js实现使元素在屏幕里上下左右居中

<script type="text/javascript" src="js/jquery-1.4.2.js"></script>

<script type="text/javascript">

$(document).ready(function() {

jQuery.fn.center = function() {

this.css("position", "absolute");

this.css("top", ($(window).height() - this.height()) / 2 + $(window).scrollTop() + "px");

this.css("left", ($(window).width() - this.width()) / 2 + $(window).scrollLeft() + "px");

return this;

}

$("#id").center();

});

</script>

//ID为要在屏幕上下左右居中的元素ID，而不是class