Understanding test-driven development

Before we go further, let's take a short detour to define some terms and understand the differences between them. It is very easy to get confused between these terms, and they are often used with different meanings in different places.

In the broadest sense of the term, unit testing simply means testing a single unit of code, isolated from other code that it might be integrated with. Traditionally, unit testing was an activity that was primarily performed by test engineers. These engineers would take code given by the developers and run them through a suite of tests to verify that the code worked. Since this code was tested before integration, the process fits into the definition of a unit test. Traditional unit testing was typically a manual affair, with test engineers walking through the tests cases by hand, although some teams would go a step further and automate the tests.

An integration test is a test that involves exercising more than one unit of the system. The goal is to check whether these units have been integrated correctly. A typical integration test might be to go to a web page, fill in a form, and check whether the right message is displayed on the screen. In order for this test to pass, the UI must show the form correctly, the input must be captured correctly, and that input must be passed on to any logic processing. The steps might involve reading and writing from a database before a message is generated and the UI has to display it correctly. Only if all these interactions succeed will the integration test pass. If any one step should fail, the integration test will fail.

At this point, a valid question would be to ask why we need unit testing at all. Why not write only integration tests, where a single test could check so many parts of the application at once? The reason is that integration tests do not pinpoint the location of failure. A failing integration test could have an error in the UI, or in the logic, or somewhere in the way data is read or written. It will take a lot of investigation to see where the error is and fix it. By contrast, with well-written unit tests, a failing unit test will pinpoint exactly what is failing. Developers can go right to the point and fix the error.

Along the way, teams started moving to a process where developers themselves wrote tests for the code that they had implemented. These tests would be written after the developer had finished the implementation, and helped verify that the code worked as expected. These tests were usually automated. Such a process is generally called developer testing or developer unit testing.

TDD takes developer tests one step further, by writing the test before starting the implementation.

As we can see, unit testing is a general term, whereas developer testing is a specific subset of unit testing, and TDD is a specific form of developer testing.

On the surface, traditional unit testing, developer testing and TDD look similar. They all appear to be about writing tests for a single unit of code, with only minor variations based on who writes the test and whether the tests are written before the code or after.

However, dig deeper and differences appear. First, the intent is vastly different. Traditional unit testing and developer testing are all about writing tests to verify that the code works as it is supposed to. On the other hand, the main focus of TDD is not really about testing. The simple act of writing a test before the implementation changes the way we think when we implement the corresponding functionality. The resulting code is more testable, usually has a simple and elegant design, and is more maintainable and readable. This is because making a class easy to test also encourages good design practices, such as decoupling dependencies and writing small, modular classes.

Thus, one can say that TDD is all about writing better code, and it is just a happy side effect that we end up with a fully automated test suite as an outcome.

This difference in intent manifests itself in the type of tests. Developer testing usually results in large test cases, with a hefty part of the test code involved in test setup. By contrast, tests written using TDD are very small and numerous. Some people like to call them micro tests to differentiate them from other developer tests or traditional unit tests. TDD-style unit tests also try to be very fast to run because they are executed every few minutes during the development process.

Finally, the tests that are written in TDD are those that drive the development forward, and not necessarily those that cover all imaginable scenarios. For example, a function that is supposed to process a file might have tests to handle cases when the file exists or it doesn't exist, but probably won't have tests to see what happens if the file is 1 terabyte in size. The latter is something that a tester might conceivably test for, but would be an unusual test in TDD unless the function is clearly expected to work with such a file.

This really highlights the difference between TDD and other forms of unit testing.