liskov-substitution-principle-3.ts

interface EmailBroadcaster {
  broadcast(event: Event);
}

class AnonymousEmailBroadcaster implements EmailBroadcaster {
  constructor(protected user: User) {}
  broadcast(event: Event) {
    // Send email with event details to user.
  }
}

class SubscriptionEmailBroadcaster implements EmailBroadcaster {
  constructor(protected user: SubscribedUser) {}
  broadcast(event: Event) {
    // Send email with subscription information 
    // and event details to user.
  }
}

function onAccountCreated(event: Event) {
  const user = getCurrentUser(); // returns type User or SubscribedUser
  (new AnonymousEmailBroadcaster(user)).broadcast(event);
}