test runners, with shrinking to clarify failures. It is free, but version 5.x has specific runtime requirements to check before adopting it.

Where Specifiction places it

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