Write tests. Not too many. Mostly integration (2017)

The thing you should be thinking about when writing tests is how much confidence they bring you that your project is free of bugs. This is because most of my open source projects are smaller libraries and tools that are reusable in many different situations (a breakage could lead to a serious problem in a lot of consuming projects) and they’re relatively easy to get 100% code coverage on anyway. So while E2E tests may be slower and more expensive than unit tests, they bring you much more confidence that your application is working as intended.

Source: blog.kentcdodds.com