Property based testing is another way to test programs. Instead of relying on hard-coded inputs and outputs, it checks characteristics of the output given the whole range of possible inputs.

Programming language: TypeScript
License: MIT License
Latest version: v2.25.0

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Property based testing framework for JavaScript/TypeScript

Getting started

Hands-on tutorial and definition of Property Based Testing: ๐Ÿ 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(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.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
  • 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)
3.x โ‰ฅ8(1) ES2017 โ‰ฅ4.1
2.x โ‰ฅ8(1) ES2017 โ‰ฅ3.2
1.x โ‰ฅ0.12(1) ES3 โ‰ฅ3.0

(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

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*Note that all licence references and agreements mentioned in the fast-check README section above are relevant to that project's source code only.