What is a Test Double? | Types & Best Practices

Testing is often a heavily dependent process; there are instances where one component (sign-up) could have dependencies (UI elements), but the latter is not fully developed. Alternatively, these components might not be suitable for specific test environments, may not return the desired test outcome, or could have undesirable side effects upon execution. And since the focus is on validating a particular feature, you shouldn’t have to wait for the internal or external dependencies to carry out the tests. But then, what should you do if you encounter such cases? Replace the missing components with test doubles and continue with the test process.

We will tell you how.

What are Test Doubles?

Test doubles are stand-ins for the real components of your software under test. They mimic the behavior of these components, allowing you to test the code in isolation without relying on any dependencies. Their use often comes up during unit testing when developers need to verify a standalone feature, which might require another component or a complex input or data to run. Test double fills the latter component gap without developers needing to run the actual code.

There are different types of test doubles, like mocks, stubs, spies, and fakes, each with its own unique purpose and role in the testing process. We will discuss these types in the upcoming sections.

Why Use Test Doubles?

You might be wondering, why bother with these test doubles when you can just test your code with the real components? Because they help you recognize and test modules without actually waiting for separate dependencies to become available. Some of the other reasons are:

• Test doubles provide a way to isolate the code you’re testing from its dependencies.
• It ensures that any failures or errors are due to the code being tested and not caused by external factors.
• They make testing more efficient by allowing you to control the behavior of dependencies and focus on specific scenarios without worrying about the entire system.
• Test doubles give you more control, flexibility, and reliability in your testing efforts.
• Test doubles are essential for practicing test-driven development (TDD), where tests are written before the actual implementation code.

Test Doubles Examples

In a web application’s development, consider the scenario of integrating with a payment gateway. Testing the checkout process with the actual payment gateway can be cumbersome and time-consuming. In this case, a test double for the payment gateway API can be created. This test double emulates the real payment gateway’s behavior within the testing environment, simulating various responses like successful payments, failures, network errors, or timeouts. This approach facilitates comprehensive testing of the checkout process, including error handling and edge cases, in a controlled and efficient manner. It additionally reduces external dependencies and ensures fast, reliable, and cost-effective tests.

Here’s how the test double might look in Javascript:

<styled-code>
// Define a test double for the payment gateway API
const paymentGatewayTestDouble = {
  // Simulate a successful payment response
  successfulPayment: () => {
    return {
      status: 'success',
      transactionId: '123456789',
      amount: 100.00,
      currency: 'USD'
    };
  },
  // Simulate a failed payment response
  failedPayment: () => {
    return {
      status: 'failure',
      errorMessage: 'Insufficient funds'
    };
  },
  // Simulate a network error response
  networkError: () => {
    throw new Error('Network error: Unable to connect to payment gateway');
  }
};
// Example usage of the test double
function processPayment(amount) {
  // Simulate making a payment request to the payment gateway API
  // In real testing, this would be replaced with an actual API call
  if (amount > 0 && amount < 1000) {
    return paymentGatewayTestDouble.successfulPayment();
  } else if (amount >= 1000) {
    return paymentGatewayTestDouble.failedPayment();
  } else {
    return paymentGatewayTestDouble.networkError();
  }
}
// Example usage of the test double
console.log(processPayment(50)); // Simulated successful payment
console.log(processPayment(1500)); // Simulated failed payment

</styled-code>

When To Use Test Doubles?

Only during the testing process would you encounter scenarios where test doubles come in handy. Here are some situations where you would need to use test doubles:

• When simulating responses from external APIs or third-party services to verify the application’s behavior when interacting with external systems.
• Test doubles are useful in simulating error conditions, exceptions, or edge cases to verify that the application handles errors gracefully 
• When functions consume too much memory or time, especially without asynchronous implementation, testing becomes challenging. Test doubles can replace these long-running functions with predefined behaviors, optimizing CI/CD execution time.
• They are helpful in creating mock database objects to replicate database operations, such as data retrieval, insertion, updating, or deletion, without accessing the actual database.
• Test doubles in software testing and development create mock authentication and authorization objects to duplicate different user roles, permissions, or authentication states for testing access control and security features.

Types of Test Doubles

Different testing types require varying functioning features of the software. There might be cases where a certain test run may need the function to return a numeric value. Choosing the right test double for such test scenarios is critical. This is the list of different types of test doubles available:

Fakes

A fake is a simplified version of a real component. Fakes are often used when the real component is too complex, unavailable, or impractical to use in testing. For example, a fake database implementation may store data in memory rather than a persistent storage medium. This type of test double is essential for speeding up test execution, reducing dependencies on external systems, and creating controlled environments for testing.

Stubs

A stub is a basic implementation of a component or interface that returns predetermined responses to method calls. Stubs are used to simulate the behavior of real components, which allows testers to control the responses returned during testing. For instance, a stub HTTP client may return predefined HTTP responses without actually making network requests. Stubs are valuable for testing components that rely on external dependencies.

Mocks

Mocks are objects that mimic the behavior of a real component, allowing testers to set expectations and verify interactions with the component during testing. They are useful in behavior-driven testing to verify method calls, parameters, and return values. Consider a mock email service, which may expect to receive specific email messages during testing. Mocks are indispensable for verifying the interactions between components, ensuring that the system behaves as per specifications and meets the requirements.

Dummies

Dummies are placeholder objects that satisfy parameter requirements in method calls without affecting the behavior of the system under test. Unlike other types of test doubles, dummies do not have any functional implementation and are simply placeholders. They come in handy when a parameter is required but not relevant to the test scenario. For instance, a dummy user object may be passed as a parameter to a method that requires a user object but does not use it in the test case. Dummies help maintain code structure and fulfill method signatures without introducing unnecessary complexity or behavior.

Spies

A Spy is a test double that records and tracks interactions with a real component. They are helpful in monitoring method calls, parameters, and return values. For example, a spy on a logging service may record log messages generated during testing for later analysis. Spies are essential for gathering data about component usage, identifying potential issues, and ensuring that the system functions correctly in different scenarios.

Bonus Reading: Test reporting tools to make test results analysis easy

Test Doubles – The Difference Between Stubs and Mocks

Quite possibly, testers can confuse test double types while working even after knowing their definitions. Among the five discussed here, stubs and mocks are the most widely used test doubles by developers and testers. You know their definition; now, let’s look at how they differ from each other.

Stubs	Mocks
Stubs mimic the behavior of the component, which has a predetermined response.	Mocks records and checks the interactions between the application under test and its integrations.
The focus is on providing predictable outcomes.	The focus is on simply documenting and validating the flow of information between the AUT and its collaborators.
Testing happened without any external dependencies.	The testing process is done to verify interactions.
Behavior-agnostic	Behavior-focused
Stub HTTP client returns pre-defined responses.	Mock email service expects specific email messages.
It does not throw any exceptions during the testing process.	Mocks raise exceptions for unexpected method calls.

How to Create Test Doubles?

There are several ways to create test doubles, each with its own advantages and use cases. Here are the different methods commonly used to create test doubles:

Manual Implementation

You can annually implement test doubles by writing custom classes or functions that mimic the behavior of real components. This approach provides full control over the behavior of the test double. However, they are often time-consuming and require significant effort, especially for complex dependencies.

Using Testing Frameworks

Many testing frameworks, such as JUnit, Mockito, Jasmine, and Sinon.js, provide built-in support for creating test doubles, including mocks, stubs, and spies. These frameworks offer convenient APIs and utilities for creating and configuring test doubles, simplifying the testing process and reducing manual effort.

Custom Implementations

In some cases, custom implementations of test doubles may need to meet specific testing requirements. Testers can leverage programming languages’ features, such as inheritance, interfaces, and lambda expressions, to create custom test doubles tailored to their testing scenarios.

Using Mocking Libraries

Mocking libraries, such as Mockito for Java, Moq for .NET, and Sinon.js for JavaScript, provide powerful tools for creating test doubles, including mocks, stubs, and spies. These libraries offer advanced features, such as method verification, behavior definition, and automatic stubbing, to streamline the creation and configuration of test doubles.

Dependency Injection

Dependency injection frameworks, such as Spring for Java and Angular for TypeScript, facilitate the creation of test doubles by injecting mock or stub objects into the components being tested. This approach allows testers to easily replace real dependencies with test doubles for more flexible and modular testing setups.

Using Third-Party Tools

Some third-party tools and services, such as WireMock for HTTP stubbing and Pact for consumer-driven contract testing, provide specialized support for creating test doubles in specific scenarios, such as testing interactions with external systems or services. These tools offer pre-built functionalities and integrations to simplify the creation and management of test doubles.

How Do Test Doubles Work?

They simply simulate the real dependencies, so the application under test thinks the actual components are in motion. They can be controlled and configured to behave in specific ways for testing purposes. These test doubles are either hardcoded early on to function in a specific way or configured to match the test case at hand.

Test doubles allow testers to isolate components under test, control interactions, simulate various scenarios, and verify the behavior of the system under different conditions. This enables comprehensive and efficient testing, ultimately improving the software’s reliability and quality.

Best Practices with Test Doubles

When you know the best ways to use a certain feature, you get the most out of it. These best practices to create and implement test doubles will save you tons of time and mental load.

• It is a good practice to not share the test doubles behavior among tests. This approach is likely to create dependencies between the tests and lead to erroneous results.
• If you absolutely need to share one double among various tests, go with fakes. They help reduce duplication of code and ensure consistency in behavior across tests.
• Do not rely on using the same test double type for all your tests. Give proper attention to the test double that would work best for the test case you are currently working on.
• Only stub the behavior that you need at the moment. Avoid wasting your time on creating and implementing test doubles that are not needed for the test at hand.

What are the Advantages and Disadvantages of Test Doubles?

A test double has a single role—to hold the place of or mimic the behavior of an actual component. But even with a sole purpose, test doubles come with a myriad of advantages and disadvantages, which are extremely important for developers and testers to recognize.

Advantages of Test Doubles

• Test doubles isolate the components under test, allowing for focused testing without interference from external dependencies.
• They provide a controlled environment for testing.
• Use of test doubles enables quick test execution by overcoming the complexities of real components.
• They eliminate flakiness and unpredictability in tests by providing consistent and reproducible behavior.
• Testers can run parallel testing by removing dependencies on shared resources or external systems when using test doubles.

Disadvantages of Test Doubles

• Test doubles require maintenance, which can become cumbersome as the complexity of the test suite grows.
• Over-specifying test behavior with test doubles may lead to brittle tests that break with minor changes to the implementation code.
• Setting up and configuring test doubles may introduce complexity.
• Test doubles may not accurately replicate the behavior of real dependencies in all scenarios.
• Incorrectly configured or poorly implemented test doubles may produce misleading test results.

Role of Test Doubles in Test Automation

When it comes to test automation, test doubles play a crucial role; they simulate the behavior of actual dependencies, such as APIs, databases, or external services. This capability enables testers to isolate components under test and control their interactions during automated web application testing and mobile testing. Consider an example of a test double while running the testing process for an e-commerce website. In this scenario, test doubles can be useful in simulating responses from payment gateways or inventory systems, allowing testers to verify the application’s behavior without relying on actual external services. The rest of the automation testing process would stay the same to provide accurate test results.

Testsigma is an AI-driven test automation platform with the capability to run test cases without the need for coding. The tool comes with the in-built capabilities to simulate various testing scenarios and interactions.

Testers can simulate different behaviors and responses by leveraging its features, such as parameterization, conditional statements, data-driven testing, and advanced test flows in Testsigma. You can also create and manage test data right within the tool. For example, testers can parameterize test inputs and conditions to simulate different scenarios, create conditional test steps to handle various outcomes and use data-driven testing to execute tests with different datasets.

Learn more about Automated Testing with Testsigma

Conclusion

No testing is done only on a single component. Even when the software under test is isolated, it would still require connecting components to function. However, validating such features does not always come with a fully-functional external dependencies. Test doubles replace such missing links or complex components and act as the placeholder in structure and functionality.

Use them correctly in your test cases, manual, and automation.

Frequently Asked Questions

What is the difference between mock and test doubles?

The main difference between mocks and test doubles lies in their purpose and functionality. Mocks are specific types of test doubles that focus on verifying interactions between components, such as method calls and parameters, during testing. Test doubles, on the other hand, encompass a broader category of simulated objects used in testing to replace real dependencies and control behavior, including mocks, stubs, fakes, and spies.