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

nginx.conf

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;
    }
}