Serving SPA with Golang and Chi router enters a loop

I´m trying to serve a SPA with Golang and solving the 404 error became a challenge.

• In the init() func I buffered into memory the index.html file.
• I created a indexHandler that returns the index file buffered.
• I call the indexHandler in the router.NotFound() func so the route is returned to the SPA.
• I serve the rest of the static files with FileServer.

The problem is that it seems to be in a loop, when I try to access the app in the browser, it reloads itself indefinitely.

package main

import (
	"log"
	"net/http"
	"os"
	"path/filepath"
	"time"

	"github.com/go-chi/chi/v5"
	"github.com/go-chi/chi/v5/middleware"

	"github.com/joho/godotenv"
)

var indexBuffer []byte

func main() {	
	r := chi.NewRouter()
	
	r.Use(middleware.RequestID)
	r.Use(middleware.RealIP)
	r.Use(middleware.Logger)
	r.Use(middleware.Recoverer)

	fs := http.FileServer(http.Dir(os.Getenv("FRONTEND_PATH")))
	r.Handle("/app/static/*", http.StripPrefix("/app/static/", fs))

	apiRouter := chi.NewRouter()
	apiRouter.Get("/cargas", handlerCargas)

	r.Mount("/app/api", apiRouter)

	r.NotFound(indexHandler)

	log.Fatal(http.ListenAndServe(":8000", r))
}

func init() {
	err := godotenv.Load()
	if err != nil {
		log.Fatal("Erro ao ler .env verifique!")
	}

	indexBuffer, err = os.ReadFile(filepath.Join(os.Getenv("FRONTEND_PATH"), "index.html"))
	if err != nil {
		log.Fatal("Erro ao tentar bufferizar a index na init().")
	}
}

func indexHandler(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/html")
	w.Write(indexBuffer)
}

I´m expecting index.html to be served by the indexHandler and the rest of the files be served by the FileServer and errors of page not found be handled by the SPA.

I found a solution here

I adapted my code and used the FileServer as below:

// FileServer para SPA
func FileServerSPA(r chi.Router, public string, static string) {

	if strings.ContainsAny(public, "{}*") {
		panic("FileServer does not permit URL parameters.")
	}

	root, _ := filepath.Abs(static)
	if _, err := os.Stat(root); os.IsNotExist(err) {
		panic("Static Documents Directory Not Found")
	}

	fs := http.StripPrefix(public, http.FileServer(http.Dir(root)))

	if public != "/" && public[len(public)-1] != '/' {
		r.Get(public, http.RedirectHandler(public+"/", 301).ServeHTTP)
		public += "/"
	}

	r.Get(public+"*", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		file := strings.Replace(r.RequestURI, public, "/", 1)
		if _, err := os.Stat(root + file); os.IsNotExist(err) {
			http.ServeFile(w, r, path.Join(root, "index.html"))
			return
		}
		fs.ServeHTTP(w, r)
	}))
}

and, in main():

// ...
// after all the endpoints of the api

frontendPath := os.Getenv("FRONTEND_PATH")
FileServerSPA(r, "/app", frontendPath)

log.Fatal(http.ListenAndServe(":8000", r))

Hope it helps