Test-driven development (TDD) is an extreme programming practice. In TDD, we start with a test case and incrementally write the production code that is required to make the test case succeed. The idea is that one should focus on one test case or scenario at a time and once the test case passes, they can then move on to the next scenario. In this process, if the new test case passes, we shouldn't modify the production code. In other words, in the process of developing a new feature or while fixing a bug, we can modify the production code only for two reasons: either to ensure the test case passes or to refactor the code. The primary focus of TDD is unit testing; however, it can be extended to integration and interaction testing to some extent.
The following figure demonstrates the TDD process visually:
When TDD is followed religiously, one can achieve both functional and structural quality of the code. It is very crucial that you write the test case first before writing the production code...