liskov-substitution-principle-1.ts

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

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

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