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What's new in fast-check 4.7.0?

Β· 5 min read
Nicolas Dubien
Nicolas Dubien
fast-check maintainer

Until now, Unicode property escapes of the form \p{...} or \P{...} were not implemented in stringMatching. This release adds support for them so stringMatching can generate matching values directly.

This release also makes fc.json() reversible, allowing arbitrary JSON strings to be shrunk even when they did not originate from fast-check itself.

Continue reading to explore the detailed updates it brings.

Unicode properties in stringMatching​

Regular expressions can restrict their character set with Unicode property escapes such as \p{Emoji} or \P{ASCII}. Before 4.7.0, feeding such a regex to stringMatching threw a "not implemented yet" error.

Starting with 4.7.0, stringMatching recognises both \p{...} (positive) and \P{...} (negated) forms:

// Strings made of emoji characters
// e.g. "πŸ¦ŽπŸ‘­πŸ‘‡πŸš€πŸ•", "πŸ»πŸ˜πŸ§€πŸͺ πŸ•Ή"
fc.stringMatching(/^\p{Emoji}+$/u);

// One uppercase letter followed by one or more lowercase letters
// e.g. "Gα»₯", "F𞀻applyg"
fc.stringMatching(/^\p{Uppercase_Letter}\p{Lowercase_Letter}+$/u);

// Strings made only of non-ASCII characters
// e.g. "𸉂τΏΌ", "πΏ˜π–ˆ"
fc.stringMatching(/^\P{ASCII}+$/u);

Under the hood, each property is expanded into the set of codepoints it covers and plugged into the existing machinery backing character classes. The resulting ranges are computed once and cached, so subsequent uses of the same property stay cheap.

Support covers binary properties like Emoji, Alphabetic or Math, general categories such as Letter, Decimal_Number or Punctuation, plus scripts through Script=Greek, sc=Han and friends. This should let you keep your regex expressive when testing code that handles internationalised inputs.

Reversible json arbitrary​

The json arbitrary is built by serialising the output of jsonValue through JSON.stringify. Until this release, that mapping was one-way: fast-check could produce JSON strings but could not recognise externally-provided ones. As a consequence, scenarios relying on replaying or recomposing values (for instance shrinking a payload captured from production logs) could not leverage fc.json().

Starting with 4.7.0, the arbitrary carries an unmapper based on JSON.parse. Any valid JSON string is now accepted and shrinkable:

const arb = fc.json();
arb.canShrinkWithoutContext('{"a":{"b":[1,2,3]}}'); // now true

This unlocks composition with other arbitraries that expect a reversible building block and makes fc.json() a drop-in candidate wherever you need to replay a JSON input without re-generating it from scratch.

Changelog since 4.6.0​

The version 4.7.0 is based on version 4.6.0.

Features​

  • (PR#6866) Reversible json arbitrary
  • (PR#6868) Parse \p{} and \P{} in stringMatching
  • (PR#6870) Support for \p{UnicodeProperty} in stringMatching
  • (PR#6871) Support negated unicode properties in stringMatching

Fixes​

  • (PR#6710) CI: Pass explicit string to make_latest
  • (PR#6714) CI: Remove unused vite dependency from multiple packages
  • (PR#6780) CI: Silent zizmor issues (as they used to be)
  • (PR#6786) CI: Configure release workflow settings for announcements
  • (PR#6787) CI: Add force-build-status-execution label trigger to CI workflow
  • (PR#6818) CI: Push tag after creating draft release
  • (PR#6827) CI: Update CSP for our playgrounds backed by stackblitz
  • (PR#6832) CI: Add format/lint/typecheck hooks for Claude Code
  • (PR#6834) CI: Fix Claude's session start hook
  • (PR#6852) CI: Skip website prebuild remote fetches on cloud Claude Code
  • (PR#6869) CI: Add workflow to clean up GitHub Actions caches
  • (PR#6789) Clean: Remove unused code identified by knip
  • (PR#6711) Doc: Release note for version 4.6.0
  • (PR#6756) Doc: Fix typo in the documentation
  • (PR#6758) Doc: Add rugk as doc contributor
  • (PR#6764) Doc: Document gitmoji PR naming
  • (PR#6776) Doc: Add nielk as code contributor
  • (PR#6753) Doc: Migrate playgrounds in documentation to StackBlitz
  • (PR#6830) Doc: Switch to ?raw imports for advents
  • (PR#6836) Doc: Add Vitest documentation guide for setting up property-based testing
  • (PR#6833) Doc: Remove dead doc hub pages
  • (PR#6855) Doc: Integrate API reference natively into our doc
  • (PR#6867) Doc: Simplify examples
  • (PR#6835) Script: Migrate from ESLint to oxlint
  • (PR#6872) Script: Rework hooks for Claude Code
  • (PR#6754) Test: Migrate race condition tests to Vitest
  • (PR#6859) Test: Stabilize flaky timeout tests on Windows