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    Fast Check

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

    Build Status npm version monthly downloads

    Coverage Status (unit tests) Package quality Snyk Package quality Semver stability

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    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 property(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

    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


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