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What's new in fast-check 4.4.0?

· 6 min read
Nicolas Dubien
Nicolas Dubien
fast-check maintainer

We expand fast-check's collection of arbitraries with two data structures: Map and Set. These native collections are now first-class citizens in fast-check, making it easier to test code that relies on them. We've also improved the flexibility of fc.dictionary to support the full range of property keys.

Continue reading to explore the detailed updates it brings.

New fc.map arbitrary​

With this release, fast-check now provides a dedicated fc.map arbitrary for generating JavaScript's Map.

The new arbitrary works similarly to fc.dictionary and fc.object, accepting key and value arbitraries:

// Generate a map with string keys and number values
fc.map(fc.string(), fc.nat());

// Generate a map with specific constraints
fc.map(fc.string(), fc.nat(), { minKeys: 1, maxKeys: 10 });

Like other arbitraries in fast-check, fc.map comes with sensible defaults while allowing you to customize it deeply.

New fc.set arbitrary​

Alongside fc.map, we're also introducing fc.set for generating instances of Set.

// Generate a set of strings
fc.set(fc.string());

// Generate a set with size constraints
fc.set(fc.nat(), { minLength: 2, maxLength: 20 });

The fc.set arbitrary ensures all elements in the generated set are unique according to equality semantics of a Set.

Full PropertyKey support in fc.dictionary​

Prior to this release, fc.dictionary only supported string keys. However, JavaScript objects can have properties keyed by strings, numbers or symbols. With 4.4.0, fc.dictionary now accepts the full range of property keys.

Changelog since 4.3.0​

The version 4.4.0 is based on version 4.3.0.

Features​

  • (PR#6232) Support full PropertyKey in fc.dictionary(...)
  • (PR#6267) Add fc.map arbitrary
  • (PR#6040) Add circular option to fc.letrec
  • (PR#6270) Add fc.set arbitrary
  • (PR#6334) REVERT-6040: Self-referencing capabilities from letrec

Fixes​

  • (PR#6138) CI: Force OTP at publication time
  • (PR#6170) CI: Stop running tests against Windows
  • (PR#6178) CI: Add GH Action to reformat code in PRs or branches
  • (PR#6205) CI: Add GH Action to add contributors to the project
  • (PR#6246) CI: Fix PR mode in format-pr workflow
  • (PR#6184) CI: Add provenance attestation to npm package publishing
  • (PR#6248) CI: Add workflow to resolve pnpm lock file merge conflicts on PRs
  • (PR#6251) CI: Restrict Format workflow to PRs
  • (PR#6253) CI: Fix PR number type in workflows
  • (PR#6254) CI: Fix PR workflows
  • (PR#6255) CI: Fix PNPM conflicts workflow
  • (PR#6256) CI: Scope permissions of PR Format to job
  • (PR#6257) CI: Scope permissions of PR PNPM to job
  • (PR#6258) CI: Job level permissions for add contributor
  • (PR#6269) CI: Add GitHub Action to validate PR titles
  • (PR#6281) CI: Add back Windows runners for tests
  • (PR#6280) CI: Add back latest node for tests
  • (PR#6282) CI: Bump test matrix to Node 24
  • (PR#6283) CI: Shard tests producing coverage
  • (PR#6301) CI: Downgrade node in tests
  • (PR#6300) CI: Enforce trust-policy for pnpm
  • (PR#6307) CI: Add back latest Node in test matrix
  • (PR#6136) Doc: Release note for version 4.3.0
  • (PR#6169) Doc: Preserve links on Sponsors for the website
  • (PR#6172) Doc: Update CSP to properly display sponsors.svg
  • (PR#6173) Doc: Update CSP to properly display sponsors.svg
  • (PR#6174) Doc: Better support of mobile display for sponsors
  • (PR#6175) Doc: Better accessibility on website
  • (PR#6192) Doc: Add @traversable/zod-test to ecosystem
  • (PR#6204) Doc: Add ahrjarrett as doc contributor
  • (PR#6238) Doc: Add jamesbvaughan as code contributor
  • (PR#6250) Doc: Add GitHub Copilot instructions for gitmoji PR naming convention
  • (PR#6180) Doc: Generate llms.txt and related for AI crawlers
  • (PR#6279) Doc: Add emilianbold as code contributor
  • (PR#6287) Doc: Fix example in quick start guide
  • (PR#6288) Doc: Add russbiggs as doc contributor
  • (PR#6278) Performance: Use Math.imul and shifts in perf-critical paths
  • (PR#6275) Refactor: Remove unnecessary npm install steps from publish workflows
  • (PR#6311) Refactor: Factorize letrec implementations
  • (PR#6318) Refactor: Extract logic building lazy arbs
  • (PR#6320) Refactor: Iterate on own props array in letrec
  • (PR#6321) Refactor: Explicit null check in LazyArbitrary
  • (PR#6277) Script: Fix script updating the documentation for fast-check