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 within 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?

(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: (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: 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.

How to read and write a file using JavaScript?

I/O operations like reading or writing a file are 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?

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.

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.

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);

We can declare an array like this: var scripts = new Array(); We can add elements to this array like this: Now our array script has 4 elements inside it and we can print or access them using their index number. Note that the index number starts from 0. To get the third element of the array we need to use 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 are a fixed-width table and its advantages?