JavaScript Interview Questions

1) What is JavaScript?
Ans:JavaScript is a scripting language most often used for client-side web development.

2) What is the difference between JavaScript and Jscript?
Ans:Both JavaScript and Jscript are almost similar. JavaScript was developed by Netscape. Microsoft reverse engineered Javascript and called it JScript.

3) How do we add JavaScript onto a web page?
Ans:There are several way for adding JavaScript on a web page, but there are two ways which are commonly used by developers
If your script code is very short and only for single page, then following ways are the best:
a) You can place <script type="text/javascript"> tag inside the <head> element.

<title>Page Title</title>
<script language="JavaScript" type="text/javascript">
   var name = "Vikas Ahlawta"

b) If your script code is very large, then you can make a JavaScript file and add its path in the following way:

<title>Page Title</title>
<script type="text/javascript" src="myjavascript.js"></script>

4) Is JavaScript case sensitive?
A function getElementById is not the same as getElementbyID.

5) What are the types used in JavaScript?

6) What are the boolean operators supported by JavaScript? And Operator: &&
Or Operator: ||
Not Operator: !

7) What is the difference between “==” and “===”?
“==” checks equality only,
“===” checks for equality as well as the type.

8) How to access the value of a textbox using JavaScript?
Ans: ex:-

<!DOCTYPE html>
Full name: <input type="text" id="txtFullName" 
name="FirstName" value="Vikas Ahlawat">

There are following ways to access the value of the above textbox:

var name = document.getElementById('txtFullName').value;



we can use the old way:


var name = document.forms[0].FirstName.value;


Note: This uses the “name” attribute of the element to locate it.

9) What are the ways of making comments in JavaScript?

// is used for line comments
ex:- var x=10; //comment text

*/  is used for block comments


var x= 10; /* this is
block comment example.*/

10) How will you get the Checkbox status whether it is checked or not?

var status = document.getElementById('checkbox1').checked; 

will return true or false.

11) How to create arrays in JavaScript?
Ans:There are two ways to create array in JavaScript like other languages:

a) The first way to create array
Declare Array:

var names = new Array(); 
Add Elements in Array:-
names[0] = "Vikas";
names[1] = "Ashish";
names[2] = "Nikhil";

b) This is the second way:

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var names = new Array("Vikas", "Ashish", "Nikhil");

12) If an array with name as “names” contain three elements, then how will you print the third element of this array?
Ans: Print third array element document.write(names[2]);
Note:- Array index starts with 0.

13) How do you submit a form using JavaScript?
Ans:Use document.forms[0].submit();

14) What does isNaN function do?
Ans: It returns true if the argument is not a number.

document.write(isNaN("Hello")+ "<br>");
document.write(isNaN("2013/06/23")+ "<br>");
document.write(isNaN(123)+ "<br>");

The output will be:


15) What is the use of Math Object in JavaScript?
Ans: The math object provides you properties and methods for mathematical constants and functions.

var x = Math.PI; // Returns PI
var y = Math.sqrt(16); // Returns the square root of 16
var z = Math.sin(90);    Returns the sine of 90

16) What do you understand by this keyword in JavaScript?
Ans:”this” keyword refers to the current object like other language.

17) What does “1”+2+4 evaluate to?
Ans: Since is a string, everything is a string, so the result is 124.

18) What does 3+4+”7″ evaluate to?
Ans: Since and are integers, this is number arithmetic, since is a string, it is concatenation, so 77 is the result.

19) How do you change the style/class on any element using JavaScript?

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document.getElementById(“myText”).style.fontSize = “10";


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

20) Does JavaScript support foreach loop?
Ans: Yes, See example here


21) What looping structures are there in JavaScript?
Ans: forwhiledowhile loops

22) What is an object in JavaScript, give an example?
Ans: An object is just a container for a collection of named values:

// Create the man object

var man = new Object(); = 'Vikas Ahlawat'; = true;
man.age = 27;

23) How you will add function as a property in a JavaScript object? Give an example.

var man = new Object(); = 'Vikas Ahlawat'; = true;
man.age = 27;
man.getName = function() { return;}
console.log(man.getName()); // Logs 'Vikas Ahlawat'.

24) What is the similarity between the 1st and 2nd statement?
1st:- var myString = new String('male'); // An object.
2nd:- var myStringLiteral = 'male'; // Primitive string value, not an object.
Ans: Both will call String() constructor function
You can confirm it by running the following statement:

console.log(myString.constructor, myStringLiteral.constructor);

25) What will be the output of the following statements?

var myString = 'Vikas' // Create a primitive string object.
var myStringCopy = myString; // Copy its value into a new variable.
var myString = null; // Manipulate the value
console.log(myString, myStringCopy);
Ans: // Logs 'null Vikas'

26) Consider the following statements and tell what would be the output of the logs statements?

var price1 = 10;
var price2 = 10;
var price3 = new Number('10'); // A complex numeric object because new was used.
console.log(price1 === price2); 
console.log(price1 === price3);


console.log(price1 === price2); // Logs true.
console.log(price1 === price3); /* Logs false because price3 
contains a complex number object and price 1
is a primitive value. */

27) What would be the output of the following statements?

var object1 = { same: 'same' };
var object2 = { same: 'same' };
console.log(object1 === object2);

Ans: // Logs false, JavaScript does not care that they are identical and of the same object type.
When comparing complex objects, they are equal only when they reference the same object (i.e., have the same address). Two variables containing identical objects are not equal to each other since they do not actually point at the same object.

28) What would be the output of the following statements?

var object1 = { same: 'same' };
var object2 = object1;
console.log(object1 === object2);

Ans: // Logs true

29) What is this?

var myArray = [[[]]];

Ans: Three dimensional array

30) Name any two JavaScript functions which are used to convert nonnumeric values into numbers?

var n1 = Number(“Hello world!”); //NaN
var n2 = Number(“”);             //0
var n3 = Number(“000010”);       //10
var n4 = Number(true);           //1
var n5 = Number(NaN);            //NaN

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