Unit Testing Event Listeners

Notes Used to Craft this Lesson

If you'll recall, our welcome email is being sent via an event listener.

export default class UserService {
  /**
   * Sends a welcome email to the user.
   * The email is sent asynchronously and will not block the execution of the current code.
   * @param data - An object containing the user's full name and email address
   * @returns A promise resolving when the email is sent
   */
  async sendWelcomeEmail(data: { fullName: string | null; email: string }) {
    await emitter.emit('mail:send_welcome_email', data)
  }
}

  

    

      Copied!
      

        

      
    
  
  
    
    
    
    
  
app
services
user_service.ts

What if we also wanted to test the event listener itself?

We only have one listener in our project, so let's create a "listeners" specification for now. You can always split this into a folder with separate specifications per listener, should your number of listeners grow.

node ace make:test listeners --suite=unit

  

    

      Copied!
      

        

      
    
  
  
    
    
    
    
  

For this one, we're just going to name the test after the listener itself. Listeners are pretty specific in purpose as-is, so it'll be descriptive enough.

test.group("Listeners", () => {
  test("SendWelcomeEmail listener", async ({ assert }) => {});
});

  

    

      Copied!
      

        

      
    
  
  
    
    
    
    
  

Okay, again, the listener sends an email, so we'll want to fake the mailer, and we can go ahead and add our cleanup as well.

test.group("Listeners", () => {
  test("SendWelcomeEmail listener", async ({ cleanup }) => {
    const { mails } = mail.fake();
    cleanup(() => mail.restore());

    // todo
  });
});

  

    

      Copied!
      

        

      
    
  
  
    
    
    
    
  

Then, we want to instantiate an instance of our event listener and call the method we want to test.

test.group("Listeners", () => {
  test("SendWelcomeEmail listener", async ({ cleanup }) => {
    const { mails } = mail.fake();
    cleanup(() => mail.restore());

    const user = {
      fullName: "Phillip Fry",
      email: "phillip.fry@test.com",
    };

    const listener = new SendWelcomeEmail();

    await listener.handle(user);
  });
});

  

    

      Copied!
      

        

      
    
  
  
    
    
    
    
  

Finally, we'll make our assertions.

test.group("Listeners", () => {
  test("SendWelcomeEmail listener", async ({ cleanup }) => {
    const { mails } = mail.fake();
    cleanup(() => mail.restore());

    const user = {
      fullName: "Phillip Fry",
      email: "phillip.fry@test.com",
    };

    const listener = new SendWelcomeEmail();

    await listener.handle(user);

    mails.assertSentCount(1);
    mails.assertSent(WelcomeEmailNotification, ({ message }) => {
      return message.hasTo(user.email);
    });
  });
});

  

    

      Copied!