Jest

It's the default testing framework for React, and most JavaScript projects use it whether they chose it or not: Create React App, Next.js, and dozens of starter templates include it.

Jest bundles everything: test runner, assertion library, mocking, code coverage, and snapshot testing. You don't need to wire together Mocha + Chai + Sinon + Istanbul; Jest includes all of it. Write a file ending in `.test.js`, run `jest`, and it finds and runs your tests automatically.

Snapshot testing is Jest's signature feature. Render a component, Jest saves the output. Next run, it compares. If something changed, the test fails and shows you the diff. Great for catching unintended UI changes.

The catch: Jest is showing its age. It's slow compared to Vitest, which uses Vite's transform pipeline and runs significantly faster on modern projects. Jest's config can be painful, especially with ESM modules, TypeScript transforms, and custom module resolution. The project's development pace has slowed since Meta reduced its open source investment. For new projects in 2026, Vitest is the better choice. For existing projects with thousands of Jest tests, the migration cost probably isn't worth it yet.