Noodles, Poodles and More!

    TypeScript icon, indicating that this package has built-in type declarations

    2.25.0 • Public • Published

    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

    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:

    const fc = require('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(, 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.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
    • Smart: ability to shrink on fc.oneof [more] - surprisingly some frameworks don't
    • Smart: biased by default [more] - 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] - easier troubleshooting with verbose mode enabled
    • Debug: replay directly on the minimal counterexample [more] - 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] - 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.


    Here are the minimal requirements to use fast-check properly without any polyfills:

    fast-check node ECMAScript version TypeScript (optional)
    2.x ≥8(1) ES2017 ≥3.2
    1.x ≥0.12(1) ES3 ≥3.0

    (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.

    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.

    Issues found by fast-check in famous packages

    fast-check has been able to find some unexpected behaviour among famous npm packages. Here are some of the errors detected using fast-check:


    Issue detected: toStrictEqual fails to distinguish 0 from 5e-324 [more]

    Code example: expect(0).toStrictEqual(5e-324) succeeds


    Issue detected: enabling !!int: binary style when dumping negative integers produces invalid content [more]

    Code example: yaml.dump({toto: -10}, {styles:{'!!int':'binary'}}) produces toto: 0b-1010 not toto: -0b1010


    Issue detected: enabling the bracket setting when exporting arrays containing null values produces an invalid output for the parser [more]

    Code example:

    m.stringify({bar: ['a', null, 'b']}, {arrayFormat: 'bracket'}) //=> "bar[]=a&bar&bar[]=b"
    m.parse('bar[]=a&bar&bar[]=b', {arrayFormat: 'bracket'})       //=> {bar: [null, 'b']}

    MORE: Issues detected thanks to fast-check


    Code Contributors

    This project would not be the same without them 💖 - Become one of them


    Thank you to all our backers! 🙏 [Become a backer] and help us sustain our community.


    Support this project by becoming a sponsor. Your logo will show up here with a link to your website. [Become a sponsor]


    npm i fast-check

    DownloadsWeekly Downloads






    Unpacked Size

    1.05 MB

    Total Files


    Last publish


    • ndubien