请教一个跨域的问题： No 'Access-Control-Allow-Origin' header ，参考了很多教程还是解决不了。

参考的教程如下： http://serverfault.com/questions/162429/how-do-i-add-access-control-allow-origin-in-nginx 和 http://www.tuicool.com/articles/3aaQB3b 和 https://segmentfault.com/q/1010000003116113

自己尝试在 nginx.conf 文件中添加了
location / {
add_header Access-Control-Allow-Origin *;
}

和



location / {
if ($request_method = 'OPTIONS') {
add_header 'Access-Control-Allow-Origin' '*';
#
# Om nom nom cookies
#
add_header 'Access-Control-Allow-Credentials' 'true';
add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
#
# Custom headers and headers various browsers *should* be OK with but aren't
#
add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';
#
# Tell client that this pre-flight info is valid for 20 days
#
add_header 'Access-Control-Max-Age' 1728000;
add_header 'Content-Type' 'text/plain charset=';
add_header 'Content-Length' 0;
return 204;
}
if ($request_method = 'POST') {
add_header 'Access-Control-Allow-Origin' '*';
add_header 'Access-Control-Allow-Credentials' 'true';
add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';
}
if ($request_method = 'GET') {
add_header 'Access-Control-Allow-Origin' '*';
add_header 'Access-Control-Allow-Credentials' 'true';
add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';
}
}

丝毫不起作用，求助后端伙伴们，怎么搞？
console 里面的错误（不知道如何贴图，就只能贴 error 了）如下：

XMLHttpRequest cannot load https://www.mydomain.com/?/ac... No 'Access-Control-Allow-Origin' header is present on the https://www.mydomain.com/?/ac... requested resource. Origin 'https://www.mydomain.com' is therefore not allowed access.