Golang : Allow Cross-Origin Resource Sharing request

Problem:

You have a client JavaScript code that want to make XHR(XMLHttpRequest) request to another Golang server which is also controlled by you. However, on the client's browser console shows CORS (Cross-Origin Resource Sharing) error messages. How to fix the issue?

Solution:

Normally, a server should not allow XMLHttpRequest made by another unauthorized client. However, to enable Cross-Origin Resource Sharing, you will need to set the server header with Access-Control-Allow-Origin:* to the response.

w.Header().Set("Access-Control-Allow-Origin", "*") // usually, this line is enough, but you can add the following options.

If you want to add more specific headers, these are some of the options

w.Header().Set("Access-Control-Allow-Credentials", "true")

w.Header().Set("Access-Control-Allow-Methods", "OPTIONS, GET, POST")

w.Header().Set("Access-Control-Allow-Headers", "Content-Type, Depth, User-Agent, X-File-Size, X-Requested-With, If-Modified-Since, X-File-Name, Cache-Control")

If you're using Nginx, Apache or Caddy as proxy instead of directly serving up from your Golang program. You can add the allow CORS and XHR headers in the proxy server configuration. For example, this tutorial shows how to do that with Caddy. (https://www.socketloop.com/tutorials/golang-setup-api-server-or-gateway-with-caddy-and-http-listenandserve-function-example)

Here you go!

 package main

 import (
 "net/http"
 "strings"
 )

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

 func ReplyName(w http.ResponseWriter, r *http.Request) {
 w.Header().Set("Access-Control-Allow-Origin", "*") // usually, this line is enough, but you can add the following options.
 w.Header().Set("Access-Control-Allow-Credentials", "true")
 w.Header().Set("Access-Control-Allow-Methods", "OPTIONS, GET, POST")
 w.Header().Set("Access-Control-Allow-Headers", "Content-Type, Depth, User-Agent, X-File-Size, X-Requested-With, If-Modified-Since, X-File-Name, Cache-Control")

 URISegments := strings.Split(r.URL.Path, "/")

 if URISegments[2] != "" {
 w.Write([]byte(URISegments[2]))
 } else {
 w.Write([]byte("You gotta give a name for me to reply...."))
 }
 }

 func main() {
 // http.Handler
 mux := http.NewServeMux()
 mux.HandleFunc("/", SayHelloWorld)
 mux.HandleFunc("/replyname/", ReplyName)

 http.ListenAndServe(":8080", mux)
 }