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douglascrockford/JSCheck
A random property testing tool for JavaScript
A minimal QuickCheck-style property testing library for JavaScript, written by Douglas Crockford. Given a spec of generators and a predicate function, it generates random inputs and checks the property holds, useful for testing functions with wide input domains rather than hand-picked examples.
Genuinely tiny footprint - the whole thing is one file, no build step, no dependencies. The generator composition model (integer, string, sequence, one_of, etc. as composable claim functions) is a clean, faithful port of the QuickCheck idea into JS. Public domain license means zero friction to vendor the source directly into a project.
Last meaningful activity is years stale and there's no shrinking of failing cases - when it finds a counterexample you get the raw random input, not a minimal reproduction, which is the single most useful feature of QuickCheck-style tools. No TypeScript types, no npm-idiomatic API (it predates ESM/CommonJS conventions), and no integration story for Jest/Vitest - you're on your own wiring it into a modern test runner. Documentation lives on Crockford's personal site rather than in-repo, so it's easy to end up out of sync with the actual code.