javascript - 发送请求时，时常在发起前多一条请求，会有哪些原因引起？

Question

接口都是CORS跨域

多出的一条请求：Request Method:OPTIONS、没有传递参数

多出的条请求的response headers

Access-Control-Allow-Credentials:true
Access-Control-Allow-Headers:Access-Control-Allow-Headers, Origin,Accept, X-Requested-With, Content-Type, Access-Control-Request-Method, Access-Control-Request-Headers
Access-Control-Allow-Methods:POST, GET, PUT, OPTIONS, DELETE
Access-Control-Allow-Origin:*
Access-Control-Max-Age:3600
Allow:GET, HEAD, POST, PUT, DELETE, TRACE, OPTIONS, PATCH
Connection:keep-alive
Content-Length:0
Date:Fri, 05 May 2017 09:27:40 GMT
Server:WildFly/10
X-Powered-By:Undertow/1

---

正常请求的response headers

Access-Control-Allow-Credentials:true
Access-Control-Allow-Headers:Access-Control-Allow-Headers, Origin,Accept, X-Requested-With, Content-Type, Access-Control-Request-Method, Access-Control-Request-Headers
Access-Control-Allow-Methods:POST, GET, PUT, OPTIONS, DELETE
Access-Control-Allow-Origin:*
Access-Control-Max-Age:3600
Connection:keep-alive
Content-Type:application/json;charset=UTF-8
Date:Fri, 05 May 2017 09:27:41 GMT
Server:WildFly/10
Set-Cookie:JSESSIONID=Zv7WiKxKZFNkE6ihLsGfGwYF8ZEFNEFxENl7rNLW.localhost; path=/bridge
Transfer-Encoding:chunked
X-Powered-By:Undertow/1

Answer 1

跨域 post请求，需要option一次

Answer 2

楼上正解，如果是简单的跨域请求，建议不要使用POST

Answer 3

你curl一下你的请求，应该就不会有这样的预请求

Answer 4

1楼正解，跨域请求会先option一次到服务端，经过服务端验证通过后，在发送请求