js浮动DIV

$(window).bind("scroll", function test() {
var bodyTop = 0;
	if (typeof window.pageYOffset != 'undefined') {
	bodyTop = window.pageYOffset;
}
else if (typeof document.compatMode != 'undefined' && document.compatMode != 'BackCompat')
{
	bodyTop = document.documentElement.scrollTop;
}
else if (typeof document.body != 'undefined') {
	bodyTop = document.body.scrollTop;
}
var num = 0 + bodyTop;
$("#submenu").css("top",  num + "px");
});