Our library should also manage AUDIOBOOKS alongside regular books – with almost the same properties, but a runtime length instead of a page count. Inheritance avoids duplicating the shared code.

A shared base class: Medium

import { Author } from './Author.js';
import { Genre } from './Genre.js';
import { BookStatus } from './Book.js';

export class Medium {
  constructor(
    public readonly isbn: string,
    public title: string,
    public author: Author,
    public publicationYear: number,
    public genres: Genre[],
    public status: BookStatus,
  ) {}

  description(): string {
    return `${this.title} by ${this.author.name} (${this.publicationYear})`;
  }
}

Medium uses constructor property promotion from chapter 12 for ALL shared properties – deliberately a CLASS instead of an interface, since we already want to bundle BEHAVIOR (description()) with the data, shared by ALL derived types.

Book and Audiobook as derived classes

import { Medium } from './Medium.js';
import { Author } from './Author.js';
import { Genre } from './Genre.js';

export type BookStatus = 'available' | 'borrowed' | 'reserved';

export class Book extends Medium {
  constructor(
    isbn: string,
    title: string,
    author: Author,
    publicationYear: number,
    genres: Genre[],
    status: BookStatus,
    public pageCount: number,
  ) {
    super(isbn, title, author, publicationYear, genres, status);
  }
}

import { Medium } from './Medium.js';
import { Author } from './Author.js';
import { Genre } from './Genre.js';
import { BookStatus } from './Book.js';

export class Audiobook extends Medium {
  constructor(
    isbn: string,
    title: string,
    author: Author,
    publicationYear: number,
    genres: Genre[],
    status: BookStatus,
    public runtimeMinutes: number,
    public narrator: string,
  ) {
    super(isbn, title, author, publicationYear, genres, status);
  }

  // overrides the inherited method with an extended version
  override description(): string {
    return `${super.description()}, narrated by ${this.narrator}`;
  }
}

extends, super, and override in detail

• extends Medium – Book/Audiobook INHERIT ALL properties and methods of Medium, without rewriting them.
• super(...) – MUST be called as the FIRST statement in a derived class's constructor, calls the BASE class's constructor to initialize the inherited properties.
• override description() – REPLACES the inherited method with a custom implementation. The override keyword is an explicit STATEMENT OF INTENT: "I'm deliberately overriding an inherited method" – TypeScript shows an error if Medium had NO method called description at all (typo protection).
• super.description() – calls the ORIGINAL, inherited version of the method before Audiobook APPENDS its own text – reuse instead of duplication.

Polymorphism: treating different media uniformly

const media: Medium[] = [
  new Book('123', 'The Hobbit', tolkien, 1937, ['Fantasy'], 'available', 310),
  new Audiobook('456', 'The Hobbit', tolkien, 1937, ['Fantasy'], 'available', 620, 'Rob Inglis'),
];

media.forEach((medium) => console.log(medium.description()));
// Book uses the INHERITED description(), Audiobook the OVERRIDDEN one -
// same call, different behavior, depending on the ACTUAL type

media: Medium[] can hold BOTH types, because Book AND Audiobook both ARE Medium (an "is-a" relationship) – that's EXACTLY "polymorphism": the same method call (description()) behaves differently depending on the actual object type, without the calling code having to check that itself.