Why Abstract classes in Typescript? - Purpose & Use Cases

The Big Idea

What if you could create a perfect plan that every part of your program must follow, saving you from messy, repeated code?

The Scenario

Imagine you are building a game with different types of characters like warriors and mages. You want each character to have some common actions like moving and attacking, but each type does these actions differently. Without a clear plan, you might write the same code again and again for each character, or forget to add important actions.

The Problem

Writing repeated code for each character wastes time and causes mistakes. If you forget to add an action to one character, the game breaks. It's hard to keep track of what every character should do, and changing common behavior means updating many places manually.

The Solution

Abstract classes let you create a blueprint for characters with common actions defined but not fully implemented. Each character class then fills in the details. This way, you write shared code once and ensure every character follows the same rules, reducing errors and saving time.

Before vs After

✗ Before

class Warrior {
  move() { /* code */ }
  attack() { /* code */ }
}
class Mage {
  move() { /* code */ }
  attack() { /* code */ }
}

✓ After

abstract class Character {
  abstract move(): void;
  abstract attack(): void;
}
class Warrior extends Character {
  move() { /* code */ }
  attack() { /* code */ }
}
class Mage extends Character {
  move() { /* code */ }
  attack() { /* code */ }
}

What It Enables

Abstract classes make your code organized and safe by forcing all related classes to share the same structure while allowing their own unique behavior.

Real Life Example

Think of a car factory where every car must have an engine and wheels, but each model has different designs. An abstract class is like the factory blueprint ensuring every car has these parts, while letting each model customize how they work.

Key Takeaways

Abstract classes provide a shared blueprint for related classes.

They prevent code duplication and enforce consistent structure.

They allow different classes to implement their own specific behavior.

Practice

1. What is true about an abstract class in TypeScript?

easy

A. Abstract classes are the same as interfaces.

B. Abstract classes cannot have any methods implemented.

C. You cannot create an instance of an abstract class directly.

D. You must use the new keyword to instantiate an abstract class.

Solution

1. Step 1: Understand abstract class instantiation rules

   Abstract classes cannot be instantiated directly, so you cannot create objects from them using new.

2. Step 2: Compare other options

   Abstract classes can have implemented methods, are different from interfaces, and cannot be instantiated with new.

3. Final Answer:

   You cannot create an instance of an abstract class directly. -> Option C

4. Quick Check:

   Abstract class instantiation = not allowed [OK]

Hint: Remember: abstract classes can't be directly instantiated [OK]

Common Mistakes:

• Thinking abstract classes are interfaces
• Assuming abstract classes can't have implemented methods
• Trying to instantiate abstract classes directly

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

easy

A. abstract class Vehicle {}

B. class abstract Vehicle {}

C. abstract Vehicle class {}

D. class Vehicle abstract {}

Solution

1. Step 1: Recall TypeScript abstract class syntax

   The correct syntax places the keyword abstract before class, followed by the class name.

2. Step 2: Check each option

   Only abstract class Vehicle {} correctly uses abstract class Vehicle {}. Others have incorrect keyword order.

3. Final Answer:

   abstract class Vehicle {} -> Option A

4. Quick Check:

   Abstract class syntax = abstract class Name {} [OK]

Hint: Put 'abstract' before 'class' keyword [OK]

Common Mistakes:

• Placing 'abstract' after 'class'
• Mixing keyword order
• Omitting 'abstract' keyword

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

abstract class Animal {
  abstract makeSound(): void;
  move(): void {
    console.log('Moving');
  }
}

class Dog extends Animal {
  makeSound(): void {
    console.log('Bark');
  }
}

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

medium

A. Moving\nBark

B. No output

C. Error: Cannot create instance of abstract class

D. Bark\nMoving

Solution

1. Step 1: Understand class inheritance and method calls

   The class Dog extends Animal and implements makeSound(). Calling dog.makeSound() prints 'Bark'.

2. Step 2: Check the move() method call

   The move() method is implemented in Animal and prints 'Moving'. Calling dog.move() prints 'Moving'.

3. Final Answer:

   Bark Moving -> Option D

4. Quick Check:

   Subclass methods run, abstract class methods inherited [OK]

Hint: Abstract methods must be implemented; inherited methods run normally [OK]

Common Mistakes:

• Thinking abstract class instance is created
• Confusing method call order
• Expecting errors on calling inherited methods

4. Identify the error in the following TypeScript code:

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

class Circle extends Shape {
  radius: number;
  constructor(radius: number) {
    super();
    this.radius = radius;
  }
}

medium

A. Circle must implement the abstract method area()

B. Abstract class cannot have abstract methods

C. radius property should be private

D. Missing call to super() in Circle constructor

Solution

1. Step 1: Check abstract method implementation

   The abstract class Shape declares an abstract method area(). Any subclass must implement this method.

2. Step 2: Verify Circle class methods

   The Circle class does not implement area(), so this causes a compile-time error.

3. Final Answer:

   Circle must implement the abstract method area() -> Option A

4. Quick Check:

   Abstract methods require subclass implementation [OK]

Hint: All abstract methods must be implemented in subclasses [OK]

Common Mistakes:

• Forgetting to implement abstract methods
• Thinking super() call is not mandatory when base has no constructor
• Confusing abstract methods with optional methods

5. Given the abstract class below, which implementation correctly uses it to calculate the area of a rectangle?

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

// Choose the correct Rectangle class implementation

hard

A. class Rectangle extends Shape {
width: number;
height: number;
constructor(width: number, height: number) {
this.width = width;
this.height = height;
}
area(): number {
return this.width + this.height;
}
}

B. class Rectangle extends Shape {
width: number;
height: number;
constructor(width: number, height: number) {
super();
this.width = width;
this.height = height;
}
area(): number {
return this.width * this.height;
}
}

C. class Rectangle extends Shape {
width: number;
height: number;
constructor(width: number, height: number) {
super();
this.width = width;
this.height = height;
}
area(): number {
return this.width + this.height;
}
}

D. class Rectangle extends Shape {
width: number;
height: number;
constructor(width: number, height: number) {
super();
this.width = width;
this.height = height;
}
}

Solution

1. Step 1: Check constructor and super() call

   Since Shape has no constructor, calling super() is required before using this in the subclass constructor.

2. Step 2: Verify area() method correctness

   class Rectangle extends Shape { width: number; height: number; constructor(width: number, height: number) { super(); this.width = width; this.height = height; } area(): number { return this.width * this.height; } } correctly implements area() returning width * height, which is the rectangle's area.

3. Step 3: Check other options for errors

   class Rectangle extends Shape { width: number; height: number; constructor(width: number, height: number) { this.width = width; this.height = height; } area(): number { return this.width + this.height; } } return width + height, which is incorrect for area. class Rectangle extends Shape { width: number; height: number; constructor(width: number, height: number) { super(); this.width = width; this.height = height; } } lacks area() implementation, causing error.

4. Final Answer:

   class Rectangle extends Shape { width: number; height: number; constructor(width: number, height: number) { super(); this.width = width; this.height = height; } area(): number { return this.width * this.height; } } -> Option B

5. Quick Check:

   Correct area method and constructor = class Rectangle extends Shape { width: number; height: number; constructor(width: number, height: number) { super(); this.width = width; this.height = height; } area(): number { return this.width * this.height; } } [OK]

Hint: Area of rectangle = width * height; implement abstract method [OK]

Common Mistakes:

• Returning sum instead of product for area
• Omitting abstract method implementation
• Forgetting to assign constructor parameters