如何让一个方块上下左右居中

//构造函数;(function($, window, document,undefined){    var MaskLayer = function(ele, opt){        this.$element = ele,        this.defaults = {            width:‘500px‘,            height:"500px",            backgroundColor:"#00baff",            display:"block"        },        this.options = $.extend({}, this.defaults, opt)    };//方法    MaskLayer.prototype = {        mask: function() {            var _left,_top,_newWidth,_newHeight;            if($(‘div‘).is(‘.big-mask‘)){                $(".big-mask").css({display:"block"})            }else{                $("body").append("<div class=‘big-mask‘></div>");                $(".big-mask").css({                    position:"fixed" ,                    width:"100%",                    height:"100%",                    top:0,                    left:0,                    backgroundColor:"#000",                    opacity: "0.5",                    filter: "progid:DXImageTransform.Microsoft.Alpha(opacity=50)",                    zIndex: "99999"                });            }            console.log(this.$element);           this.$element.css({                backgroundColor:this.options.backgroundColor,                width:this.options.width,                height:this.options.height,                display:this.options.display,                zIndex: "999999",                position:"fixed"            });            _left =  parseInt($(window).width()/2);            _top =  parseInt($(window).height()/2);            _newWidth =_left-parseInt(this.options.width)/2;            _newHeight =_top-parseInt(this.options.height)/2;

            return this.$element.css({                ‘left‘: _newWidth,                ‘top‘:_newHeight            });        },        removeMask:function(){            this.$element.css({display:"none"});            $(".big-mask").css({display:"none"});        }    };//myMaskLayer对象使用    $.fn.createMaskLayer = function(options) {        var masklayer = new MaskLayer(this, options);        return masklayer.mask();    }

    $.fn.maskRemove = function(options) {        var masklayer = new MaskLayer(this, options);        return masklayer.removeMask();    }

})(jQuery, window, document);

//调用方法

$(".aaa").createMaskLayer({    backgroundColor:"#fff",    width:"300px",    height:"300px"});

$(".aaa").maskRemove();