JS变量作用域给搞糊涂了，求人解答：报错

var jsFieldLoading = function(fields) {
            var jsStr = "";
            for ( var i = 0; i < fields.length; i++) {
                Ext.Ajax.request( {
                    method : 'POST',
                    url : fields[i].js,
                    success : function(response, option) {
                        jsStr += response.responseText;
                        alert("test1" + jsStr);
                    },
                    failure : function(response, option) {
                        Ext.MessageBox.alert('错误信息提示',
                                '加载页面超时 或 页面连接错误！');
                    }
                });
            }
            alert("test:" + jsStr);
            return jsStr;

        }