Jest

Jest is a JavaScript testing framework developed by Facebook. It is a complete and easy-to-set-up JavaScript testing solution that works with projects using Babel, TypeScript, Node.js, React, Angular, Vue.js, and more. Jest is used to test JavaScript code and it can run on any JavaScript environment, including a web browser or Node.js. 

Jest allows you to write tests with an approachable, familiar, and feature-rich API that gives you results quickly. Jest is well-documented, requires little configuration, and can be extended to match your requirements.

Jest provides a rich set of features for testing JavaScript code, including:

• A built-in test runner, which can automatically discover and run tests.
• Support for snapshot testing, which allows developers to capture the current state of a component or module and assert that it hasn’t changed unexpectedly.
• An assertion library, which makes it easy to write test assertions.
• A mocking library, which allows developers to mock functions and modules in their tests
• Support for parallel test execution, which can help to speed up test runs.

Jest is a popular choice for JavaScript testing, as it’s easy to set up and provides a lot of features that make it easy to write and run tests. With its built-in assertion library, mocking library, and support for snapshot testing, it’s a powerful tool that can help developers to improve the quality and reliability of their code.

Jest is particularly popular for testing React components. It is built in the create-react-app package and it is well integrated with other React testing tools such as Enzyme. The increasingly popular React Testing Library is built on Jest too. But having said that, Jest can also be used for testing any JavaScript code, whether it’s a Node.js application or a simple JavaScript utility library.