Sending HTTP Responses

In this lesson, explore sending http responses through one small Go program, and connect its output to the language rules behind it.

Set response headers before calling WriteHeader or writing the first body byte. If you do not explicitly send a status, Go defaults to 200 when the handler first writes the body.

Return clear content types and status codes. JSON responses should be encoded with json.Encoder so strings are escaped properly.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

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

func main() {
	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("Content-Type", "application/json")
		w.WriteHeader(http.StatusAccepted)
		json.NewEncoder(w).Encode(map[string]string{"status": "queued"})
	})
	rec := httptest.NewRecorder()
	handler.ServeHTTP(rec, httptest.NewRequest("GET", "/", nil))
	fmt.Println(rec.Code, rec.Header().Get("Content-Type"))
	fmt.Print(rec.Body.String())
}

Output

202 application/json
{"status":"queued"}

The response carries status 202 and the application/json content type. The JSON encoder serializes a one-key map with a stable value.

• Status must be written before the response body.
• Encoding a JSON response can fail; handle errors where appropriate.

Change the response to 201 with a Task object containing an ID and title.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Sending HTTP Responses in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.