As we have discussed in the Types of Software Testing guide, unit testing is performed by isolating a small section of code (e.g. a function or a method) and validating it for correctness. In this manner, individual units or components of the software are tested for their functionality. It is the first level of software testing done before the code is integrated, and is usually performed by developers by writing code to test a specific function of the application in isolation.
It can be performed either manually or through an automated tool, though most developers prefer to use coded automation. While it may appear to be time-consuming and resource-intensive, unit testing ensures that developers can catch and identify bugs early on in the development process, refine their code, and ensure that all modules work as intended, thereby improving overall product quality and reducing time spent on later, more costly fixes.
Unit Testing is required to ensure code correctness and fix bugs earlier on in the Software Development Lifecycle (SDLC). By isolating the code into different sections, each section’s function can be tested, allowing developers to make the required changes faster, while also helping with code reusability.
There are two types of unit testing:
Manual unit tests are usually performed before automated unit testing. While it may be time-consuming and involves plenty of effort, manual unit testing helps uncover hidden bugs that cause the application to behave in unexpected ways. Here, the tester runs all test cases manually to ensure that the application is working as intended. Once this is done, the programmer fixes all bugs and returns the application to the tester for further checks.
Here, the tester assumes the role of the end user, guaranteeing that there are no defects in the application. However, these tests are hard to reuse, as each new unit requires its own unique set of test cases. Also to be noted is the fact that each tester would approach manual unit testing slightly differently, therefore test coverage is not an absolute in this case.
Automated unit testing involves leveraging automation tools that use pre-existing software to script tests automatically, therefore providing greater test coverage and maintenance. This type of unit testing is more widely accepted as it is more robust and less prone to errors.
Here, the developer would first create test cases, review the test code, arrive at a baseline, and then execute the test cases. For example, they would write a piece of code in the application in order to test a particular function. They would then remove the test code before the application is deployed. Alternatively, they would isolate the code by copying and pasting it to its own test environment, in order to identify any unwanted dependencies between the code and other units within the application. One point to note is that automated unit testing would require professional-level developers.
In this manner, repetitive tests can be automated, freeing up the time of QA engineers for more complex test cases. This approach provides greater consistency, with earlier bug detection and tests that can run 24/7 with lesser human intervention. It is more cost, resource, and time-effective than manual unit testing. It aids in test reusability across different versions of the application and provides enhanced test case coverage for each of the application’s features. What’s more, programmers can refactor the code at a later stage to ensure that the application still works as intended (regression testing).
As unit testing is performed early on in the SDLC, it has the potential to uncover high-cost defects much earlier than other tests which may be carried out in later stages of development. It has both time and cost-saving benefits that are irrefutable.
Unit testing allows developers to get familiar with the code base, enabling them to gather project knowledge faster while making required changes sooner. These unit tests will also serve as crucial documentation, enabling smoother implementation of changes in later stages of the project.
As unit testing involves writing code that is modular, it becomes easier to reuse these modules as required. Both the code and unit tests can be migrated to the new project, allowing testers to reuse the code and tweak tests till they are ready to run once more.
Debugging is easier at the unit testing level, as only the latest changes need to be debugged. Defects detected at later stages of testing would involve evaluating updates made over the past weeks and months.
Unit testing does not guarantee the discovery of all bugs in the application code. As it involves isolating the code into multiple sections, it does not cover the functionality of these sections once integrated (integration testing).