Chai Assertions Count

Plugin for ChaiJS allows checking how many assertions or expects were run per each test.

Why do we need to check it?

Let’s look at the test:

import InstanceGenerator from '../lib/instances-generator';

describe('suite #1', () => {
  it('test #1', () => {
    class S1T1A {
      /* Other props and methods are skipped */
      /**
       * I'm called after any instance of S1T1A is created
       */
      afterCreate(...args) {
        // I need to check `args` here
        // chai.expect(args)... 
      }
    }
    InstanceGenerator.create(S1T1A, 3); // create 3 instances of S1T1A
  });
});

Test looks pretty dummy, but its main idea that it’s not possible to figure out was afterCreate called or not without dummy flag. It must be initialized on the top of the test. Then it must be toggle inside afterCreate and another expect must be added at the end of the test.

Tests becomes less readable.

Better way is to check how many expect were done.

Install

npm i -D chai-assertions-count

or

yarn add -D chai-assertions-count

Plugin

Use this plugin as you would all other Chai plugins.

const chai = require('chai');
const chaiAssertionsCount = require('chai-assertions-count');

chai.use(chaiAssertionsCount);

Usage

const chai = require('chai');
const chaiAssertionsCount = require('chai-assertions-count');
chai.use(chaiAssertionsCount);

describe('suite #2', () => {
  beforeEach(() => {
    chai.Assertion.resetAssertsCheck();
  });
  afterEach(() => {
    // you don't need both of them
    chai.Assertion.checkAssertionsCount();
    chai.Assertion.checkExpectsCount();
  });
});

Method resetAssertsCheck just drops internal counters and must be used before each test.

Method checkExpectsCount calculated how many times chai.expect was called. Use it in case when your tests use Expect style.

Method checkAssertionsCount calculated how many assertions were done. Main difference between this method and previous one is that single expect may do more than one assertion. Example below illustrates this:

const chai = require('chai');
const chaiAssertionsCount = require('chai-assertions-count');
chai.use(chaiAssertionsCount);

describe('suite #3', () => {
  it('test #1', () => {
    chai.Assertion.expectAssertions(3);
    chai.Assertion.expectExpects(2);

    chai.expect(1).to.be.equal(1);
    chai.expect([]).to.have.property('length', 0);
  });
});

Here are two expects and we “expect” that two of them will be executed. In the same time there are three assertions “under the hood”. First expect has a single assertion. However, second expect has two of them. First one checks that property length exists and another one checks its value. So, be aware with expectAssertions counter.

Method expectExpects can cover most cases, so expectAssertions won’t be used in 99.9%.

Restrictions

• Works only with Expect style.
• Stop other tests in the current suite on expectExpects or expectAssertions fail.