Abstract methods in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an abstract method named display.

Typescript

abstract class Vehicle {
  abstract [1](): void;
}

Drag options to blanks, or click blank then click option'

Aprint

Bshow

Cdisplay

Drun

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different method name than display.

Trying to provide a method body in the abstract method.

The abstract method must be declared with the exact name display as required.

2fill in blank

medium

Complete the code to make Vehicle an abstract class.

Typescript

abstract class [1] {
  abstract display(): void;
}

Drag options to blanks, or click blank then click option'

AVehicle

BCar

CTransport

DMachine

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different class name than Vehicle.

Not declaring the class as abstract (though not asked here).

The class must be named Vehicle to match the abstract method declaration context.

3fill in blank

hard

Fix the error in the subclass by correctly implementing the abstract method display.

Typescript

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

class Car extends Vehicle {
  [1] display(): void {
    console.log('Car display');
  }
}

Drag options to blanks, or click blank then click option'

Aoverride

Babstract

Cprivate

Dpublic

Attempts:

3 left

💡 Hint

Common Mistakes

Using abstract keyword in subclass method implementation.

Using private which restricts access and causes errors.

The method must be declared public to correctly implement the abstract method from the base class.

4fill in blank

hard

Fill both blanks to declare an abstract method calculateSpeed that returns a number.

Typescript

abstract class Vehicle {
  abstract [1](): [2];
}

Drag options to blanks, or click blank then click option'

AcalculateSpeed

Bstring

Cnumber

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong return type like string or void.

Using a different method name.

The method name is calculateSpeed and it returns a number type.

5fill in blank

hard

Fill all three blanks to implement the abstract method calculateSpeed in subclass Bike that returns a number.

Typescript

abstract class Vehicle {
  abstract calculateSpeed(): number;
}

class Bike extends Vehicle {
  [1] [2](): [3] {
    return 50;
  }
}

Drag options to blanks, or click blank then click option'

Apublic

BcalculateSpeed

Cnumber

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using void as return type instead of number.

Using wrong method name or missing access modifier.

The method must be declared public, named calculateSpeed, and return type number.

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