How to write user login error in login page - go

Question

I'm new one for Golang programming. In Golang how to display the username error.
This is my code:
func LoginHandler(w http.ResponseWriter, req *http.Request){
if req.Method == http.MethodPost{
un := req.FormValue("username")
p := req.FormValue("password")
u, ok := dbUsers[un]
if !ok{
var body, _ = helpers.LoadFile("sss/login.html")
fmt.Fprintf(response, body)
//fmt.Fprintln(w, "incorrect user name")
//return
}
if u.Password != p{
fmt.Fprintln(w, "incorrect password")
return
}
http.Redirect(w, req, "/index.html", http.StatusSeeOther)
return
}
}

Answer 1

In go You have different ways for error handling
func SomeHandler(w http.ResponseWriter, r *http.Request) (int, error) {
if req.Method == http.MethodPost{
un := req.FormValue("username")
p := req.FormValue("password")
u, ok := dbUsers[un]
if !ok{
var body, _ = helpers.LoadFile("sss/login.html")
return 400, errors.New("can't load sss/login")
}
if u.Password != p{
return 400, errors.New("wrong password")
}
http.Redirect(w, req, "/index.html", http.StatusSeeOther)
return 302, nil
}
or
func SomeHandler(w http.ResponseWriter, r *http.Request) {
un := req.FormValue("username")
p := req.FormValue("password")
u, ok := dbUsers[un]
if !ok{
var body, _ = helpers.LoadFile("sss/login.html")
http.Error(w, "can't load sss/login", 400)
return
}
if u.Password != p{
http.Error(w, "wrong password", 400)
return
}
http.Redirect(w, req, "/index.html", http.StatusSeeOther)
return
}
For more information You can read here very helpful blogpost about middlewares:
https://www.nicolasmerouze.com/middlewares-golang-best-practices-examples/
Post with simple example:
https://blog.questionable.services/article/http-handler-error-handling-revisited/
Http package docs: https://golang.org/pkg/net/http/

Answer 2

You could do something a little bit more elegant.
Pass a context object to the template and render the error there.
func index(w http.ResponseWriter, r *http.Request) {
context := make(map[string]string]
context["Error"] = "Username error"
t := template.Must(template.ParseFiles("./templates/index.html"))
t.Execute(w, context)
}
Then, on the template render it like this
{{.Error}}}
If no error occurred, the field will be empty.
Cheers.