screen 屏幕對象 反映了當(dāng)前用戶的屏幕設(shè)置。
width 返回屏幕的寬度（像素數(shù)）。
height 返回屏幕的高度。
availWidth 返回屏幕的可用寬度（除去了一些不自動隱藏的類似任務(wù)欄的東西所占用的寬度）。
availHeight 返回屏幕的可用高度。
colorDepth 返回當(dāng)前顏色設(shè)置所用的位數(shù) - 1：黑白；8：256色；16：增強色；24/32：真彩色
下面是英文的Navigator瀏覽器的屏幕對象：
availHeight:minus permanent or semipermanent user interface features displayed by the operating system:such as the Taskbar on Windows.
availWidth:Specifies the width of the screen, in pixels, minus permanent or semipermanent user interface:features displayed by the operating system, such as the Taskbar on Windows.
colorDepth:The bit depth of the color palette, if one is in use; otherwise, the value is derived from screen.pixelDepth.
height:Display screen height.
pixelDepth:Display screen color resolution (bits per pixel).
width:Display screen width.