Top 25 essential JavaScript OOP questions along with syntax examples

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1. What is OOP in JavaScript?

• Answer: Object-Oriented Programming (OOP) in JavaScript uses objects and classes to structure code.

   class Animal {
       speak() {
           console.log("Animal speaks!");
       }
   }

   const dog = new Animal();
   dog.speak();

2. What is a Class in JavaScript?

• Answer: A class is a blueprint for creating objects with shared properties and methods.

   class Car {
       constructor(make, model) {
           this.make = make;
           this.model = model;
       }
   }

3. What is an Object in JavaScript?

• Answer: An object is an instance of a class.

   const myCar = new Car('Toyota', 'Corolla');

4. What is a Constructor in JavaScript?

• Answer: A constructor is a special method for initializing objects created by a class.

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

   const user = new User("John");

5. What is Inheritance in JavaScript?

• Answer: Inheritance allows a class to inherit properties and methods from another class.

   class Vehicle {
       start() {
           console.log("Starting engine...");
       }
   }

   class Car extends Vehicle {}
   const car = new Car();
   car.start();

6. What is Method Overriding in JavaScript?

• Answer: A child class can override a method from the parent class.

   class Animal {
       speak() {
           console.log("Animal speaks");
       }
   }

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

   const dog = new Dog();
   dog.speak();

7. What is super in JavaScript?

• Answer: super is used to call the constructor of the parent class.

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

   class Dog extends Animal {
       constructor(name) {
           super(name);
       }
   }

8. What is this in JavaScript?

• Answer: this refers to the current object instance.

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

9. What is a Static Method in JavaScript?

• Answer: Static methods belong to the class, not instances.

   class Calculator {
       static add(a, b) {
           return a + b;
       }
   }

   console.log(Calculator.add(2, 3));

10. What is Encapsulation?

• Answer: Encapsulation restricts access to certain data and methods within an object.

   class BankAccount {
       #balance = 0; // private field

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

11. What is Polymorphism?

• Answer: Polymorphism allows objects to be treated as instances of their parent class.

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

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

12. What are Getters and Setters in JavaScript?

• Answer: Getters and Setters control access to object properties.

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

       get name() {
           return this._name;
       }

       set name(name) {
           this._name = name;
       }
   }

13. What is Prototypal Inheritance?

• Answer: JavaScript objects inherit properties from a prototype.

   function Person(name) {
       this.name = name;
   }

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

14. What is Method Chaining?

• Answer: Method chaining allows multiple methods to be called in sequence on the same object.

   class Calculator {
       constructor(value = 0) {
           this.value = value;
       }

       add(n) {
           this.value += n;
           return this;
       }

       subtract(n) {
           this.value -= n;
           return this;
       }
   }

   const calc = new Calculator();
   calc.add(5).subtract(2);

15. What is Closures in JavaScript?

• Answer: A closure is a function that retains access to its outer scope.

   function counter() {
       let count = 0;
       return function() {
           count++;
           return count;
       };
   }

16. What is a Module Pattern in JavaScript?

• Answer: The Module Pattern encapsulates related variables and functions.

   const Module = (function() {
       let privateVar = "I am private";
       return {
           publicMethod: function() {
               console.log(privateVar);
           }
       };
   })();

17. What is instanceof in JavaScript?

• Answer: Checks if an object is an instance of a specific class.

   console.log(dog instanceof Dog); // true

18. What is the Factory Pattern?

• Answer: The Factory Pattern creates objects based on criteria without exposing the instantiation logic.

   function createPerson(name, age) {
       return {
           name,
           age,
           greet() {
               console.log(`Hi, I'm ${name}`);
           }
       };
   }

19. What is the bind method?

• Answer: bind sets the this context for a function.

   const person = {
       name: 'John',
       greet() {
           console.log(`Hello, ${this.name}`);
       }
   };

   const greet = person.greet.bind(person);
   greet();

20. What are Arrow Functions and this?

• Answer: Arrow functions do not have their own this.

   const obj = {
       name: 'Arrow Example',
       showName: () => console.log(this.name) // 'this' is not bound to 'obj'
   };

21. What is call in JavaScript?

• Answer: call invokes a function with a specified this context.

   function introduce() {
       console.log(`Hi, I'm ${this.name}`);
   }

   const person = { name: 'Alice' };
   introduce.call(person);

22. What is apply in JavaScript?

• Answer: apply is similar to call but takes an array of arguments.

   introduce.apply(person, ['Hello']);

23. How does Prototypal Inheritance work?

• Answer: JavaScript uses prototypes for inheritance between objects.

   function Animal(name) {
       this.name = name;
   }

   Animal.prototype.speak = function() {
       console.log(this.name + ' makes a noise.');
   };

   const dog = new Animal('Dog');
   dog.speak();

24. What is a Mixin?

• Answer: A mixin adds properties to an object without inheritance.

   const canEat = {
       eat() {
           console.log("Eating...");
       }
   };

   const person = Object.assign({}, canEat);

25. What are Symbols in JavaScript?

• Answer: Symbols are unique and immutable identifiers.

   const sym = Symbol('unique');
   const obj = { [sym]: 'hidden data' };

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These questions cover essential JavaScript OOP concepts with explanations and examples to reinforce understanding.