jest-fast-check

Property based testing for Jest based on fast-check

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

Getting started

Install jest-fast-check and its peer dependencies:

# With yarn
yarn add -D jest fast-check jest-fast-check

# With npm
npm install --save-dev jest fast-check jest-fast-check

Example

import { testProp, fc } from "jest-fast-check";

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

Please note that the properties accepted by jest-fast-check as input can either be synchronous or asynchronous (even just PromiseLike instances).

Advanced

If you want to forward custom parameters to fast-check, testProp accepts an optional fc.Parameters (more).

jest-fast-check also comes with .only, .skip and .todo from jest.

import { itProp, testProp, fc } from "jest-fast-check";

testProp(
  "should replay the test for the seed 4242",
  [fc.nat(), fc.nat()],
  (a, b) => {
    return a + b === b + a;
  },
  { seed: 4242 }
);

testProp.skip("should be skipped", [fc.fullUnicodeString()], (text) => {
  return text.length === [...text].length;
});

describe("with it", () => {
  itProp("should run too", [fc.nat(), fc.nat()], (a, b) => {
    return a + b === b + a;
  });
});

Minimal requirements

jest-fast-check	jest	fast-check
^2.0.0	>=26.5.0(1)(2)	^3.0.0
^1.0.0	>=26.5.0(1)(2)	^2.0.0(3)

• (1) any version of jest should be great if you are using commonjs
• (2) in order to use esm build, you may need to enable experimental features of node, see here
• (3) fast-check@^2.0.0 for hybrid module support: commonjs and esm together