Classes in TypeScript

TypeScript adds type-safety and additional features to JavaScript classes, making object-oriented programming more robust and predictable.

Basic Class Structure

class User {
    // Properties with type annotations
    name: string;
    email: string;
    private loginCount: number;

    constructor(name: string, email: string) {
        this.name = name;
        this.email = email;
        this.loginCount = 0;
    }

    // Method with return type
    getInfo(): string {
        return `${this.name} (${this.email})`;
    }
}

const user = new User("Alice", "alice@example.com");

Access Modifiers

TypeScript provides three access modifiers:

class BankAccount {
    public accountHolder: string;      // Accessible everywhere
    private balance: number;           // Only inside the class
    protected accountNumber: string;   // Inside class and subclasses

    constructor(holder: string, initialBalance: number) {
        this.accountHolder = holder;
        this.balance = initialBalance;
        this.accountNumber = Math.random().toString();
    }
}

Parameter Properties

A shorthand way to define and initialize class members:

class Point {
    // Automatically creates and initializes x and y properties
    constructor(
        public x: number,
        public y: number
    ) {}

    getDistance(): number {
        return Math.sqrt(this.x ** 2 + this.y ** 2);
    }
}

Inheritance

Classes can extend other classes:

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

    makeSound(): void {
        console.log("Some sound");
    }
}

class Dog extends Animal {
    constructor(name: string, private breed: string) {
        super(name);
    }

    makeSound(): void {
        console.log("Woof!");
    }

    getInfo(): string {
        return `${this.name} is a ${this.breed}`;
    }
}

Implementing Interfaces

Classes can implement interfaces to ensure they have required properties and methods:

interface Vehicle {
    start(): void;
    stop(): void;
    speed: number;
}

class Car implements Vehicle {
    speed: number = 0;

    start(): void {
        console.log("Engine started");
    }

    stop(): void {
        this.speed = 0;
        console.log("Car stopped");
    }
}

Best Practices

1. Use access modifiers to control encapsulation
2. Keep classes focused and single-purpose
3. Prefer composition over inheritance
4. Use parameter properties for simpler constructors
5. Document public APIs with JSDoc comments

Practice Exercise

Create a Library class that manages a collection of books. Include:

1. Methods to add and remove books
2. A way to search for books
3. Different access levels for properties
4. Implementation of an interface