How to mock es6 class using Jest

Question

I am attempting to mock a class Mailer using jest and I can't figure out how to do it. The docs don't give many examples of how this works. The process is the I will have a node event password-reset that is fired and when that event is fired, I want to send an email using Mailer.send(to, subject, body). Here is my directory structure:

project_root
-- __test__
---- server
------ services
-------- emails
---------- mailer.test.js
-- server
---- services
------ emails
-------- mailer.js
-------- __mocks__
---------- mailer.js

Here is my mock file __mocks__/mailer.js:

const Mailer = jest.genMockFromModule('Mailer');

function send(to, subject, body) {
  return { to, subject, body };
}

module.exports = Mailer;

and my mailer.test.js

const EventEmitter = require('events');
const Mailer = jest.mock('../../../../server/services/emails/mailer');

test('sends an email when the password-reset event is fired', () => {
  const send = Mailer.send();
  const event = new EventEmitter();
  event.emit('password-reset');
  expect(send).toHaveBeenCalled();
});

and finally my mailer.js class:

class Mailer {

  constructor() {
    this.mailgun = require('mailgun-js')({
      apiKey: process.env.MAILGUN_API_KEY,
      domain: process.env.MAILGUN_DOMAIN,
    });
  }

  send(to, subject, body) {
    return new Promise((reject, resolve) => {
      this.mailgun.messages().send({
        from: 'Securely App <[email protected]>',
        to,
        subject: subject,
        html: body,
      }, (error, body) => {
        if (error) {
          return reject(error);
        }

        return resolve('The email was sent successfully!');
      });
    });
  }

}

module.exports = new Mailer();

So, how do I successfully mock and test this class, using Jest? Many thanks for helping!

Andreas Köberle · Accepted Answer · 2017-05-03 11:29:36Z

28

You don't have to mock your mailer class but the mailgun-js module. So mailgun is a function that returns the function messages that return the function send. So the mock will look like this.

for the happy path

const happyPath = () => ({
  messages: () => ({
    send: (args, callback) => callback()
  })
})

for the error case

const errorCase = () => ({
  messages: () => ({
    send: (args, callback) => callback('someError')
  })
})

as you have this 2 cases it make sense to mock the module inside your test. First you have to mock it with a simple spy where we later can set the implementation for our cases and then we have to import the module.

jest.mock('mailgun-js', jest.fn())
import mailgun from 'mailgun-js'
import Mailer from '../../../../server/services/emails/mailer'

As your module uses promises we have 2 options either return the promise from the test or use async/await. I use the later one for more info have a look here.

test('test the happy path', async() => {
 //mock the mailgun so it returns our happy path mock
  mailgun.mockImplementation(() => happyPath)
  //we need to use async/awit here to let jest recognize the promise
  const send = await Mailer.send();
  expect(send).toBe('The email was sent successfully!')
});

If you would like to test that the mailgun send method was called with the correct parameter you need to adapt the mock like this:

const send = jest.fn((args, callback) => callback())
const happyPath = () => ({
  messages: () => ({
    send: send
  })
})

Now you could check that the first parameter for send was correct:

expect(send.mock.calls[0][0]).toMatchSnapshot()

answered May 3, 2017 at 11:29

Andreas Köberle

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2 Comments

dericcain Over a year ago

I can't wait to get home and try this out. Thank you so much for the super-great explanation.

muthu Over a year ago

Tried with the same way to mock mailgun-js, but endup with receiving the error mailgun_js_1.default is not a function

Justus Romijn · Accepted Answer · 2018-01-09 16:40:45Z

19

Just for Googlers and future visitors, here's how I've setup jest mocking for ES6 classes. I also have a working example at github, with babel-jest for transpiling the ES module syntax so that jest can mock them properly.

__mocks__/MockedClass.js

const stub = {
  someMethod: jest.fn(),
  someAttribute: true
}

module.exports = () => stub;

Your code can call this with new, and in your tests you can call the function and overwrite any default implementation.

example.spec.js

const mockedClass = require("path/to/MockedClass")(); 
const AnotherClass = require("path/to/AnotherClass");
let anotherClass;

jest.mock("path/to/MockedClass");

describe("AnotherClass", () => {
  beforeEach(() => {
    mockedClass.someMethod.mockImplementation(() => {
      return { "foo": "bar" };
    });

    anotherClass = new AnotherClass();
  });

  describe("on init", () => {
    beforeEach(() => { 
      anotherClass.init(); 
    });

    it("uses a mock", () => {
      expect(mockedClass.someMethod.toHaveBeenCalled();
      expect(anotherClass.settings)
        .toEqual(expect.objectContaining({ "foo": "bar" }));
    });
  });

});

edited Jan 9, 2018 at 16:40

answered Oct 18, 2017 at 14:28

Justus Romijn

16.1k5 gold badges54 silver badges64 bronze badges

10 Comments

stone Over a year ago

Not working for me. mockedClass.someMethod is undefined. Using jest 21.2.1.

Justus Romijn Over a year ago

@PacketTracer you are correct! I've updated the code example.

Flo Over a year ago

@JustusRomijn, awesome - I have been searching for a solution for this for quite some time, but returning a function that always returns the same instance is pretty smart. Thanks!

stone Over a year ago

@JustusRomijn, how can you call new on an arrow function? I get error 'mockedClass is not a constructor' when AnotherClass calls new mockedClass().

Justus Romijn Over a year ago

I've updated my answer to point to a working example, a github repo.

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