How to Handle 404 and Error Pages in Go

Go does not automatically serve 404 or error pages; you must explicitly register a handler for the 404 Not Found status code in your http.ServeMux.

package main

import (
	"log"
	"net/http"
)

func main() {
	http.HandleFunc("/not-found", func(w http.ResponseWriter, r *http.Request) {
		w.WriteHeader(http.StatusNotFound)
		w.Write([]byte("404: Page Not Found"))
	})

	log.Fatal(http.ListenAndServe(":8080", nil))
}

For global error handling, wrap your ServeMux with a custom http.Handler that checks w.WriteHeader status codes or use http.NotFound as a fallback handler.

Go's web server doesn't guess what to show when a page is missing; you have to tell it exactly what to do. Think of it like a receptionist who only answers calls for specific extensions; if no one answers, the call just hangs unless you program a specific 'no one is here' message. You write a small function to catch missing requests and send a custom message back to the user.