有的时候我们需要统一给所有的ajax请求添加请求头，或者错误处理回调函数，这个时候可以通过jquery来设置全局的ajax参数，如下

$.ajaxSetup({
                headers: {
                    "Authorization":token,
                error: function(jqXHR){
                    debugger
                    switch (jqXHR.status){
                        case(500):
                            alert(jqXHR.message);
                            break;
                        case(401):
                            alert("未登录");
                            break;
                        case(403):
                            alert("无权限执行此操作");
                            break;
                        case(408):
                            alert("请求超时");
                            break;
                        default:
                            alert("未知错误");
            });

这样就不用单独给每个ajax添加这些参数了