Property-based testing in JavaScript

Property-based testing in JavaScriptJavaScript is a very powerful and flexible language. But javaScripts flexibility comes at a cost. You will often end up with problems like adding strings to numbers when doing calculations or similar strange behaviors. I think every experienced developer has its own anecdotes. (Mine could fill another article. Leave me an eMail if I should write it! :) ) This sometimes strange behavior makes it even more important to have strong tests and find those situations. Property-based testing is a testing method with the ability to do that with its randomized test data generation you\`re chances are high to find such errors.
Categoriestestingproperty-based testingjavaScriptfastCheck

Selection of the right JavaScript testing Libary

For JavaScript, there exist several Property-based testing Libraries, all of them are following Quick-Check [6] which was implemented in Haskell. Some of them to mentions are JsVerify [12], fast-check [8], and test check [4]. Another option would be using scala.js and ScalaCheck, but for this approach, you have to import the SUT as a Js. Object into an external scala project and there is no jest support, so the test program must be run by hand every time, and there is no native support for Vue.js which makes UI testing very hard. Also, JsVerifyhas no out of the box jest support, according to its documentation[12] they just have support for Mocca and jasmine but not for Jest, and since the last commit was in 2018 and according to this article [14] by default there are just very small integers created and not normally distributed values I will not consider using it. In Table 1 fast check test check and scalaCheck are compared about their features and how they will match the needs of the project. TestCheck and FactCheck.js are supported by Jest out of the box. Both of them provide nearly the same functionality of quick-Check and are very well documented. TestCheck is a little bit easier to understand because they use the same naming conventions quickCheck,scalaCheck or FsCheck are using, fastCheck.js is still being maintained since there are still commits on the Github Repository. TestChecks last commit was in 2018, but still, have some issues with test data generation [14].

feature fast-check testcheck scalaCheck
combine Properties no no
Generators ✔ but different syntax ✔ but issues with objet Gen
Arbitaries no
Model Based Testing no
Jest support no
CI integrable no
Stars on GitHub 1.5k 1.1k 1.6k
Last GitHub Commit (checked on 14.07.2020) 14.07.2020 26.03.2018 07.07.2020

Based on the findings from above I have chosen fast-check.js for testing my JavaScript code.

Setting up fast-check

First, the fast check Package needs to be installed, this is done by running the following command

npm  install  --save -dev fast-check

Then to use fast check import it like following into your test files.

 import * as fc from "fast -check";

Now everything is ready to use.

Getting started with fast check

To get started I want to describe a simple example from the fast check documentation.

const contains = (text, pattern) => text.indexOf(pattern) >= 0;

// Properties
describe('properties', () => {
	// string text always contains itself
	it('should always contain itself', () => {
		fc.assert(, text => contains(text, text)));
	// string a + b + c always contains b, whatever the values of a, b and c
	it('should always contain its substrings', () => {
		fc.assert(, fc.string(), fc.string(), (a,b,c) => {
			// Alternatively: no return statement and direct usage of expect or assert
			return contains(a+b+c, b);

Describe in the above example defines a test Class for Jest and it(... defines the test to run.

fc.assert runs the property-based test as the first parameter it needs the fc.propery for the test and as the second parameter, it takes the actual code to run. The number of executions could be passed as a parameter, 100 by default.

fc.string is an example of a built-in Arbitrary, it generates a random string, this string could be further parameterized like specifying a maximal and minimal length or the string encoding.

These arbitraries exist for every simple Datatype.

If the result is true the test will succeed but if one of the runs evaluates to false the test fails and the output will be shrunk to the failing test case.


When it comes to testing, unit testing is like asking for the time, property-based testing is having a watch and no testing is like being somewhere alone without a clock. You can roughly estimate the time but no one knows if the estimation was right.

Property-based testing is also a very effective way to get strong test data since you do not have to write it on your own. Think about the time it takes to provide a unit test with all the data it needs. With property-based testing, you just provide the structure of your data and the frameworks will handle the rest, hundreds of tests with different data will be executed and you will get confidence that your code works for any case.

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