Running tests for a module without requiring a full site installation

Question

I'm building a custom module (for use on multiple projects) with code that relies on stuff from Drupal core and third party modules (e.g. extending classes, implementing interfaces, annotating as plugins, etc.). I would like to start writing tests for them, with unit tests to begin with. But so far, all of the resources point to putting the module in a Drupal installation and test in that setup, as if I'm running a full Drupal site - even for a unit test.

I was more imagining that my module would have the runner, the dependencies, and Drupal core be dependencies (or dev dependencies) - just like how JavaScript tests are usually written, whether it's unit tests or headless browser tests. That is, JavaScript testing doesn't require you to pop your code in some other code before you can start testing.

Can this be achieved? Or is putting the my modules code in a Drupal install really the only way to test Drupal modules?

Answer 1

Unit tests do not bootstrap the whole system, and run very fast. Kernel tests, functional tests, and JavaScript tests require bootstrapping the system, and take longer.

With unit testing, you don't declare dependencies upon foreign code. This is because tests unit test a piece of code, not the foreign code. If an error was introduced into the foreign code, and the code being tested depended upon that code, the tests would fail, even though the code being tested was still correct. This would be a false fail, and a developer would probably spend time trying to figure out why their code had failed, even though it had not.

The solution is to use mocks to stub the data. A mock is a fake service, created within the test, that simulates the actual dependent service. So when the code being tested makes a call to foreign code, the mock returns a response (aka a stub) as if the foreign code had responded, taking away any risk of false positives from the foreign code failing, since your test will always respond, and will always respond exactly as you set it to.