fast-check
v4.5.3TypeScript
Property based testing framework for JavaScript (like QuickCheck)
0/weekUpdated 1 months agoMITUnpacked: 1.7 MB
Published by Nicolas DUBIEN
npm install fast-check
Property based testing framework for JavaScript/TypeScript
Getting started
Hands-on tutorial and definition of Property Based Testing: π 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:
``js
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:
- π Introduction to Property Based & Hands On
- π£ Built-in arbitraries
- π§ Custom arbitraries
- πββοΈ Property based runners
- π₯ Tips
- π API Reference
- β 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 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_chain
- Extendable: kind of flatMap-operation called \[more\] - _able to bind the output of an arbitrary as input of another one while keeping the shrink working_fc.pre(...)
- Extendable: precondition checks with \[more\] - _filtering invalid entries can be done directly inside the check function if needed_fc.gen()
- Extendable: easily switch from fake data in tests to property based with \[more\] - _generate random values within your predicates_fc.oneof
- Smart: ability to shrink on \[more\] - _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.
$3
fast-check has been trusted for years by big projects like: jest, jasmine, fp-ts, io-ts, ramda, js-yaml, query-string...
$3
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 | β₯12.17.0(1) | ES2020 | β₯5.0 |
| 3.x | β₯8(2) | ES2017 | β₯4.1(3) |
| 2.x | β₯8(2) | ES2017 | β₯3.2(4) |
| 1.x | β₯0.12(2) | ES3 | β₯3.0(4) |
More details...
1. 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.
2. 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.@types/node
3. Require either lib or target β₯ ES2020 or to be installed.@types/node` to be installed.
4. Require either lib or target β₯ ES2015 or
Contributors β¨
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 πΈ
Many individuals and companies offer their financial support to the project, a huge thanks to all of them too π
You can also become one of them by contributing via GitHub Sponsors or OpenCollective.