jest

Jest is an open-source JavaScript testing framework maintained by Meta (formerly Facebook) and a wide community of contributors. While designed with React in mind, it is framework-agnostic and can be used to test any JavaScript project, including Node.js backends and libraries. Jest aims to provide a complete, zero-configuration testing experience, with a built-in test runner, assertion library, and mocking utilities included out of the box.

A core strength of Jest is its fast, parallel test execution across worker processes, which helps deliver quick feedback on large codebases. It includes module mocking, with automatic and manual options, and a robust snapshot testing feature that stores serialized UI output in .snap files for regression checks. Additional capabilities include code coverage reporting and an intelligent watch mode that focuses re-runs on relevant tests as files change.

Jest runs tests in a configurable environment, commonly jsdom for browser-like tests or Node for server-side code. It supports transforming modern JavaScript and TypeScript syntax through Babel or TypeScript preprocessors, and can be configured via a jest.config.js file or the jest field in package.json. The framework integrates well with popular tooling, including Create React App, and works with various continuous integration systems.

Historically, Jest was released by Facebook in 2016 to address testing needs for React applications and has since evolved into a widely adopted, actively maintained testing solution with a large ecosystem of plugins and presets.

See also: unit testing, snapshot testing, test coverage, jsdom, Babel.