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What is TDD (Test-Driven Development)?

Posted Jan 25, 2024 Updated Jan 25, 2024
In (CS) Learning Note
7 min read

Test-Driven Development is a powerful methodology that, when implemented correctly, can lead to more reliable, clean, and maintainable code. While it does come with challenges, particularly around the initial time investment and learning curve, the long-term benefits in code quality and maintainability can be substantial. As with any methodology, the key is finding the right balance and adapting it to your project’s specific needs and context.

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What is Test-Driven Development?

Test-Driven Development is a software development approach where tests are written before the actual code. The typical TDD cycle involves three stages, often referred to as the Red-Green-Refactor cycle:

  1. Red: Write a test that defines a function or improvements of a function, which should fail initially.
  2. Green: Write the minimum amount of code necessary to pass the test.
  3. Refactor: Improve the existing code while ensuring that it still passes all the tests.

The Red-Green-Refactor Cycle

Red: Write a Failing Test

  • Purpose: The first step in TDD is to write a test for the next bit of functionality you want to add. This test should fail initially – hence the term ‘Red’. Writing a failing test first helps ensure that the test is actually testing the right thing and that it genuinely works.

  • Example: If you’re adding a new function to calculate the sum of two numbers, you write a test that calls this function with two numbers and checks if the output is as expected. Since the function isn’t implemented yet, the test will fail.

Green: Make the Test Pass

  • Purpose: The next step is to write the minimal amount of code required to make the test pass. This phase is called ‘Green’ because the goal is to get a green light from your testing framework, indicating success.

  • Example: Implement the sum function so that when the test runs, it passes. The implementation doesn’t need to be perfect. The primary goal here is to quickly go from red to green.

Refactor: Improve the Code

  • Purpose: Once the test is passed, the next step is to refactor the code. Refactoring in this context means improving the code without changing its behavior.

  • Example: After getting the sum function to pass the test, you might notice redundancy or complexity in the code. Now is the time to clean up the code, knowing that you can run the test again to ensure you haven’t changed its behavior.

Implementing TDD

To effectively implement TDD, consider the following steps:

  1. Understand the Requirements: Clearly define what needs to be developed before starting the TDD cycle.
  2. Start Small: Write a small test for a small part of the functionality.
  3. Run the Tests: Initially, the new test will fail (the Red phase).
  4. Write the Code: Implement the code to pass the test.
  5. Refactor: Clean up the code while keeping the tests green.
  6. Repeat: Continue with the next test.

The Three Laws of TDD

There are three important laws to follow:

  1. First Law: You may not write production code until you have written a failing unit test.
  2. Second Law: You may not write more of a unit test than is sufficient to fail, and not compiling is failing.
  3. Third Law: You may not write more production code than is sufficient to pass the currently failing test.

Advantages of TDD

  1. Improved Code Quality
    • TDD leads to a more thorough testing process, which in turn results in higher quality, more robust code.
  2. Enhanced Design
    • Writing tests first forces developers to consider the design of the code, potentially leading to cleaner and more modular designs.
    • Writing tests forces you to decouple, thereby creating a better design.
  3. Simplified Debugging
    • When a test fails, only the latest changes need to be checked, simplifying the debugging process.
    • Your debug time will drop to near zero.
  4. Documentation Benefits
    • The tests themselves serve as documentation for the system, making it easier for new developers to understand the codebase.
    • Your tests will provide examples of how every part of the system works.
  5. Confidence in Refactoring
    • With a comprehensive suite of tests, developers can refactor code more confidently, ensuring that the functionality remains intact.

If you follow TDD, and use it to build a test suite that you trust. And if that test suite executes in seconds (a design goal). Then you will not be afraid of the code. You will not be afraid to clean it. You will not be afraid to fix it. You won’t be afraid to do anything to the code, because your tests will tell you if you’ve broken it.

Disadvantages of TDD

  1. Initial Learning Curve
    • TDD can be challenging to implement initially, especially for teams accustomed to traditional development methodologies.
    • With TDD, developers need to understand the requirements very well.
  2. Increased Time for Development
    • Writing tests before implementation can lead to longer development times, though this is often offset by reduced maintenance time.
  3. Possibility of Over-Reliance on Unit Tests
    • There is a risk of focusing too much on passing tests rather than ensuring overall system functionality.
  4. Ineffective Without Proper Discipline
    • TDD requires discipline and adherence to its principles. Without this, the benefits of TDD can be lost.

Some Questions about TDD

Is more testing better?

  • One of the most important things to avoid in the TDD process is writing too many tests, especially tests that have nothing to do with the requirements.
  • here is a cost to writing tests, and introducing too many tests into the TDD process means that there are three possibilities:
    1. you don’t have a good understanding of the requirements;
    2. you’re over-designing;
    3. you don’t know what you’re doing.

Doesn’t TDD require software design?

Software design is very important in TDD:

  • In TDD, decoupling is crucial as it allows testing each component separately. Good design helps to achieve decoupling, which makes testing easier and more reliable.
  • In the TDD process, good architectural design guides developers on how to organise code and tests. A clear architecture helps define the boundaries and components of the system, which makes testing more focused and effective.
  • TDD encourages the creation of small modules that can be tested independently. Good code and architectural design makes modularity possible, which in turn improves the testability and maintainability of the code.

Is TDD applicable to all scenarios?

Test-driven development (TDD) is not applicable to all scenarios. the applicability of TDD depends on a variety of factors, including the nature of the project, the experience of the team, and the constraints of time and resources.

  • For UI/UX projects, TDD may be less applicable because user interface design often requires iterations based on user feedback that are not always easy to validate with automated tests.
  • If the whole team is not familiar with TDD, then using TDD may be a struggle. In such a team, compulsory use of TDD may not be beneficial either. For teams unfamiliar with TDD, it may take time and training to get used to this type of development.
  • Implementing TDD can be a challenge when resources and time are constrained, as writing tests in the initial phase adds some overhead.

From my experience with using TDD, using TDD requires a higher level of competence from the developer. Developers need to have the following qualities:

  1. Developers need to have a good understanding of the requirements and the ability to divide the requirements.
  2. Developers need to have good code design skills and clear design ideas before writing code, TDD requires testing before coding, which encourages developers to think before they code, so that they can design a clearer and more maintainable code structure.
  3. Developers need to have good refactoring skills. The TDD process involves refactoring code to keep it clean and maintainable.
  4. Developers need to be able to write effective unit tests. This includes not only which points of functionality need to be tested, but also how to test them and how to write readable tests, etc. Tests should not be either too many or too few.
  5. Developers also need to have good patience. Using TDD can make the development process more overwhelming. TDD is an iterative process that involves writing tests, coding to pass them, and then refactoring the code. This cycle may need to be repeated several times, especially when dealing with complex functionality.
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