HTTP Error Handling

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

Differentiate bad client requests from server failures. Send a clear status code and safe explanation; log internal details separately rather than leaking them in API responses.

Return promptly after writing an error. Otherwise a handler may accidentally append a success response to the error body.

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"
	"net/http"
	"net/http/httptest"
)

func handler(w http.ResponseWriter, r *http.Request) {
	if r.URL.Query().Get("id") == "" {
		http.Error(w, "missing id", http.StatusBadRequest)
		return
	}
	fmt.Fprint(w, "found")
}
func main() {
	rec := httptest.NewRecorder()
	http.HandlerFunc(handler).ServeHTTP(rec, httptest.NewRequest("GET", "/", nil))
	fmt.Println("Status:", rec.Code)
}

Output

Status: 400

Because the URL omits id, the handler writes a 400 response and returns before printing found.

• Never expose database credentials or stack traces in client-facing error text.
• Check errors returned by downstream I/O before reporting success.

Add a not-found branch that sends status 404 for an unknown task ID.

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

You have practiced HTTP Error Handling 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.