Set Header to Response in Golang (HTTP Handler)

On the response writer variable call the Header() method and then Set() to set the header you want to add:

w.Header().Set("Content-Type", "text/plain")

This snippet is complete and help you to check how it works:

package main

import (
	"fmt"
	"net/http"
	"time"
)

func greetHandler(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/plain")
	w.Header().Set("Access-Control-Allow-Origin", "*")

	w.WriteHeader(http.StatusOK)

	fmt.Fprintf(w, "Hello World! %s", time.Now())
}

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