JavaScript Classes — Interview Questions & Answers

Ques: What are classes in JavaScript?

Ans: A class is a template for creating objects. It simplifies working with objects and inheritance in an object-oriented way.

class Person {
  constructor(name) {
    this.name = name;
  }
  greet() {
    console.log(`Hello, ${this.name}`);
  }
}

Ques: Why were classes introduced in ES6?

Ans: To make OOP (Object-Oriented Programming) more readable and structured. They provide cleaner syntax for prototype-based inheritance in JavaScript.

Ques: What is the difference between a class and a constructor function?

Ques: How do you create an instance of a class?

Ans: Use the new keyword:

class Car {
  constructor(model) { this.model = model; }
}
const c1 = new Car("Tesla");

Ques: What is a constructor method?

Ans: A special method automatically called when a new object is created from a class.

class User {
  constructor(name) {
    this.name = name;
  }
}

Ques: Can a class have multiple constructors?

Ans: No, A class can have only one constructor.

However, you can handle multiple initialization patterns using default or conditional logic.

Ques: What are instance methods and static methods?

• Instance methods: Belong to the object instance.
• Static methods: Belong to the class itself.

class MathUtils {
  static add(a, b) { return a + b; }
  multiply(a, b) { return a * b; }
}
MathUtils.add(2, 3); // 5
new MathUtils().multiply(2, 3); // 6

Ques: What is inheritance in classes?

Ans: It allows one class to derive properties and methods from another using extends.

class Animal {
  speak() { console.log("Animal sound"); }
}
class Dog extends Animal {
  speak() { console.log("Bark"); }
}
new Dog().speak(); // "Bark"

Ques: What is the role of super()?

Ans: Used inside the child class constructor to call the parent class constructor.

class Parent {
  constructor(name) { this.name = name; }
}
class Child extends Parent {
  constructor(name, age) {
    super(name);
    this.age = age;
  }
}

Ques: What are getters and setters in classes?

Ans: They provide controlled access to object properties.

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

Ques: What are private fields in JavaScript classes?

Ans: Private fields (prefixed with #) are accessible only inside the class.

class BankAccount {
  #balance = 0;
  deposit(amount) { this.#balance += amount; }
}

Ques: What are public fields in classes?

Ans: They are declared directly in the class body and can be accessed from anywhere.

class Car {
  color = "blue";
}
console.log(new Car().color); // "blue"

Ques: What is the extends keyword used for?

Ans: It sets up inheritance — allowing a subclass to access the parent’s properties and methods.

class Employee extends Person {}

Ques: Can you override parent class methods?

Ans: Yes, by redefining the same method name in the child class.

Ques: What happens if you forget to call super() in a subclass constructor?

Ans: You’ll get a ReferenceError, because this cannot be used before calling super() in derived classes.

Ques: What are mixins in JavaScript classes?

Ans: Mixins allow combining multiple behaviors into one class.

const canRun = {
  run() { console.log("Running..."); }
};
class Animal {}
Object.assign(Animal.prototype, canRun);

Ques: What is the difference between prototype and class syntax?

Ans: Both create objects with shared methods. Classes are syntactic sugar over prototype-based inheritance.

Ques: What are best practices with JavaScript classes?

• Keep methods small and focused.
• Prefer composition over deep inheritance.
• Use private fields for encapsulation.
• Use getters/setters for data validation.