fast-check
Advanced tools
fast-check is a property-based testing library for JavaScript and TypeScript. It allows developers to define properties that should hold true for a wide range of inputs, and then automatically generates test cases to verify those properties. This helps in identifying edge cases and ensuring the robustness of the code.
Property-based Testing
This feature allows you to define properties that should hold true for a wide range of inputs. In this example, the property being tested is the commutativity of addition.
const fc = require('fast-check');
fc.assert(
fc.property(fc.integer(), fc.integer(), (a, b) => {
return a + b === b + a;
})
);Custom Arbitraries
Custom arbitraries allow you to define complex data structures for your tests. In this example, a custom arbitrary is created that generates objects with a number and a string.
const fc = require('fast-check');
const myArbitrary = fc.tuple(fc.integer(), fc.string()).map(([num, str]) => ({ num, str }));
fc.assert(
fc.property(myArbitrary, ({ num, str }) => {
return typeof num === 'number' && typeof str === 'string';
})
);Shrinkable Values
Shrinkable values help in minimizing the size of failing test cases to make debugging easier. In this example, if the property fails, fast-check will try to find the smallest array that causes the failure.
const fc = require('fast-check');
fc.assert(
fc.property(fc.array(fc.integer()), (arr) => {
return arr.length < 100;
}),
{ verbose: true }
);jsverify is another property-based testing library for JavaScript. It offers similar functionality to fast-check, such as defining properties and generating test cases. However, fast-check is generally considered to have a more modern API and better TypeScript support.
testcheck is a property-based testing library inspired by QuickCheck. It provides similar capabilities for generating test cases and defining properties. Compared to fast-check, testcheck is less actively maintained and has fewer features.
Hypothesis is a property-based testing library for Python, but it has inspired several JavaScript libraries, including fast-check. While not a direct competitor, it offers similar concepts and is often used as a reference for property-based testing.
Property based testing framework for JavaScript/TypeScript
Hands-on tutorial and definition of Property Based Testing: 🏁 see tutorial. Or directly try it online on our pre-configured CodeSandbox.
Property based testing frameworks check the truthfulness of properties. A property is a statement like: for all (x, y, ...) such that precondition(x, y, ...) holds predicate(x, y, ...) is true.
Install the module with: yarn add fast-check --dev or npm install fast-check --save-dev
Example of integration in mocha:
import fc from 'fast-check';
// Code under test
const contains = (text, pattern) => text.indexOf(pattern) >= 0;
// Properties
describe('properties', () => {
// string text always contains itself
it('should always contain itself', () => {
fc.assert(fc.property(fc.string(), (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.property(fc.string(), fc.string(), fc.string(), (a, b, c) => {
// Alternatively: no return statement and direct usage of expect or assert
return contains(a + b + c, b);
}),
);
});
});
In case of failure, the test raises a red flag. Its output should help you to diagnose what went wrong in your implementation. Example with a failing implementation of contain:
1) should always contain its substrings
Error: Property failed after 1 tests (seed: 1527422598337, path: 0:0): ["","",""]
Shrunk 1 time(s)
Got error: Property failed by returning false
Hint: Enable verbose mode in order to have the list of all failing values encountered during the run
Integration with other test frameworks: ava, jasmine, jest, mocha and tape.
More examples: simple examples, fuzzing and against various algorithms.
Useful documentations:
fast-check has initially been designed in an attempt to cope with limitations I encountered while using other property based testing frameworks designed for JavaScript:
map method to derive existing arbitraries while keeping shrink [more] - some frameworks ask the user to provide both a->b and b->a mappings in order to keep a shrinkerchain [more] - able to bind the output of an arbitrary as input of another one while keeping the shrink workingfc.pre(...) [more] - filtering invalid entries can be done directly inside the check function if neededfc.gen() [more] - generate random values within your predicatesfc.oneof [more] - surprisingly some frameworks don'tFor more details, refer to the documentation in the links above.
fast-check has been trusted for years by big projects like: jest, jasmine, fp-ts, io-ts, ramda, js-yaml, query-string...
It also proved useful in finding bugs among major open source projects such as jest, query-string... and many others.
Here are the minimal requirements to use fast-check properly without any polyfills:
| fast-check | node | ECMAScript version | TypeScript (optional) |
|---|---|---|---|
| 3.x | ≥8(1) | ES2017 | ≥4.1(2) |
| 2.x | ≥8(1) | ES2017 | ≥3.2(3) |
| 1.x | ≥0.12(1) | ES3 | ≥3.0(3) |
bigint-related ones - all the capabilities of fast-check should be usable given you use at least the minimal recommended version of node associated to your major of fast-check.@types/node to be installed.@types/node to be installed.Bindings to use fast-check in ReScript are available in package rescript-fast-check. They are maintained by @TheSpyder as an external project.
Thanks goes to these wonderful people (emoji key):
This project follows the all-contributors specification. Contributions of any kind welcome! Become one of them
Many individuals and companies offer their financial support to the project, a huge thanks to all of them too 💓
You can also become one of them by contributing via GitHub Sponsors or OpenCollective.
FAQs
Property based testing framework for JavaScript (like QuickCheck)
The npm package fast-check receives a total of 1,208,846 weekly downloads. As such, fast-check popularity was classified as popular.
We found that fast-check demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 1 open source maintainer collaborating on the project.
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