当前页面的脚本发生错误 为空或不是对象 JavaScript 中的 HTTP 跨域请求

OPTIONS 请求会将当前的跨域请求所使用的特殊 HTTP 请求头和 HTTP 请求方法发送给服务器端，如 Access-Control-Request-Method 和 Access-Control-Request-Headers 。服务器端接收到 OPTIONS 请求后返回相应的响应头。浏览器根据返回的响应头再来判断该跨域请求是否被允许的。当浏览器判定 OPTIONS 请求通过了，真正的请求才会发出。如以下则是一个带有 OPTIONS 请求以及真正的 GET 请求的响应头和请求头：

OPTIONS /api4 HTTP/1.1Host: us1.serenader.me:3333Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache
Access-Control-Request-Method: PUT
Origin: http://us1.serenader.me:3334User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.95 Safari/537.36Accept: */*
Referer: http://us1.serenader.me:3334/
Accept-Encoding: gzip, deflate, sdch
Accept-Language: zh-CN,zh;q=0.8,en;q=0.6,zh-TW;q=0.4,fr;q=0.2

HTTP/1.1 200 OK
X-Powered-By: Express
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: GET,POST,PUT,DELETE

Content-Type: text/html; charset=utf-8Content-Length: 2ETag: W/"2-REvLOj/Pg4kpbElGfyfh1g"Date: Thu, 19 Jan 2017 15:21:15 GMT
Connection: keep-alive

PUT /api4 HTTP/1.1Host: us1.serenader.me:3333Connection: keep-alive
Content-Length: 0Pragma: no-cache
Cache-Control: no-cache
Origin: http://us1.serenader.me:3334User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.95 Safari/537.36Accept: */*
Referer: http://us1.serenader.me:3334/
Accept-Encoding: gzip, deflate, sdch
Accept-Language: zh-CN,zh;q=0.8,en;q=0.6,zh-TW;q=0.4,fr;q=0.2

HTTP/1.1 200 OK
X-Powered-By: Express
Access-Control-Allow-Origin: *
Content-Type: text/html; charset=utf-8Content-Length: 2ETag: W/"2-REvLOj/Pg4kpbElGfyfh1g"Date: Thu, 19 Jan 2017 15:21:15 GMT
Connection: keep-alive