Vitest and happy-dom are not direct competitors but complementary tools in the JavaScript testing ecosystem. Vitest is a modern, Vite-powered testing framework that provides Jest-compatible APIs for writing and running tests. Happy-dom is a lightweight DOM implementation that simulates browser APIs during test execution, acting as an environment option within test runners like Vitest or Jest.
This comparison matters because developers often conflate these tools when setting up testing infrastructure. Vitest targets engineers seeking a fast, ESM-native alternative to Jest with built-in TypeScript support and Vite integration. Happy-dom serves teams prioritizing test execution speed and low memory overhead for DOM manipulation tests, trading full browser API coverage for performance gains over traditional jsdom environments.
Choose Vitest as your testing framework when building modern JavaScript/TypeScript applications, especially if using Vite for bundling. Its ESM-native architecture, zero-config TypeScript support, and Jest-compatible APIs make it the superior choice over Jest for new projects. The question isn't whether to use Vitest—it's which DOM environment to pair with it.
Configure Vitest with happy-dom (environment: 'happy-dom') when testing component logic with simple DOM manipulation where speed is critical and you don't rely on advanced Web APIs. Switch to jsdom if you encounter missing browser APIs or need higher standards compliance for complex interactions. For actual browser testing with real APIs, use Vitest's browser mode instead of any simulated environment. Most teams should start with happy-dom and only migrate specific test files to jsdom when hitting API limitations.