在Main.go中连接ReactJS文件

Question

I am trying to develop a web application with ReactJS and a Go server. The web app works when I connect ReactJS file with Node.js, but it shows an error on the output when I connect ReactJS file with the Go server.

output:

"404 page not found"

main.go:

package main

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

var items = []*Item{}

// Item representing the todo
type Item struct {
    ID   string `json:"item_id"`
    Name string `json:"item_name"`
}

func main() {
    //r := http.NewServeMux()
     r := mux.NewRouter()

    r.PathPrefix("client/").Handler(http.StripPrefix("client/", http.FileServer(http.Dir(".client/"))))
    r.HandleFunc("/api/new-item", addItemHandler)
    fmt.Println("Running server on port :3000")
    http.ListenAndServe(":3000", r)
}

func addItemHandler(w http.ResponseWriter, r *http.Request) {
    item := &Item{}
    json.NewDecoder(r.Body).Decode(item)
    items = append(items, item)
}

my file structure:

appName/main.go
appName/client/src/App.js

I would like the web app to load App.js when I run main.go

What am I missing?