1. Unit Testing

• Tests individual functions, components, or modules in isolation.
• Focuses on verifying the smallest pieces of code (e.g., a function returns the expected result).
• Usually fast to run since no external dependencies are involved.
• Tools: Vitest, Jest, Mocha.
• Helps catch bugs early during development.

2. Integration Testing

• Tests how multiple units or modules work together.
• Ensures correct communication between components (e.g., frontend calls backend API properly).
• More realistic than unit tests, but still less complex than full system tests.
• Tools: React Testing Library, Supertest, Playwright (for APIs).
• Harder than unit testing since we may need to mock server data.

3. End-to-End (E2E) Testing

• Simulates real user workflows across the entire application.
• Tests from frontend → backend → database (full stack).
• Ensures that critical paths (like login, checkout, form submission) work as expected.
• Usually slower and more expensive to run than unit/integration tests.
• Tools: Cypress, Playwright, Selenium.