General Principles

Write tests no matter which library or framework is being used!
Strive to write many small pure functions, and minimize where mutations occur.
Be cautious about stubs and mocks - they can make tests more brittle.
100% test coverage is a good goal to strive for, even if it's not always practical to reach it.
Whenever a bug get fixed, write a regression test. A bug fixed without a regression test is almost certainly going to break again in the future.

Testing