JavaScript Interview Question and Answers

Javascript Interview Questions and Answers

Introduction

Preparing for a JavaScript interview can be daunting, whether you’re a fresher or experienced professional. This guide covers latest Javascript interview questions and answers to help you succeed.

Table of Contents

Here’s a list of the frequently asked javascript questions with answers on the technical concept which you can expect to face during an interview.

Variables and data types

1)What are the different ways to declare variables in javascript?

				
					var name = 'John';
let age = 30;
const isStudent = true;
				
			

2)What are primitive data types give some examples?

– String

– Number

– Boolean

– Null

– Undefined

– Symbol (ES6)

– BigInt (ES2020)

Primitive data types in javascript

3)How do you check data type of the variable?

				
					typeof variable;
				
			

4) Differentiate between var, let, and const?

Featurevarletconst
ScopeFunction scopeBlock scopeBlock scope
HoistingHoisted and initialized with undefinedHoisted but not initializedHoisted but not initialized
ReassignmentCan be reassignedCan be reassignedCannot be reassigned
RedeclarationCan be redeclaredCannot be redeclaredCannot be redeclared
Temporal Dead Zone (TDZ)NoYesYes
InitializationOptional (can be initialized later)Optional (can be initialized later)Must be initialized during declaration

Functions

5)How do you define functions in javascript?

				
					function greet(name) {
  return 'Hello ' + name;
}

// Arrow function
const greet = (name) => 'Hello ' + name;
				
			

6)What is a higher order function?

A function that takes another function as an argument or returns a function as a result.

Control Structures

7)What are the different types of loops in JavaScript?

-for loop

– while loop

– do-while loop

– for…in loop

– for…of loop

8)How do you write an if-else statement in JavaScript?

				
					if (condition) {
  // code to be executed if condition is true
} 

else {
// code to be executed if condition is false
}
				
			

9)Differentiate between  == and === comparison operators?

== compares values while === compares data types also.

For eg:

				
					Var a=10
Var b=”10”
console.log(a==b)
				
			

The above will return true because it compares only values.

				
					console.log(a===b)
				
			

The above will return false because it compares  values and data types also and both variables have values of different data types.

Arrays

10)How to create array in javascript?

				
					let fruits = ['Apple', 'Banana', 'Cherry'];
				
			

11)What are some common array methods in javascript?

– push()

– pop()

– shift()

– unshift()

– map()

– filter()

– reduce()

– forEach()

– find()

– indexOf()

Objects

12)How to create object in javascript?

				
					let person = {
  name: 'John',
  age: 30,
  isStudent: true
};
				
			

13)How do you access object properties?

				
					console.log(person.name); // Dot notation

console.log(person['age']); // Bracket notationStudent: true
				
			

DOM Manipulation

14)How do you select an HTML element in JavaScript?

				
					document.getElementById('elementId');

document.querySelector('.className');

document.querySelectorAll('tagName');
				
			

15 )How do you change the content of an HTML element?

				
					document.getElementById('elementId').innerHTML = 'New Content';
				
			

Closures

16)What is a closure in JavaScript?

A closure is a function that remembers the variables from the place where it was defined, regardless of where it is executed later.

Closures in JavaScript

17)Give an example of a closure.

				
					function outerFunction() {
  let outerVariable = 'I am outside!';

    function innerFunction() {
    console.log(outerVariable);
  }

   return innerFunction;
}

const myClosure = outerFunction();

myClosure(); // Logs: I am outside!
				
			

Asynchronous JavaScript

18)What is the event loop in JavaScript?

The event loop is a mechanism that handles asynchronous operations in JavaScript, allowing non-blocking code execution.

19)How do you write a callback function?

				
					function fetchData(callback) {
  setTimeout(() => {
    callback('Data fetched');
  }, 1000);
}

fetchData((data) => {
  console.log(data); // Logs: Data fetched
});
				
			

Promises and Async/Await

20)What is a Promise in JavaScript?

A Promise is an object representing the eventual completion or failure of an asynchronous operation.

21)How do you use async/await in JavaScript?

				
					async function fetchData() {
  try {
    let response = await fetch('https://api.example.com/data');
    let data = await response.json();
    console.log(data);
  } catch (error) {
    console.error('Error:', error);
  }
}
				
			

Prototypes and Inheritance

22)What is prototypal inheritance in javascript?

Prototypal inheritance is a feature in JavaScript where objects can inherit properties and methods from other objects.

23)How do you create a prototype chain?

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

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

function Dog(name) {
  Animal.call(this, name);
}

Dog.prototype = Object.create(Animal.prototype);

Dog.prototype.constructor = Dog;
const myDog = new Dog('Rex');
myDog.speak(); // Logs: Rex makes a noise.
				
			

ES6 Features

24)What are some new features introduced in ES6?

– let and const

– Arrow functions

– Template literals

– Default parameters

– Destructuring

– Rest and spread operators

– Classes

– Modules

25)How do you use destructuring in JavaScript?

				
					let [a, b] = [1, 2];

let {name, age} = {name: 'John', age: 30};
				
			

26) How do you handle errors in JavaScript?

				
					try {
 // code that may throw an error
}

catch (error) {
  console.error(error);
}
finally {
// code that will run regardless of error
}
				
			

27)What is the purpose of the finally block?

The finally block contains code that will be executed regardless of whether an error occurs or not.

Conclusion

Mastering the questions and concepts outlined in this guide is essential for excelling in javascript interviews. By thoroughly preparing with these common interview questions and their answers, you’ll be well-equipped to showcase your expertise and confidence during the interview process.

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