javascript

Chapter 8: The Browser Object Model
Whenever frames are used, multiple Global objects exist in the browser. Global
variables defined in each frame are defined to be properties of that frame ’ s window
object. Since each window object contains the native type constructors, each frame
has its own version of the constructors, which are not equal. For example,
top.Object is not equal to top.frames[0].Object ,which affects the use of
instanceof when objects are passed across frames.
Window Position
The position of a window object may be determined and changed using various properties and methods.
Internet Explorer (IE), Safari, Opera, and Chrome all provide screenLeft and screenTop properties
that indicate the window ’ s location in relation to the left and top of the screen, respectively. Firefox
provides this functionality through the screenX and screenY properties, which are also supported in
Safari and Chrome. Opera supports screenX and screenY , but you should avoid using them in Opera,
because they don ’ t correspond to screenLeft and screenTop . The following code determines the left
and top positions of the window across browsers:
var leftPos = (typeof window.screenLeft == “number”) ?
window.screenLeft : window.screenX;
var topPos = (typeof window.screenTop == “number”) ?
window.screenTop : window.screenY;
This example uses the ternary operator to determine if the screenLeft and screenTop properties exist.
If they do (which is the case in IE, Safari, Opera, and Chrome), they are used. If they don ’ t exist (as in
Firefox), screenX and screenY are used.
There are some quirks to using these values. In IE, Opera, and Chrome, screenLeft and screenTop
refer to the space from the left and top of the screen to the page view area represented by window . If the
window object is the topmost object and the browser window is at the very top of the screen (with a
y - coordinate of 0), the screenTop value will be the pixel height of the browser toolbars that appear
above the page view area. Firefox and Safari treat these coordinates as being related to the entire browser
window, so placing the window at y - coordinate 0 on the screen returns a top position of 0.
To further confuse things, Firefox, Safari, and Chrome always return the values of top.screenX and
top.screenY for every frame on the page. Even if a page is offset by some margins, these same values
are returned every time screenX and screenY are used in relation to a window object. IE and Opera
give accurate coordinates for the location of frames in relation to the screen edges.
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