Welcome back! I am Mihir, and in this lesson we will learn abstract classes in TypeScript.

An abstract class is a base class that cannot be created directly. It is meant to be extended by other classes.

Basic Abstract Class

abstract class Animal {
  abstract makeSound(): void;

  move(): void {
    console.log("Moving...");
  }
}

makeSound is abstract, so child classes must implement it.

move is a normal method and can be reused.

Extending an Abstract Class

class Dog extends Animal {
  makeSound(): void {
    console.log("Woof");
  }
}

const dog = new Dog();
dog.makeSound();
dog.move();

Dog must implement makeSound.

You Cannot Instantiate Abstract Classes

Invalid:

const animal = new Animal();

Abstract classes are only for inheritance.

Abstract Methods

Abstract methods have no body.

abstract class PaymentProcessor {
  abstract pay(amount: number): void;
}

Child classes provide the actual behavior.

class StripeProcessor extends PaymentProcessor {
  pay(amount: number): void {
    console.log(`Paid ${amount} with Stripe`);
  }
}

Abstract Properties

You can also require properties.

abstract class Shape {
  abstract name: string;

  abstract getArea(): number;
}

class Square extends Shape {
  name = "square";

  constructor(public size: number) {
    super();
  }

  getArea(): number {
    return this.size * this.size;
  }
}

Abstract Class vs Interface

Use an interface when you only need a contract.

Use an abstract class when you need a contract plus shared implementation.

abstract class BaseService {
  protected log(message: string) {
    console.log(`[Service] ${message}`);
  }

  abstract run(): void;
}

Child classes get the shared log method.

Quick Recap

• Abstract classes cannot be instantiated directly.
• They are meant to be extended.
• Abstract methods and properties must be implemented by child classes.
• Abstract classes can include shared concrete methods.
• Use them when subclasses need common behavior.

Next up, we will learn Static Members →.