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Property based testing framework for JavaScript (like QuickCheck) written in TypeScript

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fast-check logo

Property based testing framework for JavaScript/TypeScript

Build Status npm version monthly downloads

Coverage Status (unit tests) Package quality Snyk Package quality OpenSSF Scorecard OpenSSF Best Practices

PRs Welcome License Twitter

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: 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:

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 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.

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)
3.x β‰₯8(1) ES2017 β‰₯4.1(2)
2.x β‰₯8(1) ES2017 β‰₯3.2(3)
1.x β‰₯0.12(1) ES3 β‰₯3.0(3)
More details...
  1. 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.
  2. Require either lib or target β‰₯ ES2020 or @types/node to be installed.
  3. Require either lib or target β‰₯ ES2015 or @types/node to be installed.

ReScript bindings

Bindings to use fast-check in ReScript are available in package rescript-fast-check. They are maintained by @TheSpyder as an external project.

Contributors ✨

Thanks goes to these wonderful people (emoji key):

Nicolas DUBIEN
Nicolas DUBIEN

πŸ’» πŸ“– ⚠️ πŸš‡ 🎨 🚧
Aaron Elligsen
Aaron Elligsen

πŸ’» πŸ“– ⚠️
Will Heslam
Will Heslam

πŸ“–
kazchimo
kazchimo

πŸ’» πŸ“–
Brandon Chinn
Brandon Chinn

πŸ’» πŸ“–
Irakli Safareli
Irakli Safareli

πŸ“–
Andrew Herron
Andrew Herron

πŸ“– πŸ”Œ
Eric Crosson
Eric Crosson

πŸ“– πŸ’»
burrscurr
burrscurr

πŸ“–
JC (Jonathan Chen)
JC (Jonathan Chen)

πŸ“–
Larry Botha
Larry Botha

πŸ“– πŸ’» ⚠️
Roman Gusev
Roman Gusev

πŸ“–
Tim Wisniewski
Tim Wisniewski

πŸ“–
Brais PiΓ±eiro
Brais PiΓ±eiro

πŸ’» ⚠️
Renaud-Pierre Bordes
Renaud-Pierre Bordes

🎨
Jemma Nelson
Jemma Nelson

πŸ“–
John Haugeland
John Haugeland

πŸ“–
Trey Davis
Trey Davis

🎨
Leon Si
Leon Si

πŸ“–
Gorgi Kosev
Gorgi Kosev

πŸš‡
mayconsacht
mayconsacht

πŸ’»
Simon Friis Vindum
Simon Friis Vindum

πŸ’» ⚠️
Richard Gibson
Richard Gibson

πŸ“–
Alan Harper
Alan Harper

πŸ“–
Makien Osman
Makien Osman

πŸ’»
David Sommerich
David Sommerich

πŸ’» ⚠️
Diego Pedro
Diego Pedro

πŸ’» ⚠️
Borui Gu
Borui Gu

πŸ“–
Brian Donovan
Brian Donovan

πŸ“–
volrk
volrk

πŸ’» πŸ“– ⚠️
tinydylan
tinydylan

πŸ’» ⚠️
Caleb Jasik
Caleb Jasik

πŸ“–
Rulai Hu
Rulai Hu

πŸ“–
Afonso Jorge Ramos
Afonso Jorge Ramos

πŸ“–
Tom Jenkinson
Tom Jenkinson

πŸ“–
phormio
phormio

πŸ“–
Giovanni Gonzaga
Giovanni Gonzaga

πŸ’» ⚠️
Tomas Carnecky
Tomas Carnecky

πŸ’»
Kirill Romanov
Kirill Romanov

πŸ’» πŸ“– ⚠️
Giovanny GonzΓ‘lez
Giovanny GonzΓ‘lez

πŸ“–
Mark Kulube
Mark Kulube

πŸš‡
Peter Hamilton
Peter Hamilton

πŸ’»
Chinedu Ozodi
Chinedu Ozodi

πŸ“–
Gunar Gessner
Gunar Gessner

πŸ“–
Christian Batchelor
Christian Batchelor

⚠️
Tomer Aberbach
Tomer Aberbach

πŸ’» πŸ“– ⚠️
0xflotus
0xflotus

πŸ“–
Ryan Leonard
Ryan Leonard

πŸ’» πŸ“– ⚠️
Jason Dreyzehner
Jason Dreyzehner

πŸ’» ⚠️
Matin Zadeh Dolatabad
Matin Zadeh Dolatabad

πŸ’»
Juan JuliΓ‘n Merelo GuervΓ³s
Juan JuliΓ‘n Merelo GuervΓ³s

πŸ“–
Simen Bekkhus
Simen Bekkhus

πŸ“–
Tarjei Skjærset
Tarjei Skjærset

πŸ“–
Denis Gorbachev
Denis Gorbachev

πŸ“–
Trevor McCauley
Trevor McCauley

πŸ“–
Grant Kiely
Grant Kiely

πŸ“–
Attila Večerek
Attila Večerek

πŸ’» πŸ“– ⚠️
Zach Bjornson
Zach Bjornson

πŸ’» πŸ“–
Bennett Perkins
Bennett Perkins

πŸ“–
Alexandre Oger
Alexandre Oger

πŸ“–
ej shafran
ej shafran

πŸ“–
Niklas Gruhn
Niklas Gruhn

πŸ’»
Patrick Roza
Patrick Roza

πŸ’»
Cindy Wu
Cindy Wu

πŸ“–
Noah
Noah

πŸ“–
James Vaughan
James Vaughan

πŸ“–

This project follows the all-contributors specification. Contributions of any kind welcome! Become one of them

Sponsors πŸ’Έ

Many individuals and companies offer their financial support to the project, a huge thanks to all of them too πŸ’“

all sponsors

You can also become one of them by contributing via GitHub Sponsors or OpenCollective.