扩展jquery.fn，增加获取或设置zIndex的function

/*扩展 jQuery.fn 的方法*/
jQuery.fn.extend({
    /*
    获取或设置zIndex
    */
    zIndex: function (zIndex) {
        if (zIndex !== undefined) {
            return this.css("zIndex", zIndex);
        }
        if (this.length) {
            var elem = $(this[0]), position, value;
            while (elem.length && elem[0] !== document) {
                // Ignore z-index if position is set to a value where z-index is ignored by the browser
                // This makes behavior of this function consistent across browsers
                // WebKit always returns auto if the element is positioned
                position = elem.css("position");
                if (position === "absolute" || position === "relative" || position === "fixed") {
                    // IE returns 0 when zIndex is not specified
                    // other browsers return a string
                    // we ignore the case of nested elements with an explicit value of 0
                    // <div style="z-index: -10;"><div style="z-index: 0;"></div></div>
                    value = parseInt(elem.css("zIndex"), 10);
                    if (!isNaN(value) && value !== 0) {
                        return value;
                    }
                }
                elem = elem.parent();
            }
        }
        return 0;
    }
});