Episode 7 — DSA with JavaScript / 7.4 — Object Oriented Programming

7.4 — Interview Questions: OOP in JavaScript

<< Overview

Beginner

Q1. What is a class in JavaScript?

Answer: A class is a blueprint for creating objects with shared structure and behavior. Introduced in ES6, it's syntactic sugar over JavaScript's prototype-based inheritance.

class Person {
    constructor(name, age) {
        this.name = name;
        this.age = age;
    }
    greet() {
        return `Hi, I'm ${this.name}`;
    }
}

class Person {
public:
    string name;
    int age;
    Person(const string& n, int a) : name(n), age(a) {}
    string greet() const { return "Hi, I'm " + name; }
};

Q2. What does the this keyword refer to?

Answer: this refers to the current object instance. In a method, it points to the object the method was called on.

class Counter {
    constructor() { this.count = 0; }
    increment() { this.count++; return this; }
}
const c = new Counter();
c.increment().increment();  // method chaining with this
console.log(c.count);       // 2

Pitfall: Arrow functions inherit this from enclosing scope; regular functions get their own this.

Q3. What is inheritance in OOP?

Answer: Inheritance lets a child class reuse and extend a parent class's properties and methods.

class Animal {
    constructor(name) { this.name = name; }
    speak() { return `${this.name} makes a sound`; }
}

class Dog extends Animal {
    speak() { return `${this.name} barks!`; }
}

Q4. What is the super keyword?

Answer: super calls the parent class's constructor or methods.

class Cat extends Animal {
    constructor(name, color) {
        super(name);        // must call before using this
        this.color = color;
    }
    speak() {
        return super.speak() + " (meow)";
    }
}

Q5. How do you make a property private in JavaScript?

Answer: Use the # prefix (ES2022):

class Secret {
    #value;
    constructor(v) { this.#value = v; }
    getValue() { return this.#value; }
}
const s = new Secret(42);
s.getValue();  // 42
// s.#value;   // SyntaxError!

In C++: use the private: access modifier.

Intermediate

Q6. Explain polymorphism with an example.

Answer: Polymorphism means objects of different classes respond to the same method call differently.

class Shape {
    area() { throw new Error("Not implemented"); }
}
class Circle extends Shape {
    constructor(r) { super(); this.r = r; }
    area() { return Math.PI * this.r ** 2; }
}
class Rect extends Shape {
    constructor(w, h) { super(); this.w = w; this.h = h; }
    area() { return this.w * this.h; }
}

// Polymorphic behavior
[new Circle(3), new Rect(4, 5)].forEach(s => console.log(s.area()));

class Shape {
public:
    virtual double area() const = 0;  // pure virtual
    virtual ~Shape() = default;
};
class Circle : public Shape {
    double r;
public:
    Circle(double r) : r(r) {}
    double area() const override { return 3.14159 * r * r; }
};

Q7. What is encapsulation and why does it matter?

Answer: Encapsulation bundles data and methods together, hiding internal state to prevent external code from putting the object in an invalid state.

Benefits:

  1. Data integrity (validation in setters)
  2. Loose coupling (can change internals without breaking users)
  3. Security (sensitive data not directly accessible)

Q8. Explain static methods. When should you use them?

Answer: Static methods belong to the class, not instances. Use them for utility functions that don't need instance state.

class ArrayUtils {
    static sum(arr) { return arr.reduce((a, b) => a + b, 0); }
    static avg(arr) { return ArrayUtils.sum(arr) / arr.length; }
}
ArrayUtils.sum([1, 2, 3]);  // 6

Q9. What is the difference between composition and inheritance?

Answer:

  • Inheritance: "is-a" relationship (Dog is-a Animal)
  • Composition: "has-a" relationship (Car has-a Engine)
// Composition (preferred for flexibility)
class Engine { start() { return "vroom"; } }
class Car {
    constructor() { this.engine = new Engine(); }
    start() { return this.engine.start(); }
}

Rule of thumb: Favor composition over inheritance for flexibility.

Q10. How does JS class inheritance differ from C++?

Answer:

FeatureJavaScriptC++
Single/MultipleSingle onlyMultiple supported
Virtual dispatchAlways (prototype chain)Only with virtual keyword
Abstract classesNo built-in (throw in base)Pure virtual = 0
Access modifiers# for privatepublic/private/protected
MemoryGarbage collectedManual/RAII/smart pointers

Advanced

Q11. Implement the Singleton pattern in both JS and C++.

Answer:

class Logger {
    static #instance = null;
    #logs = [];

    static getInstance() {
        if (!Logger.#instance) Logger.#instance = new Logger();
        return Logger.#instance;
    }

    log(msg) { this.#logs.push({ time: Date.now(), msg }); }
    getLogs() { return [...this.#logs]; }
}

class Logger {
    static Logger* instance;
    vector<string> logs;
    Logger() {}
public:
    static Logger& getInstance() {
        static Logger inst;
        return inst;
    }
    void log(const string& msg) { logs.push_back(msg); }
};

Q12. What is the prototype chain in JavaScript?

Answer: Every JS object has an internal [[Prototype]] link. When a property is not found on the object, JS looks up the chain.

myDog → Dog.prototype → Animal.prototype → Object.prototype → null

const dog = new Dog("Rex");
dog.speak();      // found on Dog.prototype
dog.toString();   // found on Object.prototype
dog.foo;          // not found → undefined

Q13. Implement the Observer pattern.

Answer:

class EventEmitter {
    #listeners = {};

    on(event, fn) {
        (this.#listeners[event] ??= []).push(fn);
    }

    emit(event, ...args) {
        for (const fn of this.#listeners[event] ?? []) {
            fn(...args);
        }
    }

    off(event, fn) {
        this.#listeners[event] = (this.#listeners[event] ?? []).filter(f => f !== fn);
    }
}

const emitter = new EventEmitter();
emitter.on("data", (d) => console.log("Got:", d));
emitter.emit("data", 42);  // "Got: 42"

Q14. How do you implement abstract classes in JavaScript?

Answer: JavaScript has no abstract keyword. Throw an error in the base class:

class Shape {
    constructor() {
        if (new.target === Shape) throw new Error("Cannot instantiate abstract class");
    }
    area() { throw new Error("area() must be implemented"); }
}

class Circle extends Shape {
    constructor(r) { super(); this.r = r; }
    area() { return Math.PI * this.r ** 2; }
}

// new Shape();     // Error!
new Circle(5).area(); // 78.53...

Quick-fire table

#QuestionAnswer
1Class vs object?Class is blueprint, object is instance
2this in arrow fn?Inherited from enclosing scope
3super() required when?In child constructor before using this
4Private in JS?#field prefix
5Static method accessed how?ClassName.method()
6Polymorphism?Same method, different behavior per type
7virtual in C++?Enables runtime polymorphism
8Composition vs inheritance?has-a vs is-a; prefer composition
9Prototype chain?Object → Proto → Proto → ... → null
10Singleton?One instance, global access point

Rapid self-check cards

SC-001

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-002

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-003

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-004

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-005

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-006

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-007

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-008

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-009

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-010

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-011

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-012

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-013

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-014

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-015

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-016

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-017

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-018

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-019

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-020

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-021

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-022

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-023

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-024

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-025

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-026

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-027

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-028

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-029

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-030

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-031

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-032

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-033

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-034

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-035

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-036

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-037

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-038

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-039

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-040

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-041

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-042

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-043

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-044

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-045

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-046

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-047

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-048

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-049

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-050

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-051

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-052

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-053

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-054

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-055

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-056

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-057

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-058

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-059

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-060

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-061

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-062

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-063

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-064

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-065

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-066

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-067

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-068

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-069

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-070

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-071

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-072

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-073

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-074

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-075

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-076

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-077

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-078

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-079

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-080

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-081

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-082

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-083

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-084

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-085

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-086

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-087

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-088

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-089

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-090

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-091

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-092

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-093

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-094

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-095

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-096

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-097

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-098

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-099

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-100

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-101

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-102

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-103

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-104

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-105

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-106

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-107

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-108

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-109

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-110

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-111

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-112

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-113

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-114

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-115

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-116

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-117

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-118

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-119

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-120

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-121

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-122

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-123

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-124

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-125

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-126

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-127

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-128

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-129

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-130

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-131

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-132

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-133

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-134

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-135

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-136

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-137

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-138

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-139

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-140

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-141

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-142

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-143

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-144

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-145

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-146

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-147

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-148

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-149

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-150

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-151

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-152

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-153

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-154

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-155

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-156

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-157

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-158

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-159

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-160

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-161

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-162

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-163

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-164

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-165

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-166

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-167

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-168

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-169

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-170

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-171

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-172

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-173

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-174

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-175

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-176

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-177

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-178

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-179

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-180

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-181

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-182

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-183

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-184

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-185

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-186

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-187

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-188

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-189

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-190

  • Q: What is method chaining?
  • A: Return this from methods to chain calls

SC-191

  • Q: What is a class?
  • A: A blueprint for creating objects with shared properties and methods

SC-192

  • Q: What does this refer to?
  • A: The current object instance the method is called on

SC-193

  • Q: What is inheritance?
  • A: Child class inherits properties/methods from parent class

SC-194

  • Q: What is super?
  • A: Calls the parent class constructor or methods

SC-195

  • Q: How to make a field private in JS?
  • A: Prefix with # (e.g., #balance)

SC-196

  • Q: What is polymorphism?
  • A: Same interface, different behavior depending on the type

SC-197

  • Q: What is encapsulation?
  • A: Bundling data+methods and hiding internals

SC-198

  • Q: What is a static method?
  • A: Belongs to the class, not instances

SC-199

  • Q: Composition vs inheritance?
  • A: has-a vs is-a; composition is more flexible

SC-200

  • Q: What is method chaining?
  • A: Return this from methods to chain calls