Testing is crucial for developing high-quality software that can withstand change. Testing approaches include unit, integration, and end-to-end testing. Unit tests focus on individual units of code, such as functions or React components, ensuring they work correctly without relying on expensive operations like network requests. Integration tests verify how cohesive components work together to realize a feature, covering a large surface area while being relatively fast and efficient. End-to-end tests act like real users, engaging in operations that cut across the entire stack, providing confidence in critical features but running slower due to rendering, network requests, and infrastructure considerations. A testing strategy depends on time, team skill level, practice of Test-Driven Development (TDD), and application testability. Effective testing balances speed, surface area coverage, and confidence, with unit tests complementing integration and end-to-end tests for a comprehensive approach.