@fast-check/jest

What is @fast-check/jest?

@fast-check/jest is a package that integrates fast-check, a property-based testing library, with Jest, a popular JavaScript testing framework. It allows developers to write tests that automatically generate a wide range of inputs to ensure code behaves correctly under various conditions.

What are @fast-check/jest's main functionalities?

Property-based Testing

This feature allows you to define properties that your code should satisfy for a wide range of automatically generated inputs. The code sample demonstrates a simple property-based test that checks if the absolute value of any integer is always non-negative.

const { testProp, fc } = require('@fast-check/jest');

testProp('should always return a positive number', [fc.integer()], (n) => {
  expect(Math.abs(n)).toBeGreaterThanOrEqual(0);
});

Custom Arbitraries

This feature allows you to create custom arbitraries, which are generators for your test inputs. The code sample shows how to create a custom arbitrary that generates tuples of integers and strings, and then tests a property using these tuples.

const { testProp, fc } = require('@fast-check/jest');

const customArbitrary = fc.tuple(fc.integer(), fc.string());

testProp('should handle custom arbitrary', [customArbitrary], ([num, str]) => {
  expect(typeof num).toBe('number');
  expect(typeof str).toBe('string');
});

Other packages similar to @fast-check/jest

jest-check

jest-check is another package that integrates property-based testing with Jest. It is similar to @fast-check/jest in that it allows for property-based testing within Jest, but it is less popular and not as actively maintained as fast-check.

testcheck

testcheck is a property-based testing library inspired by QuickCheck from Haskell. It can be used with Jest, but it requires more setup compared to @fast-check/jest, which is specifically designed to integrate seamlessly with Jest.

README

@fast-check/jest

Bring the power of property based testing framework fast-check into Jest. @fast-check/jest simplifies the integration of fast-check into Jest testing framework.

Getting Started

Install @fast-check/jest:

npm install --save-dev @fast-check/jest

In order to work properly, @fast-check/jest requires jest to be installed.

We also highly recommend users to launch their tests using the --show-seed option provided by Jest. It ensures Jest will always print the seed by itself (requires Jest ≥29.2.0).

jest --show-seed

Example

import { test, fc } from '@fast-check/jest';

// for all a, b, c strings
// b is a substring of a + b + c
test.prop([fc.string(), fc.string(), fc.string()])('should detect the substring', (a, b, c) => {
  return (a + b + c).includes(b);
});

// Or the exact same test but based on named parameters
test.prop({ a: fc.string(), b: fc.string(), c: fc.string() })('should detect the substring', ({ a, b, c }) => {
  return (a + b + c).includes(b);
});

The it and test functions returned by @fast-check/jest are just enriched versions of the ones coming from jest itself. They both come with .prop.

Please note that the properties accepted by @fast-check/jest as input can either be synchronous or asynchronous (even just PromiseLike instances). In other words, the predicate passed as the last argument can be asynchronous.

Remark: it and test have been introduced in 1.4.0. You have to refer to Deprecated API if you are using a version of @fast-check/jest <1.4.0.

Advanced

Support for variations of test and it

If you want to forward custom parameters to fast-check, test.prop and its variants accept an optional fc.Parameters (more).

@fast-check/jest also comes with support for .only, .skip, .todo and .concurrent from jest. It also accepts more complex ones such as .concurrent.failing or .concurrent.only.failing.

import { it, test, fc } from '@fast-check/jest';

// With custom `fc.Parameters`, here { seed: 4242 }
test.prop([fc.nat(), fc.nat()], { seed: 4242 })('should replay the test for the seed 4242', (a, b) => {
  return a + b === b + a;
});

// With .skip
test.skip.prop([fc.string()])('should be skipped', (text) => {
  return text.length === [...text].length;
});

// With it version
describe('with it', () => {
  it.prop([fc.nat(), fc.nat()])('should run too', (a, b) => {
    return a + b === b + a;
  });
});

Experimental worker-based runner

The following feature is experimental! When used it makes runners able to kill long running synchonous code. Meaning that it will make fast-check able to kill infinite loops blocking the main thread. So far, the feature does not fully support transformations performed via transform steps defined with jest.

The CommonJS approach would be:

const { init, fc } = require('@fast-check/jest/worker');
const { pathToFileURL } = require('node:url');

const { test, expect } = init(pathToFileURL(__filename));
// can also be passed options such as isolationLevel: init(pathToFileURL(__filename), {})

test.prop([fc.constant(null)])('should pass', (value) => {
  expect(value).toBe(null);
});

The ES Modules approach would be:

import { init, fc } from '@fast-check/jest/worker';

const { test, expect } = await init(new URL(import.meta.url));
// can also be passed options such as isolationLevel: init(new URL(import.meta.url), {})

test.prop([fc.constant(null)])('should pass', (value) => {
  expect(value).toBe(null);
});

⚠️ Do not forget to add the await before init for the ES Module version!

Minimal requirements

@fast-check/jest	jest	fast-check	node
^2.0.0	>=26.5.0(1)(2)	^3.0.0	>=14.15.0(3) and <18, >=18.17.0 and <19(4), >=20
^1.0.0	>=26.5.0(1)(2)	^3.0.0	>=14.15.0(3) and <18, >=18.17.0 and <19(4), >=20

• (1) any version of jest should be greater or equal than 26.5.0 if you are using commonjs
• (2) in order to use esm build, you may need to enable experimental features of node, see here
• (3) >=14.15.0 is the minimal requirements for jest, >=12.17.0 is the one for @fast-check/jest
• (4) timeout defined on jest might not be properly applied to fast-check for node 18 (until 18.17.0) and node 19, see #4004