Integration tests

Where a unit test isolates one function, an integration test lets real parts talk to each other. It might start the API, send a request to /orders, and check both the response and the row written to a test database. On the frontend, rendering a whole component with Testing Library and clicking through it is a common form.

They catch the bugs that live where parts meet: a wrong column name, a missing header, a validation rule that disagrees with the server. They are slower and need setup (a throwaway database, seeded data, often run in Docker), so most suites have fewer of them than unit tests, focused on the paths that matter most.