Benefits of unit testing

Why unit test?

Functional tests are expensive

Functional tests are expensive. They typically involve opening up the application and performing a series of steps that you (or someone else) must follow in order to validate the expected behavior. These steps might not always be known to the tester. They'll have to reach out to someone more knowledgeable in the area in order to carry out the test.

Testing itself could take seconds for trivial changes, or minutes for larger changes. Lastly, this process must be repeated for every change that you make in the system.

Unit tests, on the other hand, take milliseconds, can be run at the press of a button, and don't necessarily require any knowledge of the system at large. Whether or not the test passes or fails is up to the test runner, not the individual.

Protection against regression

Regression defects are defects that are introduced when a change is made to the application. It's common for testers to not only test their new feature but also test features that existed beforehand in order to verify that previously implemented features still function as expected.

With unit testing, it's possible to rerun your entire suite of tests after every build or even after you change a line of code. Giving you confidence that your new code doesn't break existing functionality.

Executable documentation

It might not always be obvious what a particular method does or how it behaves given a certain input. You might ask yourself: How does this method behave if I pass it a blank string? Null?

When you have a suite of well-named unit tests, each test should be able to clearly explain the expected output for a given input. In addition, it should be able to verify that it actually works.

Less coupled code

When code is tightly coupled, it can be difficult to unit test. Without creating unit tests for the code that you're writing, coupling might be less apparent.

Writing tests for your code will naturally decouple your code, because it would be more difficult to test otherwise.