In the last video, I said that Typescript doesn't have a final keyword that can make sure that a given property does not get changed.

That was a mistake on my part.

Typescript does have a keyword of readonly.  It prevents a property of a class from being changed.  You can read more about readonly here: https://www.typescriptlang.org/docs/handbook/classes.html#readonly-modifier

With this in mind, we can update the TicketCreatedListener class to the following:

export class TicketCreatedListener extends Listener<TicketCreatedEvent> {
  readonly subject = Subjects.TicketCreated;

  // ...everything else
}

This change can be made in all other listeners that we create in this course.

Credit for this fix goes to Sergio Municio (https://www.udemy.com/course/microservices-with-node-js-and-react/learn/#questions/11049788/ )