Determining an arguments type from an argument (typescript)

example.ts

interface Author {
  id: string;
  firstName: string;
  lastName: string;
  address: Address;
  email: string;
  posts: Post[]
}

interface Post {
  id: string;
  author: Author;
  content: string;
}



function instanceOfPerson(object: any): object is Persone {
  return "firstName" in object
}

function instanceOfPost(object: any): object is Post {
  return "content" in object
}



function determinePostOrPerson(obj: Post | Page){
  if(instanceOfPerson(obj)){
    return "Person"
  }
  if(instanceOfPage(obj)){
    return "Page"
  }
}