Monday, 29 July 2013


Here I come with latest JavaScript questions and answers. As you know JavaScript is the most popular scripting language, If you are prepare for interview then you must need to go through following impotent set of questions, this will be helpful for clear interview.
This set contain top 60 most frequently asked questions in JavaScript interview. This set of questions suitable for both fresher and experienced developer.

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

2).Difference between JavaScript and Jscript?
Ans:-Both JavaScript and Jscript are almost similar. Java script was developed by Netscape. Microsoft reverse engineered Javascript and called it JScript

3).How do we add JavaScript onto a web page?
There are serveral way for adding javascript on a web page but there are two way with is commonly userd by developers
If your script code is very short and only for single page then following ways is 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?
Ans:-String, Number, Boolean, Function, Object, Null, Undefined.

6).What are the boolean operators sported 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?
<!DOCTYPE html>
Full name: <input type="text" id="txtFullName" name="FirstName" value="Vikas Ahlawat">

There are following way 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 way of make comment 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 you will get the CheckBox status whether it is checked or not?
var status = document.getElementById('checkbox1').checked;
it will return true or false

11).How to create arrays in JavaScript?
There are Two way dor create array in Javascript like other languages..
a) 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 second way
var names = new Array("Vikas", "Ashish", "Nikhil");

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

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

14).What does isNaN function do?
It Return 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>");

output will be:-

15).What is the use of Math Object in Javascript?
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 1 is a string, everything is a string, so the result is 124.

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

19).How do you change the style/class on any element using javascript?
document.getElementById(“myText”).style.fontSize = “10";
document.getElementById(“myText”).className = “anyclass”;

20).Does javascript support foreach loop?

21).What looping structures are there in JavaScript?
Ans:-for, while, do-while loops

22).what is an object in JavaScript, give an example?
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 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 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 run 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, JavaScipt 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 dimantional array

30).Name any two JavaScript functions which are used for 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
31). What is the use of IsNan() function in JavaScript?
Ans:-isNaN() function accepts a single argument, which can be of any data type, to determine if the value is “not a number.”.
For more:-
alert(isNaN(“red”)); //true - cannot be converted to a number
alert(isNaN(true)); //false - can be converted to number 1
alert(isNaN(NaN)); //true
alert(isNaN(5)); //false - 5 is a number
alert(isNaN(“5”)); //false - can be converted to number 5

32). Can you describe "===" operator?
The identically equal operator is represented by three equal signs "===" and returns true only if the operands are equal without
conversion, as in this example
var result1 = (“55” == 55); //true - equal because of conversion
var result2 = (“55” === 55); //false - not equal because different data

33). DOM method "document.getElementById()" didn’t exist in Internet Explorer
prior to version 5. What is the way to detect that "document.getElementById()" is supported by the browser or not?

if (document.getElementById){
return document.getElementById("txtBox1");
else {
throw new Error(“getElementById function not supported by browser”);

34). How do you check if a variable is an array in JavaScript?
Ans:- We can use "instanceof Array"
if (v1 instanceof Array) {
alert('v1 is Array!');
} else {
alert('v1 is not an array');

35). What is the difference between following two
var myfunc = function() {}
function myfunc() {}

var myfunc = function() {} defines a variable that references an anonymous function.
function myfunc() {} defines a named function "myfunc".

36). When we try to convert a sting into Boolean then in JavaScritp it will show error/true/false?
var message = "Hello!!";
var messageAsBoolean = Boolean(message);
Ans:-Alert box will show "true"
Other Conversion
<!DOCTYPE html>
var SimpleString = "Hello!!";
var EmptyString = "";
var ZeroNumber = 0;
var NoneZeroNumber = 7;
var NanString = NaN;
var Obj = new Object();
var Undifined;

//Convert into Boolean
var SimpleStringAsBoolean = Boolean(SimpleString); //true
var EmptyStringAsBoolean = Boolean(EmptyString); //fasle
var ZeroNumberAsBoolean = Boolean(ZeroNumber); //false
var NoneZeroNumberAsBoolean = Boolean(NoneZeroNumber); //true
var NanStringAsBoolean = Boolean(NanString); //false
var UndifinedAsBoolean = Boolean(Undifined); //false

//See result
alert(SimpleStringAsBoolean); //Will Show "true"
alert(EmptyStringAsBoolean); //Will Show "false"
alert(ZeroNumberAsBoolean); //Will Show "false"
alert(NoneZeroNumberAsBoolean); //Will Show "true"
alert(NanStringAsBoolean); //Will Show "false"
alert(UndifinedAsBoolean); //Will Show "false"

37). What will be the output of below statement?
alert(NaN == NaN);
Ans:- it will show false
because NaN is not equal to any value, including NaN.

38). What is the difference between undefined value and null value?
Undefined value: If a value that is not defined and has no keyword then it is known as undefined. For example in the declaration, int num; the num has undefined value.
Null value: If a value that is explicitly specified by the keyword ‘null’ then it is known as null value. For example in the declaration, String str=null; the str has a null value.

39). What would be the difference if we declare two variables inside a JavaScript, one with 'var' keyword and another without 'var' keyword?
Ans:- The variable with var keyword inside a function is treated as Local variable.
and the variable without var keyword inside a function is treated a global variable.

40). Have a look on the following script and tell me which the global/local variables here are.
var n1= 10;
function my_function()
var n2 = 0;
n3 = 0;

var n1= 10; // Global
function my_function()
var n2 = 0; // is a local
n3 = 0; // is a global
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