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abstract class

Learn abstract classes.

What is an abstract class?

A class marked abstract cannot be instantiated directly; it only serves as a base class for subclasses. Subclasses inherit from it with extends. abstract methods are declared without a body and must be implemented in the subclass.

abstract class Shape {
  abstract area(): number;
}
class Circle extends Shape {
  area() { return 3.14 * 2 * 2; }
}

Abstract method and concrete method

An abstract class can contain both abstract methods (no body, implemented in subclass) and concrete methods (have a body, inherited by subclass). The subclass must implement only abstract methods; concrete methods may be overridden optionally.

abstract class Animal {
  abstract makeSound(): void;
  introduce() { console.log("I am an animal"); }
}

Practice Compute square area

Define abstract class Sekil with abstract alan(): number inside. Then make Kare extend Sekil, take a kenar field, and implement alan returning kenar * kenar. Print new Kare(4).alan() with console.log.

Starter code

index.ts
// Sekil adinda abstract sinif tanimla
// abstract alan metodu number donsun
// Kare sinifi Sekil'i genişletsin
// alan metodunu kenar*kenar olarak uygula

// console.log(new Kare(4).alan());

Sample solution

index.ts
abstract class Sekil {
  abstract alan(): number;
}
class Kare extends Sekil {
  kenar: number;
  constructor(kenar: number) { super(); this.kenar = kenar; }
  alan(): number { return this.kenar * this.kenar; }
}
console.log(new Kare(4).alan());

Expected output

16

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Which of the following is true for an abstract class?

  • Can contain only static members
  • Cannot be instantiated directly with new Correct answer
  • Can have only one subclass
  • Methods cannot be defined inside it

Explanation: Abstract classes cannot be instantiated directly; subclasses inherit with extends and implement abstract methods.

2 Abstract class declaration

Question 
 ____ class Sekil {
  abstract alan(): number;
}
Answer 
 abstract class Sekil {
  abstract alan(): number;
}

Explanation: The `abstract` keyword is used to prevent direct instantiation and make the class a base class.

3 Abstract class parts

Term Meaning
abstract class Base class declaration
abstract metot Bodyless, implemented in subclass
extends Subclass inherits
somut metot Has body, inherited by subclass

All lessons in Intermediate

1 Classes

  1. Class basics
  2. Constructor and fields
  3. Methods
  4. Static members
  5. Summary - Classes

2 Access modifiers

  1. public
  2. private
  3. protected
  4. Parameter properties
  5. Summary - Access modifiers

3 Readonly properties

  1. readonly fields
  2. Readonly parameter
  3. Readonly vs const
  4. Summary - Readonly

4 Generics

  1. What is a generic?
  2. Generic functions
  3. Generic classes
  4. Multiple type parameters
  5. Summary - Generics

5 Generic constraints

  1. extends constraint
  2. keyof constraint
  3. Default type parameter
  4. Summary - Generic constraints

6 Utility types

  1. Partial
  2. Required and Pick
  3. Omit and Record
  4. Summary - Utility types

7 keyof and typeof

  1. keyof operator
  2. typeof operator
  3. Indexing with keyof
  4. Summary - keyof and typeof

8 Mapped types

  1. What is a mapped type?
  2. Readonly mapped type
  3. Optional mapped type
  4. Summary - Mapped types

9 Conditional types

  1. What is a conditional type?
  2. Distributive conditional
  3. Logic with conditional
  4. Summary - Conditional types

10 infer

  1. infer keyword
  2. Extracting from Promise
  3. Array element type
  4. Summary - infer

11 Decorators

  1. What is a decorator?
  2. Method decorator
  3. Property decorator
  4. Decorator factories
  5. Summary - Decorators

12 Modules

  1. export and import
  2. Default export
  3. Re-export
  4. Type-only import
  5. Summary - Modules

13 Namespaces

  1. What is a namespace?
  2. Nested namespace
  3. Namespace vs module
  4. Summary - Namespaces

14 Declaration files

  1. What is .d.ts?
  2. Ambient module
  3. Triple-slash directive
  4. Summary - Declaration files

15 Advanced classes

  1. abstract class
  2. implements
  3. Get and set
  4. Summary - Advanced classes

16 Type assertion

  1. as keyword
  2. angle bracket syntax
  3. Type assertion vs casting
  4. Casting from unknown
  5. Summary - Type assertion

17 Project: Library package

  1. Package design
  2. Exporting types
  3. Generic helpers
  4. Declaration file
  5. Integration

18 Project: Quiz app

  1. Question model
  2. Question pool
  3. Answer check
  4. Score calculation
  5. Integration

19 Project: Weather app

  1. Weather model
  2. API response type
  3. Data transform
  4. Display
  5. Integration

20 Intermediate final

  1. Challenge 1: Classes
  2. Challenge 2: Generics
  3. Challenge 3: Type systems
  4. Challenge 4: Mini app