What to test, what to skip

Test the things that would hurt if they broke silently. Formatting, validation, any business rule, the login and checkout flows. Skip things the framework already tests for you: do not test that useState updates a value, do not test that a form submits when the user clicks the button.

Three common mistakes that waste test time. Snapshotting the entire DOM of a component and calling it a test. Testing that a component renders (the render call would have thrown otherwise). Mocking everything to the point where the test becomes a test of the mocks instead of the code. Notice the pattern: tests that do not assert on behavior are rarely worth the maintenance.