不僅僅是fetch，只要你滿足以下幾種情況，都會去發起一個 Preflighted requests，也就是options請求，參見上面連結。

• 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)

你看看你是不是觸發這幾個條件中的其中一種了吧，瀏覽器在發起跨域請求的時候會帶著一個`Origin` header，那麼其實就是個custom headers，那麼也就會先觸發一個Preflighted requests，
Fetch Standard 也有提到。