testland/fast-check-testing
Authors property-based tests in JavaScript / TypeScript using fast-check - wires `fc.assert(fc.property(arbitrary, ...))`, picks arbitraries (`fc.integer`, `fc.string`, `fc.array`, `fc.tuple`, `fc.record`), uses `.map()` / `.chain()` / `.filter()` to build domain arbitraries, and integrates with Jest / Vitest / Mocha / Jasmine / AVA / Tape. Use when a JS/TS codebase needs PBT to catch edge cases - fast-check has been used to find bugs in major libraries (`query-string`, etc.) and is trusted by Jest, Jasmine, fp-ts, Ramda.
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shrinking-and-reproducibility.mdreferences/
fast-check shrinking and reproducibility
When a property fails, fast-check prints the falsifying input + a shrunk minimal version + a seed:
Property failed after 47 tests
{ seed: 1234567890, path: "12:1:0", endOnFailure: true }
Counterexample: [{"id": "abc", "age": -1}]
Shrunk 8 time(s)
Got error: Expected age to be >= 18, got -1To reproduce, replay with the seed:
fc.assert(
fc.property(...),
{ seed: 1234567890, path: "12:1:0", endOnFailure: true }
);The seed/path is the deterministic recipe to re-derive the failure.
For CI, set a fixed seed:
import fc from 'fast-check';
fc.configureGlobal({ seed: process.env.CI ? 42 : Date.now() });