What are abstract classes?

An abstract class is a base class from which other classes can be derived. An abstract class serves as a foundational blueprint for creating more concrete classes.

1
Abstract classes cannot be instantiated directly.
2
They can implement methods of their own.
3
They can define methods that inheriting classes must implement.
4
They are primarily used for inheritance, allowing other classes to extend from them.

Key Characteristics:

1
Abstract methods or properties: Abstract classes typically include one or more abstract methods or property declarations.
2
Inheritance: Other classes derive from abstract classes to share common functionality.
3
No direct instantiation: You cannot create an instance of an abstract class directly.

javascript

1
A property name.
2
A normal method display().
3
An abstract method find(name: string): Person.

javascript

1
Employee derives from Person.
2
Employee must implement the find() method defined in Person.
3
The abstract class enforces a contract that derived classes must fulfill.

Notes:

1
When implementing an abstract class, the derived class must call super() in its constructor.
2
Abstract classes can also include abstract properties.
3
Remember, abstract classes provide a powerful way to structure and share functionality while ensuring adherence to a specific contract.

Difficulty: 4/10

Topics: inheritance, polymorphism, type safety

Scenario Questions

0-2 years experience

1
We have a simple Shape interface with an area() method. How would you refactor it into an abstract class so that Circle and Rectangle can share a common color property and a default toString() implementation?
2
If you declare an abstract class Vehicle with an abstract startEngine() method, what error do you get when a subclass forgets to implement startEngine, and how would you fix it?
3
Can you try to instantiate an abstract class directly in TypeScript? What compile‑time error appears, and why?

2-5 years experience

1
Our codebase defines an abstract BaseApiService that provides a protected httpClient. A new service extending it crashes at runtime with Cannot read property 'get' of undefined. Walk me through how you'd debug this issue.
2
We're deciding between an abstract class and an interface for a plugin system that needs shared helper methods. What factors would guide your choice?
3
During a refactor a developer changed a concrete method in an abstract class to abstract, causing compile errors in several subclasses. How would you resolve the breakage while keeping the change minimal?

5-8 years experience

1
Our microservice framework uses an abstract RequestHandler with a synchronous handle() method. We now need to support async handlers without breaking existing subclasses. How would you evolve the abstract class design?
2
A large UI component hierarchy extends an abstract Component class. Adding a new lifecycle hook caused noticeable performance regressions. How would you evaluate the impact and decide whether to keep the hook?
3
When abstract classes are exported from separate npm packages, we sometimes see instanceof checks fail because of duplicate type definitions. Explain why this happens and how to prevent it.

8+ years experience

1
Our legacy JavaScript code uses function constructors to mimic inheritance. We're migrating to TypeScript and want to replace them with abstract classes while preserving backward compatibility. Outline a migration plan.
2
We need a contract for all data‑access objects across multiple services. Would you choose an abstract class, an interface, or a hybrid approach? Discuss the long‑term maintenance and versioning implications.
3
A cross‑team initiative is releasing a public SDK that relies on abstract classes for extensibility. What versioning strategy and compatibility guarantees would you put in place to avoid breaking downstream consumers?

Follow-up Questions

❓What trade‑offs does that introduce compared to using an interface?
❓How does TypeScript enforce the abstract contract at compile time?
❓Can you think of any scenario where making a method concrete in the abstract class could cause issues later?