A demo code to test [Handling CORS in Nginx as a reverse proxy]

main.go

package main

import (
	"net/http"

	"github.com/rs/cors"
)

func main() {
	mux := http.NewServeMux()

	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte("Hello World"))
	})

	r := cors.AllowAll().Handler(mux)
	http.ListenAndServe(":3000", r)
}

server.conf

# Server Configrations
#
server {
    listen 80;
    server_name example.com;


    location / {
        proxy_pass http://localhost:3000;
        
        # Handling OPTIONS for this location directive.
        if ($request_method = 'OPTIONS') {
          add_header Access-Control-Allow-Origin "$http_origin";
          add_header Access-Control-Allow-Credentials 'true';
          add_header Access-Control-Allow-Headers 'Authorization, Content-Type, Origin, X-Requested-With, Accept';
          add_header Access-Control-Allow-Methods 'GET, POST, OPTIONS, PUT, DELETE';
          add_header Content-Type 'text/plain; charset=utf-8';
          return 204;
        }

        proxy_set_header X-Original-URI $request_uri;
        proxy_set_header X-Original-Remote-Addr $remote_addr;
        proxy_set_header X-Original-Host $host;
    }
}

# vim: syntax=nginx