How do you mock dependencies in Go without a mocking framework?

Question

Accepted Answer

// Production interface
type EmailSender interface {
    Send(ctx context.Context, to, subject, body string) error
}

// Hand-written mock — simple and readable
type mockEmailSender struct {
    sentEmails []struct{ To, Subject string }
    err        error // preset error to return
}

func (m *mockEmailSender) Send(_ context.Context, to, subject, body string) error {
    if m.err != nil { return m.err }
    m.sentEmails = append(m.sentEmails, struct{ To, Subject string }{to, subject})
    return nil
}

// Test using the mock
func TestUserRegistration(t *testing.T) {
    sender := &mockEmailSender{}
    svc := NewUserService(mockUserRepo{}, sender)

err := svc.Register(context.Background(), &RegisterRequest{
        Email: "alice@example.com",
        Name:  "Alice",
    })

if err != nil {
        t.Fatalf("Register() error: %v", err)
    }
    if len(sender.sentEmails) != 1 {
        t.Errorf("expected 1 email sent, got %d", len(sender.sentEmails))
    }
    if sender.sentEmails[0].To != "alice@example.com" {
        t.Errorf("email sent to wrong address")
    }
} Hand-written mocks: preferred for small interfaces (1-3 methods) — simple and readable testify/mock: adds call expectations and assertion helpers for complex interaction testing gomock (mockgen): generates mocks from interfaces — useful for large interfaces with many methods Avoid over-mocking: if your test mocks everything, it tests nothing — prefer integration tests for complex scenarios Functional options on mocks: allow test-specific behavior without modifying the mock struct

How do you mock dependencies in Go without a mocking framework?

Define interfaces for dependencies and implement them with simple structs in tests that return preset values or record calls. This is idiomatic Go and requires no reflection.

How do you mock dependencies in Go without a mocking framework?

Define interfaces for dependencies and implement them with simple structs in tests that return preset values or record calls. This is idiomatic Go and requires no reflection.

Scenario Questions

Follow-up Questions