vitest-dev/vitest

Vitest is a testing framework for JavaScript and TypeScript projects, built on top of Vite (a fast build tool for web development). A testing framework is a tool that lets developers write automated checks, called tests, that verify their code works as expected. You describe expected behavior in code, run the tests, and immediately see which parts pass and which fail. Because Vitest is built on Vite, it reuses the same configuration and setup you already have for your project. If you're already using Vite for your app, you don't need a separate testing configuration. Tests run fast because Vite handles the file transformation under the hood. The README's code example shows the basic structure: you use describe to group related tests, it to define individual test cases, and expect or assert to check that values are what you think they should be. The API is compatible with Jest (another popular testing tool), so teams familiar with Jest can switch with minimal changes. Key features include a watch mode that re-runs only affected tests when you save a file (similar to how Vite handles hot-reload for apps), built-in code coverage reporting (showing which lines of your code are actually exercised by tests), snapshot testing (capturing the shape of a value and checking it hasn't changed), mocking capabilities (replacing real dependencies with fake ones during tests), browser-mode testing for UI components, and TypeScript support out of the box. It is written in TypeScript and available via npm.