[reactjs] Axios having CORS issue

May help to someone:

I'm sending data from react application to golang server.

Once I change this, w.Header().Set("Access-Control-Allow-Origin", "*"). Error has fixed.

React form submit function:

async handleSubmit(e) {
    e.preventDefault();

    const headers = {
        'Content-Type': 'text/plain'
    };

    await axios.post(
        'http://localhost:3001/login',
        {
            user_name: this.state.user_name,
            password: this.state.password,
        },
        {headers}
        ).then(response => {
            console.log("Success ========>", response);
        })
        .catch(error => {
            console.log("Error ========>", error);
        }
    )
}

Go server got Router,

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

    router.HandleFunc("/login", Login.Login).Methods("POST")

    log.Fatal(http.ListenAndServe(":3001", router))
}

Login.go,

func Login(w http.ResponseWriter, r *http.Request)  {

    var user = Models.User{}
    data, err := ioutil.ReadAll(r.Body)

    if err == nil {
        err := json.Unmarshal(data, &user)
        if err == nil {
            user = Postgres.GetUser(user.UserName, user.Password)
            w.Header().Set("Access-Control-Allow-Origin", "*")
            json.NewEncoder(w).Encode(user)
        }
    }
}