models.ts

interface Movie {
  id: number;
  title: string;
}

interface User {
  firstName: string;
  lastName: string;
  age: number;
  favoriteMovies: Array<Movie | number>;
  /*
   notice how we supposed that this property
   may be either an Array of Movie objects
   or of numerical identificators
  */
}

class UserModel implements User {
  firstName: string;
  lastName: string;
  age: number;
  favoriteMovies: Array<Movie | number>;

  constructor(source: User){
    this.firstName = source.firstName;
    this.lastName = source.lastName;
    this.age = source.age;
    this.favoriteMovies = source.favoriteMovies.map((movie: Movie) => movie.id);
    /*
     we moved the data manipulation to this separate class,
     which is also a valid representation of a User model,
     so no unnecessary clutter here
    */
  }

}