To wrap up Phase 2, let's explore arrays and tuples from chapter 4 further – including readonly arrays, named tuple elements, and optional tuple positions.

readonly arrays: protection against accidental mutation

function showGenres(genres: readonly string[]): void {
  console.log(genres.join(', '));
  genres.push('New'); // error - push() mutates the array, readonly forbids that
}

const allGenres: readonly string[] = ['Fantasy', 'Mystery'];
allGenres[0] = 'Novel'; // error - direct index assignment is forbidden too

EXACTLY like readonly on interface properties (chapter 7), readonly string[] prevents the array from being mutated AFTER being passed – useful as a "contract" when a function should only READ an array, never modify it.

An important difference: readonly array vs. const array

const genres: string[] = ['Fantasy'];
genres.push('Mystery'); // ✓ ALLOWED - const only prevents REASSIGNING the variable,
                        // NOT modifying the array's CONTENTS

genres = ['Novel'];     // ✗ error - THAT'S what const forbids

Named tuple elements

type ShelfPosition = [shelf: string, slot: number, position: number];

function formatPosition(pos: ShelfPosition): string {
  const [shelf, slot, position] = pos; // destructuring as usual
  return `${shelf}, slot ${slot}, position ${position}`;
}

The names shelf/slot/position in the type definition are pure DOCUMENTATION – they change NOTHING about the actual behavior (the tuple remains a normal array with three elements), but considerably improve readability, especially in autocompletion when hovering over the type.

Optional tuple elements

type SearchFilter = [query: string, genre?: string];

const filter1: SearchFilter = ['Tolkien'];
const filter2: SearchFilter = ['Tolkien', 'Fantasy'];
const filter3: SearchFilter = ['Tolkien', 'Fantasy', 'extra']; // error - too many elements

In practice: organizing library data with typed arrays

import { Book } from '../models/Book.js';
import { Author } from '../models/Author.js';

export const tolkien: Author = {
  name: 'J.R.R. Tolkien',
  birthYear: 1892,
  nationality: 'British',
};

export const orwell: Author = {
  name: 'George Orwell',
  birthYear: 1903,
  nationality: 'British',
};

export const libraryStock: readonly Book[] = [
  {
    isbn: '978-0-618-64015-7',
    title: 'The Lord of the Rings',
    author: tolkien,
    publicationYear: 1954,
    pageCount: 1216,
    genres: ['Fantasy'],
    status: 'available',
  },
  {
    isbn: '978-0-452-28423-4',
    title: '1984',
    author: orwell,
    publicationYear: 1949,
    pageCount: 328,
    genres: ['NonFiction', 'Novel'],
    status: 'borrowed',
  },
];

libraryStock: readonly Book[] – the ENTIRE library stock is deliberately modeled as a read-only array. That's intentional: ADDING or REMOVING books should later go through dedicated, controlled functions (chapter 13, classes), not through direct array manipulation from anywhere in the code.