Paul SerbanSoftware Engineer

When to Use Test Doubles in Software TestingNavigating the World of Stubs, Mocks, and Spies

Delve into the intricacies of test doubles and discern when to employ stubs, mocks, and spies to bolster your software testing voyage.

Introduction

  1. The Genesis of Test Doubles: In the realm of software testing, realism often clashes with practicality. While the desire is to test software in the exact environment it will operate in, the reality is that dependencies often make this impractical. Enter the world of Test Doubles - the stunt doubles of software testing. They mimic the behavior of complex objects and provide controlled responses to the system under test, making them indispensable tools in a tester's toolkit.

  2. The Spectrum of Test Doubles: The domain of test doubles is vast, encompassing variations such as stubs, mocks, and spies, each with a distinct role in the testing landscape. The choice among them hinges on the testing context and the feedback required. This blog post endeavors to elucidate the scenarios where each type of test double shines, aiding in informed decisions in your testing strategy.

The Landscape of Stubs

  1. Defining Stubs: Stubs are the simplest form of test doubles that return hard-coded responses. They are particularly useful when you need to isolate the system under test (SUT) from external dependencies like databases or network calls. Stubs excel in providing a quick, straightforward way to test the SUT's behavior with different inputs.
// Example: Using a stub to isolate a function from its dependency
function fetchData(callback) {
  // Simulated network call to fetch data
  setTimeout(() => callback('Fetched data'), 2000);
}

// Stub for the fetchData function
const fetchDataStub = callback => callback('Stubbed data');

// Now your test can run instantly, without waiting for the network call
  1. When to Use Stubs: The ideal playground for stubs is unit testing, where isolating the SUT from external elements is paramount. Stubs offer a straightforward, efficient method to ensure that the SUT behaves as expected when it receives specific inputs. They are the go-to choice when the focus is on testing the SUT's behavior in a controlled, predictable environment.

The Domain of Mocks

  1. Defining Mocks: Mocks are a step beyond stubs, offering not only controlled responses but also expectations on how they are used. They are equipped to verify interactions, such as method calls with specific arguments, making them invaluable for testing side effects and interactions between objects.
// Example: Using a mock to verify interactions
const mockFunction = jest.fn();
someFunction(mockFunction);
expect(mockFunction).toHaveBeenCalledWith('expected argument');
  1. When to Use Mocks: Mocks are the tool of choice when the testing focus shifts from the SUT's behavior to its interactions with dependencies. They are the sentinel, ensuring that the SUT communicates with external elements as expected. Utilize mocks in situations where verifying interactions is crucial, and the emphasis is on the communication protocol rather than the returned values.

The Realm of Spies

  1. Defining Spies: Spies reside in a sweet spot between stubs and mocks, offering the ability to observe function calls while not altering the behavior of the function. They are adept at reporting how they were called, allowing for precise verification of the interactions.
// Example: Using a spy to observe function calls
const spy = jest.spyOn(someObject, 'someMethod');
someFunction(someObject);
expect(spy).toHaveBeenCalled();
  1. When to Use Spies: Spies thrive in scenarios where monitoring function calls is essential, but the genuine behavior of the function is still desired. They are the silent observers, reporting the interactions without interfering, making them ideal for scenarios where verification of behavior and interactions is required without altering the function's natural behavior.

Conclusion

  1. The Prolific World of Test Doubles: Test Doubles are the unsung heroes in software testing, offering the dexterity to simulate, control, and observe interactions in a controlled environment. Their usage, be it stubs, mocks, or spies, is dictated by the testing context and the feedback loop required.

  2. Navigating the Test Double Landscape: The journey through the land of test doubles illuminates the path to robust, reliable, and efficient testing. By discerning when to employ each type of test double, you not only enhance the testing strategy but also edge closer to the ultimate goal - software that stands the test of time and change.

Unravel the tapestry of test doubles, and harness the power of stubs, mocks, and spies to elevate your software testing game to the zenith of precision and reliability.