How to fix 'Access to XMLHttpRequest at 'http://localhost:8000/api/companies' from origin 'http://localhost:3000' has been blocked by CORS policy'

Question

I am currently working on a web app with Go/Postgres backend and React frontend. I am running into some trouble with CORS despite trying many solutions found here and around the internet.

Currently, I am using the package github.com/rs/cors to handle CORS on the server side and, checking with a curl request like

curl -D - -H 'Origin: http://localhost:3000' \
-H 'Authorization: Bearer mytoken' \
http://localhost:8000/api/companies

I get the correct response:

HTTP/1.1 200 OK
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: http://localhost:3000
Content-Type: application/json
Vary: Origin
Date: Mon, 26 Aug 2019 18:19:32 GMT
Content-Length: 273

{"data":[{"ID":1,"CreatedAt":"2019-08-23T17:54:23.43867+01:00","UpdatedAt":"2019-08-23T17:59:26.898246+01:00","DeletedAt":null,"name":"Goldman Sachs","url":"https://goldmansachs.com","industry":"Investment Banking","hq":"New York, US"}],"message":"success","success":true}

But the request fails when it is done by the React frontend with an Access to XMLHttpRequest at 'http://localhost:8000/api/companies' from origin 'http://localhost:3000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. message.

main.go

func main() {
    router := mux.NewRouter()

    db := models.GetDB()
    defer db.Close()

    port := os.Getenv("PORT")
    if port == "" {
        port = "8000" //localhost
    }


    router.HandleFunc("/api/companies", handlers.GetAllCompanies).Methods("GET")
    ...


    c := cors.New(cors.Options{
        AllowedOrigins: []string{"http://localhost:3000"},
        AllowedMethods: []string{"POST", "GET", "OPTIONS", "PUT", "DELETE"},
        AllowCredentials: true,
    })

    router.Use(app.JwtAuthentication)

    handler := c.Handler(router)

    err := http.ListenAndServe(":"+port, handler)
    if err != nil {
        panic(err.Error())
    }
}

endpoints.js

...
export const token = () => {
  const localToken = getLocalAccessToken()
  //getLocalAccessToken returns the token correctly
  return localToken
}
...
const headers = {
  Authorization: `Bearer ${token()}`,
  'Content-Type': 'application/json',
};

export const getCompanies = () => axios.get(`http://localhost:8000/api/companies`, { headers }, { crossDomain: true });
...

I don't really know what to do to fix this because it works with curl but not in the browser and at the moment I am clueless to why.