Abstract classes in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of abstract class method call

What is the output of this TypeScript code?

Typescript

abstract class Animal {
  abstract makeSound(): string;
  move(): string {
    return "Moving";
  }
}

class Dog extends Animal {
  makeSound(): string {
    return "Bark";
  }
}

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

A Bark\nundefined

B Bark\nMoving

C Error: Cannot create an instance of an abstract class

D Moving\nBark

💡 Hint

Remember that abstract classes cannot be instantiated directly, but their subclasses can be.

The class Dog extends the abstract class Animal and implements the abstract method makeSound. Calling dog.makeSound() returns "Bark" and dog.move() returns "Moving".

🧠 Conceptual

intermediate

Purpose of abstract classes

What is the main purpose of using an abstract class in TypeScript?

A To create objects directly without subclassing

B To allow multiple inheritance of classes

C To prevent any class from inheriting from it

D To provide a base class with some implemented methods and some methods that must be implemented by subclasses

💡 Hint

Think about how abstract classes help organize code and enforce method implementation.

Abstract classes allow you to define some common behavior and force subclasses to implement specific methods, ensuring a consistent interface.

🔧 Debug

advanced

Identify the error in abstract class usage

What error will this TypeScript code produce?

Typescript

abstract class Vehicle {
  abstract startEngine(): void;
}

const v = new Vehicle();

A Error: Cannot create an instance of an abstract class

B Error: Abstract method must have an implementation

C No error, instance created successfully

D Error: Missing constructor in abstract class

💡 Hint

Can you create an object directly from an abstract class?

Abstract classes cannot be instantiated directly. Trying to create an instance of Vehicle causes a compile-time error.

📝 Syntax

advanced

Correct syntax for abstract method

Which option shows the correct syntax for declaring an abstract method in a TypeScript abstract class?

Typescript

abstract class Shape {
  // Choose the correct abstract method declaration
}

A abstract calculateArea(): number;

B abstract calculateArea() { return 0; }

C calculateArea(): abstract number;

D abstract calculateArea: () => number;

💡 Hint

Abstract methods do not have a body and end with a semicolon.

Abstract methods are declared with the abstract keyword and have no implementation (no body). Option A is correct.

🚀 Application

expert

Number of methods implemented in subclass

Given this abstract class and subclass, how many methods does the Circle class implement?

Typescript

abstract class Shape {
  abstract area(): number;
  abstract perimeter(): number;
  description(): string {
    return "A shape";
  }
}

class Circle extends Shape {
  radius: number;
  constructor(radius: number) {
    super();
    this.radius = radius;
  }
  area(): number {
    return Math.PI * this.radius ** 2;
  }
  perimeter(): number {
    return 2 * Math.PI * this.radius;
  }
}

A 0

B 3

C 2

D 1

💡 Hint

Count only the methods that must be implemented by the subclass.

The abstract class Shape has two abstract methods: area and perimeter. The subclass Circle implements both. The description method is already implemented in the abstract class and does not need to be redefined.