Abstract classes in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an abstract class in TypeScript?

An abstract class is a class that cannot be instantiated directly. It is meant to be a base class for other classes and can contain abstract methods that must be implemented by subclasses.

beginner

Can you create an object from an abstract class?

No, you cannot create an object directly from an abstract class. You must create an object from a subclass that extends the abstract class and implements its abstract methods.

beginner

What is an abstract method?

An abstract method is a method declared in an abstract class without an implementation. Subclasses must provide their own implementation of this method.

beginner

How do you declare an abstract class and an abstract method in TypeScript?

Use the keyword <code>abstract</code> before the class name to declare an abstract class. Use <code>abstract</code> before a method signature inside the class to declare an abstract method.

intermediate

Why use abstract classes instead of interfaces in TypeScript?

Abstract classes can have both implemented methods and abstract methods, allowing shared code and enforced method implementation. Interfaces only declare method signatures without implementation.

Which keyword is used to declare an abstract class in TypeScript?

A abstract

B interface

C class

D implements

The keyword abstract is used before the class name to declare an abstract class.

Can you instantiate an abstract class directly?

A Yes, always

B No, never

C Only if it has no abstract methods

D Only inside subclasses

Abstract classes cannot be instantiated directly; you must instantiate a subclass.

What must a subclass do if it extends an abstract class with abstract methods?

A Override only some abstract methods

B Ignore abstract methods

C Implement all abstract methods

D Convert abstract methods to properties

Subclasses must implement all abstract methods declared in the abstract class.

Which of these can an abstract class contain?

A Both abstract and implemented methods

B Only implemented methods

C Only properties

D Only abstract methods

Abstract classes can have both abstract methods (without body) and implemented methods (with body).

What happens if a subclass does not implement all abstract methods?

A It automatically implements methods

B It compiles without errors

C It throws a runtime error

D It becomes an abstract class itself

If a subclass does not implement all abstract methods, it must be declared abstract itself.

Explain what an abstract class is and why you would use it in TypeScript.

Think about a blueprint that cannot be built but guides other buildings.

Describe the difference between an abstract class and an interface in TypeScript.

One can have code inside, the other only rules.