Do you want to deliver software faster and of better quality? Sure, everyone wants to. We know a methodology that will help you achieve that goal. The method is test-driven development (TDD), focusing on testing the first approach.
In this article, you’ll learn what test-driven development is, its process, and also some pros and cons to give you a full understanding of TDD.
Test-driven development (TDD) meaning
Test-driven development concentrates on building unit test cases as a first step of coding new feature functions. In this iterative approach, you mix the traditional order of unit tests, refactoring, and programming. So how does it work?
Developers make small tests for every feature they build. For each feature, developers gather requirements, and these requirements are used first in testing, not in coding (like in many usual methodologies). The main goal is to write new code only when the test fails. Firstly, the developer makes the simplest implementation that makes the test stop failing. Then the development team refactors the code to make a clean and effective solution, and they can proceed with further development. This helps to maintain consistency and good coding practices because you don’t duplicate code.
What is a unit test?
Unit tests are crucial in understanding the TDD approach, so let’s begin with the concept of a single unit. A unit is the smallest testable part, and it can be, for example, a function, class, subroutine, method, or property. Usually, the smaller the part you test, the better because it gives you well-measured results. Also, small units can be run faster when testing.
So what is unit testing, then? Unit tests check the above-mentioned units of the code independently and individually to ensure that they work as expected. Unit tests take the already existing data, run tests on the specific portion of code, and ensure that the result is as it should be. The graph below shows the process:
Why is unit testing used in the TDD approach? Unit tests are used because they check small parts of the code, which is one of the main assumptions of TDD: it is better to test small features. It allows for early bug detection, helps save money in the future, and avoids code duplication.
Test-driven development process
The TDD process, sometimes called test-first development, consists of specific steps. Let’s see how to approach TDD.
- Make precise tests - To check how features of your code will perform, developers, create precise unit tests based on specific requirements for a particular feature. In many cases, the test should fail in a proper way as the code for the feature is not ready yet. It’s a meaningful failure because tests are prepared based on assumptions. So, it helps verify functionality.
- Make changes in the code - When a test fails, the development team has to correct the code to run it successfully.
- Refactor the code - In this step, when the test runs successfully, you can make some improvements to the code to improve the performance, but make sure that the refactoring doesn’t affect the software externally.
The graph below shows the TDD process in more detail:
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Unit tests in TDD
Unit tests are a core part of test-driven development. So it’s important to remember that without traditional testing, TDD isn’t possible to perform as it’s integrated into the process. For example, unit testing is performed again and again until the code functions accordingly.
Unit testing is more about testing the functions, classes, and procedures, but you can also write functional and acceptance tests. Other important tests are automated tests that might help you accelerate the overall process and improve continuous integration (CI).
Agile model-driven development
Writing tests first is considered to be a practice first used by the C3 team in the 90s. They were famous for practicing test-first programming called “eXtreme programming”, which they explained in the case study Chrysler Goes to “Extremes” 
.
Three members of the C3 team wrote and signed the Agile Manifesto. The Agile Manifesto especially highlights the importance of one value: “Working software over comprehensive documentation”. In test-driven development, this happens naturally. Writing tests first, you protect the software, and documentation is a natural product of this approach. That’s why test-driven development fits into the Agile software development methodology. It meets the condition of one of the core Agile values, which emphasizes simplicity and working software rather than complicated processes.
Benefits of test-driven development (TDD)
The undoubted advantage of TDD is the ability to detect and eliminate errors early. Also, already having a set of tests that have gone through the TDD process increases the system's resistance to regression. If, in the early stage, the running code causes another test to fail, this is obvious evidence of regression in the code.
Another benefit is that, according to TDD, every line of code you have created is tested, giving code coverage of 100%. Usually, to create clean code, you need special tools because TDD is based on the principle that no line of code is written without testing, giving you 100% coverage by default (if you implement this approach correctly). Moreover, for many business owners, 100% code coverage can be an important factor. It can sometimes be an attractive factor for potential customers.
Using TDD also increases programmers ’ awareness of what they are creating and how the system works. Starting programming with tests gives a broader perspective not only on the business value but also on the architecture of the developed system.
Disadvantages of test-driven development
Undoubtedly, for beginners, writing a test for non-existent code can be a challenge (especially at the unit test level). Nevertheless, after a few tests have been written with the TDD approach, this should not be much of a problem.
Also, many developers find it difficult to constantly remember not to add more code than needed, and TDD helps avoid this situation
Conclusion
Test-driven development is a methodology for developing production code, not writing tests. This means that after TDD is over, there might still be features of the code that you will want to verify.
It's also important to remember that we use this technique most often when adding new functionality, which helps determine its final form and easier implementation. However, TDD should not be considered the only right software product development technique. It has its advantages and disadvantages, and you need to choose the tools and techniques to suit the individual needs of the project.
