What is a 404 and what other 4xx codes do you know?

Code 404 is an HTTP status indicating "Not Found." It is returned when the client requests a resource that does not exist on the server.

Other common 4xx codes:

• 400 Bad Request — incorrect request, such as a syntax error.
• 401 Unauthorized — authentication is required.
• 403 Forbidden — access is denied, even if the user is authenticated.
• 405 Method Not Allowed — the HTTP method is not allowed for this resource.
• 408 Request Timeout — the request timeout has expired.
• 429 Too Many Requests — the request limit has been exceeded.

In Go, when working with HTTP, you can use constants from the net/http package, for example:

import (
    "net/http"
)

func handler(w http.ResponseWriter, r *http.Request) {
    http.Error(w, "Page not found", http.StatusNotFound) // 404
}