@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!

Extra setup for @fast-check/worker 0.6.0 and later

Since @fast-check/worker is an ESM-only package, running it under Jest in CommonJS mode requires babel-jest to transpile it. Install the following dependencies:

npm install --save-dev babel-jest @babel/core @babel/preset-env

Then add a babel.config.cjs at the root of your project:

module.exports = {
  presets: [['@babel/preset-env', { targets: { node: 'current' }, modules: 'commonjs' }]],
};

And update your Jest configuration to use babel-jest while excluding @fast-check/worker from the ignore patterns:

module.exports = {
  transform: {
    '^.+\\.[t|j]sx?$': 'babel-jest',
  },
  transformIgnorePatterns: ['/node_modules/(?!(?:@fast-check/worker)/)'],
};

This ensures @fast-check/worker gets transpiled to CommonJS by Babel instead of being skipped. Without it, Jest will fail to load the ESM-only package.

For ES Modules mode, no extra setup is needed — just make sure to run Jest with --experimental-vm-modules:

node --experimental-vm-modules node_modules/jest/bin/jest.js

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