创建遮罩层

//创建遮罩层
function createMask() {
var height = document.documentElement.clientHeight;
var width = document.documentElement.clientWidth;
var bodyHeight = $("body").height();


if (bodyHeight > height) {
height = bodyHeight;
}


var mask = {};
if ($("#mask_div").length == 0) {
$("body").append('<div id="mask_div" style="position:absolute;top:0;left:0;filter:alpha(opacity=20);-moz-opacity:0.2;opacity:.2;"></div>')
}
mask = $("#mask_div");
mask.css({
"width": width,
"height": height,
"background": "#000"
});
}


//移除遮罩层
function removeMask() {
$("#mask_div").remove();
}