Javascript Interview Questions and Answers

To prepare yourself for these job positions you must be aware of the Commonly asked JavaScript Interview Questions and Answers. Here in this blog, we have compiled a set of Important Javascript Interview Questions and Answers and these questions have been compiled in the perspectiveto give equal emphasis for both fresher and experienced candidates

JavaScript was created to give little more pep to the web pages and improve them as the basic tag was not fascinating enough. Ever since its evolution, JavaScript has proven its capabilities and hence, big IT giants like Google and Facebook are using JavaScript. Now, it is used to make desktop-like and complex web-applications. And in addition to it, Node.js has made it more popular to design server-side software. And since, it’s just the beginning, there is going to be a huge growth in the requirement of JavaScript professionals in the industry. 

Javascript Interview Questions and Answers

We know it is hard to face an interview, even when you have the desired knowledge and skill set. So for that we have brought up a complete reference for you to have confidence on your JavaScript skills. 

So go through this collection of a 101 most common and predicted interview question and answer series that will help you clear your basics and will help you face the interview with confidence. 

Q1. What is JavaScript?

JavaScript is an interpreted programming language with great object-oriented capabilities and is very lightweight. It is used to manipulate HTML as well as CSS to make web pages interactive. The functionality of JavaScript lets you do calculations, validate, and manipulate data. 

Q2. Name the founder of JavaScript and in which year it was created?

JavaScript was founded by Brandan Eich in 1995.

List some of the data types supported by JavaScript.

Here are some of the data types supported by JavaScript:

Null

Boolean

String

Number

Object

Symbol

Undefined

Q3. Mention some features of the JavaScript Language.

Although JavaScript has a lot of features, a few of the important ones are:

JavaScript is interpreted and a very lightweight programming language

JavaScript is an open and a cross-platform scripting language

By using JavaScript, it is easy to design network-centric applications

It is complementary to Java

Q4. Is JavaScript case-sensitive in nature?

Yes, JavaScript is a case-sensitive language. The variable names, keywords, function names, and other phrases must be written in a correct combination of capitals and smalls.

Q5. List some advantages of using JavaScript

Some of the advantages of working with JavaScript are:

Increased interactivity

Rich interfaces

Less interaction with the server

Visitors can see immediate feedback

Q6. Is there any connection or similarity between Java and JavaScript?

No, there is no similarity between Java and JavaScript. Even though their names are similar, there is nothing in common for their working. JavaScript is just coded programs that can be added to HTML pages whereas Java is a complete full-stack programming language.

Q7. How can you debug JavaScript?

JavaScript can be debugged using various methods like:

By using JavaScript debuggers

The console.log() method

By using the ‘debugger’ keyword

Q8. How will you create an Object in JavaScript?

We can easily create an object in JavaScript as it is also an object-oriented language. We use the following syntax for creating an object: 

Var emp = {

Name: “O7 Solutions”

};

Q9. How will you create an array in JavaScript?

Creating an array is very simple in JavaScript, you can use the array literals for creating arrays.

var x = [];

var y = [1, 2, 3, 4, 5];

Q10 . Can you assign an anonymous function to a variable in JavaScript?

Yes, we can assign an anonymous function to a variable. We can also pass it as an argument for a different function.

Q11. What is called the scope of a variable in JavaScript?

The scope of any variable is the defined region in which that particular variable can be used. 

Q12. How many scopes of a variable does JavaScript have?

JavaScript has two scopes of any variable, local and global

Local Scope: A variable having local scope can be assessed only within the function where it is defined. Parameters of the functions are local for the variable.

Global Scope: A variable having global scope in visible and accessible anywhere in the JavaScript code.

These are commonly asked JavaScript Interview Questions and Answers for Fresher and Experienced candidates.

Q13. Explain the use of ‘this’ operator in JavaScript

‘this’ is a keyword in JavaScript and it is used to refer to the object where it belongs. Depending on where it is used, it can have different values. For example, when used in a function, it refers to the global object and when used in a method, it refers to the owner object.

Q14. What is the ‘Callback’ function?

The ‘Callback’ function is a very simple JavaScript function that is passed as an argument to a method. This function is executed after another function has completed its execution. In JavaScript, functions are objects and hence they can accept other functions as arguments and also, they can be returned as the output of any function.

Q15. What do you understand by ‘Closure’ in JavaScript?

Closures are automatically created whenever there is an incident in which a variable defined outside the scope is being accessed. It is used to give access to the outer function’s scope from the inner function. In JavaScript, a closure is created every time a function is created.

Q16. Explain the use of the ‘TypeOf’ operator in JavaScript.

The prime use of the ‘TypeOf’ operator is to fetch the data type of its operand. It is a unary operator and is placed before the operand to fetch its type. The operand is allowed to be any data structure or literal. It returns a string that indicates the data type of the operand.

Q17. Depict how to create a cookie in JavaScript.

In JavaScript, a cookie can be created easily with the help of document.cookie object, syntax of which is as follows:

document.cookie = “key1 = value1; key2 = value2, key3 = value3; expires = date”;

Q18. Explain how you can read a cookie with the help of JavaScript?

Very similar to writing, reading a cookie in JavaScript is also quite simple, as the cookie is the value returned by document.cookie, and this string is useful every time you want to access a cookie.

Q19. Explain how you can delete a cookie with the help of JavaScript?

For deleting a cookie in JavaScript, you just need to set the timer of the cookie to any time in the past and it will be deleted automatically. Also, you should carefully define the path of the cookie to ensure that the right cookie is being deleted.

Q20. In how many ways you can include the JavaScript code in an HTML file?

There are 3 ways of doing so:

Inline

External

Internal

Q21. List the ways to define a variable in JavaScript.

Here are 3 ways to define a variable:

Var – The JavaScript variables statement is employed to declare a variable and, optionally, we will initialize the worth of that variable. For instance: var x =60; Before executing the code, all the variable declarations are processed at first.

Const – the thought of const functions doesn’t allow them to switch the thing on which they’re called. When a function is said as const, it is often called on any sort of object.

Let – It is a signal that the variable may be reassigned, such as a counter in a loop, or a value swap in an algorithm. It also signals that the variable is going to be used only within the block it’s defined in.

Q22. Name the looping structures that can be used in JavaScript.

While

Do-while

For

Q23. What keywords are used to handle exceptions in JavaScript?

Try-catch-finally is the keywords that are used for exception handling in JavaScript. Here is the syntax:

Try {

Try to execute this piece of code

}

Catch (Exception) {

Run this code if an exception occurs

}

Finally {

This piece of code runs irrespective of the exception.

}

Q24. Name some of the JavaScript frameworks.

Some of the common JavaScript frameworks are:

React

Angular

Vue

Q25. Explain the difference between a document and a window in JavaScript.

The window in JavaScript is a global object that is used to hold functions, variables, location, and history. Document in JavaScript is also a kind of window that is considerably called a property of the window.

Tell the difference between innerHTML and innerText.

Basically, if an HTML tag is found in a string, innerHTML will process it and innerTEXT would not be able to process it.

Q26. What do you know about NaN in JavaScript?

NaN is the abbreviation of Not a Number. Generally, NaN appears when we compare unequal to a number. It is used for showing the error condition that occurs when a function returns an absurd value when it is expected to return a valid numeral. NaN is also seen in conditions where we convert a string or anything to a number and the process cannot be completed and results in error. 

Q27. How are objects/data types passed in functions?

In JavaScript, objects are passed by reference into a function and primitive data types are passed by value into the function.

Q28. What do you understand by ‘strict’ mode in JavaScript?

Strict mode is enabled to ensure better error checking of the code. You can enable this strict mode by adding the ‘use strict’ keyword at the beginning of any function, program, or file.

Explain what is a prompt box in JavaScript.

The prompt box is just like a text box which allows the user to give inputs. It appears when the user uses the execute prompt() method. After entering the desired values, the user needs to click on either the ‘Ok’ or ‘Cancel’ button to continue further.

Explain the difference between apply() and call().

The apply() method is used to call any function provided with ‘this’ value along with an array that contains the arguments, whereas the call() methods are used to call any function with a mentioned ‘this’ value and individual arguments.

Q29. What is the JavaScript command used to print text onto the screen?

A very simple command document.write(“Hello”) is used to print Hello on the screen.

These are commonly asked JavaScript Interview Questions and Answers for Fresher candidates in an interview

Q30. What is the use of a ‘blur’ function?

The ‘blur’ function in JavaScript is used to disconnect the focus from a particular object.

What do you understand by variable typing in JavaScript?

Variable typing is done in JavaScript when first a number is assigned to a variable and then a string is assigned to that same variable.

Ex:

a = 4;

a = ‘O7 Solutions’;

Q31. What command of JavaScript is used to find the OS in the client machine?

 There is a special command in JavaScript for doing this job. It’s the ‘Navigator.appversion’ command.

Q32. List the types of errors that can occur in JavaScript.

The types of errors are:

Load time errors

Logical errors

Run-time errors

Q33. Explain the use of the Unshift method in JavaScript.

The unshift methods work at the starting of an array. It is very similar to the push method. Using these methods, we can prepend some elements from the beginning of an array.

Q34. Explain the use of the push method.

The push method works at the endpoint of an array. It appends some elements at the very end of an array. Also, multiple arguments will append multiple elements onto the array by using this push command.

Q35. Is there any similarity between JavaScript and JScript?

Yes, both of them are very similar, the main difference is that JavaScript was developed and released by Netscape, and Jscript is developed by Microsoft.

Q36. How are the properties of objects assigned in JavaScript?

There are two ways to assign properties to any object:

obj.class = 15;

Or

obj[“class”] =15;

These are commonly asked JavaScript Interview Questions and Answers for Fresher candidates in an interview

Q37. How to retrieve the status of a checkbox in JavaScript?

alert(document.getElementById(‘checkbox1’).checked);

The above query is used to get the status of the checkbox. By running this query, if the checkbox is checked, the query returns TRUE.

Q38. Explain the role of break and continue statements

The break statement is used to break the current loop and jump onto the next consecutive line. A continue statement is used to continue within the same loop but with a new recurrence.

Q39. What is the method used for reading or writing a specific file in JavaScript?

This action can be performed by using the mentioned JavaScript extensions, which are available through JavaScript editors.

For opening a file, the extension used is:

fh = fopen(getScriptPath(), 0);

Q40. What are screen objects?

To get and read the information from a client’s screen, various screen objects are used.

Q41. Name some of the properties of screen objects

Height

Width 

ColorDepth

AvailWidth

AvailHeight

Q42. What does encodeURI() do?

EncodeURI() is the method used for converting the URL into Hex coding

Q43. What does decodeURI() do?

DecodeURI() is the methods used to convert the Hex code or the encoded URL back to normal 

Q44. How will you declare a three-dimensional array in JavaScript?

The following code is used

Var array1 = [[[]]];

Q45. What is the reason that innerHTML is not advised to use in JavaScript

Since there is no scope of validation in innerHTML and the content of innerHTML is refreshed each time, making it much slower, it is advised to not use innerHTML. Also, it makes the webpage unstable.

Q46. What are timers in JavaScript?

In JavaScript, timers are higher-order functions that are supposed to be used to delay the function or to repeat the execution of other functions, which are received as the very first argument. 

Q47. How to write comments in JavaScript

There are two methods of writing comments in JavaScript.

Inline comments (//)

Multiline comments (/……/)

Q48. What is the meaning of === operator in JavaScript

It’s an equity operator. And is used without type coercion. In simple words, if triple equal is used, then the compared values must be equal in their types also. 

Is there automatic type conversion in JavaScript

Yes, automatic type conversion is done in JavaScript.

Q49. How to convert a string to integer of any base in JavaScript

There are multiple methods used to convert a string to an integer

Parse.int function

Unary plus operator

Math.round

Using number object

Q50. What will be the result of the equation 3 + 9 + “5” in JavaScript?

“125”

Q51. What is meant by NULL in JavaScript?

THere, to represent the absence of the value of any object, NULL is used. 

Q52. List the types of pop-up boxes available in JavaScript

Alert

Confirm

Promp

Q53. Explain the use of void (0) in JavaScript.

The void (0) operator in JavaScript is used to evaluate any given expression and returns an undefined value as the result. Generally, it is used when there is a certainty of having a side effect while inserting any expression into the web page.

These are commonly asked JavaScript Interview Questions and Answers for Fresher and experienced candidates in an interview

Q54. Define what is namespacing and how it is used in JavaScript.

Namespacing is a JavaScript feature that is used for grouping the required variables and functions with unique names. It just provides the names to attach to particular object properties or even functions. it is useful for using the specific piece of code and is also important to maintain the modularity of the code. 

Q55. What do you understand by Escape Characters of JavaScript?

Basically, the escape characters that are used in JavaScript are a special feature that allows the coder to write special characters in the code without disturbing the application. To make the special characters like double quotes, singles quotes, apostrophes, etc. visible in your text just write the escape character which is the backslash character (/) before the characters to display them.

Q56. How will you empty the array?

There are multiple methods that can be used to empty the array:

arraylist = []

arrayList.length = 0;

arrayList.splice(0, arrayList.length);

while (arrayList.length)

{

arrayList.pop();

}

Q57. What do you understand by event bubbling in JavaScript?

Event propagation is also known as event bubbling in HTML DOM API. It is a process when an event is targeted to the innermost element and then further triggered until it reaches the outermost layer of DOM. 

Q58. Explain the difference between ‘undefined’ and ‘null’ in JavaScript.

The primary difference is that the keyword ‘undefined’ is used to mention that a variable has been declared earlier but no value has been assigned to it yet. And ‘null’ is a value in itself that can be assigned to any variable. The other difference between ‘undefined’ and ‘null’ is that undefined is a type and null is said to be an object.

Q59. Explain the difference between ‘undefined’ and ‘undeclared’ in JavaScript.

lared somewhere but no value has been assigned to it and the ‘undeclared’ variables are the variable that is not even declared and they do not exist in a particular program. A runtime error occurs when the program reads the value of the undeclared variable whereas an undefined value is returned when the program reads the value of an undefined variable.

Q60. List the different types of errors that can occur in a JavaScript program.

Some of the errors that can occur in a JavaScript program are as follows

EvalError

RangeError

SyntaxError

TypeError

URIError

ReferenceError

Q61. What does the window.onload command do in JavaScript?

This command is used to execute a particular piece of code when a browser has completely loaded the DOM tree along with all the resources such as objects, images and more.

Q62. What does the onDocumentReady command do in JavaScript?

This command allows the developer to execute a particular piece of code when the browser has loaded only the DOM tree and does not wait for any images of objects to load. ,

Q63. Does JavaScript support Boolean operators?

es, JavaScript is made to support Boolean operators which can generally return the value of true or false. The Boolean () function is defined to find if the provided expression is true or false. 

Q64. Explain the difference between a web farm in a web garden.

When a web application is hosted over multiple web servers in order to balance the load of it comet is called a web farm and when there are several worker processes of an application, it is called a web garden. 

Q65. Are there any functional components in JavaScript?

There are two functional components in JavaScript. These are:

First-class functions: Normal functions are recognized as first-class functions in JavaScript. They can be stored easily in data structures, assigned to variables, can be passed as arguments to other functions, and can also be retrieved as a result of other functions.

Nested functions: These are the functions that are defined inside another function in a nested format and are called every time the main is executed.

Q66. Is JavaScript strong enough to work with regular expressions?

Yes, JavaScript provides support for regular expressions as well

Q67. What is an Image Map?

Basically, a JavaScript Image Map is a simple image on a web page which has different links that navigate the user to different web pages for different sections of that particular web page. 

Q68. What methods are used to redirect the URL?

There are two different methods that can be used to redirect user from one web page to another web page:

location.href

location.replace

Q69. If there is an infinite loop running in JavaScript what can happen?

If the program is stuck in an infinite loop running based on javascript, it will result in crashing the browser. 

Q70. Is there any method to convert the normal JavaScript date to ISO standard?

Yes, the tolSOString() method is specially designed to convert the JavaScript date to ISO standard. Basically what it does is it converts the JavaScript date object into a string with the help of the ISO standard. 

Q71. What will you do if you need to call a particular function in every e x seconds in JavaScript?

There is a separate function called setInterval() pictures used to call a function repetitively in the defined time of interval. The syntax is

setInterval(function, milliseconds, param1, param2, … )

Q72. Which logical operators are allowed to use in JavaScript?

the and [&&] operator or [ || ]operator and the not [! ] operators are allowed to be used in JavaScript. 

Q73. How to clone an object in JavaScript?

you can simply clone the object in JavaScript by using the code below

var obj = {a : 1, b: 2}

var objclone = Object.assign({}, obj);

Q74. How can you manipulate the DOM using JavaScript?

The document represents the web page loaded in the browser and it is an entry point to the page content. This provides a powerful API to interact with the DOM like getElementsById, getElementsByClassName, getElementsByTagName, and querySelector. Through this, we can select one or group of elements and manipulate them.

These are commonly asked JavaScript Interview Questions and Answers for Experienced candidates in an interview

Q75. List a few things you can build with the help of JavaScript.

Server applications

Websites

Web applications

Presentations

Games

Smartwatch apps

Mobile apps

Flying robots

Q76. Which browsers support JavaScript?

Nowadays, JavaScript is supported by all the modern web browsers that are available and have an inbuilt interpreter.

Q77. Is JavaScript multithreaded?

No, JavaScript is a single-threaded language and because of this, some developers who are majorly working with multithreaded languages think that JavaScript is slow.

Q78. Can someone hack JavaScript?

Yes, hackers can try to hack JavaScript in order to extract sensitive information from the victim’s website. The CSRF attack, also known as JavaScript Hijacking is done by the hackers to damage a page of the victim’s website in order to extract sensitive information like JSON and JavaScript.

Q79. What is the output of the following code?

Math.max() > Math.Min

False.

It sounds quite weird but the fact is that, in JavaScript, when no argument is passed in min() and max() function they return the values Infinity and -Infinity respectively. And hence, min is proved greater than max.

Q80. Does parenthesis position matter in JavaScript?

Yes, the parenthesis position is very important in JavaScript. It needs every syntax to be perfect and everything to be in its right place in order to run without any errors. 

Q81. What do you understand about the currying function?

Currying function is a process that takes a function as an input along with multiple arguments and then returns it by turning it into a sequence of functions in which each function has only one single argument. This function type is named after Haskell Curry, a mathematician. With the help of this function we can create unary functions from n-ary functions.

Q82. Explain the meaning of the ‘let’ keyword.

The ‘let’ keyword statement of JavaScript is used to declare block scope of a local variable. The statement or a variable which is defined with the keyword ‘let’ has its scope limited to the block or statement or expression where it is used. It is contrary to the var keyword.

Q83. What do you understand about temporal dead zones in JavaScript?

The Temporal dead zone which occurs when the developer declares a variable using the const and let keywords, but not with var keyword. Reference-Error is cost in ECMAScript 6 when we try to access a let or const variable before it is declared. And the time duration when this happens is known as the temporal dead zone. 

Q84. What do you know about IIFE?

IIFE is an acronym for immediately invoked function expression and it is a JavaScript function with the property that it runs as soon as this function is defined. The main reason for using this function is for data privacy.

Here is the syntax of this function:

(function ()

{

// Logic here

}

();

Q85. List the benefits of using modules.

The main three benefits of using modules for sprawling are

Maintainability

Reusability

Namespacing

Q86. List some of the events that are for the server-sent events.

A different method is used to mitigate any SQL Injection.

Here are some of the server sent events:

Onopen – Used when connection to the server is being opened

Onmessage – Used whenever a message is received

Onerror – Used when an error occurs

State some of the prime rules of ‘promise’.

Here are some of the main rules of promise:

Its an object that is used to supply .then() method

The promise in the pending state might change its status to either rejected or fulfilled.

One the promise reaches the rejected or fulfilled stage, it should not change its state again.

Once a promise is settled, the value must not change

Q87. JavaScript is placed in which element of the HTML code?

JavaScript is embedded in the

Q88. How can you detect the browser name of the client, through JavaScript?

We can use the ‘Navigator.appName’ command to get information about the browser of the Client.

Q89. How can you read properties of an object?

In JavaScript, we can use the (.) dot operator to read as well as write properties of an object.

Q90. List some of the built-in methods of JavaScript.

anchor()

concat()

ceil()

constructor()

Date()

Date.parse()

link()

fontcolor()

filter()

match()

pop()

toLocaleString()

valueOf()

Explain what is Negative Infinity in JavaScript.

Whenever a negative number is divided by zero in JavaScript, the negative infinity occurs. 

Q91. Is there any way to import all the exports of a file in JavaScript?

Yes, to import all the exports of a file as objects, the following syntax is used:

Q92. What is Event Capturing in JavaScript?

It’s a rarely used method in which the process is started with the outermost element to capture the event and then mitigating it to the innermost element. It is completely contrary to Event Bubbling.

Q93. Draw a very simple DOM (Document Object Model)

The above are the commonly asked JavaScript Interview Questions and Answers in an Interview for Freshers and Experienced candidates. In the coming days, we will also add more Interview Questions and Answers to this blog that are prevalent in the Industry.We provide the best-in-class JavaScript Training in jalandhar or JavaScript Training in jalandhar under expert guidance with certification.