The testing pyramid guides test distribution: many fast unit tests at the base, fewer integration tests in the middle, and a small number of slow E2E tests at the top. Each layer catches different types of bugs.
Unit tests verify individual functions and components in isolation. Test pure functions, utility helpers, and component rendering with various props. Use Jest or Vitest with React Testing Library.
React Testing Library philosophy: test components the way users interact with them. Query by role, label, and text content, not by CSS classes or component internals. If your test resembles your implementation, it is testing the wrong thing.
Integration tests verify that multiple components work together correctly. Test form submissions that trigger API calls, navigation flows, and state management across component boundaries.
End-to-end tests (Playwright, Cypress) simulate real user interactions in a browser. Test critical paths: user registration, login, purchase flow, and key feature workflows. E2E tests are slow but catch integration issues unit tests miss.
Mock strategies: mock external APIs and services, not internal implementation. Use MSW (Mock Service Worker) to intercept network requests, providing realistic test environments without external dependencies.
Test data management: create factory functions that generate test data with sensible defaults. Allow overrides for specific test scenarios. Consistent test data makes tests readable and maintainable.
Snapshot testing captures component output and alerts on changes. Use sparingly for stable components. Snapshot tests break easily and often lead to developers blindly updating snapshots rather than reviewing changes.
CI/CD integration: run unit and integration tests on every pull request. Run E2E tests before deployment to staging and production. Fast feedback (under 10 minutes for unit/integration) prevents blocking development.
Test maintenance: delete tests that break frequently without catching real bugs. Update tests when requirements change. Treat test code with the same quality standards as production code.
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