//写成了插件形式 
(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; 
} 
})(jQuery); 
$(document).ready(function () { 
//调用 
$("#somediv").center(); 
}); 
//javascript/4232