What are golden file tests in Go and when should you use them?

Golden file tests compare output against a saved reference file. They are ideal for testing complex output (JSON API responses, generated SQL, HTML) where manually writing the expected value in code is tedious and error-prone.

import "github.com/sebdah/goldie/v2"

// Golden file test — output compared against testdata/*.golden
func TestUserToJSON(t *testing.T) {
    g := goldie.New(t,
        goldie.WithFixtureDir("testdata"),
        goldie.WithUpdateFlag("update"), // flag: -update
    )

    user := User{
        ID:        1,
        Name:      "Alice",
        CreatedAt: time.Date(2024, 1, 15, 0, 0, 0, 0, time.UTC),
    }
    data, err := json.MarshalIndent(user, "", "  ")
    if err != nil { t.Fatal(err) }

    // Compares with testdata/TestUserToJSON.golden
    g.Assert(t, "TestUserToJSON", data)
}

// Manual golden file implementation
func assertGolden(t *testing.T, name string, got []byte) {
    t.Helper()
    path := filepath.Join("testdata", name+".golden")

    if *update {
        // go test -run TestUserToJSON -args -update
        os.MkdirAll("testdata", 0755)
        os.WriteFile(path, got, 0644)
        return
    }

    want, err := os.ReadFile(path)
    if err != nil {
        t.Fatalf("golden file missing: %s (run with -args -update to create)", path)
    }
    if !bytes.Equal(got, want) {
        diff := diffStrings(string(want), string(got))
        t.Errorf("golden file mismatch:\n%s", diff)
    }
}

var update = flag.Bool("update", false, "update golden files")

// Commit testdata/*.golden files to version control
// Update when intentional output changes:
// go test -run TestUserToJSON -args -update

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