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Classes

This lesson covers classes, access modifiers, readonly, implements, and the differences between classes and interfaces.

Class basics

class Lesson {
  id: string
  title: string

  constructor(id: string, title: string) {
    this.id = id
    this.title = title
  }

  summary(): string {
    return `${this.id}: ${this.title}`
  }
}

Create an instance with new.

const lesson = new Lesson('1', 'TypeScript basics')

Parameter properties

TypeScript can declare and assign properties directly in the constructor.

class Course {
  constructor(
    public readonly id: string,
    public title: string,
    private lessons: string[] = []
  ) {}

  addLesson(lesson: string): void {
    this.lessons.push(lesson)
  }
}

Access modifiers

Modifier Access
public Anywhere
private Only inside the class
protected Inside the class and subclasses 
class User {
  public name: string
  private passwordHash: string
  protected role: string
}

implements

A class can implement an interface.

interface Printable {
  print(): string
}

class Report implements Printable {
  constructor(public content: string) {}

  print(): string {
    return this.content
  }
}

readonly

A readonly property can be assigned once during initialization.

class Point {
  constructor(public readonly x: number, public readonly y: number) {}
}

Inheritance

class Animal {
  constructor(public name: string) {}

  speak(): string {
    return `${this.name} makes a sound`
  }
}

class Dog extends Animal {
  speak(): string {
    return `${this.name} barks`
  }
}

Classes versus interfaces

  • Use class when you need runtime behavior, constructors, or instances.
  • Use interface when you only need to describe a shape.

Hands-on: library catalog

  1. Create src/library.ts.
  2. Define interface Borrowable { borrow(): void; returnItem(): void }.
  3. Create an abstract-ish base class CatalogItem with id, title, and a description() method.
  4. Create class Book extends CatalogItem implements Borrowable with author, isBorrowed, and methods borrow/returnItem.
  5. Create class DVD extends CatalogItem implements Borrowable with runtimeMinutes.
  6. Create an array of CatalogItem[], add a Book and a DVD, and call description() on each.
  7. Try to access a private property from outside the class and observe the error.