Wirte JSON and Error message for net/http ResponseWriter

jsonAPI.go

package jsonAPI

import (
	"encoding/json"
	"fmt"
	"net/http"
)

type MessageResponse struct {
	Message string `json:"message"`
}

type JSONWriter interface {
	Write([]byte) (int, error)
	WriteHeader(int)
	Header() http.Header
}

func WriteJSON(jw JSONWriter, data interface{}, code ...int) {
	optionalStatusCode(jw, code...)

	jw.Header().Set("Content-Type", "application/json")
	var (
		b   []byte
		err error
	)
	switch t := data.(type) {
	case []byte:
		b = t
	case string:
		b = []byte(t)
	default:
		b, err = json.Marshal(data)
		if err != nil {
			WriteError(jw, err)
			return
		}
	}
	jw.Write(b)
}

func WriteError(w http.ResponseWriter, message interface{}, code ...int) {
	var msg string
	switch m := message.(type) {
	case string:
		msg = m
	case error:
		msg = m.Error()
	}

	w.Header().Set("Content-Type", "application/json")
	s := fmt.Sprintf(`{"message":"%s"}`, msg)
	optionalStatusCode(w, code...)
	w.Write([]byte(s))
}

func optionalStatusCode(w http.ResponseWriter, code ...int) {
	switch len(code) {
	case 0:
	default:
		w.WriteHeader(code[0])
	}
}