"100% Code Coverage" is a vanity metric that doesn't guarantee your app works. I see teams writing unit tests for simple button components that verify "if true is true." This item promotes Kent C. Dodds' "Testing Trophy": Static Analysis: TypeScript and ESLint catch typos and basic errors (Fast & Cheap). Integration Tests: The bulk of your tests. Render the page, click the button, check if the database updated. Use React Testing Library or Vitest. E2E Tests: A few critical flows (Login, Checkout) using Playwright or Cypress. Don't test implementation details (e.g., "function name is X"). Test user behavior (e.g., "user sees success message").