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.

Quality

97%

Does it follow best practices?

Impact

—

Average score across 3 eval scenarios

Securityby

Passed

No findings from the security scan

fast-check stateful and async testing

Name: testland/fast-check-testing
Rating: 97 (1 reviews)
Author: testland

Advanced fast-check for concurrent and stateful systems.

Race condition detection

Per fast-check-overview: "Race condition detection for async code."

import { test } from 'vitest';
import fc from 'fast-check';

test('concurrent counter increments are atomic', async () => {
  await fc.assert(
    fc.asyncProperty(fc.scheduler(), async (s) => {
      const counter = new AsyncCounter();
      const tasks = [
        s.schedule(counter.increment()),
        s.schedule(counter.increment()),
        s.schedule(counter.increment()),
      ];
      await s.waitAll();
      await Promise.all(tasks);
      expect(counter.value).toBe(3);
    })
  );
});

fc.scheduler exhaustively explores task interleavings; s.schedule queues an async operation; s.waitAll() advances. fast-check finds interleavings that cause the property to fail - the canonical race-condition catcher.

Model-based testing

Per fast-check-overview: "Model-based testing for stateful systems."

class CounterModel {
  count = 0;
  increment() { this.count++; }
  decrement() { this.count--; }
}

const allCommands = [
  fc.constant({ run: (c, real) => { c.increment(); real.increment(); expect(real.value).toBe(c.count); } }),
  fc.constant({ run: (c, real) => { c.decrement(); real.decrement(); expect(real.value).toBe(c.count); } }),
];

it('counter behaves per model', () => {
  fc.assert(
    fc.property(fc.commands(allCommands), (cmds) => {
      const model = new CounterModel();
      const real = new RealCounter();
      fc.modelRun(() => ({ model, real }), cmds);
    })
  );
});

fast-check generates random sequences of commands; the model stays in sync with the real implementation; any divergence is a bug in the real implementation.