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:

• 🏁 Introduction to Property Based & Hands On
• 🐣 Built-in arbitraries
• 🔧 Custom arbitraries
• 🏃‍♂️ Property based runners
• 💥 Tips
• 🔌 API Reference
• ⭐ Awesome 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 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 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.

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:

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):

Nicolas DUBIEN 💻 📖 ⚠️ 🚇 🎨 🚧	Aaron Elligsen 💻 📖 ⚠️	Will Heslam 📖	kazchimo 💻 📖	Brandon Chinn 💻 📖	Irakli Safareli 📖	Andrew Herron 📖 🔌
Eric Crosson 📖 💻	burrscurr 📖	JC (Jonathan Chen) 📖	Larry Botha 📖 💻 ⚠️	Roman Gusev 📖	Tim Wisniewski 📖	Brais Piñeiro 💻 ⚠️
Renaud-Pierre Bordes 🎨	Jemma Nelson 📖	John Haugeland 📖	Trey Davis 🎨	Leon Si 📖	Gorgi Kosev 🚇	mayconsacht 💻
Simon Friis Vindum 💻 ⚠️	Richard Gibson 📖	Alan Harper 📖	Makien Osman 💻	David Sommerich 💻 ⚠️	Diego Pedro 💻 ⚠️	Borui Gu 📖
Brian Donovan 📖	volrk 💻 📖 ⚠️	tinydylan 💻 ⚠️	Caleb Jasik 📖	Rulai Hu 📖	Afonso Jorge Ramos 📖	Tom Jenkinson 📖
phormio 📖	Giovanni Gonzaga 💻 ⚠️	Tomas Carnecky 💻	Kirill Romanov 💻 📖 ⚠️	Giovanny González 📖	Mark Kulube 🚇	Peter Hamilton 💻
Chinedu Ozodi 📖	Gunar Gessner 📖	Christian Batchelor ⚠️	Tomer Aberbach 💻 📖 ⚠️	0xflotus 📖	Ryan Leonard 💻 📖 ⚠️	Jason Dreyzehner 💻 ⚠️
Matin Zadeh Dolatabad 💻	Juan Julián Merelo Guervós 📖	Simen Bekkhus 📖	Tarjei Skjærset 📖	Denis Gorbachev 📖	Trevor McCauley 📖	Grant Kiely 📖
Attila Večerek 💻 📖 ⚠️	Zach Bjornson 💻 📖	Bennett Perkins 📖	Alexandre Oger 📖	ej shafran 📖	Niklas Gruhn 💻	Patrick Roza 💻

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.