Another thing to mention here: if something goes wrong in your application which (obviously) wasn't covered by any test then try to add a unit test for this particular case. If that isn't an option then you may write an integration test. If that isn't viable either than you are allowed to write and e2e test.
Why? Because unit tests are the cheapest tests you can run. Everything else is exepnsive in various ways.
You should keep in mind that if writing a unit test seems to be really hard your code may need some refactoring. This of course always sounds trivial but it's sometimes a hard thing to grasp at first. But it will pay off because you'll learn a lot. Just try!
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