50 Most Important JavaScript Interview Questions And Answers

By – HtmlCssMonk

List of 50 Most Important JavaScript Interview Questions For Freshers and Experienced Professionals:

JavaScript also abbreviated as JS, is a high level server side programming language. JavaScript is widely used all over the world to build various web applications, which means there are huge opportunities available for the JavaScript programming. To build a career in JavaScript programming, candidates need to crack the interview in which 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.

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1- What is JavaScript?

JavaScript is a scripting language developed by Netscape. It can be used to program web browsers or even servers. It can dynamically update the contents of the webpage, which is the beauty of this language.

2- List some features of JavaScript.

Some of the features of JavaScript are:

  • Lightweight
  • Interpreted programming language
  • Good for the applications which are network-centric
  • Complementary to Java
  • Complementary to HTML
  • Open source

3- List some of the advantages of JavaScript.

Some of the advantages of JavaScript are:

  • Server interaction is less
  • Feedback to the visitors is immediate
  • Interactivity is high
  • Interfaces are richer

4- List some of the disadvantages of JavaScript.

Some of the disadvantages of JavaScript are:

  • No support for multithreading
  • No support for multiprocessing
  • Reading and writing of files is not allowed
  • No support for networking applications.

5- What is the difference between test () and exec () methods?

Both test () and exec () are RegExp expression methods.

By using a test (), we will search a string for a given pattern, if it finds the matching text then it returns the Boolean value ‘true’ or else it returns ‘false’.

But in exec (), we will search a string for a given pattern, if it finds the matching text then it returns the pattern itself or else it returns ‘null’ value.

6- In the following code snippet can you please predict the output or If you get an error; please explain the error?

<!DOCTYPE html>
<h2> <strong> Sample: Software Testing Help</strong> </h2>
<p style='text-decoration:underline'>Example Const Variable </p>
<p id="display"></p>
const first_num;
first_num =1000;
document.getElementById("display").innerHTML = "First Number:"+ first_num;

Answer: The ‘const’ variable ’first_num’ is not initialized with a value, so the code will produce a syntax error.

Output of the code snippet:

Error: Uncaught SyntaxError: Missing initializer in the const declaration

7- Have you used any browser for debugging? If yes, how is it done?

By pressing the ‘F12’ key in the keyboard we can enable debugging in the browser. Chose the ‘Console’ tab to view the results.

In Console, we can set breakpoints and view the value in variables. All modern browsers have a built-in debugger with them (For example: Chrome, Firefox, Opera, and Safari). This feature can be turned ON and OFF.

8- What is the use of the ‘debugger’ keyword in JavaScript code?

Using the ‘debugger’ keyword in the code is like using breakpoints in the debugger.

To test the code, the debugger must be enabled for the browser. If debugging is disabled for the browser, the code will not work. During debugging of the code, the remaining part should stop executing, before it goes to the next line.

9- What are the data types supported by JavaScript?

The data types supported by JavaScript are:

  • Undefined
  • Null
  • Boolean
  • String
  • Symbol
  • Number
  • Object

10- Is JavaScript a case-sensitive language?

Yes, JavaScript is a case sensitive language.  The language keywords, variables, function names, and any other identifiers must always be typed with a consistent capitalization of letters.

11- How can you create an object in JavaScript?

JavaScript supports Object concept very well. You can create an object using the object literal as follows −

var emp = {
name: "Daniel",
age: 23

12- How can you create an Array in JavaScript?

You can define arrays using the array literal as follows-

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

13- What is the use of isNaN function?

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

14- What is the difference between Java & JavaScript?

Java is an OOP programming language.JavaScript is an OOP scripting language.
It creates applications that run in a virtual machine or browser.The code is run on a browser only.
Java code needs to be compiled.JavaScript code are all in the form of text.

15- What is negative infinity?

Negative Infinity is a number in JavaScript which can be derived by dividing negative number by zero.

16- 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.

17- What is JavaScript Hoisting?

While using the ‘JavaScript Hoisting’ method, when an interpreter runs the code, all the variables are hoisted to the top of the original /current scope. If you have a variable declared anywhere inside the code, then it is brought to the top.

This method is only applicable to the declaration of a variable and is not applicable for the initialization of a variable. Functions are also hoisted to the top, whereas function explanations are not hoisted to the top.

Basically, where we declared the variable inside the code doesn’t matter much.

18- What are Self Invoking Functions?

They are also known as ‘Immediately Invoked Function Expressions’ or ‘Self Executing Anonymous Functions’. These functions are invoked automatically in the code, hence they are named as ‘Self Invoking Functions’.

Usually, we define a function and invoke it, but if we want to execute a function automatically where it is explained, and if we are not going to call it again, we can use anonymous functions. And these types of functions have no name

19- What is the syntax of ‘Self Invoking Function’? Give an example?

The syntax for the Self-Invoking function:

(function () {
return () } () ;

Here, the last ‘()’ parenthesis in the syntax states that it is a function expression.

20- Define a named function in JavaScript.

The function which has named at the time of definition is called a named function. For example-

function msg()  
  document.writeln("Named Function");  

21- Define anonymous function.

It is a function that has no name. These functions are declared dynamically at runtime using the function operator instead of the function declaration. The function operator is more flexible than a function declaration. It can be easily used in the place of an expression. For example:

var display=function()  
  alert("Anonymous Function is invoked");  

22- What are the scopes of a variable in JavaScript?

The scope of a variable is the region of your program in which it is defined. JavaScript variable will have only two scopes.
• Global Variables − A global variable has global scope which means it is visible everywhere in your JavaScript code.
 Local Variables − A local variable will be visible only within a function where it is defined. Function parameters are always local to that function.

23- What is the purpose of ‘This’ operator in JavaScript?

The JavaScript this keyword refers to the object it belongs to. This has different values depending on where it is used. In a method, this refers to the owner object and in a function, this refers to the global object.

24- What is Callback?

callback is a plain JavaScript function passed to some method as an argument or option. It is a function that is to be executed after another function has finished executing, hence the name ‘call back‘. In JavaScript, functions are objects. Because of this, functions can take functions as arguments, and can be returned by other functions.

25- What is Closure? 

Closures are created whenever a variable that is defined outside the current scope is accessed from within some inner scope. It gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is created. To use a closure, simply define a function inside another function and expose it.

26- How to use external JavaScript file?

I am assuming that js file name is message.js, place the following script tag inside the head tag.

<script type="text/javascript" src="message.js"></script>  

27- What is the use of window object?

The window object is created automatically by the browser that represents a window of a browser. It is not an object of JavaScript. It is a browser object.

The window object is used to display the popup dialog box. Let’s see with description.

alert()displays the alert box containing the message with ok button.
confirm()displays the confirm dialog box containing the message with ok and cancel button.
prompt()displays a dialog box to get input from the user.
open()opens the new window.
close()closes the current window.
setTimeout()performs the action after specified time like calling function, evaluating expressions.

28- What is a prompt box?

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

29- Which symbol is used for comments in Javascript?

// for Single line comments and

/* Multi




30- What is === operator?

=== is called as strict equality operator which returns true when the two operands are having the same value without any type conversion.

31- What is the difference between ‘var’ and ‘let’ keyword?

The differences are as follows:

’var’ keyword was introduced in JavaScript code from the beginning Stage itself.‘let’ keyword is introduced in 2015 only.
’Var’ keyword has function scope. The variable defined with var is available anywhere within the functionA variable declared with ‘let’ keyword has a scope only with in that block. So, let has a Block Scope.
The variable declared with ‘var’ be hoistedThe variable declared with ‘let’ be hoisted

32- What is the difference between ‘==’ and ‘===’?

Both ‘==’ and ‘===’ are comparison operators.

‘==’ operator‘===’ operator
It is known as ‘Type Converting Operator’It is known as ‘Strict Equality Operator’
It compares Value, do not compare typeIt compares both value and type.

33- How does TypeOf Operator work?

The typeof operator is used to get the data type of its operand. The operand can be either a literal or a data structure such as a variable, a function, or an object. It is a unary operator that is placed before its single operand, which can be of any type. Its value is a string indicating the data type of the operand.

34- How to create a cookie using JavaScript?

The simplest way to create a cookie is to assign a string value to the document.cookie object, which looks like this-

document.cookie = "key1 = value1; key2 = value2; expires = date";

35- What is the difference between Attributes and Property?

Attributes-  provide more details on an element like id, type, value etc.

Property-  is the value assigned to the property like type=”text”, value=’Name’ etc.

36- What are the ways to define a variable in JavaScript?

The three possible ways of defining a variable in JavaScript are:

  • Var – The JavaScript variables statement is used to declare a variable and, optionally, we can initialize the value of that variable. Example: var a =10; Variable declarations are processed before the execution of the code.
  • Const – The idea of const functions is not allow them to modify the object on which they are called. When a function is declared as const, it can be called on any type 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 will be used only in the block it’s defined in.

37- What is the difference between null & undefined?

Undefined means a variable has been declared but has not yet been assigned a value. On the other hand, null is an assignment value. It can be assigned to a variable as a representation of no value. Also, undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

38- What are JavaScript Frameworks ?

JavaScript framework is an application framework written in JavaScript. It differs from a JavaScript library in its control flow. There are many JavaScript Frameworks available but some of the most commonly used frameworks are:

  • Angular
  • React
  • Vue

39- What is the difference between window & document in JavaScript?

JavaScript window is a global object which holds variables, functions, history, location.The document also comes under the window and can be considered as the property of the window.

40- What is the use of history object?

The history object of a browser can be used to switch to history pages such as back and forward from the current page or another page. There are three methods of history object.

  1. history.back() – It loads the previous page.
  2. history.forward() – It loads the next page.
  3. history.go(number) – The number may be positive for forward, negative for backward. It loads the given page number.

41- What is the output of 10+20+”30″ in JavaScript?

3030 because 10+20 will be 30. If there is numeric value before and after +, it treats as binary + (arithmetic operator).

function display()  

42- Difference between Client side JavaScript and Server side JavaScript?

Client-side JavaScript comprises the basic language and predefined objects which are relevant to running JavaScript in a browser. The client-side JavaScript is embedded directly by in the HTML pages. The browser interprets this script at runtime.

Server-side JavaScript also resembles client-side JavaScript. It has a relevant JavaScript which is to run in a server. The server-side JavaScript are deployed only after compilation.

43- What are all the looping structures in JavaScript?

Following are looping structures in Javascript:

  • For
  • While
  • do-while loops

44- What are break and continue statements?

Break statement exits from the current loop.

Continue statement continues with next statement of the loop.

45- 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 codes, 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 to work 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";

46- What will be the output of the following code?

var Output = (function(x)
Delete X;
return X;

The output would be 0. The delete operator is used to delete properties from an object. Here x is not an object but a local variable. delete operators don’t affect local variables.

47- How can a value be appended to an array?

A value can be appended to an array in the given manner –

arr[arr.length] = value;

48- Define event bubbling?

JavaScript allows DOM elements to be nested inside each other. In such a case, if the handler of the child is clicked, the handler of parent will also work as if it were clicked too.

49- What are escape characters in JavaScript?

JavaScript escape characters enable you to write special characters without breaking your application. 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.

50- Write about the errors shown in JavaScript?

JavaScript gives a message if it encounters an error. The recognized errors are –

  • Load-time errors: The errors shown at the time of the page loading are counted under Load-time errors. These errors are encountered by the use of improper syntax, and thus are detected while the page is getting loaded.
  • Run-time errors: This is the error that comes up while the program is running. It is caused by illegal operations, for example, division of a number by zero, or trying to access a non-existent area of the memory.
  • Logic errors: It is caused by the use of syntactically correct code, which does not fulfill the required task. For example, an infinite loop.

With this, we have come to the end of JavaScript interview questions blog. I Hope these JavaScript Interview Questions will help you in your interviews. In case you have attended any JavaScript interview in the recent past, do paste those interview questions in the comments section and we’ll answer them. You can also comment below if you have any questions in your mind, which you might face in your JavaScript interview.

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