How to solve the preflight request encountered after adding header in the fetch method

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Today, using the fetch method

fetch('xxx.com',{header:{bbbbbbb:111}})

In the request information returned by the browser, header It became

:authority:koss.nocorp.me
:method:OPTIONS
:path:/?a=1
:scheme:https
accept:*/*
accept-encoding:gzip, deflate, br
accept-language:zh-CN,zh;q=0.8
access-control-request-headers:bbbbbbbbbbb
access-control-request-method:GET
origin:http://localhost:3333
referer:http://localhost:3333/
user-agent:Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36

and the Request Method of this request also became OPTION. Whether it is the strange request header generated or the OPTION method, I have never seen it before. .

This request is different from the requests sent during normal development in the following points

1. The website targeted for this request is Cross-domain address

2. The server will not obtain the request header added in this request

, so it will query based on the request information field name returned by the browser. , the query found that

access-control-request-headers:bbbbbbbbbbb
access-control-request-method:GET

is called the preflight header, which is a step in the CORS request to obtain permission when sending a request to the server. The server's response is mainly reflected in the Access-Control-Allow-Origin field in Response.

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