Different tests answer different questions
Unit tests check a small function in isolation. Integration tests verify that multiple modules work together. End-to-end tests validate a real user journey through the running application.
Keep the base fast
Most business rules should be covered by quick unit tests. They are cheap to run locally and make failures easy to diagnose. Use integration tests for database queries, API contracts, and important boundaries.
Choose end-to-end tests carefully
Browser tests are valuable for login, checkout, or other critical flows, but they are slower and more sensitive to environment changes. A small set of stable journeys is better than testing every visual detail through a browser.
Test behavior, not implementation
Assertions should describe what a user or API consumer can observe. Tests that depend on private variables or exact component structure become expensive whenever the implementation improves.