Classes & OOP: Building Robust Architectures

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TypeScript provides full support for Object-Oriented Programming (OOP). While JavaScript has classes, TypeScript enhances them with type safety, access modifiers, and advanced patterns like abstract classes and interfaces.

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1. Class Structure and Properties

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A class in TypeScript is a blueprint for creating objects. You must define the types of the class members (properties and methods).

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class BankAccount {
    accountNumber: string;
    balance: number;

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constructor(accountNumber: string, initialBalance: number) {
    this.accountNumber = accountNumber;
    this.balance = initialBalance;
}

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deposit(amount: number): void {
    this.balance += amount;
}

}

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2. Access Modifiers: Encapsulation

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Access modifiers control which parts of your code can see and modify class members.

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• public (Default): Accessible from anywhere.
• private: Only accessible within the class itself.
• protected: Accessible within the class and its subclasses (inheritance).

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class Employee {
    public name: string;
    private salary: number;
    protected department: string;

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constructor(name: string, salary: number, dept: string) {
    this.name = name;
    this.salary = salary;
    this.department = dept;
}

}

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3. Parameter Properties: The Pro Shortcut

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TypeScript provides a shorthand syntax to declare and initialize properties directly in the constructor parameters.

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class User {
    // This automatically creates 'public username' and 'private id' properties
    constructor(public username: string, private id: number) {}
}

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const user = new User("alice_dev", 101); console.log(user.username); // OK // console.log(user.id); // Error: id is private

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4. Getters and Setters

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You can use get and set to intercept access to a property. This is useful for validation or logging.

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class Circle {
    private _radius: number = 0;

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get radius(): number {
    return this._radius;
}

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set radius(value: number) {
    if (value < 0) throw new Error("Radius cannot be negative");
    this._radius = value;
}

}

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5. Inheritance and Polymorphism

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Inheritance allows one class to derive from another. Use the extends keyword.

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class Shape {
    constructor(public color: string) {}
    draw() { console.log("Drawing shape..."); }
}

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class Square extends Shape { constructor(color: string, public size: number) { super(color); // Must call the parent constructor }

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// Overriding a method
override draw() {
    console.log(`Drawing a ${this.color} square of size ${this.size}`);
}

}

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6. Abstract Classes

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An Abstract Class cannot be instantiated directly. It serves as a base class for other classes. It can contain abstract methods which must be implemented by subclasses.

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abstract class Logger {
    abstract log(message: string): void; // No implementation here

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printInfo() {
    console.log("System Logger Initialized");
}

}

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class FileLogger extends Logger { log(message: string) { // Implementation for writing to a file } }

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7. Summary

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• Classes encapsulate data and behavior.
• Access Modifiers implement the OOP principle of Encapsulation.
• Inheritance allows for code reuse and logical hierarchies.
• Abstract Classes define templates for other classes to follow.

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