Writing Testable Code: Best Practices and Patterns to Follow for Developers

Writing Testable Code in Software Development

Writing testable code is essential in software development as it ensures that the codebase is maintainable and adaptable to change. Testable code simplifies automated testing, a vital component for developers and QA testers. Understanding specific best practices and design patterns is crucial for writing testable code. I will discusses and showcase some coding practices that strengthen code testability, providing clear examples of good and bad approaches, and shares practical tips for testing. I have been following the development practice of shifting left which essentially means moving the testing practices earlier on in the development phase. Since I started working at a startup we do not have access to dedicated QA, we are expected to create tests ourselves and maintain the test suite. I found that building tests in a fast paced environment often saves me a lot of development time when new features are requested, it allows me to quickly pivot.

Understanding Testable Code

Testable code is designed for easy testing through automated unit tests. This means it is modular, has minimal dependencies, and is simple to grasp at first glance.

Characteristics of Testable Code

• Simplicity: Code should be straightforward and easy to read.

• Modularity: Functions and classes should carry out one specific task each.

  
  

• Loose Coupling: Reducing dependencies between components allows for straightforward updates.

  
  

• Explicitness: Function outputs should be clear and verifiable, which aids in understanding behavior.

  
  

The journey towards writing testable code leads to a more manageable and predictable codebase.

Best Practices for Writing Testable Code

1. Favor Smaller Functions

Smaller functions that execute a single task are easier to test. When functions are overloaded with responsibilities, they become challenging to test thoroughly. I will showcase a two function both I have written and thought once it was "Great" code. One thing to note here is there needs to be a balance between speed and maintainability, I believe there is a good middle ground. There isn't necessarily right and wrong way of coming up with a solution, I think of it as a solution that is more flexible to change and the other end not flexible to change and more difficult to extend with new requests.

Bad Example

Good Example

In the good example, each function has a distinct purpose, making it easier to test them independently and ensuring that they behave as expected. This approach emphasizes the importance of writing small, focused functions that are easier to test, extend and reuse.

2. Dependency Injection

Using dependency injection allows you to swap out dependencies easily while testing. This approach minimizes coupling between components.

Bad Example

Good Example

Using interfaces in your codebase introduces a trade-off of having a bit more boilerplate code, but it offers significant benefits in terms of flexibility and testability. By defining interfaces such as `INotificationChannel`, `IUserPreferences`, `IUserRepository`, and `IAnalytics`, you create clear contracts that classes can implement.

One major advantage of using interfaces is that they enable you to easily swap out implementations. For example, the `UserRepository` can be replaced with a mock repository during testing, allowing you to isolate and test the `UserService` without relying on a real database or external dependencies.

While interfaces introduce more code I believe the tradeoff is well worth the cost. Allowing the ease of swapping out implementation can greatly improve speed. Personally I found that working like this helps me with the always changing requests I get not to mention writing testable code helps prove my code works as expected.

4. Isolate External Dependencies

When dealing with external APIs or databases, isolate these calls in specific layers. This enables easy mocking of dependencies during testing. This also applies to global variables or global state, instead of creating global state move it into the class. Generally I tend to avoid any type of global state if possible, when in doubt utilize dependency injection.

Bad Example

Good Example

Encapsulating dependencies in their classes simplifies testing with mock APIs.

Final Thoughts

Writing testable code goes beyond merely enabling tests. It embodies a commitment to quality coding principles. By following best practices such as writing smaller functions, utilizing dependency injection, and isolating external dependencies, developers can produce code that is resilient, easy to maintain, and simple to test.

Adopting these practices will lead to long-term improvements in both your projects and your role as a developer or QA tester.

An illustration demonstrating the importance of testing code for software reliability.

As you advance in your software development or testing journey, remember that the objective is to write code that stands the test of time—both in functionality and maintainability. Embrace these principles, and you will notice positive changes in your current tasks and future projects.

Visual representation of best practices for writing clean and testable code.