An experimental Mocha BDD wrapper for fast-check's property based testing.

• mocha-foam
  • Contents
  • Install
  • Usage
  • Build
  • License

Add mocha-foam to your project as a dev dependency, along with fast-check and mocha, if you are not already using them:

> yarn add -D fast-check mocha mocha-foam

This is a BDD-style wrapper around fc.check(fc.asyncProperty(...)), and supports all of the same Arbitraries that fast-check normally provides.

Property based suites are defined with over and the library is provided as an ES module:

import { expect } from 'chai';
import { integer } from 'fast-check';
import { over } from 'mocha-foam';

describe('example properties', () => {
  over('some numbers', integer(), (it) => {
    it('should be even', (n: number) => {
      return n % 2 === 0;
    });

    it('should be odd', async (n: number) => {
      expect(n % 2).to.equal(1);
    });
  });
});

Assertions can be made through expect (and the other Chai assertion styles) or by returning a boolean, and may be async functions. The test callbacks are called with a Mocha context in this, so the usual caveats apply to arrow functions.

Note the wrapped it passed to the suite callback by over. This wrapped it calls through to Mocha's it, after wrapping the test in a fast-check property, while over calls through to Mocha's describe. Most Mocha features, like beforeEach and afterEach hooks, work correctly within the suite defined by over.

You can pass additional parameters to the fast-check Runner after the suite callback:

describe('more examples', () => {
  over('some UUIDs', uuid(), (it) => {
    // beforeEach hooks work normally, since the wrapped it calls through to real it
    beforeEach(() => {
      console.log('before each UUID test');
    });

    it('should be a good one', (id: string) => {
      return id[9] !== 'a';
    });

    it('should be long enough', (id: string) => {
      expect(id).to.have.length.greaterThan(2);
    });
  }, {
    // fast-check parameters are supported, like examples
    examples: ['a', 'b'],
    numRuns: 1_000,
  });

  over('mapped properties', tuple(lorem(), integer()).map(([a, b]) => a.substr(b)), (it) => {
    it('should have content', (text: string) => {
      return text.length > 0;
    });
  }, {
    // error formatting can be overridden with a custom handler, or fast-check's default reporter
    errorReporter: defaultReportMessage,
  });
});

The default reporter, briefReporter, will print the number of runs, number of shrinks, and any counterexamples found:

  5) example properties
       some UUIDs
         should be a good one:
     Error: Property failed by returning false after 3 runs and 8 shrinks, failing on: "00000000-a000-300d-8000-000000000000" (seed: -886543855, path: '2:0:10:0:10:0:2:3:3')
      at /home/ssube/code/ssube/mocha-foam/out/src/index.js:20:385
      at processTicksAndRejections (internal/process/task_queues.js:97:5)

To build mocha-foam, run make. The build depends on:

• Node 14+
• Yarn 1.x

To run small tests with coverage, run make cover.

The make help target will print help for the available targets.

mocha-foam is released under the MIT license.