# Testing the Event

Recall how we implemented the `userHasRegisteredEvent`.

*File: src/events/userHasRegisteredEvent.js*

```javascript
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*

```javascript
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: 'john@john.com'
    })

    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.


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