Testing the Event

Recall how we implemented the userHasRegisteredEvent.

File: src/events/userHasRegisteredEvent.js

const mailer = require('../utils/mailer')

/**
 *
 * @param {*} user
 *
 * @returns boolean
 */
const userHasRegisteredEvent = (user) => {
  const bodyText = `Hello ${user.firstName}. Thanks for registering!`
  return mailer.sendEmailToUser(user.email, 'Welcome aboard!', bodyText)
}

module.exports = userHasRegisteredEvent

Now for this test, we actually don't care that much about the mailer utility and whether or not we actually sent an email. What we are really interested in is whether or not the mailer.sendEmailToUser was called or not. For this particular test, it would be sufficient to just mock up the mailer and the sendEmailToUser method to check if they've been called.

The Test

The mocking part for this test is a lot tricky, but notice how we mock both the mailer and the sendEmailToToUser below.

File: src/events/__tests__/userHasRegisteredEvent.test.js

const event = require('../userHasRegisteredEvent')
const mailer = require('../../utils/mailer')

const sentEmailSuccessful = !!Math.round(Math.random(0, 1))
jest.mock('../../utils/mailer')
mailer.sendEmailToUser.mockImplementation(() => sentEmailSuccessful)

describe('Test Suite: userHasRegisteredEvent', () => {
  test("Email has (un)successfully been sent to the user's email address", () => {
    const eventResult = event({
      firstName: 'John',
      email: '[e-mail]'
    })

    mailer.sendEmailToUser.mockImplementation(() => true)

    expect(mailer.sendEmailToUser).toHaveBeenCalledTimes(1)
    expect(eventResult).toBe(sentEmailSuccessful)
  })
})

This might look a little strange at first, but one aspect to consider is that you should not be sending an actual email when running the test. Imagine a scenario in which we did not mock the mailer utility and its sendEmailToUser method, and we actually stubbed in real inputs into the function, then we would be making real life calls that would send real emails.