fast-check

What is fast-check?

fast-check is a property-based testing library for JavaScript and TypeScript. It allows developers to define properties that should hold true for a wide range of inputs, and then automatically generates test cases to verify those properties. This helps in identifying edge cases and ensuring the robustness of the code.

What are fast-check's main functionalities?

Property-based Testing

This feature allows you to define properties that should hold true for a wide range of inputs. In this example, the property being tested is the commutativity of addition.

const fc = require('fast-check');

fc.assert(
  fc.property(fc.integer(), fc.integer(), (a, b) => {
    return a + b === b + a;
  })
);

Custom Arbitraries

Custom arbitraries allow you to define complex data structures for your tests. In this example, a custom arbitrary is created that generates objects with a number and a string.

const fc = require('fast-check');

const myArbitrary = fc.tuple(fc.integer(), fc.string()).map(([num, str]) => ({ num, str }));

fc.assert(
  fc.property(myArbitrary, ({ num, str }) => {
    return typeof num === 'number' && typeof str === 'string';
  })
);

Shrinkable Values

Shrinkable values help in minimizing the size of failing test cases to make debugging easier. In this example, if the property fails, fast-check will try to find the smallest array that causes the failure.

const fc = require('fast-check');

fc.assert(
  fc.property(fc.array(fc.integer()), (arr) => {
    return arr.length < 100;
  }),
  { verbose: true }
);

Other packages similar to fast-check

jsverify

jsverify is another property-based testing library for JavaScript. It offers similar functionality to fast-check, such as defining properties and generating test cases. However, fast-check is generally considered to have a more modern API and better TypeScript support.

testcheck

testcheck is a property-based testing library inspired by QuickCheck. It provides similar capabilities for generating test cases and defining properties. Compared to fast-check, testcheck is less actively maintained and has fewer features.

hypothesis

Hypothesis is a property-based testing library for Python, but it has inspired several JavaScript libraries, including fast-check. While not a direct competitor, it offers similar concepts and is often used as a reference for property-based testing.

README

Property based testing framework for JavaScript/TypeScript

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(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:

• 🏁 Introduction to Property Based & Hands On
• 🐣 Built-in arbitraries
• 🔧 Custom arbitraries
• 🏃‍♂️ Property based runners
• 💥 Tips
• 🔌 API Reference
• ⭐ Awesome fast-check
• 🤯 How fast-check works?

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.

Compatibility

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:

jest

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

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

js-yaml

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

query-string

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

Credits

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