Easy:

1. What is Object-Oriented Programming? — A paradigm that organizes code around objects containing data and behavior.
2. What are the four pillars of OOP? — Encapsulation, Abstraction, Inheritance, Polymorphism.
3. What is the difference between a class and an object? — A class is a blueprint; an object is an instance of a class.
4. What is a constructor? — A special method called when an object is created to initialize its state.

Medium: 5. What’s the difference between procedural and OOP programming? — Procedural uses functions operating on separate data; OOP bundles data and behavior in objects. 6. What is the difference between method overloading and overriding? — Overloading: same name, different params. Overriding: child redefines parent method. 7. What is the difference between static and instance members? — Static belongs to the class; instance belongs to the object.

Advanced: 8. How does JavaScript implement OOP compared to Java? — JavaScript uses prototypal inheritance; Java uses classical inheritance. 9. Explain the new keyword’s internal workings. — Creates new object, sets prototype, calls constructor with this bound to new object.

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Easy: 10. What is encapsulation? — Bundling data and methods within a class, hiding internal state. 11. What are access modifiers? — Keywords controlling visibility: public, private, protected.

Medium: 12. How is encapsulation implemented in JavaScript? — Using # private fields, closures, or WeakMap. 13. How does encapsulation improve maintainability? — Changes to internal implementation don’t affect external code.

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Easy: 14. What is abstraction? — Hiding complex implementation details and showing only essential features. 15. What is an abstract class? — A class that can’t be instantiated; may contain abstract methods. 16. What is an interface? — A contract defining methods that implementing classes must provide.

Medium: 17. How is abstraction different from encapsulation? — Abstraction hides implementation; encapsulation hides data. 18. What’s the difference between abstract class and interface? — Abstract classes can have state and concrete methods; interfaces are pure contracts.

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Easy: 19. What is inheritance? — A mechanism where a child class inherits properties and methods from a parent class. 20. What is the super keyword? — Used to call parent class constructors and methods. 21. What is method overriding? — Child class providing its own implementation of a parent method.

Medium: 22. What are the different types of inheritance? — Single, Multilevel, Hierarchical, Multiple, Hybrid. 23. What is the diamond problem? — Ambiguity when a class inherits from two classes that share a common ancestor.

Advanced: 24. How does JavaScript prototypal inheritance work? — Objects inherit from other objects through prototype chain. 25. Why is composition preferred over inheritance? — More flexible, less coupling, easier to change at runtime.

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Easy: 26. What is polymorphism? — “Many forms” — objects of different types respond to the same method call differently. 27. What’s the difference between compile-time and run-time polymorphism? — Compile-time (overloading), run-time (overriding).

Medium: 28. How does polymorphism support the Open/Closed Principle? — New types can be added without modifying existing code.

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Easy: 29. What does SOLID stand for? — Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion. 30. What is the Single Responsibility Principle? — A class should have only one reason to change.

Medium: 31. Give an example of violating LSP. — A Square extending Rectangle where setting width also changes height. 32. What is Dependency Inversion Principle? — Depend on abstractions, not concrete implementations.

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Easy: 33. What is association? — A relationship where objects know about each other. 34. What is aggregation? — A “has-a” relationship with weak ownership. 35. What is composition? — A “has-a” relationship with strong ownership.

Medium: 36. How do you represent aggregation vs composition in UML? — Aggregation: empty diamond. Composition: filled diamond.

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Easy: 37. What is DRY? — Don’t Repeat Yourself. 38. What is KISS? — Keep It Simple, Stupid. 39. What is YAGNI? — You Ain’t Gonna Need It.

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