Javascript – CORS problem with interaction between server and client: Go + JS

I’m making simple web application of uploading the products. I have a server on GO, which has only one handler on POST method. I have some demontsration below:

package main

import (
 "encoding/json"
 "fmt"
 "github.com/gorilla/mux"
 "net/http"
)

type Product struct {
 Id int json:"id"
 Desc string json:"desc"
}

func productsPostHandler(w http.ResponseWriter, r *http.Request) {
 p := &Product{}
 json.NewDecoder(r.Body).Decode(p)
 fmt.Println(p.Id, p.Desc)
}

func main() {
 router := mux.NewRouter()
 router.HandleFunc("/products", productsPostHandler).Methods("POST")
 fmt.Println("Server is listening...")
 http.ListenAndServe(":8181", router)
}

And also, I have js client, a simple query to my server. I’m trying to upload the specific Product:

fetch("http://localhost:8181/products", {
 method: "POST",
 body: JSON.stringify({
 id: 1,
 desc: "The first product",
 })
 })
 .then(response => {
 console.log(response)
 })

When server and client try to interact, it failed with the CORS error below.

I want to avoid this error. I guess, I got to fix the http headers in the request somehow. I’ve tried to add the “Access-Allow-Origin” and nothing happened. So, I’m asking for your help.