Go Test Httptest Recorder

In this tutorial, you'll learn about Go Test: httptest.ResponseRecorder. We cover key concepts, practical examples, and best practices.

httptest.ResponseRecorder -- Use httptest.ResponseRecorder to test HTTP handlers without a server.

The Problem

http.ResponseRecorder implements http.ResponseWriter and captures the status code, headers, and body for assertion. No server startup needed.

Wrong

func TestHandler(t *testing.T) {
    // Starting a full server just to test handler
    server := httptest.NewServer(myHandler)
    defer server.Close()
    resp, _ := http.Get(server.URL)

Output:

// Works but heavier than needed for unit tests

Right

func TestHandler(t *testing.T) {
    req := httptest.NewRequest("GET", "/api/user", nil)
    rec := httptest.NewRecorder()
    myHandler(rec, req)
    resp := rec.Result()
    if resp.StatusCode != 200 {
        t.Errorf("got %d, want 200", resp.StatusCode)
    }
    body, _ := io.ReadAll(resp.Body)
    resp.Body.Close()
    if string(body) != "expected" {
        t.Errorf("got %s, want expected", body)
    }
}

Output:

// Fast, no server, direct handler testing

Prevention

• Use httptest.NewRecorder() to capture response
• Use httptest.NewRequest() to create test requests
• No server needed, faster than httptest.NewServer
• Check status code, headers, body
• Use for unit testing handlers and middleware

Common Mistakes with test httptest recorder

1. Forgetting deriving (Show, Eq) on custom data types needed for debugging
2. Placing the wildcard pattern first in case expressions, making all subsequent patterns unreachable
3. Using head and tail instead of pattern matching, causing runtime errors on empty lists

These mistakes appear frequently in real-world GO code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

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