Top 100 JavaScript Interview Questions and Answers

Top 100 JavaScript Interview Questions and Answers for 2020

JavaScript Interview Questions and Answers PDF file: Download Here!..

JavaScript, also abbreviated as JS, is a high-level server-side programming language. JavaScript is widely used worldwide to build various web applications, which means there are huge opportunities for JavaScript programming.

To build a JavaScript programming career, candidates need to crack the interview. They are asked for various JavaScript interview questions and answers.

Following is a list of JavaScript interview questions and answers, which are likely to be asked during the interview. Candidates are likely to be asked basic JavaScript interview questions to advance JS interview questions depending on their experience and various other factors.

The below list covers all the JavaScript questions for freshers and JavaScript interview questions for professional-level candidates. This JS interview questions guide will help you crack the interview and help you get your dream job for JavaScript Programming.


Frequently Asked JavaScript Interview Questions and Answers for Freshers and Experienced

  1. What is JavaScript?

JavaScript is a client-side and server-side scripting language inserted into HTML pages and is understood by web browsers. JavaScript is also an Object-based Programming language

  1. Enumerate the differences between Java and JavaScript?

Java is a complete programming language. In contrast, JavaScript is a coded program that can be introduced to HTML pages. These two languages are not at all inter-dependent and are designed for different intent. Java is an object-oriented programming (OOPS) or structured programming languages like C++ or C, whereas JavaScript is a client-side scripting language.

  1. What are JavaScript Data Types?

Following are the JavaScript Data types:

  • Number
  • String
  • Boolean
  • Object
  • Undefined


  1. What is the use of isNaN function?

isNan function returns true if the argument is not a number; otherwise, it is false.

  1. Which is faster between JavaScript and an ASP script?

JavaScript is faster. JavaScript is a client-side language,, and thus it does not need the assistance of the webserver to execute. On the other hand, ASP is a server-side language and hence is always slower than JavaScript. Javascript now is also a server-side language (nodejs).

  1. Is it possible to break JavaScript Code into several lines?

Breaking within a string statement can be done by using a backslash, ‘,’ at the end of the first line.


document. Write ("This is a program,");

And if you change to a new line when not within a string statement, then javaScript ignores the break in the line.



var x=1, y=2,



The above code is perfectly fine, though not advisable as it hampers debugging.


  1. Which company developed JavaScript?

Netscape is the software company that developed JavaScript.

  1. What are undeclared and undefined variables?

Undeclared variables are those that do not exist in a program and are not declared. If the program tries to read the value of an undeclared variable, then a runtime error is encountered.

Undefined variables are those that are declared in the program but have not been given any value. If the program tries to read the value of an undefined variable, an undefined value is returned.

  1. Write the code for adding new elements dynamically?




<script type="text/javascript"> 

    function addNode () { var newP = document. createElement("p"); 

    var textNode = document.createTextNode(" This is a new text node"); 

    newP.appendChild(textNode); document.getElementById("firstP").appendChild(newP); } 

</script> </head> 

<body> <p id="firstP">firstP<p> </body> 



  1. What are global variables? How are these variable declared?

Global variables are available throughout the length of the code so that it has no scope. The var keyword is used to declare a local variable or object. If the var keyword is omitted, a global variable is declared.


// Declare a global: globalVariable = "Test";

The problems faced by using global variables are the clash of variable names of local and global scope. Also, it is difficult to debug and test the code that relies on global variables.


  1. What is a prompt box?

A prompt box is a box that allows the user to enter input by providing a text box. A label and box will be provided to enter the text or number.

  1. What is ‘this’ keyword in JavaScript?

‘This’ keyword refers to the object from where it was called.

  1. What is the working of timers in JavaScript?

Timers are used to execute a piece of code at a set time or repeat the code in a given interval. This is done by using the functions setTimeout, setInterval, and clearInterval.

The setTimeout(function, delay) function is used to start a timer that calls a particular function after the mentioned delay. The setInterval(function, delay) function repeatedly executes the given function in the mentioned delay and only halts when canceled. The clearInterval(id) function instructs the timer to stop.

Timers are operated within a single thread, and thus events might queue up, waiting to be executed.


  1. Which symbol is used for comments in Javascript?

// for Single line comments and

/* Multi




  1. What is the difference between ViewState and SessionState?

‘ViewState’ is specific to a page in a session.

‘SessionState’ is specific to user-specific data that can be accessed across all web application pages.

  1. What is === operator?

=== is called a strict equality operator, which returns true when the two operands have the same value without conversion.

  1. How you can submit a form using JavaScript?

To submit a form using JavaScript use




  1. How can the style/class of an element be changed?

It can be done in the following way:

document.getElementById(“myText”). style. fontSize = “20”;


document. getElementById (“myText”). className = “anyclass”;


  1. How to read and write a file using JavaScript?

There are two ways to read and write a file using JavaScript

Using JavaScript extensions

Using a web page and Active X objects


  1. What are all the looping structures in JavaScript?

Following are looping structures in Javascript:

  • For
  • While
  • Do-while loops


  1. What is called Variable typing in Javascript?

Variable typing is used to assign a number to a variable. The same variable can be assigned to a string.


i = 10;

i = "string;"

This is called variable typing.

  1. How can you convert the string of any base to an integer in JavaScript?

The parseInt() function is used to convert numbers between different bases. parseInt() takes the string to be converted as its first parameter. The second parameter is the base of the given string.

To convert 4F (or base 16) to integer, the code used will be –

parseInt (“4F”, 16);

  1. Difference between “==” and “===”?

“==” checks only for equality in value, whereas “===” is a stricter equality test and returns false if either the value or the type of the two variables are different.

  1. What would be the result of 3+2+”7″?

Since 3 and 2 are integers, they will be added numerically. And since 7 is a string, its concatenation will be done. So the result would be 57.

  1. How to detect the operating system on the client machine?

In order to detect the operating system on the client machine, the navigator. Platform string (property) should be used.

  1. What do you mean by NULL in Javascript?

The NULL value is used to represent no value or no object. It implies no object or null string, no valid boolean value, no number, and no array object.

  1. What is the function of the delete operator?

The delete keyword is used to delete the property as well as its value.


var student= {age:20, batch:"ABC"};

Delete student. age;


  1. What is an undefined value in JavaScript?

Undefined value means the

Variable used in the code doesn’t exist

Variable is not assigned to any value

Property does not exist.


  1. What are all the types of Pop up boxes available in JavaScript?


Confirm and



  1. What is the use of Void (0)?

Void(0) is used to prevent the page from refreshing, and parameter “zero” is passed while calling.

Void(0) is used to call another method without refreshing the page.

  1. How can a page be forced to load another page in JavaScript?

The following code has to be inserted to achieve the desired effect:

<script language="JavaScript" type="text/javascript" >

<!-- location. href="http://newhost/newpath/newfile.html"; //--></script>

  1. What is the data type of variables in JavaScript?

All variables in JavaScript are object data types.

  1. What is the difference between an alert box and a confirmation box?

An alert box displays only one button, which is the OK button.

But a Confirmation box displays two buttons, namely OK and cancel.

  1. What are escape characters?

Escape characters (Backslash) is used when working with special characters like single quotes, double quotes, apostrophes, and ampersands. Place backslash before the characters to make it display.


document. write "I m a "good" boy."

document. write "I m a "good" boy."


  1. What are JavaScript Cookies?

Cookies are the small test files stored in a computer, and they get created when the user visits the websites to store information that they need. Examples could be User Name details and shopping cart information from previous visits.

  1. What a pop()method in JavaScript is?

The pop() method is similar to the shift() method, but the difference is that the Shift method works at the array’s start. The pop() method takes the last element off of the given array and returns it. The array on which it is called is then altered.


var cloths = ["Shirt", "Pant", "TShirt"];


//Now cloth becomes Shirt,Pant


  1. Does JavaScript has concept level scope?

No. JavaScript does not have concept-level scope. The variable declared inside the function has scope inside the function.

  1. What are the disadvantages of using innerHTML in JavaScript?

If you use innerHTML in JavaScript, the disadvantage is

Content is replaced everywhere

We cannot use it like “appending to innerHTML

Even if you use +=like “innerHTML = innerHTML + ‘html'” still the old content is replaced by html

The entire innerHTML content is re-parsed and builds into elements. Therefore, it’s much slower

The innerHTML does not provide validation, and therefore we can potentially insert valid and broken HTML in the document and break it


  1. What is break and continue statements?

Break statement exits from the current loop.

Continue statement continues with next statement of the loop.

  1. What are the two basic groups of data types in JavaScript?

They are as—Primitive

Reference types

Primitive types are number and Boolean data types. Reference types are more complex types like strings and dates.

  1. What is variable typing?

Variable typing assigns a number to a variable and then assigns a string to the same variable. An example is as follows:

i= 8;


  1. What are the different types of errors in JavaScript?

There are three types of errors:

Load time errors: Errors that come up when loading a web page, like improper syntax errors, are known as Load time errors and generate the errors dynamically.

Runtime errors: Errors that come due to misuse of the command inside the HTML language.

Logical Errors: These are the errors that occur due to the bad logic performed on a function with a different operation.

  1. What is the ‘Strict Mode in JavaScript, and how can it be enabled?

Strict Mode adds certain compulsions to JavaScript. Under the strict Mode, JavaScript shows errors for a piece of code, which did not show an error before, but might be problematic and potentially unsafe. Strict Mode also solves some mistakes that hamper the JavaScript engines from working efficiently.

Strict mode can be enabled by adding the string literal “use strict” above the file. This can be illustrated by the given example:

function myfunction() {

    "use strict;"

    var v = "This is a strict mode function";


  1. What is the difference between .call() and .apply()?

The function .call() and .apply() are very similar in their usage except a little difference. .call() is used when the number of the function’s arguments are known to the programmer, as they have to be mentioned as arguments in the call statement. On the other hand, .apply() is used when the number is not known. The function .apply() expects the argument to be an array.

The basic difference between .call() and .apply() is in the way arguments are passed to the function. Their usage can be illustrated by the given example.

var someObject = {

myProperty : 'Foo',

myMethod : function(prefix, postfix) {

    alert(prefix + this.myProperty + postfix);


someObject.myMethod('<', '>'); // alerts '<Foo>'

var someOtherObject  = {

    myProperty : 'Bar.'

};, '<', '>'); // alerts '<Bar>'

someObject.myMethod.apply(someOtherObject, ['<', '>']); // alerts '<Bar>'

  1. What are Screen objects?

Screen objects are used to read the information from the client’s screen. The properties of screen objects are –

AvailHeight: Gives the height of the client’s screen

AvailWidth: Gives the width of the client’s screen

ColorDepth: Gives the bit depth of images on the client’s screen

Height: Gives the total height of the client’s screen, including the taskbar

Width: Gives the total width of the client’s screen, including the taskbar


87.How to use Loops in Javascript?

There are mainly four types of loops in JavaScript.

  • for loop
  • for/in a loop (explained later)
  • while loop
  • do…while loop

for loop


for(statement1; statement2; statment3)

lines of code to be executed


Statement1 is executed first, even before executing the looping code. So, this statement is normally used to assign values to variables used inside the loop.

The statement2 is the condition to execute the loop.

The statement3 is executed every time after the looping code is executed.




    <script type="text/javascript">

        var students = new Array("John", "Ann", "Aaron", "Edwin", "Elizabeth");

        document.write("<b>Using for loops </b><br />");

        for (i=0;i<students.length;i++)


        document.write(students[i] + "<br />");







while loop



lines of code to be executed


The “while loop” is executed as long as the specified condition is true. Inside the while loop, you should include the statement that will end the loop at some point in time. Otherwise, your loop will never end, and your browser may crash.

do…while loop





block of code to be executed

} while (condition)

The do…while loop is very similar to the while loop. The only difference is that in do…while loop, the block of code gets executed once even before checking the condition.




    <script type="text/javascript">

        document.write("<b>Using while loops </b><br />");

        var i = 0, j = 1, k;

        document.write("Fibonacci series less than 40<br />");

        while(i<40)        {

            document.write(i + "<br />");

            k = i+j;

            i = j;

            j = k;








  1. What are the important JavaScript Array Method explain with example?

JavaScript Array Methods

The Array object has many properties and methods which help developers to handle arrays easily and efficiently. You can get the value of a property by specifying and the output of a method by specifying arrayname.method().

length property --> If you want to know the number of elements in an array, you can use the length property.

prototype property --> If you want to add new properties and methods, you can use the prototype property.

reverse method --> You can reverse the order of items in an array using a reverse method.

sort method --> You can sort the items in an array using sort method.

pop method --> You can remove the last item of an array using a pop method.

shift method --> You can remove the first item of an array using shift method.

push method --> You can add a value as the last item of the array.




    <script type="text/javascript">

        var students = new Array("John", "Ann", "Aaron", "Edwin", "Elizabeth");


            for (i=0;i<this.length;i++){

                document.write(this[i] + "<br />");



        document.write("students array<br />");


        document.write("<br />The number of items in students array is " + students.length + "<br />");

        document.write("<br />The SORTED students array<br />");



        document.write("<br />The REVERSED students array<br />");



        document.write("<br />THE students array after REMOVING the LAST item<br />");



document.write("<br />THE students array after PUSH<br />");

        students.push("New Stuff");








People also ask for JavaScript Interview Questions and Answers

Q1. How are JavaScript and Java different from each other?

Q2. What is JavaScript?

Q3. What data types do JavaScript support?

Q4. What are the different features of JavaScript,

Q5. Is JavaScript case-sensitive,

Q6. What advantages does JavaScript have,

Q7: How can one create an object using JavaScript,

Q8. How can one use JavaScript to create an Array,

Q9. What is the name function in JavaScript and how can you define it,

Q10. Can one assign an anonymous function to a variable and then pass it as an argument to another function?

Leave a Comment