Important Javascript Interview Questions And Answer

JavaScript Interview Questions :

JavaScript interview questions and answers for provides a list of top 20 interview questions. The frequently asked JavaScript interview questions with answers for beginners and professionals are given below.

1) What is JavaScript?

• JavaScript is a scripting language.

• It is different from Java language. It is object-based, lightweight, cross-platform translated language.

• It is widely used for client-side validation.

• The JavaScript Translator (embedded in the browser) is responsible for translating the JavaScript code for the web browser

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

• Cross-platform

3) Who developed JavaScript, and what was the first name of JavaScript?

• JavaScript was developed by Brendan Eich, who was a Netscape programmer.

• Brendan Eich developed this new scripting language in just ten days in the year September 1995.

• At the time of its launch, JavaScript was initially called Mocha.

• After that, it was called Live Script and later known as JavaScript.

4) 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

5) 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.

6) 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");  
}  
msg();

7) Name the types of functions?

The types of function are:

• Named - These type of functions contains name at the time of definition. For Example:

    function display()  
    {  
      document.writeln("Named Function");  
    }  
    display();
  

Anonymous - These type of functions doesn't contain any name. They are declared dynamically at runtime.

var display=function()  
{  
  document.writeln("Anonymous Function");  
}  
display();

8) 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");  
}  
display();

9) Can an anonymous function be assigned to a variable?

Yes, you can assign an anonymous function to a variable.

10) In JavaScript what is an argument object?

The variables of JavaScript represent the arguments that are passed to a function.

11) What is the difference between JavaScript and JScript?

• Netscape provided the JavaScript language.

• Microsoft changed the name and called it JScript to avoid the trademark issue.

• In other words, you can say JScript is the same as JavaScript, but Microsoft provides it.

12)What are the key differences between Java and JavaScript? / How is JavaScript different from Java?

JavaScript is a lightweight programming language (most commonly known as scripting language) developed by Netscape, Inc. It is used to make web pages interactive. It is not a part of the Java platform. Following is a list of some key differences between Java and JavaScript

A list of key differences between Java and JavaScript

13) What is BOM?

• BOM stands for Browser Object Model.

• It provides interaction with the browser.

• The default object of a browser is a window. So, you can call all the functions of the window by specifying the window or directly.

• The window object provides various properties like document, history, screen, navigator, location, innerHeight, innerWidth.

14) What is DOM? What is the use of document object?

• DOM stands for Document Object Model.

• A document object represents the HTML document.

• It can be used to access and change the content of HTML.

15) 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.

16) How to write a comment in JavaScript?

There are two types of comments in JavaScript.

1. Single Line Comment: It is represented by // (double forward slash)

2. Multi-Line Comment: Slash represents it with asterisk symbol as /* write comment here */

17) What is the difference between == and ===?

The == operator checks equality only whereas === checks equality, and data type, i.e., a value must be of the same type.

18) How to write HTML code dynamically using JavaScript?

The innerHTML property is used to write the HTML code using JavaScript dynamically. Let's see a simple example:

document.getElementById('mylocation').innerHTML="<h2>This is heading using JavaScript</h2>";

19) How to create objects in JavaScript?

There are 3 ways to create an object in JavaScript.

1. By object literal

2. By creating an instance of Object

By Object Constructor

20) How to create an array in JavaScript?

There are 3 ways to create an array in JavaScript.

1. By array literal

2. By creating an instance of Array

3. By using an Array constructor

21) 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

• 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.

22) In which location cookies are stored on the hard disk?

• The storage of cookies on the hard disk depends on the OS and the browser.

• The Netscape Navigator on Windows uses a cookies.txt file that contains all the cookies.

• The path is c:\Program Files\Netscape\Users\username\cookies.txt

• The Internet Explorer stores the cookies on a file [email protected]

• The path is: c:\Windows\Cookies\[email protected]

23) What is the real name of JavaScript?

• The original name was Mocha, a name chosen by Marc Andreessen, founder of Netscape.

• In September of 1995, the name was changed to LiveScript.

• In December 1995, after receiving a trademark license from Sun, the name JavaScript was adopted.

24) What is the difference between undefined value and null value?

Undefined value: A value that is not defined and has no keyword is known as undefined value.

For example:

1. int number;//Here, a number has an undefined value.

Null value: A value that is explicitly specified by the keyword "null" is known as a null value.

For example:

1. String str=null;//Here, str has a null value.

25) What is this [[[]]]?

This is a three-dimensional array.

1. var myArray = [[[]]];

26) Are Java and JavaScript same?

No, Java and JavaScript are the two different languages. Java is a robust, secured and object-oriented programming language whereas JavaScript is a client-side scripting language with some limitations.

27) What are the pop-up boxes available in JavaScript?

• Alert Box

• Confirm Box

• Prompt Box

28) How can we detect OS of the client machine using JavaScript?

The navigator.appVersion string can be used to detect the operating system on the client machine.

29) How to handle exceptions in JavaScript?

By the help of try/catch block, we can handle exceptions in JavaScript. JavaScript supports try, catch, finally and throw keywords for exception handling.

30) 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

31) What is the role of a strict mode in JavaScript?

The JavaScript strict mode is used to generates silent errors. It provides "use strict"; expression to enable the strict mode. This expression can only be placed as the first statement in a script or a function. For example:

"use strict"; 
x=10;    
console.log(x)

32) What is the use of Math object in JavaScript?

The JavaScript math object provides several constants and methods to perform a mathematical operation. Unlike date object, it doesn't have constructors.

For example:

function display()  
{  
var map=new Map();    
map.set(1,"jQuery");    
map.set(2,"AngularJS");    
map.set(3,"Bootstrap");    
document.writeln(map.get(1)+"<br>");    
document.writeln(map.get(2)+"<br>");    
document.writeln(map.get(3));    
}  
display();

33)Important Differences:

Difference between JavaScript and PHP

Following lists the various differences between JavaScript and PHP.

Different between Dart and JavaScript

The following comparison chart explains the head-to-head comparison between Dart and JavaScript:

Comparison between JavaScript and Angular JS

Here, we are going to discuss a head-to-head comparison between JavaScript and Angular JS: