Example Nginx configuration for adding cross-origin resource sharing (CORS) support to reverse proxied APIs

nginx.conf

#
# CORS header support
#
# One way to use this is by placing it into a file called "cors_support"
# under your Nginx configuration directory and placing the following
# statement inside your **location** block(s):
#
#   include cors_support;
#
# As of Nginx 1.7.5, add_header supports an "always" parameter which
# allows CORS to work if the backend returns 4xx or 5xx status code.
#
# For more information on CORS, please see: http://enable-cors.org/
# Forked from this Gist: https://gist.github.com/michiel/1064640
#

set $cors '';
if ($http_origin ~ '^https?://(localhost|www\.yourdomain\.com|www\.yourotherdomain\.com)') {
        set $cors 'true';
}

if ($cors = 'true') {
        add_header 'Access-Control-Allow-Origin' "$http_origin" always;
        add_header 'Access-Control-Allow-Credentials' 'true' always;
        add_header 'Access-Control-Allow-Methods' 'GET, POST, PUT, DELETE, OPTIONS' always;
        add_header 'Access-Control-Allow-Headers' 'Accept,Authorization,Cache-Control,Content-Type,DNT,If-Modified-Since,Keep-Alive,Origin,User-Agent,X-Requested-With' always;
        # required to be able to read Authorization header in frontend
        #add_header 'Access-Control-Expose-Headers' 'Authorization' always;
}

if ($request_method = 'OPTIONS') {
        # 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=UTF-8';
        add_header 'Content-Length' 0;
        return 204;
}

https://gist.github.com/Stanback/7145487?permalink_comment_id=2824437#gistcomment-2824437

My solution is:

map $http_origin $cors_origin_header {
    default "";
    "~(^|^http:\/\/)(localhost$|localhost:[0-9]{1,4}$)" "$http_origin";
    "~^https://test-.-dev.example.pl$" "$http_origin"; # https://test-7-dev.example.pl
    "https://test.example.com" "$http_origin";
}

map $http_origin $cors_cred {
    default "";
    "~(^|^http:\/\/)(localhost$|localhost:[0-9]{1,4}$)" "true";
    "~^https://test-.-dev.example.pl$" "true"; # https://test-7-dev.example.pl
    "https://test.example.com" "true";
}

server {
    listen 443 ssl http2;
    server_name api.example.com;

    include ssl/wildcard;

    add_header Access-Control-Allow-Origin $cors_origin_header always;
    add_header Access-Control-Allow-Credentials $cors_cred;
    add_header "Access-Control-Allow-Methods" "GET, POST, OPTIONS, HEAD";
    add_header "Access-Control-Allow-Headers" "Authorization, Origin, X-Requested-With, Content-Type, Accept";

    if ($request_method = 'OPTIONS' ) {
      return 204 no-content;
    }

return 204 there is because this is configuration for load-balancer, and I don't want to send OPTIONS into certain host after load-balancer. If you haven't load-balancer than you can remove this line
I hope, it helps someone