javascript - 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 <friendly-robot@securelyapp.com>',
        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!

Answer 1

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()