🟢 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
