Vitest

If you're working with Vite-based projects or just tired of slow Jest startup times, this is the testing framework you want. Vitest gives you instant HMR-based test execution, native TypeScript support without configuration gymnastics, and a Jest-compatible API that makes migration straightforward. The skill covers the full testing stack: mocking with vi.mock and vi.spyOn, timer manipulation, type testing with expectTypeOf, and React Testing Library integration with proper jsdom setup. Coverage is handled by v8 instead of Istanbul, which is noticeably faster. The watch mode and optional UI are excellent for development workflow, and the examples show real patterns like beforeEach cleanup and mock implementation swapping that you'll actually use.