前端http获取状态码为 -1，而不是500等状态。

可在github上找到相关问题：

Angular 1.4.6 sporadic $http errors with status -1：

$http -1 status return on 404/500 OPTIONS ：

文档说明：

第一种、是因为当请求中止或超时时，请求将作为错误返回状态 -1。

angular的开发者say：the xhr spec defines this behavior. Failed options requests always return -1, theres nothing we can do in Angular about this. You can try the same with a plain XMLHttpRequest() and you should get the same behavior.

第二种、是跨站点请求时，未添加header："Access-Control-Allow-Origin"。

一些200、400、401、404的响应状态都能成功获取，但是当状态为500时，获取的状态为 -1 。

Object {data: null, status: -1, config: Object, statusText: ""}

在postman和浏览器中，看到它们返回的状态码是500，并且提示：

XMLHttpRequest cannot load http://localhost:3000/users. No Access-Control-Allow-Origin header is present on the requested resource. Origin http://localhost:3000 is therefore not allowed access. The response had HTTP status code 500。

显然在postman和浏览器中，它们都能从服务器获取状态码，但是它去没有出现在Angularjs。

When doing cross-site requests, the server has to add to ALL responses a valid CORS header. This includes error responses like 500.

Make sure you can see the "Access-Control-Allow-Origin" header in your 500 response, its most likely missing.