Advanced JavaScript Interview Questions

Last updated on Feb 07, 2024
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Advanced JavaScript Interview Questions

Over the years, JavaScript has become one of the most influential programming languages in the software industry. Due to this, there are a number of job applications and applicants in this industry. Due to this, we decided to come up with the actual Advanced JavaScript Interview Questions, questions that actually matter while you create codes for your company. Have a read on these questions to cement your knowledge in JavaScript and get that dream job. But, over time, we have found out that many developers lack a basic understanding of JavaScript such as working on objects and working on obsolete patterns like deeply layered inheritance hierarchies.

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JavaScript Coding Interview Questions

Here in this article, we will be listing frequently asked Advanced JavaScript Interview Questions and Answers with the belief that they will be helpful for you to gain higher marks. Also, to let you know that this article has been written under the guidance of industry professionals and covered all the current competencies.

Q1. What is the difference between let, var and const in JavaScript?

With JavaScript users can declare variables using three keywords: let, var as well as const. The words var let and const can be a bit confusing. We will go over all three variables using examples. The difference between the variables var, let as well as const is described below in different ways.

  • Reassign the value
  • Re-declaration of the variable
  • Scope
  • Hoisting

NOTE: If you would like to learn more about the differences between Let, Var, and Const, you should visit this site.

Q2. What is the difference between filter and map?

They both return a new array.

1. Map

The map function returns the same number of elements as present in the original array but the value of elements will be transformed in some way.

const items = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const result = => {
    return item*item;
// [ 1, 4, 9, 16, 25, 36, 49, 64, 81, 100 ]

2. Filter

On the other hand, the filter function can return fewer or more elements than the original array but the value of the original elements will not change.

const items = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const result = items.filter((item) => {
    return item > 5;
// [ 6, 7, 8, 9, 10 ]

Q3. What is the output of below code?

let x = 10;
var x = 20;


It gives an syntax error like SyntaxError: Identifier 'x' has already been declared.

Q4. What is a callback function in JavaScript with an example?

A function passed as an argument to another function is called a callback function and this technique allows a function to call another function. A callback function gets executed after the execution of another function gets finished.


function isOdd(number) {
  return number % 2 != 0;
function filter(numbers, fn) {
  let results = [];
  for (const number of numbers) {
    if (fn(number)) {
  return results;
let numbers = [1, 2, 4, 7, 3, 5, 6];
console.log(filter(numbers, isOdd));

Answer // [ 1, 7, 3, 5 ]

The benefit of using this feature is that you can wait for the result of a previous method call and then execute another method call.

Q5. What is the output of the res in given scenario?

const res = {};
res.x = 5;


{ x: 5 }

Q6. What is the difference between arrow and normal functions in JavaScript?

The main differences between arrow function and normal function are based on some parameters like.

  • Syntax
  • No duplicates of identified parameters.
  • "this" keyword is used to describe the use.
  • Utilizing a keyword that is new.
  • Arguments that are binding.

Syntax of normal functions

// Function declaration
function printHello(name) {
   return `Hey ${name}`;

NOTE: If you want to read more about Regular vs Arrow functions then you can visit here.

// Function expression
const printHello = function(name) {
  return `Hey ${name}`;

Syntax of arrow functions

const printHello = (name) => `Hey ${name}`;

Q7. What is promise in JavaScript with example?

An object that shows the eventual completion or failure of asynchronous options along with its resultant value is called a promise in JavaScript. The possible three states of a promise are pending, fulfilled, or rejected. It allows handling multiple asynchronous operations in JavaScript.

We have a list of JavaScript Multiple Choice Questions. It's really helpful to crack your interviews easily.

A Promise has 3 states:

  • Pending: initial state, neither fulfilled nor rejected.
  • Fulfilled: meaning that the operation was completed successfully.
  • Rejected: meaning that the operation failed.
javascript promise

How to create Promises?

let getDataFromAPI = function(params) {
    return new Promise((resolve, reject) => {
        const data = {
            name: "Umesh",
            phone: "9971083635"
        if(data) {
            resolve(data); // get data from API
        } else {
            reject('get error from API Call');
getDataFromAPI().then( async function (result) {
}).catch(err => {

How to call multiple promise (with dependency) methods?

callApi().then(function(result) {
    return callApi2() ;
}).then(function(result2) {
    return callApi3();
}).then(function(result3) {
    // do work
}).catch(function(error) {
    //handle any error that may occur before this point 


Promise.all([callApi, callApi2, callApi3]).then((values) => {


Promise.allSettled([callApi, callApi2, callApi3]).then((values) => {

Q8. How to check whether a string contains a sub string in JavaScript?

Example 1

const string = "JavaScript Interview Questions";
const substring = "Questions";
console.log(string.includes(substring)); Answer // true

Example 2

const string = "JavaScript Interview Questions";
const substring = "questions";
console.log(string.includes(substring)); Answer // false

Q9. What is callback hell and how can it be avoided?

When a JavaScript developer tries to execute multiple asynchronous operations then sometimes issues arise and it is termed callback hell. 

Note: If you are a frontend developer with React.JS then these Top 20 React Questions will help you to crack your interviews easily.

How to escape from a callback hell?

JavaScript gives an easy way of avoiding callback hell. This is performed by event queue and promises.

  • Promises
  • Async/Await

Example of Callback Hell

    getDataFromFunction2(x, function(y){
        getDataFromFunction3(y, function(z){

Q10. What is currying in JavaScript?

It is an advanced technique of working with functions that can accept multiple arguments. It will transform this function into a series of functions, where every function will accept one argument:

Non-curried version //
const sum = (a, b, c)=>{
    return a+ b + c
console.log(sum(2, 3, 5)) // 10

Curried version //
const sumCurry =(a) => {
    return (b)=>{
        return (c)=>{
            return a+b+c
console.log(sumCurry(2)(3)(5)) // 10


Top 10 Advanced JavaScript Interview Questions

Here you will find the list of Top Advanced JavaScript Interview Questions, which were written under the supervision of industry experts. These are the most common questions and usually asked in every interview for the role of the JavaScript Developers.

  • What is prototype?
  • What is Callback hell?
  • What is callback?
  • What are the difference between arrow functions & normal functions?
  • What is JavaScript Hoisting?
  • What are the difference between let, var & const?
  • What is Promises?
  • What is the difference between Map & filter function?
  • How to remove duplicate values in array?
  • What are Closure?
  • What is rest & spread operators?
  • What are the data types used in JavaScript?
Reviewed and verified by Umesh Singh
Umesh Singh

My name is Umesh Singh having 11+ experience in Node.JS, React JS, Angular JS, Next JS, PHP, Laravel, WordPress, MySQL, Oracle, JavaScript, HTML and CSS etc. I have worked on around 30+ projects. I lo...