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.
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Abstract classes cannot be instantiated directly.
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They can implement methods of their own.
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They can define methods that inheriting classes must implement.
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They are primarily used for inheritance, allowing other classes to extend from them.
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Abstract methods or properties: Abstract classes typically include one or more abstract methods or property declarations.
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Inheritance: Other classes derive from abstract classes to share common functionality.
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No direct instantiation: You cannot create an instance of an abstract class directly.
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A property name.
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A normal method display().
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An abstract method find(name: string): Person.
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Employee derives from Person.
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Employee must implement the find() method defined in Person.
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The abstract class enforces a contract that derived classes must fulfill.
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When implementing an abstract class, the derived class must call super() in its constructor.
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Abstract classes can also include abstract properties.
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Remember, abstract classes provide a powerful way to structure and share functionality while ensuring adherence to a specific contract.
Scenario Questions
0-2 years experience
- 1We 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?
- 2If 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?
- 3Can you try to instantiate an abstract class directly in TypeScript? What compile‑time error appears, and why?
2-5 years experience
- 1Our 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.
- 2We're deciding between an abstract class and an interface for a plugin system that needs shared helper methods. What factors would guide your choice?
- 3During 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
- 1Our 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?
- 2A 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?
- 3When 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
- 1Our 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.
- 2We 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.
- 3A 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?