How to enable CORS in Nginx proxy server?

As my title, here is the config file located in conf.d/api-server.conf

server {
  listen 80;
  server_name api.localhost;

  location / {
    add_header 'Access-Control-Allow-Origin' 'http://api.localhost';
    add_header 'Access-Control-Allow_Credentials' 'true';
    add_header 'Access-Control-Allow-Headers' 'Authorization,Accept,Origin,DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
    add_header 'Access-Control-Allow-Methods' 'GET,POST,OPTIONS,PUT,DELETE,PATCH';

    if ($request_method = 'OPTIONS') {
      add_header 'Access-Control-Max-Age' 1728000;
      add_header 'Content-Type' 'text/plain charset=UTF-8';
      add_header 'Content-Length' 0;
      return 204;
    }

    proxy_redirect off;
    proxy_set_header host $host;
    proxy_set_header X-real-ip $remote_addr;
    proxy_set_header X-forward-for $proxy_add_x_forwarded_for;
    proxy_pass http://127.0.0.1:3000;
  }
}

The nginx.conf file stay the same as default.

After I send request to api.localhost (api.localhost/admin/login), I still receive 405 error:

XMLHttpRequest cannot load http://api.localhost/admin/login. Response 
to preflight request doesn't pass access control check: No 'Access-
Control-Allow-Origin' header is present on the requested resource. 
Origin 'http://admin.localhost:3000' is therefore not allowed access. 
The response had HTTP status code 405.

The issue is that your if condition is not going to send the headers in the parent in /. If you check the preflight response headers it would be

HTTP/1.1 204 No Content
Server: nginx/1.13.3
Date: Fri, 01 Sep 2017 05:24:04 GMT
Connection: keep-alive
Access-Control-Max-Age: 1728000
Content-Type: text/plain charset=UTF-8
Content-Length: 0

And that doesn't give anything. So two possible fixes for you. Copy the add_header inside if block also

server {
  listen 80;
  server_name api.localhost;

  location / {
    add_header 'Access-Control-Allow-Origin' 'http://api.localhost';
    add_header 'Access-Control-Allow-Credentials' 'true';
    add_header 'Access-Control-Allow-Headers' 'Authorization,Accept,Origin,DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
    add_header 'Access-Control-Allow-Methods' 'GET,POST,OPTIONS,PUT,DELETE,PATCH';

    if ($request_method = 'OPTIONS') {
      add_header 'Access-Control-Allow-Origin' 'http://api.localhost';
      add_header 'Access-Control-Allow-Credentials' 'true';
      add_header 'Access-Control-Allow-Headers' 'Authorization,Accept,Origin,DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
      add_header 'Access-Control-Allow-Methods' 'GET,POST,OPTIONS,PUT,DELETE,PATCH';
      add_header 'Access-Control-Max-Age' 1728000;
      add_header 'Content-Type' 'text/plain charset=UTF-8';
      add_header 'Content-Length' 0;
      return 204;
    }

    proxy_redirect off;
    proxy_set_header host $host;
    proxy_set_header X-real-ip $remote_addr;
    proxy_set_header X-forward-for $proxy_add_x_forwarded_for;
    proxy_pass http://127.0.0.1:3000;
  }
}

Or you can move it outside the location block, so every request has the response

server {
   listen 80;
   server_name api.localhost;

   add_header 'Access-Control-Allow-Origin' 'http://api.localhost';
   add_header 'Access-Control-Allow-Credentials' 'true';
   add_header 'Access-Control-Allow-Headers' 'Authorization,Accept,Origin,DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
   add_header 'Access-Control-Allow-Methods' 'GET,POST,OPTIONS,PUT,DELETE,PATCH';

  location / {

    if ($request_method = 'OPTIONS') {
      add_header 'Access-Control-Max-Age' 1728000;
      add_header 'Content-Type' 'text/plain charset=UTF-8';
      add_header 'Content-Length' 0;
      return 204;
    }

    proxy_redirect off;
    proxy_set_header host $host;
    proxy_set_header X-real-ip $remote_addr;
    proxy_set_header X-forward-for $proxy_add_x_forwarded_for;
    proxy_pass http://127.0.0.1:3000;
  }
}

If you only want to allow certain locations in your config for CORS. like /api then you should create a template conf with your headers

 add_header 'Access-Control-Allow-Origin' 'http://api.localhost';
 add_header 'Access-Control-Allow-Credentials' 'true';
 add_header 'Access-Control-Allow-Headers' 'Authorization,Accept,Origin,DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
 add_header 'Access-Control-Allow-Methods' 'GET,POST,OPTIONS,PUT,DELETE,PATCH';

and then use

include conf.d/corsheaders.conf;

in your OPTIONS block and /api block. So CORS are only allowed for the /api. If you don't care which location for CORS then you can use the second approach of moving core headers to server block