Overview of JavaScript Objects
By aathishankaran Viewed: 31762 times Emailed: 279 times Printed: 275 times
When you begin to look closely at
the JavaScript object hierarchy you can see that each object falls into one of
two categories: Navigator objects and built-in language objects. This section
looks at these sets and introduces you to each of the objects within them.
Navigator Objects
Most of the functionality built
into JavaScript centers around what you can do with HTML pages. The first set of
objects Navigator objects generally has a correlation to the browser and HTML
tags within it.
Window Object
A Web browser-whether it's
Netscape Navigator, Microsoft Internet Explorer, or whatever- is presented to
the user in a window. Everything a user does with the browser is performed
within that window. Moreover, every screen element is also contained inside that
window. The window object provides a direct corollary to this metaphor. It is
considered the highest-level object of all objects in the JavaScript object
hierarchy and contains all other Navigator objects (except for the Navigator
object itself). Just as you can have multiple windows open in your browser, you
can work with multiple window objects at once in your code.
The window object has no HTML tag
equivalent, although you do define its event handlers (onLoad, onUnload) in the
<BODY> tag. Within JavaScript code, you work with a window object as shown
in the following example. Suppose you wart to add text to the status bar of the
window. The code follows:
EXAMPLE for OnLoad and OnUnLoad
<head>
<html>
<script>
function ab (){
alert ("The alert is inside user defined javascript function");
}
</script>
</head>
<body onLoad="alert('WELCOME')" onUnload="alert('COME AGAIN')">
<a href=" javascript: ab () "
onMouseOver="
window.status='User defined function'"
onMouseOut="window.status="">click</a>
</body>
</html>
Frame Object
As you learn in this book, frames
are especially important objects to use to enhance the presentation of your Web
application. The frame object represents a frame within a frameset. In a
multi-frame presentation, your window object is the page that contains the
<FRAMESET>definition, whereas the other pages are considered frames in
that context.
Location Object
The Web is all about content
presentation. Every window object is designed to display content to the user,
but that content must come from somewhere. The origin of the page is thus
contained in the location object. The location object is used to store all URL
information for a given window. Although users see URL information in the
Location box on-screen, you can work with that same information with the
location object.
If you want to retrieve the
protocol portion of the current URL and evaluate it, you use the following:
function evalProtocol()
{
curProtocol= window.location.protocol
if (curProtocol == "http:")
{
alert("The document comes from the Web.")
}
else
{
if (curProtocol == "file:")
{
alert("This document comes from your hard drive.")
}
else
. {
alert("This document comes from somewhere else.")
}
}
}
2007-03-21
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