Jest

Jest is the most widely used JavaScript testing framework, built to run with zero configuration right out of the box. Created by Meta to test React, it moved under the governance of the OpenJS Foundation in 2022, which secures its future beyond any single company.

What is Jest used for?

Jest runs unit and integration tests on JavaScript and TypeScript code. It bundles everything you used to assemble by hand: a test runner, an assertion library, mocks for functions and modules, and coverage reporting. Tests run in parallel and in isolation from each other, which keeps the suite fast even as it grows.

Jest or a newer test runner?

Its signature feature is snapshot testing: Jest freezes a component's rendered output and flags any drift on the next run, which is handy for React work. Compared to newer runners like Vitest, Jest starts up more slowly but benefits from a huge ecosystem and native integration in VS Code.

What are Jest's limitations?

Configuration gets unwieldy fast on large monorepos, and poorly maintained snapshots turn into tests that verify nothing. On a Vite or pure ESM project, Vitest often integrates more cleanly. Jest still remains the safe default choice for most React and Node codebases.