Enable CORS in Golang callback function

I have an AJAX call to Google's oAuth2 api looks like this:

$(document).on('click', '#signup', function() {
  var OAUTHURL        =  'https://accounts.google.com/o/oauth2/auth?';
  var SCOPE           =   'email profile';
  var STATE 		  =   'profile';
  var REDIRECT_URI    =   'https://localhost:8080/callback';
  var RESPONSE_TYPE   =   'token';
  var CLIENT_ID       =   '554886359117-8icq0dc9halr8rjd6bdtqcmagrdql9lr.apps.googleusercontent.com';
  var _url            =    OAUTHURL + 'scope=' + SCOPE + '&state=' + STATE + '&redirect_uri=' + REDIRECT_URI + '&response_type=' + RESPONSE_TYPE + '&client_id=' + CLIENT_ID;
  $.ajax({
  	type: "POST",
  	dataType: "text",
  	url: _url,
  	success: function(response)
  	{
  		console.log(response.url);
  	},
  	error: function(error)
  	{
  		console.log("ERROR: ", error);
  	}
  });
});

This is supposed to redirect back to the server running on http://localhost/callback.

Redirect URIs:	http://localhost:8080/callback
Javascript Origins:	http://localhost:8080

I also have the callback function defined as below:

func init() {
	r := mux.NewRouter()

	r.HandleFunc("/", rootHandler)
	r.HandleFunc("/callback", callbackHandler)
	http.Handle("/", r)
}
func callbackHandler(w http.ResponseWriter, r *http.Request) {
	fmt.Fprint(w, "I am sent back from the server!"+time.Now().Format("Mon, 02 Jan 2006 15:04:05 MST"))
}

everything looks good in debugging mode, except I get this error back from google's api:

> XMLHttpRequest cannot load
> https://accounts.google.com/o/oauth2/auth?scope=email%20profile&state=profi…d=554886359117-8icq0dc9halr8rjd6bdtqcmagrdql9lr.apps.googleusercontent.com.
> No 'Access-Control-Allow-Origin' header is present on the requested
> resource. Origin 'http://localhost:8080' is therefore not allowed
> access.

I have tweaked a little around but I can't seem to find my way through.
Any thought on this?

Like Not_a_Golfer said your backend is not setting correct headers to response. To handle CORS problem in every cases I made this little handler that wraps your router so you don't have to manually set headers in every callback.

Use it like this:

import "github.com/heppu/simple-cors"

func init() {
    r := mux.NewRouter()
    r.HandleFunc("/", rootHandler)
    r.HandleFunc("/callback", callbackHandler)
    http.Handle("/", cors.CORS(r))
}
func callbackHandler(w http.ResponseWriter, r *http.Request) {
    fmt.Fprint(w, "I am sent back from the server!"+time.Now().Format("Mon, 02 Jan 2006 15:04:05 MST"))
}

Just add CORS headers to your callbackHandler:

// make this configurable via command line flags, env var, or something
var MyServerName = "https://localhost:8080" 

func callbackHandler(w http.ResponseWriter, r *http.Request) {
    w.Header().Add("Access-Control-Allow-Origin", MyServerName)

    fmt.Fprint(w, "I am sent back from the server!"+time.Now().Format("Mon, 02 Jan 2006 15:04:05 MST"))
}