用fetch向后端发送post请求，为什么会产生两个请求（OPTIONS和POS?400报错

fetch的实现机制导致的结果。当发生跨域请求时，fetch会先发送一个OPTIONS请求，来确认服务器是否允许接受请求。服务器同意后，才会发送真正的请求。

Additionally, for HTTP request methods that can cause side-effects on user data (in particular, for HTTP methods other than GET, or for POST usage with certain MIME types), the specification mandates that browsers "preflight" the request, soliciting supported methods from the server with an HTTP OPTIONS request method, and then, upon "approval" from the server, sending the actual request with the actual HTTP request method. Servers can also notify clients whether "credentials" (including Cookies and HTTP Authentication data) should be sent with requests.