Fast Check

Fast-Check Property-Based Testing

Verify programmatic properties and logic invariants by generating randomized inputs and automatically shrinking failing assertions to their minimal reproducing case.

When to Use

• Use when designing property-based tests, boundary verification suites, and fuzz testing parameters for parsers, calculators, algorithms, codecs, or critical business domains.
• Use when you need to verify that an invariant holds for all valid inputs (e.g., sort preserves length, encode/decode roundtrips, discount never produces negative pricing).
• Use when you want automated shrinking to reduce a failing case to its minimal reproduction.
• Route elsewhere (e.g., standard Vitest/Jest unit tests) when validating fixed API responses, rendering components, or checking exact integration scenarios.

Prerequisites

• Node.js installed and available on PATH.
• A JavaScript or TypeScript project with a test runner (Vitest recommended).
• Windows host is primary. Use PowerShell for all shell commands. Adjust path separators if running on macOS/Linux.

Procedure

1. Install fast-check and runner integration

npm install -D fast-check @fast-check/vitest

For Jest-based projects, install @fast-check/jest instead:

npm install -D fast-check @fast-check/jest

2. Write a basic invariant test

Assert a property that must hold for all generated inputs. Example: sorting preserves array length and produces ordered elements.

import fc from "fast-check";
import { describe, it, expect } from "vitest";

describe("sorting algorithm logic", () => {
  it("maintains the original array length after sorting", () => {
    fc.assert(
      fc.property(fc.array(fc.integer()), (arr) => {
        const sorted = [...arr].sort((a, b) => a - b);
        return sorted.length === arr.length;
      })
    );
  });

  it("produces sequentially ordered elements", () => {
    fc.assert(
      fc.property(fc.array(fc.integer()), (arr) => {
        const sorted = [...arr].sort((a, b) => a - b);
        for (let i = 1; i < sorted.length; i++) {
          expect(sorted[i]).toBeGreaterThanOrEqual(sorted[i - 1]);
        }
      })
    );
  });
});

3. Write a roundtrip verification test

Test codec, serialization, and translation functions by executing encode-decode and comparing against the original input.

import fc from "fast-check";
import { encode, decode } from "./codec";

it("ensures decode(encode(val)) strictly equals val for all strings", () => {
  fc.assert(
    fc.property(fc.string(), (original) => {
      const encoded = encode(original);
      const decoded = decode(encoded);
      expect(decoded).toEqual(original);
    })
  );
});

it("ensures JSON serialization preserves complex objects", () => {
  fc.assert(
    fc.property(fc.jsonValue(), (value) => {
      const json = JSON.stringify(value);
      const parsed = JSON.parse(json);
      expect(parsed).toEqual(value);
    })
  );
});

4. Build custom arbitrary generators

Construct complex domain objects by combining base arbitraries with fc.record.

import fc from "fast-check";

const userArbitrary = fc.record({
  id: fc.uuid(),
  name: fc.string({ minLength: 1, maxLength: 100 }),
  email: fc.emailAddress(),
  age: fc.integer({ min: 13, max: 120 }),
  role: fc.constantFrom("admin", "user", "viewer"),
  createdAt: fc.date({ min: new Date("2020-01-01"), max: new Date() }),
});

const moneyArbitrary = fc.record({
  amount: fc.integer({ min: 0, max: 1_000_000 }), // represented in cents
  currency: fc.constantFrom("USD", "EUR", "GBP"),
});

it("ensures coupon discount calculations never produce negative pricing", () => {
  fc.assert(
    fc.property(
      moneyArbitrary,
      fc.integer({ min: 0, max: 100 }),
      (price, discountPercent) => {
        const discounted = applyDiscount(price, discountPercent);
        expect(discounted.amount).toBeGreaterThanOrEqual(0);
      }
    )
  );
});

5. Use preconditions to filter invalid inputs

Filter out input combinations that violate initial system prerequisites using fc.pre.

import fc from "fast-check";

it("calculates correct ratios for non-zero denominators", () => {
  fc.assert(
    fc.property(fc.integer(), fc.integer(), (numerator, denominator) => {
      fc.pre(denominator !== 0); // skip zero denominators
      const ratio = numerator / denominator;
      expect(Number.isFinite(ratio)).toBe(true);
    })
  );
});

6. Configure run limits

Scale testing size with numRuns. Default is 100 runs per assertion. Increase to 1,000 or 10,000 for critical pathways.

fc.assert(fc.property(fc.string(), (val) => { /* ... */ }), { numRuns: 1000 });

7. Reproduce failures using seeds

On failure, fast-check logs the exact seed and path (e.g., seed: 12345678, path: "0:2:1"). Feed these back into options to debug the minimal reproduction.

fc.assert(fc.property(/* ... */), { seed: 12345678, path: "0:2:1" });

8. Run the tests

npx vitest run

Pitfalls

• Excessive fc.pre filtering: Fast-check errors out with "too many runs skipped" when precondition rejection rates are high. Instead of fc.pre(val !== 0), generate values using fc.integer().map(x => x === 0 ? 1 : x) or restrict bounds directly with fc.integer({ min: 1 }).
• Test timeouts with slow functions: Large datasets or slow synchronous functions can cause runner timeouts. Adjust { numRuns: N } down or run CPU profile checks to identify performance bottlenecks.
• Unconstrained large objects: Large generated objects can cause out-of-memory overhead. Constrain sizes with minLength, maxLength, min, and max options.
• CommonJS vs ESM import paths: In legacy Node.js CommonJS setups, ensure correct import paths when migrating to pure ES modules. Use fc.assert explicitly if integration-level runner extensions are absent.
• Thinking in examples instead of properties: Property-based tests assert behavior that remains invariant under any input (e.g., x + y === y + x), not exact outputs for specific inputs. Combine property suites with edge-case example tests—use examples to pin known boundaries and properties to surface unknown gaps.
• Fuzzing vs property testing confusion: Use property testing to assert strict output constraints (expect). Use fuzzing to verify that input parsing does not crash the Node/V8 process.

Verification

• Confirm the package is installed:

npm ls fast-check @fast-check/vitest

• Run the test suite and confirm all property tests pass:

npx vitest run

• On failure, confirm fast-check logs the counterexample values, original seed, path index, and failure trace. Copy the seed and path into the assertion options to reproduce deterministically.

• Quality checklist:

  • Seeds logged on failure are added to a regression unit test.
  • No fc.pre calls are used where direct constraint configuration (e.g., fc.integer({ min: 1 })) is possible.
  • Large objects are constrained to reasonable sizes to prevent out-of-memory overhead during generation.

Related Skills

• vitest — Standard unit and integration test runner; pair with fast-check for combined example and property suites.
• jest — Alternative test runner; use @fast-check/jest integration package.

Source Anchors

• Official Fast-Check Site
• Fast-Check GitHub Repository

Changelog

• 2026-05-30: Modernized skill documentation, updated setup steps for Vitest runner integration (@fast-check/vitest), converted all instructional paragraphs to imperative style, and compiled to Developer-Tools.