Question

fetch进行post请求为什么会首先发一个options 请求

Answer 1

不仅仅是fetch，只要你满足以下几种情况，都会去发起一个 Preflighted requests，也就是options请求，参见上面链接。

• It uses methods other than GET, HEAD or POST.  Also, if POST is used to send request data with a Content-Type other than application/x-www-form-urlencoded, multipart/form-data, ortext/plain, e.g. if the POST request sends an XML payload to the server using application/xmlor text/xml, then the request is preflighted.

• It sets custom headers in the request (e.g. the request uses a header such as X-PINGOTHER)


你看看你是不是触发这几个条件中的其中一种了吧，浏览器在发起跨域请求的时候会带着一个`Origin` header，那么其实就是个custom headers，那么也就会先触发一个Preflighted requests，Fetch Standard 也有提到。