dependency-inversion-2.ts

interface Broadcaster {
  send(event: Event, user: User)
}

class EmailBroadcaster implements Broadcaster {
  send(event: Event, user: User) {
    const description = `New event ${event.name} occured at ${event.getTimestamp()}`
    emailService.send({ to: user.email, subject: event.name, body: description })
  }
}

class NotificationHandler {
  constructor(private broadcaster: Broadcaster) {}

  broadcast(event: Event, user: User) {
    this.broadcaster.send(event, user)
  }
}

function onAccountCreated(event: Event, user: User) {
  const notifications = new NotificationHandler(new EmailBroadcaster())
  notifications.broadcast(event, user)
}