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1.2.0 • Public • Published


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Bring the power of property based testing framework fast-check into AVA. @fast-check/ava simplifies the integration of fast-check into AVA testing framework.

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Getting Started

Install @fast-check/ava:

npm install --save-dev @fast-check/ava

In order to work properly, @fast-check/ava requires ava to be installed.


import { testProp, fc } from '@fast-check/ava';

// for all a, b, c strings
// b is a substring of a + b + c
testProp('should detect the substring', [fc.string(), fc.string(), fc.string()], (t, a, b, c) => {
  t.true((a + b + c).includes(b));

The property is passed AVA's t argument as its first parameter, and the value of each arbitrary for the current test case for the rest of the parameters.

@fast-check/ava supports all of AVA's assertions and like AVA, it supports synchronous and asynchronous functions, including promises, observables, and callbacks. See AVA's documentation for more information.

⚠️ WARNING: When relying on @fast-check/ava, returning true or false in your predicates is not taken into account. The library wants assertions or plans to be defined as ava itself does. Nonetheless you can still use primitives such as fc.pre to cut runs at the middle if some invariants are unmeet: the started plan (if any) will just be ignored.


fast-check Parameters

testProp accepts an optional fc.Parameters for forwarding custom parameters to fast-check (more).

import { testProp, fc } from '@fast-check/ava';

  'should detect the substring',
  [fc.string(), fc.string(), fc.string()],
  (t, a, b, c) => {
    t.true((a + b + c).includes(b));
  { numRuns: 10 }, // Example of parameters

AVA Modifiers

@fast-check/ava also comes with .only, .serial .skip, and .failing modifiers from AVA.

import { testProp, fc } from '@fast-check/ava';

  'should replay the test for the seed 4242',
  [fc.nat(), fc.nat()],
  (t, a, b) => {
    t.is(a + b, b + a);
  { seed: 4242 },

testProp.skip('should be skipped', [fc.fullUnicodeString()], (t, text) => {
  t.is([...text].length, text.length);

AVA before/after Hooks

@fast-check/ava exposes AVA's before/after hooks:

import { testProp, fc } from '@fast-check/ava';

testProp.before((t) => {

// ... omitted for brevity

testProp.after((t) => {

AVA Execution Context

@fast-check/ava mirror's AVA's procedure for customizing the test execution context:

import { fc, testProp as anyTestProp, PropertyTestInterface } from '@fast-check/ava';

type TestContext = {
  state: string;

const testProp = anyTestProp as PropertyTestInterface<TestContext>;

testProp('should reach terminal state', [fc.string()], (t, received) => {
  // here t is typed as ExecutionContext<TestContext>
  console.log(t.context.state); // logs 'uninitialized'
  // ... omitted for brevity

Minimal requirements

@fast-check/ava AVA fast-check
^1.0.0 >=4.0.0 ^3.0.0

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  • ndubien