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.

Practice

1. What is true about abstract methods in TypeScript?

easy

A. They have no body and must be implemented by subclasses.

B. They can be called directly without implementation.

C. They can only be used in interfaces, not classes.

D. They are optional to implement in child classes.

Solution

1. Step 1: Understand abstract method definition

   Abstract methods declare a method signature without a body inside an abstract class.

2. Step 2: Check subclass requirements

   Child classes must provide an implementation for all abstract methods.

3. Final Answer:

   They have no body and must be implemented by subclasses. -> Option A

4. Quick Check:

   Abstract methods require implementation [OK]

Hint: Abstract methods have no body and force subclass implementation [OK]

Common Mistakes:

• Thinking abstract methods can be called directly
• Confusing abstract methods with interface methods
• Assuming abstract methods are optional to implement

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

easy

A. abstract myMethod() { }

B. abstract myMethod(): void;

C. myMethod(): void;

D. abstract function myMethod() {}

Solution

1. Step 1: Recall abstract method syntax

   Abstract methods are declared with the abstract keyword and no body, ending with a semicolon.

2. Step 2: Analyze options

   abstract myMethod(): void; matches the correct syntax. Syntaxes that incorrectly include a body are invalid. myMethod(): void; lacks the abstract keyword.

3. Final Answer:

   abstract myMethod(): void; -> Option B

4. Quick Check:

   Abstract method syntax = abstract myMethod(): void; [OK]

Hint: Abstract methods have no body and end with a semicolon [OK]

Common Mistakes:

• Adding method body to abstract methods
• Omitting the abstract keyword
• Using function keyword inside class

3. What will be the output of the following TypeScript code?

abstract class Animal {
  abstract makeSound(): string;
}

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

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

medium

A. "Woof!"

B. Error: Cannot instantiate abstract class

C. undefined

D. Error: makeSound method not implemented

Solution

1. Step 1: Understand class hierarchy

   Animal is abstract with an abstract method makeSound. Dog extends Animal and implements makeSound returning "Woof!".

2. Step 2: Check instantiation and method call

   Dog is instantiated (allowed), and calling makeSound() returns "Woof!".

3. Final Answer:

   "Woof!" -> Option A

4. Quick Check:

   Implemented abstract method returns "Woof!" [OK]

Hint: Abstract class can't instantiate, but subclass can [OK]

Common Mistakes:

• Trying to instantiate abstract class directly
• Forgetting to implement abstract method
• Expecting undefined instead of returned string

4. Identify the error in the following TypeScript code:

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

class Car extends Vehicle {
  startEngine() {
    console.log("Engine started");
  }
}

const myCar = new Vehicle();

medium

A. startEngine method must be static.

B. Missing return type in startEngine method.

C. Cannot instantiate abstract class Vehicle.

D. Car class must be abstract.

Solution

1. Step 1: Check instantiation of abstract class

   The code tries to create an instance of Vehicle, which is abstract and cannot be instantiated.

2. Step 2: Verify method implementation and class declarations

   The Car class correctly implements startEngine. The method return type is optional in implementation. No need for static method or abstract Car class.

3. Final Answer:

   Cannot instantiate abstract class Vehicle. -> Option C

4. Quick Check:

   Abstract classes cannot be instantiated [OK]

Hint: Abstract classes cannot be instantiated directly [OK]

Common Mistakes:

• Trying to create instance of abstract class
• Thinking method must be static
• Confusing missing return type as error

5. Given the abstract class below, which implementation correctly uses abstract methods to calculate area?

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

// Choose the correct subclass implementation:

hard

A. class Square extends Shape {
constructor(private side: number) { super(); }
area(): string { return `${this.side}`; }
}

B. class Square extends Shape {
constructor(private side: number) {}
area() { return side * side; }
}

C. class Square extends Shape {
area(): number { return 0; }
}

D. class Square extends Shape {
constructor(private side: number) { super(); }
area(): number { return this.side * this.side; }
}

Solution

1. Step 1: Check constructor and super call

   class Square extends Shape { constructor(private side: number) { super(); } area(): number { return this.side * this.side; } } correctly calls super() in constructor, required for subclass of abstract class.

2. Step 2: Verify method signature and implementation

   class Square extends Shape { constructor(private side: number) { super(); } area(): number { return this.side * this.side; } } implements area() returning a number, matching abstract method signature. Other implementations have errors: one misses super() and uses an undefined variable, one returns 0 ignoring side, one returns string instead of number.

3. Final Answer:

   class Square extends Shape { constructor(private side: number) { super(); } area(): number { return this.side * this.side; } } -> Option D

4. Quick Check:

   Correct subclass calls super and matches method signature [OK]

Hint: Subclass must call super() and match abstract method signature [OK]

Common Mistakes:

• Omitting super() call in constructor
• Mismatching return types in method
• Using undefined variables inside method