javascript - 發送請求時，時常在發起前多一條請求，會有哪些原因造成？

Question

介面都是CORS跨域

多出的一條請求：Request Method:OPTIONS、沒有傳遞參數

#多出的條請求的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

---

正常要求的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請求，需要option一次

Answer 2

樓上正解，如果是簡單的跨域請求，建議不要使用POST

Answer 3

你curl一下你的請求，應該就不會有這樣的預請求

Answer 4

1樓正解，跨域請求會先option一次到服務端，經過服務端驗證通過後，在發送請求