Property based testing framework for JavaScript/TypeScript
Getting started
Hands-on tutorial and definition of Property Based Testing:
Property based testing frameworks check the truthfulness of properties. A property is a statement like: for all (x, y, ...) such as precondition(x, y, ...) holds property(x, y, ...) is true.
Install the module with: 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) => 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🔍 Generated documentation
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
mapmethod 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
For more details, refer to the documentation in the links above.
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:
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']}