Defining classes & constructors

A class in TypeScript defines a blueprint for creating objects with:

• Properties (state)

• Methods (behavior)

• A constructor for initialization

✅ Basic Class with Constructor

class Person {
  name: string;
  age: number;

  // Constructor
  constructor(name: string, age: number) {
    this.name = name;
    this.age = age;
  }

  greet(): void {
    console.log(`Hello, my name is ${this.name}.`);
  }
}

const p1 = new Person("Codeveda", 21);
p1.greet();                                // Hello, my name is Codeveda.

📦 Constructor Parameters

• You can annotate the types of constructor parameters.

• Inside the constructor, you assign those values to class properties using this.

🔐 Access Modifiers (Public, Private, Protected)

You can control access to class members:

class User {
  public name: string;
  private password: string;
  protected role: string;

  constructor(name: string, password: string, role: string) {
    this.name = name;
    this.password = password;
    this.role = role;
  }

  login(): string {
    return `${this.name} logged in`;
  }
}

const user = new User("Bhaskar", "secret123", "Admin");

console.log(user.name);     //Bhaskar
console.log(user.password); //Error: Can't be accessed because it is Private
console.log(user.role);     //Error: Can't be accessed because it is Protected
console.log(user.login());  //Bhaskar logged in

✨ Shortcut: Declare and Assign in Constructor

You can combine property declaration and initialization directly in the constructor:

class Product {
  constructor(
    public name: string,
    private price: number
  ) {}

  getPrice(): number {
    return this.price;
  }
}

const product = new Product("Laptop", 50000);

console.log(product.name);       // Laptop
console.log(product.getPrice()); // 50000

console.log(product.price);      // ❌ Error

This avoids writing this.name = name etc.

🔁 Class with Methods

class Circle {
  radius: number;

  constructor(radius: number) {
    this.radius = radius;
  }

  getArea(): number {
    return Math.PI * this.radius ** 2;
  }
}

const c = new Circle(5);
console.log(c.getArea()); // 78.54...

🧠 Optional & Readonly Properties

class Book {
  readonly id: number; //Readonly
  title: string;
  author?: string;     //Optional

  constructor(id: number, title: string, author?: string) {
    this.id = id;
    this.title = title;
    this.author = author;
  }
}

✅ Summary Table