JavaScript Interview Questions and Answers

What is JavaScript?

A1: JavaScript is a general-purpose programming language designed to let programmers of all skill levels control the behavior of software objects. The language is used most widely today in web browsers whose software objects tend to represent a variety of HTML elements in a document and the document itself. But the language can be, and is, used with other kinds of objects in other environments. For example, Adobe Acrobat Forms uses JavaScript as its underlying scripting language to glue together objects that are unique to the forms generated by Adobe Acrobat. Therefore, it is important to distinguish JavaScript, the language, from the objects it can communicate with in any particular environment. When used for web documents, the scripts go directly inside the HTML documents and are downloaded to the browser with the rest of the HTML tags and content.


How is JavaScript different from Java?

JavaScript was developed by Brendan Eich of Netscape; Java was developed at Sun Microsystems. While the two languages share some common syntax, they were developed independently of each other and for different audiences. Java is a full-fledged programming language tailored for network computing; it includes hundreds of its own objects, including objects for creating user interfaces that appear in Java applets (in web browsers) or standalone Java applications. In contrast, JavaScript relies on whatever environment it's operating in for the user interface, such as a Web document's form elements.

JavaScript was initially called LiveScript at Netscape while it was under development. A licensing deal between Netscape and Sun at the last minute let Netscape plug the "Java" name into the name of its scripting language. Programmers use entirely different tools for Java and JavaScript. It is also not uncommon for a programmer of one language to be ignorant of the other. The two languages don't rely on each other and are intended for different purposes. In some ways, the "Java" name on JavaScript has confused the world's understanding of the differences between the two. On the other hand, JavaScript is much easier to learn than Java and can offer a gentle introduction for newcomers who want to graduate to Java and the kinds of applications you can develop with it.

What's the relationship between JavaScript and ECMAScript?

ECMAScript is yet another name for JavaScript (other names include LiveScript). The current JavaScript that you see supported in browsers is ECMAScript revision 3.

How do you submit a form using JavaScript?

Use document.forms[0].submit();
(0 refers to the index of the form; if you have more than one form in a page, then the first one has the index 0, the second has index the 1 and so on).

How do we get JavaScript onto a web page?

You can use several methods of placing JavaScript in your pages. You can directly add a script element inside the body of the page.

1. For example, to add the "last updated line" to your pages in your page text, add the following:

<p>blah, blah, blah, blah, blah.</p>
<script type="text/javascript" >
<!-- Hiding from old browsers
document.write("Last Updated:" +
document.lastModified);
document.close();
// -->
</script>
<p>yada, yada, yada.</p>

(Note: the first comment, "<--" hides the content of the script from browsers that don't understand JavaScript. The "// -->" finishes the comment. The "//" tells JavaScript that this is a comment so JavaScript doesn't try to interpret the "-->". If your audience has much older browsers, you should put these comments inside your JavaScript. If most of your audience has newer browsers, the comments can be omitted. For brevity, in most examples here the comments are not shown.)

The code above will look like this in JavaScript enabled browsers.

2. JavaScript can be placed inside the <head> element. Functions and global variables typically reside inside the <head> element as in the following:

<head>
<title>Default Test Page</title>
<script language="JavaScript" type="text/javascript">
var myVar = "";
function timer(){setTimeout('restart()',10);}
document.onload=timer();
</script>
</head>

JavaScript can be referenced from a separate file. JavaScript may also be placed in a separate file on the server and referenced from an HTML page. (Don't use the shorthand ending "<script ... />). These are typically placed in the <head> element.

<script type="text/javascript" SRC="myStuff.js"></script>

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.

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.

How can JavaScript make a web site easier to use? That is, are there certain JavaScript techniques that make it easier for people to use a Web site?

JavaScript's greatest potential gift to a web site is that scripts can make the page more immediately interactive, that is, interactive without having to submit every little thing to the server for a server program to re-render the page and send it back to the client. For example, consider a top-level navigation panel that has, say, six primary image map links into subsections of the Web site. With only a little bit of scripting, each map area can be instructed to pop-up a more detailed list of links to the contents within a subsection whenever the user rolls the cursor atop a map area. Using that popup list of links, the user with a scriptable browser can bypass one intermediate menu page. The user without a scriptable browser (or who has disabled JavaScript) will need to drill down through a more traditional and time-consuming path to the desired content.

How can JavaScript be used to improve the "look and feel" of a Web site? By the same token, how can JavaScript be used to improve the user interface?

On their own, Web pages tend to be lifeless and flat unless you add animated images or more bandwidth-intensive content such as Java applets or other content requiring plug-ins to operate (ShockWave and Flash, for example). Embedding JavaScript into an HTML page can bring the page to life in any number of ways. Perhaps the most visible features built into pages recently using JavaScript are the so-called image rollovers: roll the cursor atop a graphic image and its appearance changes to a highlighted version as a feedback mechanism to let you know precisely what you're about to click on. But there are less visible yet more powerful enhancements to pages that JavaScript offers.

Interactive forms validation is an extremely useful application of JavaScript. While a user is entering data into form fields, scripts can examine the validity of the data, such as whether the user typed any letters into a phone number field, for instance. Without scripting, the user must submit the form and let a server program (CGI) check the field entry and then report back to the user. This is usually done in a batch mode (the entire form at once) and the extra transactions take a lot of time and server processing power. Interactive validation scripts can check each form field immediately after the user has entered the data, while the information is fresh in the mind.

Another helpful example is embedding small data collections into a document that scripts can look up without having to do all the server programming for database access. For instance, a small company could put its entire employee directory on a page that has its own search facility built into the script. You can cram a lot of text data into scripts no larger than an average image file, so it's not like the user must wait forever for the data to be downloaded.

Other examples abound, such as interactive tree-structure tables of contents. More modern scriptable browsers can be scripted to pre-cache images during the page's initial download to make them appear lickety-split when needed for image swapping. I've even written some multi-screen interactive applications that run entirely on the client, and never talk to the server once everything is downloaded.

What are JavaScript types?

Number, String, Boolean, Function, Object, Null, Undefined.

How do you convert numbers between different bases in JavaScript?

Use the parseInt() function, that takes a string as the first parameter, and the base as a second parameter. So to convert hexadecimal 3F to decimal, use parseInt ("3F", 16);

How to create arrays in JavaScript?

We can declare an array like this:

var scripts = new Array();

We can add elements to this array like this:

scripts[0] = "PHP";
scripts[1] = "ASP";
scripts[2] = "JavaScript";
scripts[3] = "HTML";

Now our array script has 4 elements inside it and we can print or access them using their index number. Note that index number starts from 0. To get the third element of the array we need to use the index number 2 . Here is the way to get the third element of an array.

document.write(scripts[2]);

We also can create an array like this:

var no_array = new Array(21, 22, 23, 24, 25);

How do you target a specific frame from a hyperlink?

Include the name of the frame in the target attribute of the hyperlink. <a href=”mypage.htm” target=”myframe”>>My Page</a>

What is a fixed-width table and its advantages?

Fixed width tables are rendered by the browser based on the widths of the columns in the first row, resulting in a faster display in case of large tables. Use the CSS style table-layout:fixed to specify a fixed width table.

If the table is not specified to be of fixed width, the browser must wait until all data is downloaded and then infer the best width for each of the columns. This process can be very slow for large tables.

For an example of using Regular Expressions for syntax checking in JavaScript:
...
var re = new RegExp("^(&[A-Za-z_0-9]{1,}=[A-Za-z_0-9]{1,})*$");
var text = myWidget.value;
var OK = re.test(text);
if( ! OK ) {
alert("The extra parameters need some work.\r\n Should be something like: \"&a=1&c=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 the 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, as in:

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

What can JavaScript programs do?

Generate 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, and so on. can be detected. Also date and time handling.

How to set a HTML document's background color?

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

How can JavaScript be used to personalize or tailor a web site to fit individual users?

JavaScript allows a Web page to perform "if-then" kinds of decisions based on browser version, operating system, user input and in more recent browsers, details about the screen size in which the browser is running. While a server CGI program can make some of those same kinds of decisions, not everyone has access to or the expertise to create CGI programs. For example, an experienced CGI programmer can examine information about the browser whenever a request for a page is made; thus a server so equipped might serve up one page for Navigator users and a different page for Internet Explorer users. Beyond browser and operating system version, a CGI program can't know more about the environment. But a JavaScript-enhanced page can instruct the browser to render only certain content based on the browser, operating system, and even the screen size.

Scripting can even go further if the page author desires. For example, the author may include a preference screen that lets the user determine the desired background and text color combination. A script can save this information on the client in a well-regulated local file called a cookie. The next time the user comes to the site, scripts in its pages look to the cookie info and render the page in the color combination selected previously. The server is none the wiser, nor does it need to store any visitor-specific information.

Are you concerned that older browsers don't support JavaScript and thus exclude a set of Web users or individual users?

Fragmentation of the installed base of browsers will only get worse. By definition, it can never improve unless absolutely everyone on the planet threw away their old browsers and upgraded to the latest gee-whiz versions. But even then, there are plenty of discrepancies between the scriptability of the latest Netscape Navigator and Microsoft Internet Explorer.

The situation makes scripting a challenge, especially for newcomers who may not be aware of the limitations of earlier browsers. There has been substantial effort in my books and ancillary material in suport of helping scripters know what features work in which browsers and how to either workaround limitations in earlier browsers or raise the compatibility common denominator.

Designing scripts for a Web site requires making some hard decisions about if, when, and how to implement the advantages scripting offers a page to your audience. For public web sites, I recommend using scripting in an additive way: let sufficient content stand on its own, but let scriptable browser users receive an enhanced experience, preferably with the same HTML document.

What does the isNaN function do?

Return true if the argument is not a number.

What is negative infinity?

It's a number in JavaScript, derived by dividing a negative number by zero.

In a pop-up browser window, how do you refer to the main browser window that opened it?

Use window.opener to refer to the main window from pop-ups.

What is the data type of variables in JavaScript?

All variables are of object type in JavaScript.

Methods GET and POST in HTML forms: what's the difference?

GET: Parameters are passed in the querystring. A maximum amount of data that can be sent via the GET method is limited to about 2KB.
POST: Parameters are passed in the request body. There is no limit to the amount of data that can be transferred using POST. However, there are limits on the maximum amount of data that can be transferred in one name/value pair.

How to write a script for "Select" lists using JavaScript

1. To remove an item from a list, set it to null as in the following:
mySelectObject.options[3] = null;

2. To truncate a list, set its length to the maximum size you desire as in the following:
mySelectObject.length = 2;

3. To delete all options in a select object set the length to 0 as in the following:
mySelectObject.leng

Text From Your Clipboard

It is true, text you last copied for pasting (copy & paste) can be stolen when you visit web sites using a combination of JavaScript and ASP (or PHP, or CGI) to write your possible sensitive data to a database on another server.

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 tries to access another window or frame whose document's domain is different from the document containing the script.

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 and so on.

Are Java and JavaScript the same?

No. Java and JavaScript are two distinct languages. Java is a powerful object-oriented programming language like C++ and C# whereas JavaScript is a client-side scripting language with some limitations.

How to embed JavaScript in a web page?

JavaScript code can be embedded in a web page between <script langugage="javascript"></script> tags.

What and where are the best JavaScript resources on the Web?

The Web has several FAQ areas on JavaScript. The best place to start is something called the meta-FAQ [14-Jan-2001 Editor's Note: I can't point to it anymore, it is broken!] that provides a high-level overview of the JavaScript help available on the Net. As for fact-filled FAQs, I recommend one maintained by Martin Webb and a mini-FAQ that I maintain.

For interactive help with specific problems, nothing beats the primary JavaScript Usenet newsgroup, comp.lang.javascript. Depending on my work backlog, I answer questions posted there from time to time. Netscape and Microsoft also have vendor-specific developer discussion groups as well as detailed documentation for the scripting and object model implementations.

What are the problems associated with using JavaScript, and are there JavaScript techniques that you discourage?

Browser version incompatibility is the biggest problem. It requires knowing how each scriptable browser version implements its object model. You see, the incompatibility rarely must do with the core JavaScript language (although there have been improvements to the language over time); the bulk of incompatibility issues need to do with the object models that each browser version implements. For example, scripters who started out with Navigator 3 implemented the image rollover because it looked cool. But they were dismayed to determine that the image object wasn't scriptable in Internet Explorer 3 or Navigator 2. While there are easy workarounds to make this feature work on newer browsers without disturbing older ones, it was a painful learning experience for many.

The second biggest can of worms is scripting connections among multiple windows. Many scripters like to have little windows pop-up with navigation bars or some such gizmos. But the object models, especially in the older browser versions, don't make it easy to work with these windows the minute you put a user in front of them, users who can manually close windows or change their stacking order. More recently, a glitch in some uninstall routines for Windows 95 applications can disturb vital parts of the system Registry that Internet Explorer 4 requires for managing multiple windows. A scripter can't work around this problem, because it's not possible to detect the problem in a user's machine. I tend to avoid multiple windows that interact with each other. I think many of inexperienced Web surfers can also get confused by them.

What Boolean operators does JavaScript support?

&&, || and !

What does "1"+2+4 evaluate to?

Since 1 is a string, everything is a string, so the result is 124.

What is the difference between a web-garden and a web-farm?

Web-garden: An IIS 6.0 feature where you can configure an application pool as a web-garden and also specify the number of worker processes for that pool. It can help improve performance in some cases.
Web-farm: a general term referring to a cluster of physically separate machines, each running a web-server for scalability and performance (contrast this with web-garden that refers to multiple processes on one single physical machine).

How to get the contents of an input box using JavaScript?

Use the "value" property as in the following:

var myValue = window.document.getElementById("MyTextBox").value;

How to determine the state of a checkbox using JavaScript?

var checkedP = window.document.getElementById("myCheckBox").checked;

How to set the focus in an element using JavaScript?

<script> function setFocus() { if(focusElement != null) { document.forms[0].elements["myelementname"].focus(); } } </script>

How to access an external JavaScript file that is stored externally and not embedded?

This can be done by using the following tag between head tags or between body tags.

<script src="abc.js"></script>

How to access an external JavaScript file that is stored externally and not embedded?

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

What is the difference between an alert box and a confirmation box?

An alert box displays only one button that is the OK button whereas the Confirm box displays two buttons namely OK and cancel.

What is a prompt box?

A prompt box allows the user to enter input by providing a text box.

Can JavaScript code be broken into different lines?

Breaking is possible within a string statement by using a backslash \ at the end but not within any other JavaScript statement. in other words,

document.write("Hello \ world");

Is possible but not:

document.write \
("hello world");

Taking a developer's perspective, do you think that JavaScript is easy to learn and use?

One of the reasons JavaScript has the word "script" in it is that as a programming language, the vocabulary of the core language is compact compared to full-fledged programming languages. If you already program in Java or C, you actually need to unlearn some concepts that had been beaten into you. For example, JavaScript is a loosely typed language, which means that a variable doesn't care if it's holding a string, a number, or a reference to an object; the same variable can even change what type of data it holds while a script runs.

The other part of JavaScript implementation in browsers that makes it easier to learn is that most of the objects you script are pre-defined for the author, and they largely represent physical things you can see on a page: a text box, an image, and so on. It's easier to say, "OK, these are the things I'm working with and I'll use scripting to make them do such and such," instead of having to dream up the user interface, conceive of and code objects, and handle the interaction among objects and users. With scripting, you tend to write a lot less code.

What web sites do you feel use JavaScript most effectively (in ohter words, best-in-class examples)? The worst?

The best sites are the ones that use JavaScript so transparently that I'm not aware that there is any scripting on the page. The worst sites are those that try to impress me with how much scripting is on the page.

How about 2+5+"8"?

Since 2 and 5 are integers, this is number arithmetic, since 8 is a string, it's concatenation, so 78 is the result.

What is the difference between SessionState and ViewState?

ViewState is specific to a page in a session. Session state refers to user specific data that can be accessed across all pages in the web application.