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testland/fast-check-testing

Use whenever the task mentions property-based, generative or invariant tests in JavaScript or TypeScript (fast-check): fc.assert and fc.property, arbitraries and composite inputs, map, chain and filter, shrinking and seeds, with Jest, Vitest or Mocha. Has a worked example to start from.

0.96x

Quality

95%

Does it follow best practices?

Impact

94%

0.96x

Average score across 3 eval scenarios

Securityby

Passed

No findings from the security scan

fast-check stateful and async testing

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.