🟢 Basic Level Questions

1. What is OOP in JavaScript?

👉 A programming paradigm based on objects and classes to structure code.

2. What is a class in JavaScript?

👉 A blueprint to create objects using properties and methods.

3. What is an object?

👉 An instance of a class that contains data (properties) and behavior (methods).

4. What is a constructor?

👉 A special method used to initialize object values when created.

5. What is `this` keyword?

👉 Refers to the current object.

6. Difference between class and object?

👉 Class = blueprint
👉 Object = real instance

🟡 Intermediate Level Questions

7. What is inheritance?

👉 Mechanism to reuse code from another class using extends.

8. What is encapsulation?

👉 Wrapping data and methods together and restricting direct access.

9. What are private fields in JS?

👉 Variables declared using # and cannot be accessed outside the class.

10. What is polymorphism?

👉 Same method behaves differently in different classes.

11. What is method overriding?

👉 Redefining a parent class method in a child class.

class Animal {
  speak() {
    console.log("Animal sound");
  }
}class Dog extends Animal {
  speak() {
    console.log("Dog barks");
  }
}

12. What is the difference between function constructor and class?

👉 Both create objects, but:

Class = modern, cleaner syntax
Function constructor = older way

13. What is prototype in JavaScript?

👉 Mechanism by which objects inherit features from another object.

14. What is prototype chain?

👉 Chain of objects used to resolve properties.

15. Difference between `proto` and `prototype`?

👉 prototype → property of constructor
👉 __proto__ → reference of object

🔴 Advanced Level Questions

16. What is static method?

👉 Method belongs to class, not object.

class MathUtil {
  static add(a, b) {
    return a + b;
  }
}MathUtil.add(2, 3);

17. What is abstraction?

👉 Hiding implementation details and showing only essential features.

18. Can JavaScript support multiple inheritance?

👉 ❌ No (directly)
👉 ✅ Can be done using mixins

19. What are getters and setters?

👉 Methods to get/set values safely.

class Person {
  constructor(name) {
    this._name = name;
  }  get name() {
    return this._name;
  }  set name(value) {
    this._name = value;
  }
}

20. What is method chaining?

👉 Calling multiple methods in one line.

obj.method1().method2().method3();

21. What is `super` keyword?

👉 Used to call parent class constructor/method.

22. Difference between shallow copy and deep copy?

👉 Shallow → reference copy
👉 Deep → full independent copy

23. How does JavaScript implement OOP internally?

👉 Using prototype-based inheritance

24. What is mixin?

👉 Technique to add reusable functionality to classes.

25. What are ES6 classes syntactic sugar for?

👉 Prototype-based inheritance

🔥 Bonus Practical Question

26. Create a class with inheritance example

👉 Very common in interviews:

class Vehicle {
  constructor(name) {
    this.name = name;
  }  start() {
    console.log(this.name + " starting...");
  }
}class Car extends Vehicle {
  drive() {
    console.log(this.name + " driving...");
  }
}const c = new Car("BMW");
c.start();
c.drive();

💡 Pro Tip (For Interviews)

Interviewers often check:

Clear concept understanding
Real-life examples
Ability to write small code

🇮🇳 Amazon India 🌍 Amazon Global

JavaScript OOP Interview Questions from basic to advanced