Write tests. Not too many. Mostly integration.
In a fantastic post on his blog, Kent C. Dodds makes the case for focusing on writing integration tests, and stop going for 100% code coverage. Amen to that!
You may also find yourself testing implementation details just so you can make sure you get that one line of code that’s hard to reproduce in a test environment. You really want to avoid testing implementation details because it doesn’t give you very much confidence that your application is working and it slows you down when refactoring. You should very rarely have to change tests when you refactor code.
https://blog.kentcdodds.com/write-tests-not-too-many-mostly-integration-5e8c7fff591c