February 10, 2021

Top 20 JavaScript Interview Questions and Answers


Ques. 1): What is JavaScript?


JavaScript is different from Java language. It is a scripting language. It is object-based, lightweight, cross-platform translated language. It is widely used for client-side validation. The JavaScript Translator, which is embedded in the browser, is responsible for translating the JavaScript code for the web browser.

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Ques. 2): What are the differences between Java and JavaScript?


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

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Ques. 3): What are the main advantages of JavaScript?


The main advantages of using JavaScript are as follows:

  • Less server interaction − You can validate user input before sending the page off to the server. This saves server traffic, which means less load on your server.
  • Immediate feedback to the visitors − They don’t have to wait for a page reload to see if they have forgotten to enter something.
  • Increased interactivity − You can create interfaces that react when the user hovers over them with a mouse or activates them via the keyboard.
  • Richer interfaces − You can use JavaScript to include such items as drag-and-drop components and sliders to give a Rich Interface to your site visitors.

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Ques. 4): Which is faster, JavaScript or an ASP script?


JavaScript is a client-side language and thus it does not need the assistance of the web server 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). So the JavaScript is faster.

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Ques. 5): 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.

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Ques. 6): What are global variables? How are these variables declared and what are the problems associated with using them?


Global variables are those that are available throughout the length of the code, that is, these have 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 = "GLO_VAR";

The problems that are 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.


Ques. 7): 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 pages in the web application.


Ques. 8): How can you convert the string of any base to 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, and the second parameter is the base of the given string.

In order to convert 5F (of base 16) to integer, the code used will be -

parseInt ("5F", 16);


Ques. 9): What are undefined and undeclared variables?


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.

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.


Ques. 10): What is this [[[]]]?


This is a three-dimensional array.

var myArray = [[[]]]; 


Ques. 11): Explain the working of timers in JavaScript? Also elucidate the drawbacks of using the timer, if any?


Timers are used to execute a piece of code at a set time or also to repeat the code in a given interval of time. 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 is used to repeatedly execute the given function in the mentioned delay and only halts when cancelled. 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.


Ques. 12): 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.


Ques. 13): What is the main disadvantage of using innerHTML in JavaScript?


The main disadvantages of using innerHTML in JavaScript are:

  • Content is replaced everywhere
  • We cannot use 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 build into elements, therefore its much slower
  • The innerHTML does not provide validation and therefore we can potentially insert valid and broken HTML in the document and break it


Ques. 14): What are the different types of errors in JavaScript?


There are three types of errors in JavaScript:

  • Load time errors: Errors which come up when loading a web page like improper syntax errors are known as Load time errors and it generates the errors dynamically.
  • Run time 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 which is having different operation.


Ques. 15): What is the requirement of debugging in JavaScript?


JavaScript didn't show any error message in a browser. However, these mistakes can affect the output. The best practice to find out the error is to debug the code. The code can be debugged easily by using web browsers like Google Chrome, Mozilla Firebox.

To perform debugging, we can use any of the following approaches:

•     Using console.log() method

•     Using debugger keyword


Ques. 16): 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";



Ques. 17): 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.


Ques. 18): What is Callback in JavaScript?


A 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.


Ques. 19): What are JavaScript Cookies?


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


Ques. 20): Explain what is pop()method in JavaScript?


The pop() method is similar as the shift() method but the difference is that the Shift method works at the start of the array. Also the pop() method take the last element off of the given array and returns it. The array on which is called is then altered.


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


//Now cloth becomes Shirt,Pant


Ques. 21): 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.


Ques. 22): How are event handlers utilized in JavaScript?


Events are the actions that result from activities, such as clicking a link or filling a form, by the user. An event handler is required to manage proper execution of all these events. Event handlers are an extra attribute of the object. This attribute includes event's name and the action taken if the event takes place.


Ques: 23): Explain the role of deferred scripts in JavaScript?


By default, the parsing of the HTML code, during page loading, is paused until the script has not stopped executing. It means, if the server is slow or the script is particularly heavy, then the webpage is displayed with a delay. While using Deferred, scripts delays execution of the script till the time HTML parser is running. This reduces the loading time of web pages and they get displayed faster.




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