Javascript Interview Questions

1. What is JavaScript?

JavaScript is a platform-independent,event-driven, interpreted client-side scripting and

programming language developed by Netscape Communications Corp. and Sun Microsystems.
2.How to read and write a file using javascript?

I/O operations like reading or writing a file is not possible with client-side javascript.
However , this can be done by coding a Java applet that reads files for the script.

3.How to detect the operating system on the client machine?

In order to detect the operating system on the client machine, the navigator.appVersion

string (property) should be used.

4.Where are cookies actually stored on the hard disk?

This depends on the user's browser and OS.
In the case of Netscape with Windows OS,all the cookies are stored in a single file called

cookies.txt

c:\Program Files\Netscape\Users\username\cookies.txt

In the case of IE,each cookie is stored in a separate file namely username@website.txt.

c:\Windows\Cookies\username@Website.txt

5.What can javascript programs do?

Generation of HTML pages on-the-fly without accessing the Web server.
The user can be given control over the browser like User input validation
Simple computations can be performed on the client's machine
The user's browser, OS, screen size, etc. can be detected
Date and Time Handling

6.How to set a HTML document's background color?

document.bgcolor property can be set to any appropriate color.

7.What does the "Access is Denied" IE error mean?
The "Access Denied" error in any browser is due to the following reason.
A javascript in one window or frame is tries to access another window or frame whose

document's domain is different from the document containing the script.

8.Is a javascript script faster than an ASP script?
Yes.Since javascript is a client-side script it does require the web server's help for its

computation,so it is always faster than any server-side script like ASP,PHP,etc..

9.Are Java and JavaScript the Same?
No.java and javascript are two different languages.
Java is a powerful object - oriented programming language like C++,C whereas Javascript is a

client-side scripting language with some limitations.

10.How to embed javascript in a web page?
javascript code can be embedded in a web page between tags

11.How to access an external javascript file that is stored externally and not embedded?
This can be achieved by using the following tag between head tags or between body tags.

where abc.js is the external javscript file to be accessed.

12.What is the difference between an alert box and a confirmation box?
An alert box displays only one button which is the OK button whereas the Confirm box

displays two buttons namely OK and cancel.

13.What is a prompt box?
A prompt box allows the user to enter input by providing a text box.

14.Can javascript code be broken in different lines?
Breaking is possible within a string statement by using a backslash \ at the end but not

within any other javascript statement.
that is ,
document.write("Hello \
world");
is possible but not
document.write \
("hello world");

14.How to hide javascript code from old browsers that dont run it?
Ans.
Use the below specified style of comments




or

Use the tags and code the display html statements between these and this will appear on the page if the browser does not support javascript

15. How to comment javascript code?
Ans.
Use // for line comments and
/*

*/ for block comments

16.Name the numeric constants representing max,min values
Ans.

Number.MAX_VALUE
Number.MIN_VALUE

17.What does javascript null mean?

Ans.

The null value is a unique value representing no value or no object.
It implies no object,or null string,no valid boolean value,no number and no array object.

18.What does undefined value mean in javascript?

Ans.

Undefined value means the variable used in the code doesnt exist or is not assigned any value or the property doesnt exist.


19.What is the difference between undefined value and null value?

Ans.

(i)Undefined value cannot be explicitly stated that is there is no keyword called undefined whereas null value has keyword called null
(ii)typeof undefined variable or property returns undefined whereas
typeof null value returns object

20.What is variable typing in javascript?

Ans.

It is perfectly legal to assign a number to a variable and then assign a string to the same variable as follows

example

i = 10;
i = "string";

This is called variable typing

21.Does javascript have the concept level scope?

Ans.

No.Javascript does not have block level scope,all the variables declared inside a function possess the same level of scope unlike c,c++,java.

22. What are undefined and undeclared variables?

Ans.

Undeclared variables are those that are not declared in the program (do not exist at all),trying to read their values gives runtime error.But if undeclared variables are assigned then implicit declaration is done .

Undefined variables are those that are not assigned any value but are declared in the program.Trying to read such variables gives special value called undefined value.

23. What is === operator ?

Ans.

==== is strict equality operator ,it returns true only when the two operands are having the same value without any type conversion.


24.What does the delete operator do?

Ans.

The delete operator is used to delete all the variables and objects used in the program ,but it does not delete variables declared with var keyword.

25.What does break and continue statements do?

Ans.

Continue statement continues the current loop (if label not specified) in a new iteration whereas break statement exits the current loop.

26.How to create a function using function constructor?

Ans.

The following example illustrates this
It creates a function called square with argument x and returns x multiplied by itself.

var square = new Function ("x","return x*x");