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Javascript Interview Questions and Answers
How to add Buttons in JavaScript?
The most basic and ancient use of buttons are the "submit" and "clear",
which appear slightly before the Pleistocene period. Notice when the
"GO!" button is pressed it submits itself to itself and appends the name
in the URL.
<form action="" name="buttonsGalore" method="get">
Your Name: <input type="text" name="mytext" />
<br />
<input type="submit" value="GO!" />
<input type="reset" value="Clear All" />
</form>
Another useful approach is to set the "type" to "button" and use the "onclick"
event.
<script type="text/javascript">
function displayHero(button) {
alert("Your hero is \""+button.value+"\".");
}
</script>
<form action="" name="buttonsGalore" method="get">
<fieldset style="margin: 1em; text-align: center;">
<legend>Select a Hero</legend>
<input type="button" value="Agamemnon" onclick="displayHero(this)" />
<input type="button" value="Achilles" onclick="displayHero(this)" />
<input type="button" value="Hector" onclick="displayHero(this)" />
<div style="height: 1em;" />
</fieldset>
</form>
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
How to set a HTML document's background color?
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 have to
store any visitor-specific information.
Are you concerned that older browsers don't support JavaScript and thus
exclude a set of Web users? 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. A lot of effort
in my books and ancillary material goes toward 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 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 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 of in JavaScript?
All variables are of object type in JavaScript.
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