10 Mocha and Chai Interview Questions and Answers

Mocha and Chai are essential tools in the JavaScript ecosystem for behavior-driven development (BDD) and test-driven development (TDD). Mocha is a flexible, feature-rich JavaScript test framework running on Node.js and in the browser, making asynchronous testing simple and fun. Chai, on the other hand, is a BDD/TDD assertion library that pairs seamlessly with Mocha, providing a variety of interfaces for asserting conditions in your tests.

This article offers a curated selection of interview questions focused on Mocha and Chai. By working through these questions, you will gain a deeper understanding of how to effectively use these tools to write robust, maintainable tests, thereby enhancing your readiness for technical interviews.

Mocha and Chai Interview Questions and Answers

1. Write a simple test case using Mocha to check if a function returns true.

Mocha and Chai provide a powerful combination for writing and running tests in JavaScript. Here is a simple test case using Mocha and Chai to check if a function returns true:

const { expect } = require('chai');

function returnsTrue() {
    return true;
}

describe('returnsTrue Function', function() {
    it('should return true', function() {
        expect(returnsTrue()).to.be.true;
    });
});

2. Write a test case using Chai to assert that an array contains a specific element.

One common use case is to assert that an array contains a specific element. Here’s an example:

const chai = require('chai');
const expect = chai.expect;

describe('Array', function() {
  it('should contain the element 3', function() {
    const arr = [1, 2, 3, 4, 5];
    expect(arr).to.include(3);
  });
});

3. Write a test case using Mocha and Chai to test asynchronous code.

When testing asynchronous code, it is important to ensure that the test framework waits for the operation to complete before making assertions. Here’s an example:

const chai = require('chai');
const expect = chai.expect;

describe('Asynchronous Code Test', function() {
    it('should return the correct value after a delay', function(done) {
        function asyncFunction(callback) {
            setTimeout(() => {
                callback('Hello, world!');
            }, 1000);
        }

        asyncFunction(function(result) {
            expect(result).to.equal('Hello, world!');
            done();
        });
    });
});

In this example, the asyncFunction simulates an asynchronous operation using setTimeout. The test case uses Mocha’s done callback to signal completion.

4. Write a test case using Chai’s should interface to check object properties.

The should interface in Chai allows for expressive assertions. To check object properties, you can use the property method:

const chai = require('chai');
chai.should();

describe('Object Property Test', function() {
    it('should have a property name with value John', function() {
        const obj = { name: 'John', age: 30 };
        obj.should.have.property('name').equal('John');
    });
});

5. Write a test case using Mocha and Chai to mock a function call.

To mock a function call, we can use a library like Sinon.js with Mocha and Chai. Here’s an example:

const chai = require('chai');
const sinon = require('sinon');
const expect = chai.expect;

function fetchData(callback) {
    setTimeout(() => {
        callback('data');
    }, 1000);
}

describe('fetchData', function() {
    it('should call the callback with "data"', function() {
        const callback = sinon.spy();

        fetchData(callback);

        this.clock = sinon.useFakeTimers();
        this.clock.tick(1000);

        expect(callback.calledOnce).to.be.true;
        expect(callback.calledWith('data')).to.be.true;

        this.clock.restore();
    });
});

In this example, Sinon.js is used to create a spy for the callback function.

6. Write a test case using Mocha and Chai to test a REST API endpoint.

To test a REST API endpoint, you can use Mocha and Chai with chai-http:

const chai = require('chai');
const chaiHttp = require('chai-http');
const server = require('../app');
const should = chai.should();

chai.use(chaiHttp);

describe('GET /api/endpoint', () => {
  it('it should GET all the items', (done) => {
    chai.request(server)
      .get('/api/endpoint')
      .end((err, res) => {
        res.should.have.status(200);
        res.body.should.be.a('array');
        res.body.length.should.be.eql(0);
        done();
      });
  });
});

This test checks if the GET request returns a status of 200 and an empty array.

7. Write a custom Chai assertion and use it in a Mocha test.

Custom Chai assertions allow you to extend Chai’s built-in assertions with your own logic. Here’s an example:

const chai = require('chai');
const expect = chai.expect;

chai.Assertion.addMethod('isEven', function() {
    const num = this._obj;
    this.assert(
        num % 2 === 0,
        'expected #{this} to be even',
        'expected #{this} to not be even'
    );
});

describe('Custom Chai Assertion', function() {
    it('should assert that a number is even', function() {
        expect(4).to.be.isEven();
    });

    it('should fail for an odd number', function() {
        expect(5).to.not.be.isEven();
    });
});

8. Write a test case using Mocha and Chai to mock an HTTP request.

To mock an HTTP request, you can use a library like nock:

const chai = require('chai');
const chaiHttp = require('chai-http');
const nock = require('nock');
const app = require('../app');

chai.use(chaiHttp);
const { expect } = chai;

describe('HTTP Request Mocking', () => {
    before(() => {
        nock('http://example.com')
            .get('/api/data')
            .reply(200, { success: true, data: 'Mocked Data' });
    });

    it('should mock an HTTP GET request', (done) => {
        chai.request(app)
            .get('/api/data')
            .end((err, res) => {
                expect(res).to.have.status(200);
                expect(res.body).to.be.an('object');
                expect(res.body).to.have.property('success', true);
                expect(res.body).to.have.property('data', 'Mocked Data');
                done();
            });
    });
});

9. Write a test case using Mocha and Chai to test a promise.

When testing promises, Mocha and Chai can be used together effectively:

const chai = require('chai');
const expect = chai.expect;

describe('Promise Test', function() {
    it('should resolve with the correct value', function() {
        return someAsyncFunction().then(function(result) {
            expect(result).to.equal('expected value');
        });
    });
});

In this example, someAsyncFunction is a placeholder for the actual function returning a promise.

10. How do you manage test data setup and teardown in Mocha tests?

In Mocha, managing test data setup and teardown is important for ensuring that tests are isolated. Mocha provides hooks such as before, after, beforeEach, and afterEach to handle these tasks.

const chai = require('chai');
const expect = chai.expect;

describe('Array', function() {
    let testArray;

    before(function() {
        console.log('Setting up test environment');
    });

    after(function() {
        console.log('Cleaning up test environment');
    });

    beforeEach(function() {
        testArray = [1, 2, 3];
    });

    afterEach(function() {
        testArray = [];
    });

    it('should have a length of 3', function() {
        expect(testArray).to.have.lengthOf(3);
    });

    it('should include 2', function() {
        expect(testArray).to.include(2);
    });
});

These hooks allow you to set up and clean up test data efficiently.