Fear Driven Development
If you are afraid to deploy new code on a Thursday afternoon because it might break something, your codebase lacks automated tests. Manual testing (clicking around the app yourself) scales terribly.
Unit Tests (Jest)
Unit tests check individual functions in isolation. If you have a function that calculates shopping cart taxes, a Unit Test runs 50 different scenarios through that function automatically to ensure the math is always correct.
End-to-End Tests (Cypress / Playwright)
E2E tests spin up a real browser and act like a real user. The bot automatically navigates to your site, types a password, clicks 'Login', and verifies that the dashboard loads. If the dashboard fails to load, the deployment is blocked.
Don't aim for 100% test coverage; it has diminishing returns. Aim for 70%, focusing heavily on your core revenue-generating flows (like checkout and login).