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Javascript Interview Questions and Answers
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 with the help of 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--did the user type any letters into a
phone number field?, for instance. Without scripting, the user has to
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 has to 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 scrips has 4 elements inside it and we can print or access
them by using their index number. Note that index number starts from 0.
To get the third element of the array we have 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 has to
wait till all data is downloaded and then infer the best width for each
of the columns. This process can be very slow for large tables.
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 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
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