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fast-check
Advanced tools
Property based testing framework for JavaScript/TypeScript
Getting started
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: pnpm add -D fast-check or 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:
- 🏁 Introduction to Property Based & Hands On
- 🐣 Built-in arbitraries
- 🔧 Custom arbitraries
- 🏃♂️ Property based runners
- 💥 Tips
- 🔌 API Reference
- ⭐ Awesome fast-check
Why should I migrate to fast-check?
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:
- Types: strong and up-to-date types - thanks to TypeScript
- Extendable: easy
mapmethod 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 shrinker - Extendable: kind of flatMap-operation called
chain[more] - able to bind the output of an arbitrary as input of another one while keeping the shrink working - Extendable: precondition checks with
fc.pre(...)[more] - filtering invalid entries can be done directly inside the check function if needed - Extendable: easily switch from fake data in tests to property based with
fc.gen()[more] - generate random values within your predicates - Smart: ability to shrink on
fc.oneof[more] - surprisingly some frameworks don't - Smart: biased by default - by default it generates both small and large values, making it easier to dig into counterexamples without having to tweak a size parameter manually
- Debug: verbose mode [more][tutorial] - easier troubleshooting with verbose mode enabled
- Debug: replay directly on the minimal counterexample [tutorial] - no need to replay the whole sequence, you get directly the counterexample
- Debug: custom examples in addition of generated ones [more] - no need to duplicate the code to play the property on custom examples
- Debug: logger per predicate run [more] - simplify your troubleshoot with fc.context and its logging feature
- Unique: model based approach [more][article] - use the power of property based testing to test UI, APIs or state machines
- Unique: detect race conditions in your code [more][tutorial] - shuffle the way your promises and async calls resolve using the power of property based testing to detect races
- Unique: simplify user definable corner cases [more] - simplify bug resolution by asking fast-check if it can find an even simpler corner case
For more details, refer to the documentation in the links above.
Trusted
fast-check has been trusted for years by big projects like: jest, jasmine, fp-ts, io-ts, ramda, js-yaml, query-string...
Powerful
It also proved useful in finding bugs among major open source projects such as jest, query-string... and many others.
Compatibility
Here are the minimal requirements to use fast-check properly without any polyfills:
| fast-check | node | ECMAScript version | TypeScript (optional) |
|---|---|---|---|
| 4.x | ≥12.17.0(1) | ES2020 | ≥5.0 |
| 3.x | ≥8(2) | ES2017 | ≥4.1(3) |
| 2.x | ≥8(2) | ES2017 | ≥3.2(4) |
| 1.x | ≥0.12(2) | ES3 | ≥3.0(4) |
More details...
- Even if version 12.x should support most of the ES2020 features that will be leveraged by the version 4, we recommend relying at least on version 14.x of Node as it supports all the targeted specification. In addition, we highly encourage switching to still supported LTS versions of Node and not sticking to unsupported versions for too long.
- Except for features that cannot be polyfilled - such as
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. - Require either lib or target ≥ ES2020 or
@types/nodeto be installed. - Require either lib or target ≥ ES2015 or
@types/nodeto be installed.
Contributors ✨
Thanks goes to these wonderful people (emoji key):
This project follows the all-contributors specification. Contributions of any kind welcome! Become one of them
Sponsors 💸
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.
Other packages similar to fast-check
jsverify
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
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
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.
FAQs
Property based testing framework for JavaScript (like QuickCheck)
The npm package fast-check receives a total of 29,216,652 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.
Package last updated on 15 Sep 2026
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