What to test, what to skip

New engineers often chase 100 percent code coverage. Senior engineers test the things that break and skip the things that cannot. Coverage numbers are a weak proxy for quality. Ship fewer, higher-value tests and you get more protection for less effort.

Test the behaviors that would hurt if they broke. Auth flows. Authorization checks. Money calculations. Data mutations. The happy path of every public endpoint. Anything that has bitten you before. These are the tests that pay rent.

Skip tests for things that are trivially correct or already guaranteed by the framework. Do not test that Express returns JSON when you call res.json. Do not test Drizzle's query builder. Do not test TypeScript types. The framework's job is to be correct. Your job is to test what you built on top.