Abstract methods in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of abstract method implementation

What is the output of this TypeScript code that uses an abstract method?

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 "Moving" followed by "Bark"

B "Bark" followed by "Moving"

C Compilation error because abstract method is not implemented

D Runtime error because abstract method cannot be called

💡 Hint

Remember that abstract methods must be implemented in subclasses.

The Dog class implements the abstract method makeSound, so calling dog.makeSound() returns "Bark". The move method is concrete and returns "Moving". The console logs print these in order.

❓ Predict Output

intermediate

What error occurs with missing abstract method implementation?

What error will this TypeScript code produce?

Typescript

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

class Car extends Vehicle {
  // Missing startEngine implementation
}

const myCar = new Car();

A Compilation error: Abstract class 'Vehicle' cannot be instantiated.

B Runtime error: Cannot instantiate abstract class 'Vehicle'.

C No error, code runs fine.

D Compilation error: Non-abstract class 'Car' does not implement inherited abstract member 'startEngine'.

💡 Hint

Check if all abstract methods are implemented in subclasses.

The Car class does not implement the abstract method startEngine, so TypeScript raises a compilation error.

🔧 Debug

advanced

Identify the error in abstract method usage

Find the error in this TypeScript code involving abstract methods.

Typescript

abstract class Shape {
  abstract area(): number;
}

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

const shape = new Shape();

A Property 'radius' is not initialized.

B Missing implementation of abstract method 'area' in Circle.

C Cannot create an instance of an abstract class 'Shape'.

D No error, code runs correctly.

💡 Hint

Abstract classes cannot be instantiated directly.

The code tries to create an instance of the abstract class Shape, which is not allowed and causes a compilation error.

📝 Syntax

advanced

Which option correctly declares an abstract method?

Which of the following is the correct syntax to declare an abstract method in a TypeScript abstract class?

A abstract class A { abstract method(): void; }

B abstract class A { method(): abstract void; }

C abstract class A { void abstract method(); }

D abstract class A { method(): void abstract; }

💡 Hint

Look at the placement of the 'abstract' keyword.

The correct syntax places 'abstract' before the method name and omits the method body.

🚀 Application

expert

Determine the number of abstract methods in the class

Given this TypeScript code, how many abstract methods does class B have after compilation?

Typescript

abstract class A {
  abstract foo(): void;
  abstract bar(): void;
  baz(): void {}
}

abstract class B extends A {
  foo(): void {}
  abstract qux(): void;
}

A 2

B 0

C 3

D 1

💡 Hint

Count abstract methods not implemented in class B.

Class B implements foo(), so it's no longer abstract there. bar() is not implemented in B, but it was abstract in A and not overridden, so it remains abstract in B. qux() is declared abstract in B. So B has 2 abstract methods: bar() and qux(). But since bar() is inherited abstract and not implemented, B remains abstract with 2 abstract methods. However, the question asks how many abstract methods does B have after compilation. In TypeScript, abstract methods inherited but not implemented remain abstract in subclass. So B has 2 abstract methods: bar() and qux(). The correct answer is 2.