http跨域时的options请求测试

当发起一次携带自定义请求头的http 跨域请求的时候，浏览器就会字段的先发出一个options请求，我的代码是：

    function testcors() {
        console.log(" jump");
        let location = "http://localhost:8081/emp_files/enumm/addclothesright";
        // location = "http://localhost:8081/emp_files/enumm/";
        // window.location = location;

        xhr.open("get", location, false);
        xhr.setRequestHeader("dddd", 111);
        xhr.send(null);

        if((xhr.status >=200 && xhr.status < 300) || xhr.status == 304){
            alert(xhr.responseText);
        }else{
            alert('request was unsuccessful:' + xhr.status);
        }

    }

    setTimeout(
        testcors
        , 2000 );

    var xhr;
    if(window.XMLHttpRequest){
        xhr = new XMLHttpRequest();
    }else{
        xhr = new ActiveXObject('Microsoft.XMLHTTP');
    }

跨域看到实际发出两个请求：

（不知道为什么options请求在正式请求的后面？ ———— 更正！反复测试多次发现，其实并不是一定options请求在正式请求的后面，

而是随机出现，大概是 50% 的概率。

应该来说， options请求是先于正式请求发出的，而这个窗口的顺序是排列方式可能是到达的顺序，当 options请求非常快的时候，可能浏览器也排序不准。

）

预检请求：

General：

Request URL: http://localhost:8081/emp_files/enumm/addclothesright
Request Method: OPTIONS
Status Code: 200
Remote Address: [::1]:8081
Referrer Policy: strict-origin-when-cross-origin

响应头：

Access-Control-Allow-Headers: eawww
Access-Control-Allow-Methods: GET,HEAD,POST
Access-Control-Allow-Origin: http://localhost:9999
Access-Control-Max-Age: 1800
Allow: GET, HEAD, POST, PUT, DELETE, OPTIONS, PATCH
Content-Length: 0
Date: Thu, 25 Nov 2021 10:34:04 GMT
Vary: Origin
Vary: Access-Control-Request-Method
Vary: Access-Control-Request-Headers

请求头：
Accept: */*
Accept-Encoding: gzip, deflate, br
Accept-Language: zh-CN,zh;q=0.9
Access-Control-Request-Headers: eawww
Access-Control-Request-Method: GET
Cache-Control: no-cache
Connection: keep-alive
Host: localhost:8081
Origin: http://localhost:9999
Pragma: no-cache
Referer: http://localhost:9999/
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: same-site
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.45 Safari/537.36

正式请求：

General：

Request URL: http://localhost:8081/emp_files/enumm/addclothesright
Request Method: GET
Status Code: 200
Remote Address: [::1]:8081
Referrer Policy: strict-origin-when-cross-origin

响应头：
Access-Control-Allow-Origin: http://localhost:9999
Content-Type: application/json;charset=UTF-8
Date: Thu, 25 Nov 2021 10:34:04 GMT
Transfer-Encoding: chunked
Vary: Origin
Vary: Access-Control-Request-Method
Vary: Access-Control-Request-Headers

请求头：
Accept: */*
Accept-Encoding: gzip, deflate, br
Accept-Language: zh-CN,zh;q=0.9
Cache-Control: no-cache
Connection: keep-alive
eawww: 111
Host: localhost:8081
Origin: http://localhost:9999
Pragma: no-cache
Referer: http://localhost:9999/
sec-ch-ua: " Not A;Brand";v="99", "Chromium";v="96", "Google Chrome";v="96"
sec-ch-ua-mobile: ?0
sec-ch-ua-platform: "Windows"
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: same-site
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.45 Safari/537.36

http options 请求异常情况

如果 http跨域时的options请求不通过呢？就会导致 cors error ：

可以看到请求头是 dddd， ( 请求头那么多，为什么这里只列出来这一个？估计常见的通用的请求头是默认被过滤掉了的！ )

正常请求是 Provisional，也就是被拦截了，没有发出去吧！