Making golang Gorilla CORS handler work

Please read the link Markus suggested, and also about what triggers CORS pre-flight requests.

Pre-flight requests: You may have a content type like JSON, or some other custom header that's triggering a pre-flight request, which your server may not be handling. Try adding this one, if you're using the ever-common AJAX in your front-end: https://en.wikipedia.org/wiki/List_of_HTTP_header_fields#Requested-With

Gorilla's handlers.CORS() will set sane defaults to get the basics of CORS working for you; however, you can (and maybe should) take control in a more functional manner.

Here's some starter code:

// Where ORIGIN_ALLOWED is like `scheme://dns[:port]`, or `*` (insecure)
headersOk := handlers.AllowedHeaders([]string{"X-Requested-With"})
originsOk := handlers.AllowedOrigins([]string{os.Getenv("ORIGIN_ALLOWED")})
methodsOk := handlers.AllowedMethods([]string{"GET", "HEAD", "POST", "PUT", "OPTIONS"})

// start server listen
// with error handling
log.Fatal(http.ListenAndServe(":" + os.Getenv("PORT"), handlers.CORS(originsOk, headersOk, methodsOk)(router)))

You can get more details here: Why doesn’t Postman get a "No 'Access-Control-Allow-Origin' header is present on the requested resource" error when my JavaScript code does? about this issue.

Also try this handler: Go Cors Handler which should solve your issue. I find this much cleaner and easy to resolve the issue.

package main

import (
    "log"
    "net/http"
    "github.com/rs/cors"
    "github.com/gorilla/handlers"
    "github.com/gorilla/mux"
    "myApp/src/controllers"
)

func main() {
    ac := new(controllers.AccountController)

    router := mux.NewRouter()
    router.HandleFunc("/signup", ac.SignUp).Methods("POST")
    router.HandleFunc("/signin", ac.SignIn).Methods("POST")

    c := cors.New(cors.Options{
        AllowedOrigins: []string{"http://localhost:8000"},
        AllowCredentials: true,
    })

    handler := c.Handler(router)
    log.Fatal(http.ListenAndServe(":3000", handler)
}