Abstract methods in Typescript - Step-by-Step Execution

Concept Flow - Abstract methods

Define abstract class

↓

Declare abstract method (no body)

↓

Create subclass

↓

Implement abstract method in subclass

↓

Instantiate subclass and call method

↓

Method runs with subclass implementation

Abstract methods are declared without a body in an abstract class and must be implemented in subclasses before use.

Execution Sample

Typescript

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

class Dog extends Animal {
  makeSound() { console.log('Woof!'); }
}

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

This code defines an abstract method makeSound in Animal, implements it in Dog, then calls it.

Execution Table

Step	Action	Evaluation	Result
1	Define abstract class Animal with abstract method makeSound	No method body	Animal cannot be instantiated
2	Define class Dog extending Animal	Dog must implement makeSound	Dog class created
3	Implement makeSound in Dog	Method prints 'Woof!'	Dog.makeSound ready
4	Create instance dog of Dog	dog is an object of Dog	dog created
5	Call dog.makeSound()	Runs Dog's makeSound	Output: Woof!
6	Try to instantiate Animal	Error: Cannot create instance of abstract class	Instantiation fails

💡 Execution stops after dog.makeSound() prints 'Woof!' and abstract class cannot be instantiated

Variable Tracker

Variable	Start	After Step 4	After Step 5	Final
Animal	abstract class defined	unchanged	unchanged	unchanged
Dog	class defined	unchanged	unchanged	unchanged
dog	undefined	instance of Dog	instance of Dog	instance of Dog

Key Moments - 3 Insights

Why can't we create an instance of the abstract class Animal?

Because Animal has an abstract method without implementation, TypeScript prevents instantiation to ensure subclasses provide the method (see execution_table step 6).

What happens if Dog does not implement the abstract method makeSound?

TypeScript will give an error because Dog must implement all abstract methods from Animal (see execution_table step 2).

When dog.makeSound() is called, which method runs?

The implementation in Dog runs, printing 'Woof!' (see execution_table step 5).

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 6?

A An instance of Animal is created successfully

B Error because Animal is abstract and cannot be instantiated

C Dog's makeSound method is called

D Nothing happens

💡 Hint

Check the 'Action' and 'Result' columns at step 6 in the execution_table

At which step is the dog object created?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look for the step where 'Create instance dog of Dog' happens in execution_table

If Dog did not implement makeSound, what would happen?

A TypeScript would give a compile-time error

B Dog would compile but throw error at runtime

C Dog would be abstract too

D Nothing, code runs fine

💡 Hint

Refer to key_moments about implementation requirements and execution_table step 2

Concept Snapshot

abstract class ClassName {
  abstract methodName(): returnType;
}

- Abstract methods have no body.
- Subclasses must implement all abstract methods.
- Abstract classes cannot be instantiated directly.
- Calling abstract methods runs subclass implementation.

Full Transcript

Abstract methods in TypeScript are declared inside abstract classes without a method body. These methods must be implemented by any subclass. The abstract class itself cannot be instantiated. In the example, Animal is an abstract class with an abstract method makeSound. Dog extends Animal and implements makeSound by printing 'Woof!'. We create an instance of Dog and call makeSound, which outputs 'Woof!'. Trying to instantiate Animal directly causes an error. This ensures that abstract methods enforce a contract for subclasses to provide specific behavior.