As web apps grow in complexity with new features, maintaining the codebase can be challenging, but writing tests can help ensure the app functions correctly and new features don't break it. This text provides a comprehensive guide on testing Next.js apps using Vitest and React Testing Library for unit tests and Playwright for end-to-end tests, covering various testing types, including unit, integration, performance, accessibility, and end-to-end testing. It explains how to set up a Next.js project with Vitest and React Testing Library, configure test environments, and structure test files. It also details how to write and run e2e tests with Playwright, including setting up browser environments, handling API requests, and using locators and assertions to verify test outcomes. The guide emphasizes the importance of writing clean, isolated, and reliable tests to enhance software quality and outlines best practices for effective testing.