Frequently Asked JavaScript Interview Questions and Answers

Having JavaScript in your portfolio will increase the possibilities of getting a software program developer function. That mentioned, let’s try the continuously requested JavaScript interview questions.

JavaScript is among the most used languages in internet improvement. It’s used to develop virtually any kind of utility now.

Earlier than leaping into the interview questions, let’s see some great benefits of studying JavaScript.


JavaScript is a light-weight, interpreted, or just-in-time compiled programming language. It is among the core languages of the world extensive internet. You realize the opposite two core languages of www. You higher seek for them for those who don’t.

JavaScript is especially created for the net. But it surely’s not just for the net now. With the assistance of environments like Node, Deno, and many others.., we are able to run it on virtually any platform.

Let’s try some benefits of it.

Benefits of JavaScript

  1. Simple to get began with. You may be taught it even with none coding information.
  2. Giant group round it. You’ll get all of the assist you to need if you’re caught wherever.
  3. There are plenty of libraries/frameworks construct utilizing JavaScript, which helps to develop functions quicker.
  4. We are able to develop frontend, backend, android, iOS, and many others.., functions with JavaScript. We are able to create virtually any kind of utility with it. However, it’s extra sturdy in internet improvement.

What are the info varieties in JavaScript?

The info varieties are used to retailer various kinds of knowledge. Information varieties will differ from one programming language to a different. In JavaScript, we’ve 8 knowledge varieties. Let’s see them one after the other.

  • Quantity
  • String
  • Boolean
  • Undefined
  • Null
  • BigInt
  • Image
  • Object

All the info varieties besides Object are referred to as primitive values. And they’re immutable.

What are the built-in strategies in JavaScript?

The built-in strategies in JavaScript are completely different for every knowledge kind. We are able to entry these built-in strategies utilizing the respective knowledge kind. Let’s see some built-in strategies for various knowledge varieties and knowledge buildings.

  1. Quantity
    • toFixed
    • toString
  2. String
    • toLowerCase
    • startsWith
    • chartAt
  3. Array
    • filter
    • map
    • forEach

There are plenty of built-in strategies for every knowledge kind. You may examine the references for all built-in strategies of various knowledge varieties and knowledge buildings.

The way to create an array in JavaScript?

Arrays are one of many core knowledge buildings in JavaScript. Arrays can have any kind of knowledge in them as JavaScript is dynamic. Let’s see methods to create arrays in JavaScript.

We are able to create an array utilizing sq. brackets[]. It’s easy and fast to create objects

// Empty array
const arr = [];

// Array with some random values
const randomArr = [1, "One", true];

console.log(arr, randomArr);

We are able to create an array utilizing Array constructor. Folks hardly ever use the constructor to create arrays generally initiatives.

// Empty array
const arr = new Array();

// Array with some random values
const randomArr = new Array(1, "One", true);

console.log(arr, randomArr);

JavaScript arrays are mutable i.e. we are able to modify them as we would like after creating them.

The way to create an object in JavaScript?

Apart array, the item is one other core knowledge construction in JavaScript. Objects are utilizing retailer the key-value pairs. The important thing have to be an immutable worth, whereas the worth may be something. Let’s see methods to create objects in JavaScript.

We are able to create objects utilizing curly brackets {}. It’s easy and fast to create objects.

// Empty object
const object = {};

// Object with some random values
const randomObject = { 1: 2, one: "Two", true: false };

console.log(object, randomObject);

We are able to create objects utilizing Object constructor. Folks hardly ever use this generally initiatives.

// Empty object
const object = new Object();

// Object with some random values
const randomObject = new Object();
randomObject[1] = 2;
randomObject["one"] = "Two";
randomObject[true] = false;

console.log(object, randomObject);

JavaScript objects are mutable i.e., we are able to modify them after creating, as you see within the 2nd instance.

How do you debug JavaScript code?

Debugging code just isn’t easy. And it’s completely different from one programming language to a different, one venture to a different, and many others..; let’s see the frequent issues used to debug JavaScript.

#1. Logging

We are able to use the console.log statements at a number of locations in our code to establish the bug. The code will cease operating the subsequent traces of code when there’s a bug within the earlier line.

Logging is among the previous debugging strategies, which is just about efficient for small initiatives. It’s a standard debugging method for any programming language.

#2. Developer Instruments

JavaScript is generally used for creating internet functions. So, virtually all browsers have developer instruments now that assist to debug the JavaScript code.

One of the used debugging strategies is setting breakpoints within the developer instruments. The breakpoints cease the execution of JavaScript and provides all the information in regards to the execution in the mean time.

We are able to set a number of breakpoints across the place the place we’re getting errors and see what’s inflicting it. It’s the simplest technique to debug JavaScript internet functions.

#3. IDEs

We are able to use the IDEs to debug JavaScript. VS Code helps debugging with breakpoints. The debugging function might differ based mostly on the IDE you’re utilizing. However, many of the IDEs can have that function.

The way to add JavaScript code in an HTML file?

We are able to add the JavaScript HTML file utilizing the script tag. You may examine the instance under.

<!DOCTYPE html>
<html lang="en">

      // JavaScript code goes right here
      console.log("That is JavaScript code");

What are cookies?

Cookies are key-value pairs used to retailer small data. The data may be something. We are able to set the expiry time of the cookies, which will probably be deleted after their expiry time. These are broadly used to retailer the customers’ data.

Cookies gained’t be cleared even when we refresh the web page till we delete them or they expire. You may examine the cookies of any internet app/internet web page in any browser by opening the developer instruments.

We are able to learn the cookie in JavaScript utilizing doc.cookie. It would return all of the cookies that we created.

console.log("All cookies", doc.cookie);

It would return an empty string if there are not any cookies.

We are able to create the cookies by setting the key-value pair to the doc.cookie. Let’s see an instance.

doc.cookie = "one=One;";

Within the above syntax, the one cookie key and One is its worth. We are able to add extra attributes to the cookie like area, path, expires, and many others..; every of them ought to be separated by a semicolon (;). All of the attributes are non-compulsory.

Let’s see an instance with attributes.

doc.cookie = "one=One;expires=Jan 31 2023;path=/;";

Within the above code, we’ve added an expiry date and path to the cookie. If the expiry date just isn’t offered, the cookie will probably be deleted after the session. The default path would be the file path. The expiry date format ought to in GMT.

Let’s see methods to create a number of cookies.

doc.cookie = "one=One;expires=Jan 31 2023;path=/;";
doc.cookie = "two=Two;expires=Jan 31 2023;path=/;";
doc.cookie = "three=Three;expires=Jan 31 2023;path=/;";

The cookies gained’t be overwritten if the important thing or path is completely different whereas setting a number of cookies. If the important thing and path are the identical, then it’ll overwrite the earlier cookie. Take a look at the under instance, which is able to overwrite the earlier set cookie.

doc.cookie = "one=One;expires=Jan 31 2023;path=/;";
doc.cookie = "one=Two;path=/;";

Now we have eliminated the expiry date from the cookie and adjusted the worth.

Use the expiry date a future date when you’re testing the code for it to work appropriately. In the event you maintain the identical date Jan 31 2023 even after the Jan 31 2023, cookies gained’t be created.

Now we have seen methods to create and replace cookies. Let’s see methods to delete cookies.

Deleting cookies is straightforward. Simply change the expiry date of the cookie to any previous date. Test the instance under.

// Creating cookies
doc.cookie = "one=One;expires=Jan 31 2023;path=/;";
doc.cookie = "two=Two;expires=Jan 31 2023;path=/;";
doc.cookie = "three=Three;expires=Jan 31 2023;path=/;";

// Deleting the final cookie
doc.cookie = "three=Three;expires=Jan 1 2023;path=/;";

You gained’t discover the final cookie within the cookies because it’s deleted within the final line of the code. That’s it for the min cookies tutorial.

What are the completely different JavaScript frameworks?

There are plenty of JavaScript frameworks on the market. React, Vue, Angular, and many others.., for UI improvement. Categorical, Koa, Nest, and many others.., for server-side improvement. NextJS, Gatsby, and many others.., for static website era. React Native, Ionic, and many others.., for cell app improvement. Now we have talked about a number of the JavaScript frameworks right here. You’ll find extra frameworks that may take plenty of time to discover. Discover if you want them.

Closures in JavaScript

A closure is a operate bundled with its lexical scope and its mother or father lexical setting. With closures, we are able to entry the outer scope knowledge. The closures are fashioned when the capabilities are created.

operate outer() {
  const a = 1;
  operate interior() {
    // We are able to entry all the info from the outer operate scope right here
    // The info will probably be obtainable even when we execute this operate outdoors the outer operate 
    // as inners' closure fashioned whereas creating it
    console.log("Accessing a inside interior", a);
  return interior;

const innerFn = outer();

Closures are broadly utilized in JavaScript functions. You may need used them earlier than with out realizing that they’re closures. There may be much more than this to be taught in regards to the closures. Ensure you have discovered this idea fully.

Hoisting in JavaScript

Hoisting is a course of in JavaScript the place the declaration of variables, capabilities, and courses strikes to the highest of the scope earlier than executing the code.

// Accessing `title` earlier than declaring

// Declaring and initializing the `title`
var title = "Geekflare";

In the event you run the above code, you gained’t see any error. However in most languages, you’re going to get the error. The output will probably be undefined as hoisting solely strikes the declarations to the highest, and it gained’t initialize it till line quantity 3.

Change the var to let or const as follows, and run the code once more.

// Accessing `title` earlier than declaring

// Declaring and initializing the `title`
const title = "Geekflare";

Now, you’re going to get the reference error saying we are able to’t entry the variable earlier than initializing it.

ReferenceError: Can't entry 'title' earlier than initialization

So, right here the let and const are launched in ES6, which may’t be accessed earlier than initialized, because the error suggests. It’s because the variables declared with let or const will probably be in Temporal Lifeless Zone (TDZ) till the road it’s initialized. We are able to’t entry the variables from TDZ.

Currying in JavaScript

Currying is a way to transform capabilities with many parameters into fewer parameters with a number of callables. With it, we are able to convert a operate callable add(a, b, c, d) so as to add(a)(b)(c)(d) callable. Let’s see an instance of methods to do it.

operate getCurryCallback(callback) {
  return operate (a) {
    return operate (b) {
      return operate (c) {
        return operate (d) {
          return callback(a, b, c, d);

operate add(a, b, c, d) {
  return a + b + c + d;

const curriedAdd = getCurryCallback(add);

// Calling the curriedAdd

We are able to generalize the getCurryCallback the operate which will probably be used for various capabilities to transform into currying callables. You may consult with JavaScript Information for extra particulars on it.

Distinction between doc and window

The window is the topmost object within the browser. It accommodates all the information in regards to the browser window, like historical past, location, navigator, and many others..; it’s globally obtainable in JavaScript. We are able to use it straight in our code with none imports. We are able to entry the properties and strategies of the window object with out window.

The doc is the a part of window object. All of the HTML loaded on the internet web page is transformed to the doc object. The doc object refers back to the particular HTMLDocument component, which can have completely different properties and strategies like all HTML parts.

The window the item represents the browser window and doc represents the HTML doc loaded in that browser window.

Distinction between client-side and server-side

The shopper aspect refers back to the finish consumer utilizing the appliance. The server aspect refers back to the internet server the place the appliance is deployed.

Within the frontend terminology, we are able to say browser on customers’ computer systems as shopper aspect and cloud companies as server aspect.

Distinction between innerHTML and innerText

Each the innerHTML and innerText are the properties of HTML parts. We are able to change the contents of an HTML component utilizing these properties.

We are able to assign the HTML string to innerHTML a property rendered like regular HTML. Test the under instance.

const titleEl = doc.getElementById("title");

titleEl.innerHTML = '<span fashion="shade:orange;">Geekflare</span>';

Add one component with the id title to your HTML and add the above script to the JavaScript file. Run the code and see the output. You’ll Geekflare in orange shade. And for those who examine the component, will probably be contained in the span tag. So the innerHTML will take the HTML string and render it as regular HTML.

The innerText on the opposite aspect will take a traditional string and render it as it’s. It gained’t render any HTML like innerHTML. Change the innerHTML to innerText within the above code and examine the output.

const titleEl = doc.getElementById("title");

titleEl.innerText = '<span fashion="shade:orange;">Geekflare</span>';

Now, you will notice the precise string that we offered on the internet web page.

Distinction between let and var

The let and var key phrases are used to create variables in JavaScript. The let key phrase is launched in ES6.

The let is a block-scoped and var is function-scoped.

  let a = 2;
  console.log("Inside block", a);
console.log("Exterior block", a);

Run the above code. You’ll get an error on the final line as we are able to’t entry let a outdoors the block as a result of it’s block-scoped. Now, change it to var and run it once more.

  var a = 2;
  console.log("Inside block", a);
console.log("Exterior block", a);

You gained’t get any error as we are able to entry the a variable outdoors the block as properly. Now, let’s change the block with a operate.

operate pattern() {
  var a = 2;
  console.log("Inside operate", a);
console.log("Exterior operate", a);

You’ll get a reference error for those who run the above code as we are able to’t entry var a it outdoors the operate as a result of it’s function-scoped.

We are able to redeclare the variables utilizing var key phrase however we are able to’t redeclare the variables utilizing let key phrase. Let’s see an instance.

var a = "Geekflare";
var a = "Chandan";
let a = "Geekflare";
let a = "Chandan";

The primary piece of code gained’t throw any error and the worth is a will probably be modified to the newest assigned worth. The 2nd piece of code will throw an error as we are able to’t redeclare variables utilizing let.

Distinction between session storage and native storage

The session storage and native storage are used to retailer data on the customers’ computer systems which may be accessed with out the web. We are able to retailer the key-value pairs in each session storage and native storage. Each key and worth will probably be transformed to strings for those who present another knowledge kind or knowledge construction.

The session storage will probably be cleared after the session is over (when the browser is closed). The placement storage gained’t be cleared till we clear it.

We are able to entry, replace, and delete session storage and placement storage with sessionStorage and localStorage objects respectively.

What’s NaN in JavaScript?

The NaN is abbreviated as Not-a-Quantity. It represents that one thing just isn’t a authorized/legitimate quantity in JavaScript. There are some instances the place we’ll get NaN as output like 0/0, undefined * 2, 1 + undefined, null * undefined and many others..,

What’s Lexical scoping?

The lexical scope refers to accessing the variables from its mother and father’ scope. Let’s say we’ve a operate with two interior capabilities. The innermost operate can entry its two mother or father capabilities’ scope variables. Equally, the 2nd degree operate can entry the outermost operate scope. Let’s see it in an instance.

operate outermost() {
  let a = 1;
  operate center() {
    let b = 2;
    // `a` are accessible right here
    console.log(a, b);
    operate innermost() {
      let c = 3;
      // each `a` and `b` are accessible right here
      console.log(a, b, c);

JavaScript makes use of a scope chain to search out the variable after we are accessing it someplace within the code. First, it’ll examine the variable within the present scope, after which the mother or father scope till the worldwide scope.

What’s handed by worth and handed by reference?

The move by worth and move by reference are two methods to move the arguments to a operate in JavaScript.

Go by worth: it creates a duplicate of the unique knowledge and passes it to the operate. So, after we made any adjustments within the operate it gained’t have an effect on the unique knowledge. Test the under instance.

operate pattern(a) {
  // altering the worth of `a`
  a = 5;
  console.log("Inside operate", a);
let a = 3;
console.log("Exterior operate", a);

You will notice that the unique worth of the a just isn’t modified despite the fact that we modified it contained in the operate.

Go by reference: it passes the reference of the info to the operate. So, after we made any adjustments within the operate, it’ll additionally change the unique knowledge.

operate pattern(arr) {
  // including a brand new worth to the array
  console.log("Inside operate", arr);
let arr = [1, 2];
console.log("Exterior operate", arr);

You will notice that the unique worth of the arr is modified after we change it contained in the operate.

Word: all primitive knowledge varieties are handed by worth, and non-primitive are handed by reference.

What’s memoization?

Memoization is a way that shops the computed values in caches and makes use of them after we want them once more with out computing them once more. It would velocity up the execution of the code if the computation could be very heavy. There’s a storage tradeoff which isn’t a giant difficulty in comparison with time.

const memo = {};
operate add(a, b) {
  const key = `${a}-${b}`;

  // checking whether or not we computed the worth already or not
  if (memo[key]) {
    console.log("Not computing once more");
    return memo[key];

  // including the newly computed worth to cache
  // right here cache is a straightforward international object
  memo[key] = a + b;
  return memo[key];

console.log(add(1, 2));
console.log(add(2, 3));
console.log(add(1, 2));

It’s a easy instance that demonstrates memoization. Right here, including two numbers just isn’t a heavy computation robust. It’s only for the demo.

What’s the relaxation parameter?

The relaxation parameter is used to gather all of the remaining parameters in a operate. Let’s say we’ve a operate that may settle for a minimal of two arguments and might settle for any variety of parameters at a most. As we don’t have the utmost variety of arguments, we are able to gather the primary 2 parameters with regular variables and all others with the relaxation parameter utilizing the relaxation operator.

operate pattern(a, b, ...relaxation) {
  console.log("Relaxation parameter", relaxation);

pattern(1, 2, 3, 4, 5);

The relaxation parameter will probably be an array of the final three arguments within the above instance. With this, we are able to have any variety of parameters for a operate.

One operate can have just one relaxation parameter. And the remaining parameter ought to be the final one within the order of the parameters.

What’s object destructuring?

Object destructuring is used to entry the variables from the item and assign them to variables with the identical names as object keys. Let’s see an instance.

const object = { a: 1, b: 2, c: 3 };

// Object destructuring
const { a, b, c } = object;

// Now, a, b, c will probably be used as regular variables
console.log(a, b, c);

We are able to change the variables of destructured variables in the identical line as follows.

const object = { a: 1, b: 2, c: 3 };

// Altering the names of `a` and `b`
const { a: changedA, b: changedB, c } = object;

// Now, changedA, changedB, c will probably be used as regular variables
console.log(changedA, changedB, c);

What’s array destructuring?

Array destructuring is used to entry the variables from the array and assign them to variables. Let’s see an instance.

const array = [1, 2, 3];

// Array destructuring
// It is based mostly on the index of the array
const [a, b, c] = array;

// Now, we are able to use a, b, c as regular variables
console.log(a, b, c);

What are occasion capturing and occasion effervescent?

Occasion capturing and occasion effervescent are two methods of occasion propagation in HTML DOM. Let’s say there are two HTML parts, one inside one other. And an occasion happens on the interior component. Now, the occasion propagation mode will determine the order of those occasions’ execution.

Occasion effervescent: it runs the occasion handler on the component first, then its component, after which it goes all the way in which as much as the topmost component. That is the default behaviour of all of the occasions.

Occasion capturing: we have to specify within the occasion that we have to use this sort of occasion propagation. We are able to specify it whereas including the occasion listener. The occasions will execute within the following order if we’ve the occasion capturing enabled.

  1. The occasions begins executing from the topmost component until the goal component to down.
  2. The occasion on the goal component will probably be executed once more.
  3. The effervescent occasion propagation will once more happen till the topmost component is up.

We are able to cease the occasion propagation by calling occasion.stopPropogation a way within the occasion handler.

What are the Guarantees in JavaScript?

The Promise the item is used for the asynchronous operations which is able to full in future with a hit or failure state.

A Promise may be in one of many following states.

  1. pending – when the operation remains to be in progress.
  2. fulfilled – when the operation is accomplished efficiently. We can have outcomes (if any) within the success state.
  3. rejected – when the operation is accomplished with a failure. We can have the explanation (error) why it failed.

Let’s see two examples of success and failure instances.

// Promise which is able to full efficiently
const successPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve({ message: "Accomplished efficiently" });
  }, 300);
  .then((knowledge) => {
  .catch((error) => {

// Promise which is able to full with failure state
const failurePromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    reject(new Error("Failing the promise for testing"));
  }, 300);
  .then((knowledge) => {
  .catch((error) => {

You may have multiple then chaining if required. The beforehand returned knowledge will probably be accepted within the subsequent then callback.

Clarify the various kinds of scope in JavaScript

There are two kinds of scope in JavaScript. The international scope and native scope.

You may need heard in regards to the operate scope and block scope as properly. They’re native scopes for var and let, const respectively.

What are self-invoking capabilities?

The self-invoking capabilities are anonymous capabilities that will probably be executed instantly after creation. Let’s see some examples.

// With none parameters
(operate sayHello() {
  console.log("Whats up, World!");

// With parameters
(operate add(a, b) {
  console.log("Sum", a + b);
})(1, 2);

We are able to even move the arguments to the self-invoking capabilities as you will have seen within the instance.

What are arrow capabilities?

The arrow operate is syntactic sugar to the traditional operate with some adjustments. They behave like regular capabilities generally use instances. Arrow capabilities come in useful when we’ve to have callbacks. Let’s see its syntax.

// arrow capabilities will return by default if it would not have any brackets
let add = (a, b) => a + b;

console.log(add(1, 2));

There are some variations between the arrow capabilities and regular capabilities.

  • Arrow capabilities don’t have their very own this binding. The this key phrase contained in the arrow operate refers to its mother or father scope this.
  • Arrow capabilities can’t be used as constructor capabilities

What are callbacks?

A callback is a operate that’s handed to a different operate that’s invoked inside that operate. Utilizing callbacks is a standard factor in JavaScript. Let’s see an instance.

operate pattern(a, b, callback) {
  const end result = a + b;
  callback(end result);

operate completed(end result) {
  console.log("Completed with", end result);

pattern(1, 2, completed);

The operate completed is handed as a callback to the pattern. The completed operate is invoked with the end result after performing some motion. You will notice the callbacks utilization largely in asynchronous operations like guarantees, setTimeout, and many others..,

What are the various kinds of errors?

Let’s examine some errors in JavaScript.

ReferenceError: this error will happen if the variable that we’re accessing is on the market.

TypeError: JavaScript will throw this error if the error doesn’t match with different kinds of errors. It would additionally happen after we attempt to carry out an motion which isn’t suitable with the info.

SyntaxError: this error will happen if the JavaScript syntax just isn’t right.

There are another kinds of errors as properly. However these are the frequent error varieties in JavaScript.

What are the completely different scopes of variables in JavaScript?

There are two scopes of variables in JavaScript. The variables declared utilizing the var key phrase can have operate scope, and the variables declared with the let and const can have the block scope.

Check with the seventeenth query for extra particulars about these variables’ scope.

What are escape characters in JavaScript?

The backslash is the escape character in JavaScript. It’s used to print some particular characters which we are able to’t print generally. Let’s say we wish to print apostrophe (') inside a string which we are able to’t do usually because the string will finish on the second apostrophe. In that case, we’ll the escape character to keep away from ending the string at that time.

const message = 'Hello, I am Geekflare';

We are able to obtain the above output with out utilizing escape character by changing the skin single apostrophes with double apostrophes. But it surely’s simply an instance of methods to use an escape character. There are different characters for which we undoubtedly want escape character like n, t, and many others..,

What are BOM and DOM?

Browser Object Mannequin (BOM): all browsers have BOM representing the present browser window. It accommodates our topmost window object which is used to control the browser window.

Doc Object Mannequin (DOM): browsers create the DOM when the HTML is loaded within the tree construction. We are able to manipulate the HTML parts utilizing DOM API.

What’s a display object?

The display object is among the properties of the worldwide window object. It accommodates completely different properties of the display on which the present browser window is rendered. A few of the properties are width, top, orientation, pixelDepth, and many others.


There are is likely to be follow-up questions for all of the above questions. So, you’ll want to put together the ideas round all of the above questions.

You may additionally discover some continuously requested Java interview questions and solutions.

Comfortable Studying 🙂

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