Understanding the difference from other kinds of tests
Unit tests are just one piece of a good test suite. In my opinion, they are the most important tests because when carried out correctly, they are fast, focused, and easy to understand. But to increase your confidence in your code, you should additionally add integration, UI/snapshot, and manual tests. What are those?
Integration tests
In integration tests, the feature that is being tested is not isolated from the rest of the code. With these kinds of tests, the developer tries to figure out if the different units (that are thoroughly tested with unit tests) interact with each other as required. As a result, integration tests execute real database queries and fetch data from live servers, which makes them significantly slower than unit tests. They are not run as often as unit tests and failures are more difficult to understand as the error has to be tracked down in all involved code units.
UI tests
As the name suggests, UI tests run on the UI of an app. A computer program (the test runner) executes the app as a user would do. Usually, this means that in such a test assertion, we also have to use information accessible on screen. This means a UI test can only test whether a feature works as required when the result is visible on screen. In addition, these tests are usually quite slow as the test runner often has to wait until animations and screen updates are finished.
Snapshot tests
Snapshot tests compare the UI with previously taken snapshots. If a defined percentage of pixels differs from the snapshot image, the test fails. This makes them a perfect fit for situations where the UI of one app screen is already finished and you want to make sure that it won't change for given test data.
Manual tests
The final kind of test in the development of an app is a manual test. Even if you have hundreds of unit and integration tests, real people using your app will most probably find a bug. To minimize the number of bugs your users can find, you need testers in your team or to ask some users for feedback on the beta version of your app. The more diverse the group of beta testers is, the more bugs they will find before your app ships.
In this book, we will only cover unit tests because test-driven development (TDD) only works reasonably well with fast reliable feedback.
