关于input 属性placeholder 在IE9下兼容

<script src="http://www.jq22.com/jquery/1.8.3/jquery.min.js"></script>
<script>
(function($){
    $.fn.placeholder = function(options){
        var opts = $.extend({}, $.fn.placeholder.defaults, options);
        var isIE = document.all ? true : false;
        return this.each(function(){
            var _this = this,
                placeholderValue =_this.getAttribute("placeholder"); //缓存默认的placeholder值
            if(isIE){
                _this.setAttribute("value",placeholderValue);
                _this.onfocus = function(){
                    $.trim(_this.value) == placeholderValue ? _this.value = "" : ‘‘;
                };
                _this.onblur = function(){
                    $.trim(_this.value) == "" ? _this.value = placeholderValue : ‘‘;
                };
            }
        });
    };
})(jQuery);
$("input").placeholder();
</script>