Mocking, Stubs, and Test Data

In real-world applications, your code often relies on other components or external services. These are called dependencies.

Imagine your application needs to:

• Save data to a database.
• Send emails via an email service.
• Fetch data from a third-party API.

Testing these directly can be slow, unreliable (what if the API is down?), or costly. We need a way to test our code in isolation.

When testing code that interacts with external services or complex components, we often use test doubles. These are generic objects that stand in for real dependencies during a test.

Common types include:

• Stubs: Provide predefined answers to method calls.
• Mocks: Verify that certain actions (method calls) occurred.
• Fakes: Lightweight working implementations (e.g., an in-memory database).

In this lesson, we'll focus on Stubs and Mocks, primarily using the Mox library.