Spies
Spies are a test double used in software testing to observe and record interactions with objects or functions, such as method calls, arguments, and return values, without altering their behavior. They are commonly employed in unit testing to verify that code interacts correctly with dependencies, like ensuring a function is called with specific parameters. This helps in testing side effects and integration points in a controlled manner.
Developers should use spies when writing unit tests to monitor interactions with dependencies, such as external APIs, databases, or other modules, without stubbing or mocking their actual implementation. This is particularly useful for testing that certain functions are invoked as expected in scenarios like event handling, logging, or communication between components. Spies enable non-invasive verification, making tests more focused on behavior rather than state.