Question: What is JavaScript?
Answer:JavaScript is a high-level, interpreted programming language primarily used for client-side web development. It enables dynamic content and interactive elements on web pages.
Question: What are the different data types in JavaScript?
Answer:JavaScript has several data types including:
Primitive data types:
number, string, boolean, null, undefined, and symbol.
Non-primitive data types:
object (including arrays and functions).
Question: What is the difference between null and undefined in JavaScript?
Answer: null is an intentional absence of any value, while undefined means a variable has been declared but has not yet been assigned a value.
Question: What is the difference between == and === operators in JavaScript?
Answer: == checks for equality after type coercion, whereas === checks for equality without type coercion (strict equality).
Question: What is a closure in JavaScript?
Answer:A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). It gives you access to an outer function's scope from an inner function.
Question: Explain hoisting in JavaScript.
Answer: Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during compilation phase, regardless of where they are declared.
Question: What is event bubbling and event capturing in JavaScript?
Answer: Event bubbling is the process where the event starts at the innermost target element and then bubbles up to the outer elements in the DOM hierarchy. Event capturing is the opposite, where the event is captured from the outermost element and then propagates to the target element.
Question: What is the this keyword in JavaScript?
Answer: In JavaScript, this refers to the object to which a function belongs or the context in which the function is executed.
Question: What is the difference between let, const, and var in JavaScript?
Answer: var is function-scoped, let and const are block-scoped. var can be redeclared and reassigned, let can be reassigned but not redeclared, and const cannot be reassigned or redeclared.
Question: How do you handle asynchronous operations in JavaScript?
Answer: Asynchronous operations in JavaScript are commonly handled using callbacks, promises, or async/await syntax.
Question: What are arrow functions in JavaScript?
Answer: Arrow functions are a shorthand syntax for writing anonymous functions. They have a more concise syntax compared to traditional function expressions and do not bind their own this, arguments, super, or new.target.
// Traditional function expression
function add(a, b) {
return a + b;
}
// Equivalent arrow function
const addArrow = (a, b) => a + b;
Question: Explain the difference between null, undefined, and undeclared in JavaScript.
Answer:null represents the intentional absence of any value. undefined is a type that has a single value, undefined. undeclared variables are those that have been referenced without being declared.
Question: What are the different ways to create objects in JavaScript?
Answer:Objects in JavaScript can be created using object literals, constructor functions with the new keyword, Object.create() method, and ES6 classes.
Examples :-
Object literals
// Object literal
const person = {
firstName: 'John',
lastName: 'Doe',
age: 30,
greet: function() {
return `Hello, my name is ${this.firstName} ${this.lastName}.`;
}
};
Constructor Functions
// Constructor function
function Person(firstName, lastName, age) {
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
this.greet = function() {
return `Hello, my name is ${this.firstName} ${this.lastName}.`;
};
}
// Creating an object using the constructor function
const person1 = new Person('John', 'Doe', 30);
Object.create()
// Prototype object
const personProto = {
greet: function() {
return `Hello, my name is ${this.firstName} ${this.lastName}.`;
}
};
// Creating an object using Object.create()
const person2 = Object.create(personProto);
person2.firstName = 'Jane';
person2.lastName = 'Doe';
person2.age = 25;
E6 classes
// ES6 Class
class Person {
constructor(firstName, lastName, age) {
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
}
greet() {
return `Hello, my name is ${this.firstName} ${this.lastName}.`;
}
}
// Creating an object using ES6 class
const person3 = new Person('Alice', 'Smith', 35);
Question: What is event delegation in JavaScript?
Answer:Event delegation is a technique where a parent element listens for events that occur inside its child elements. This is useful for handling events on elements that are dynamically added or removed from the DOM.
Question: What is the prototype chain in JavaScript?
Answer:The prototype chain is a mechanism for sharing properties and methods between objects in JavaScript. Each object has a prototype object, and when a property or method is accessed on an object, JavaScript looks for that property or method on the object itself, and if not found, it checks the object's prototype, and so on, until it reaches the end of the prototype chain.
Question: What are the different ways to loop through an array in JavaScript?
Answer:Arrays in JavaScript can be looped through using for loops, forEach() method, for...of loops, map() method, and reduce() method.
Question: Explain the concept of promises in JavaScript.
Answer:Promises are objects that represent the eventual completion or failure of an asynchronous operation. They allow asynchronous code to be written in a more synchronous-like manner, making it easier to manage asynchronous operations and handle errors.
Example -
let myPromise = new Promise(function(resolve, reject) {
// "some Code"
const data={id:1,email:"abc@email.com"}
resolve(data); // when successful
reject(); // when error
});
// "Consuming Code" (Must wait for a fulfilled Promise)
myPromise.then(
function(value) { /* code if successful */ },
function(error) { /* code if some error */ }
);
Question: What is the purpose of the use strict directive in JavaScript?
Answer:The 'use strict'; directive enables strict mode in JavaScript, which helps catch common coding errors and enforce stricter parsing and error handling rules. It helps prevent certain types of bugs and promotes safer and more efficient code.
Question: What are the different ways to create modules in JavaScript?
Answer:Modules in JavaScript can be created using the CommonJS pattern (Node.js), AMD (Asynchronous Module Definition), UMD (Universal Module Definition), and ES6 modules (using import and export keywords).
Question: Explain the concept of callback functions in JavaScript.
Answer:Callback functions are functions that are passed as arguments to other functions and are executed after a particular task is completed or when a specific event occurs. They are commonly used in asynchronous programming and event handling.
