Jest

Jest is the testing framework that made JavaScript testing mainstream. Snapshot testing, mocking, coverage — all built in, zero config. At 45k stars and 20M+ weekly downloads, it's the most widely deployed JavaScript test runner. Every React tutorial assumes Jest.

But Vitest is the future. 10-20x faster in watch mode, Jest-compatible API (most tests migrate without changes), native TypeScript support, and tight Vite integration. Bun's test runner is even faster but lacks jsdom and some Jest APIs. Both are better choices for new projects.

Use Jest if your codebase already uses it and migration isn't worth the effort. The ecosystem of matchers, reporters, and plugins is massive.

The catch: Jest is slow. Watch mode takes 3.4 seconds to re-run versus Vitest's 380ms. TypeScript requires additional configuration (ts-jest or SWC transform). The project's development has plateaued while Vitest iterates rapidly. For new projects in 2026, starting with Vitest is the clear call — you get Jest's API design with modern performance. Jest is for legacy, not new beginnings.