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What's new in fast-check 3.18.0?

· 3 min read
Nicolas Dubien
fast-check maintainer

This release introduces some new opt-in options on floating point number arbitraries such as float and double. They offer simpler ways to only produce non-integer numerical values.

Continue reading to explore the detailed updates it brings.

The new noInteger option

Floating point number arbitraries are responsible to generate any possible floating value. It goes from normal integers, to decimals but also includes special values like: -0, NaN, -∞ and +∞. Until now we offered our users built-in ways not to produce some specific values with options like noNaN and noDefaultInfinity but nothing for wider sets of values.

With this release we go further and allow users to choose not to generate a wider range of values. We introduced the flag noInteger to allow users to drop any integer value from the set of possible values. Before this release, it used to be done in a more manual way on the user's side:

fc.double().filter((value) => !Number.isInteger(value)); // Before version 3.18.0

Since version 3.18.0, users can rely on the shorter and more optimized construct:

fc.double({ noInteger: true }); // Starting at version 3.18.0

Better web-paths by default

According to the RFC-3986 a web-path could possibly start by //. Unfortunately with default options, it used to never be produced by webPath:

fc.statistics(fc.webPath(), (p) => (p.startsWith('//') ? 'with //' : 'without //'), { numRuns: 100_000 });
// without //..100.00%

The issue came from the fact that internally our generator for paths was configured in a way that prevented such paths from being built when using the default size. With the default setup, generating too many segments for a path was not feasible, thus we never obtained enough consecutive segments to construct a //.

fc.statistics(fc.webPath({ size: '+1' }), (p) => (p.startsWith('//') ? 'with //' : 'without //'), { numRuns: 100_000 });
// without //..92.67%
// with //......7.33%

Starting at 3.18.0, the default configuration for paths will be able to produce paths starting by // without having to tweak the size:

fc.statistics(fc.webPath(), (p) => (p.startsWith('//') ? 'with //' : 'without //'), { numRuns: 100_000 });
// without //..79.75%
// with //.....20.25%

Changelog since 3.17.0

The version 3.18.0 is based on version 3.17.2, but let see what's changed since 3.17.0 itself.


  • (PR#4917) Add option to produce non-integer on double
  • (PR#4923) Add option to produce non-integer on float
  • (PR#4935) Produce "//" in web paths


  • (PR#4842) Bug: Fix dual-packages hazard and types incompatibility
  • (PR#4853) CI: Build doc with full git history
  • (PR#4872) CI: Stop caching Jest on CI
  • (PR#4924) CI: Enable more advanced TS flags
  • (PR#4925) CI: Explicitly test against Node 22
  • (PR#4926) CI: Stabilize tests of double on small ranges
  • (PR#4836) Doc: Release note for 3.17.0
  • (PR#4844) Doc: Add new contributor patroza
  • (PR#4852) Doc: Show last update time on doc
  • (PR#4851) Doc: Add last modified date to sitemap
  • (PR#4868) Doc: Enhance SEO for homepage
  • (PR#4888) Doc: Add tutorial for PBT with Jest
  • (PR#4901) Doc: Use official doc for npm homepage
  • (PR#4921) Performance: More optimal noInteger on double
  • (PR#4933) Script: Switch on more eslint rules
  • (PR#4866) Test: Safer rewrite of Poisoning E2E
  • (PR#4871) Test: Move tests to Vitest
  • (PR#4863) Test: Explicitely import from Vitest
  • (PR#4873) Test: Move to v8 for coverage
  • (PR#4875) Test: Better mock/spy cleaning
  • (PR#4922) Test: Cover noInteger on double via integration layers