Property Based Testing with Node.js test runner

Officially made stable with Node.js v20, there is now an official test runner in Node.js since 2023. As with any other test runner, fast-check can be integrated with just a few steps.

The following tutorial provides some pointers and tricks to achieve this integration smoothly.

Basic setup

Add fast-check to your set of dependencies:

npm

npm install --save-dev fast-check

Yarn

yarn add --dev fast-check

pnpm

pnpm add --save-dev fast-check

Congratulations, everything is ready to start using Property-Based Tests with the Node.js test runner 🚀

Your first test

For our first test, we will test an algorithm computing a value of the Fibonacci sequence. Our property-based test will assess that our implementation respects the rule: fibo(n) = fibo(n-1) + fibo(n-2).

fibo.test.js

const assert = require('node:assert');
const { test } = require('node:test');
const fc = require('fast-check');

test('should follow the rule: fibo(n) = fibo(n-1) + fibo(n-2)', () => {
  fc.assert(
    fc.property(fc.integer({ min: 2, max: 78 }), (n) => {
      assert.equal(fibo(n), fibo(n - 1) + fibo(n - 2));
    }),
  );
});

// Code under test: should rather be imported from another file
function fibo(n) {
  // Note: n must be in the range 0 (included) to 78 (included)
  let a = 0;
  if (n === 0) {
    return a;
  }
  let b = 1;
  for (let i = 1; i !== n; ++i) {
    const c = a + b;
    a = b;
    b = c;
  }
  return b;
}

You can now execute it by running the following command in a terminal:

node --test

You've written and executed your first Property-Based Test using the Node.js test runner 🚀

Going further

The Fibonacci sequence isn't just about fibo(n) = fibo(n-1) + fibo(n-2). Tests can be extended much further with additional properties to confirm the implementation. You can read this article to explore more properties for it.

There are many advanced techniques to manage fast-check like a pro. Whether you want to start experimenting with asynchronous properties, detecting race conditions, building complex values, or more, our documentation is the best place to start to discover all these details and subtleties.