HTTP Client in Go

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

An http.Client sends outbound requests and manages connections. Reuse clients rather than constructing one per request, set timeouts, and close response bodies when finished.

The example uses a fake transport so the program remains self-contained without an external network or local listener.

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 (
	"fmt"
	"io"
	"net/http"
	"strings"
	"time"
)

type transportFunc func(*http.Request) (*http.Response, error)

func (f transportFunc) RoundTrip(r *http.Request) (*http.Response, error) { return f(r) }
func main() {
	client := &http.Client{Timeout: time.Second, Transport: transportFunc(func(r *http.Request) (*http.Response, error) {
		return &http.Response{StatusCode: 200, Body: io.NopCloser(strings.NewReader("pong")), Header: make(http.Header), Request: r}, nil
	})}
	resp, err := client.Get("https://example.invalid/ping")
	if err != nil {
		panic(err)
	}
	defer resp.Body.Close()
	body, err := io.ReadAll(resp.Body)
	if err != nil {
		panic(err)
	}
	fmt.Println(resp.StatusCode, string(body))
}

Output

200 pong

The custom transport returns a fixed HTTP response without making a network request. The client reads and closes its body, then prints 200 pong.

• Always close a successful response body.
• For real calls, propagate request context and set deadlines; never assume a remote server responds.

Use client.NewRequestWithContext and check a non-200 status before reading a response.

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

You have practiced HTTP Client in Go 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.