Classes, Inheritance & OOP

Question 1

What is a class in TypeScript?

Answer 1

A class is a blueprint for creating objects with shared properties and methods.

Question 2

What is a constructor in TypeScript?

Answer 2

A constructor is a special method in a class that gets called when an instance is created, used to initialize class properties.

Question 3

What are access modifiers in TypeScript?

Answer 3

Access modifiers control visibility of class members: public (default), private, protected, and readonly.

Question 4

What does ‘public’ mean in TypeScript?

Answer 4

A public member can be accessed anywhere, both inside and outside the class.

Question 5

What does ‘private’ mean in TypeScript?

Answer 5

A private member is accessible only within the class where it’s declared.

Question 6

What does ‘protected’ mean in TypeScript?

Answer 6

A protected member is accessible within the class and its subclasses.

Question 7

What does ‘readonly’ mean in TypeScript?

Answer 7

A readonly property can be assigned only during initialization or in the constructor, and cannot be changed afterward.

Question 8

What is inheritance in TypeScript?

Answer 8

Inheritance allows a class to acquire properties and methods from another class using the ‘extends’ keyword.

Question 9

What is the difference between ‘implements’ and ‘extends’?

Answer 9

‘extends’ is used for class inheritance, while ‘implements’ is used to enforce a class to follow an interface contract.

Question 10

What is an abstract class in TypeScript?

Answer 10

An abstract class can’t be instantiated and is meant to be extended by other classes. It may contain abstract methods.

Question 11

What is the purpose of abstract methods?

Answer 11

Abstract methods define method signatures without implementation and must be implemented in derived classes.

Question 12

What is a use case for using classes in TypeScript?

Answer 12

Encapsulating business logic, modeling entities like User or Order in applications.

Question 13

What are the advantages of using classes in TypeScript?

Answer 13

Encapsulation, reusability, code organization, and strong typing support.

Question 14

What is a disadvantage of classes in TypeScript?

Answer 14

They may lead to overengineering or tightly coupled code if used excessively or improperly.

Question 15

What is a best practice when using constructors?

Answer 15

Keep them simple, only use them for initializing essential properties.

Question 16

What is a best practice for using access modifiers?

Answer 16

Use ‘private’ or ‘protected’ to encapsulate internal logic and avoid exposing implementation details.

Question 17

What is a good use case for abstract classes?

Answer 17

When you want to define a common base class with default behavior and force subclasses to implement specific methods.

Question 18

How does inheritance impact system design?

Answer 18

It promotes code reuse but can lead to rigid hierarchies if not designed carefully. Composition is often preferred.

Question 19

What is the architectural implication of ‘implements’?

Answer 19

Encourages loose coupling and adherence to interfaces, enabling better testing and modularity.

Question 20

How does class inheritance affect performance?

Answer 20

Minor runtime overhead from prototype chain lookups, but not significant in most applications.

Question 21

How do you debug classes in TypeScript?

Answer 21

Use IDE breakpoints, logging in constructors/methods, and ensure source maps are configured for TS files.

Question 22

What is a real-world tradeoff between inheritance and composition?

Answer 22

Inheritance creates tight coupling and is less flexible, while composition offers better separation of concerns but may need more boilerplate.

Question 23

What is a common interview question on access modifiers?

Answer 23

Explain the difference between public, private, protected, and readonly in TypeScript.

Question 24

What is a gotcha when using private members in TypeScript?

Answer 24

Private members declared in TypeScript are only enforced at compile-time, not at runtime.

Question 25

What is a gotcha with abstract classes?

Answer 25

Forgetting to implement all abstract methods in a subclass will cause a compilation error.

Question 26

What is the impact of 'readonly' on maintainability?

Answer 26

It improves immutability and prevents accidental changes to critical data.

Question 27

What is an example of class inheritance?

Answer 27

class Dog extends Animal { bark() {} }

Question 28

What is an example of implementing an interface?

Answer 28

class Car implements Drivable { drive() { ... } }

Question 29

What happens if a class does not implement all interface methods?

Answer 29

TypeScript will throw a compile-time error indicating unimplemented members.

Question 30

What are the implications of using 'extends' excessively?

Answer 30

Can lead to deep, fragile inheritance trees that are hard to maintain and test.

Question 31

When should you prefer composition over inheritance?

Answer 31

When you need flexibility, dynamic behavior, or when classes have different concerns.

Question 32

What happens if you try to instantiate an abstract class?

Answer 32

TypeScript will show a compile-time error: abstract classes cannot be instantiated.

Question 33

How does object-oriented design help in large codebases?

Answer 33

It promotes modular, reusable, and maintainable code structures.

Question 34

What is a typical architectural pattern using classes in Node.js?

Answer 34

Service Layer pattern where services encapsulate business logic using classes.

Question 35

What does 'super' refer to in a subclass constructor?

Answer 35

It calls the constructor of the parent class and must be called before using 'this'.

Question 36

What happens if you forget to call 'super' in a subclass constructor?

Answer 36

TypeScript will throw a compilation error: 'super' must be called before accessing 'this'.

Question 37

What’s a potential issue with deep inheritance chains?

Answer 37

They make debugging and understanding code harder and introduce tight coupling.

Question 38

Why is interface-based design preferred over class inheritance?

Answer 38

It allows for better decoupling, testing, and flexibility in extending functionality.

Question 39

How can readonly and private improve system safety?

Answer 39

They reduce the chances of accidental data mutations and enforce encapsulation.