Jest is an open-source testing framework for JavaScript, a tool that finds the test files of a project, runs them and reports which tests pass and which fail. It works with projects that use Babel, TypeScript, Node, React, Angular and Vue. The site is copyrighted by the OpenJS Foundation and the Jest contributors.
A developer installs Jest with npm or yarn, writes test files such as sum.test.js with calls like expect(sum(1, 2)).toBe(3), and runs the jest command. Jest aims to work without a configuration file on most projects. It runs each test file in its own process, so tests run in parallel, and it runs previously failed tests first. Snapshot tests store the output of a function or component and fail when it changes. Mocking (replacing a module or function with a fake that records its calls) works through a custom resolver for imports. The --coverage flag adds a code coverage report, which shows the lines that no test runs.
Jest is free and open source under the MIT license.