liskov-substitution-principle-2.ts

class EmailBroadcaster  {
  broadcast(event: {}, user: SubscribedUser) {
    // Send email with subscription information 
    // and event details to user.
  }
}

class AnonymousEmailBroadcaster extends EmailBroadcaster {
  broadcast(event: {}, user: User) {
    // Send email with only event details to user.
  }
}

function onAccountCreated(event: {}) {
  const user = getCurrentSubscribedUser(); // returns type SubscribedUser
  (new EmailBroadcaster).broadcast(event, user);
}