http.Request / ResponseWriter

// Handler function signature
func handler(w http.ResponseWriter, r *http.Request) {
    // Read request information
    method := r.Method        // HTTP method (GET, POST, etc.)
    path   := r.URL.Path      // URL path
    query  := r.URL.Query()   // Query parameters (map[string][]string)

    // Write the response
    w.Header().Set("Content-Type", "text/plain")
    w.WriteHeader(http.StatusOK) // Set the status code
    fmt.Fprintln(w, "Response body")
}

package main

import (
	"fmt"
	"io"
	"log"
	"net/http"
)

func handler(w http.ResponseWriter, r *http.Request) {
	// Print the HTTP method
	fmt.Println("Method:", r.Method)

	// Get a query parameter (?name=Gopher)
	name := r.URL.Query().Get("name")
	if name == "" {
		name = "Guest"
	}

	// Branch based on the HTTP method
	switch r.Method {
	case http.MethodGet:
		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
		fmt.Fprintf(w, "Hello, %s!\n", name)

	case http.MethodPost:
		// Read the request body
		body, _ := io.ReadAll(r.Body)
		defer r.Body.Close()
		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
		fmt.Fprintf(w, "Received: %s\n", string(body))

	default:
		// Method not allowed
		w.WriteHeader(http.StatusMethodNotAllowed)
		fmt.Fprintln(w, "Method not allowed")
	}
}

func main() {
	http.HandleFunc("/", handler)
	log.Fatal(http.ListenAndServe(":8080", nil))
}

http.Request contains all information about the incoming request. You can retrieve cookies with r.Cookie(name) and headers with r.Header.Get(key). When working with form data, you must call r.ParseForm() first.

w.WriteHeader() can only be called once, before writing the body. Calling WriteHeader() after writing the body will not change the status code. Response headers must also be set before calling WriteHeader() or the first Write().