Unit testing applications is not something nice to have. It is actually a mandatory activity for having a successful software solutions that can cope with different changes across time with high stability. There is no excuse to skip unit testing of applications even for projects with a tight schedule. The importance of unit testing may not appear in the early stages of the project; however, its advantages are visible in the middle and the final stages of the project, when the code gets complicated, more features are required, and more regression defects appear (defects that appear again after a major code change).
Without unit testing, the integration of the different components in the system becomes complicated. This complexity results from the tracing of the defects of not only the integration between the components but also each "buggy" component. This complicates the life of the developers by making them spend nights in the office in order to meet the schedule.
The number of new defects and the regression defects becomes unmanageable when the code base becomes complicated and unit testing is not available. The developer can resolve a specific defect and, after a set of code changes, this defect can happen again because there is no repeatable test case to ensure that the defect will not happen again.
Having more number of defects per lines of code affects the application's quality badly, and this means that more time has to be spent on testing the application. Bad quality applications have a longer test cycle for each project deployment (or phase), because they have a high probability of having more defects for every code change, which leads to more pressure on the project management, the project developers, and the project testers.
Having good unit testing can be a good reference for the system documentation because it contains the test scenarios of the system use cases. In addition to this, unit testing shows how the system APIs are used, which reflect the current design of the system. This means that unit testing is a powerful basis of code and design refactoring for having more enhancements in the system.
Having good unit testing minimizes the number of regression defects because in good unit testing the system has a repeatable number of test cases for every relevant defect. Having a continuous integration job that runs periodically on the application unit tests will ensure that these defects will not happen again, because if a specific defect appears again due to a change in the application code, then the developer will be notified to fix the defect and ensure that the test case of this defect passes successfully.
Tip
Continuous integration (CI) is a practice that ensures automating the build and the test process of the application. In continuous integration testing, the tests of the application source code run periodically (for example many times per day) in order to identify the application's potential problems and to reduce the integration time of the application components.
As a result of reducing the regression defects, having good unit testing reduces the test cycle for each phase (or system deployment). In addition to this, the application can have more and more features per iterations or phases peacefully without worrying if these features shall break an existing module that has good unit tests.