Jest : Javascript Unit Testing Framework

Jest is delightful JavaScript Testing framework. It has below features:

• Easy Setup: Complete and easy to set-up JavaScript testing solution. Works out of the box for any React project.
• Instant Feedback: Fast interactive watch mode runs only test files related to changed files and is optimized to give signal quickly.
• Snapshot Testing: Capture snapshots of React trees or other serializable values to simplify testing and to analyze how state changes over time.
• Zero configuration testing platform: Jest is used by Facebook to test all JavaScript code including React applications. Jest is already configured when you use create-react-app or react-native init to create your React and React Native projects. Place your tests in a __tests__ folder, or name your test files with a .spec.js or .test.js extension. Whatever you prefer, Jest will find and run your tests.
• Fast and sandboxed: Jest parallelizes test runs across workers to maximize performance. Console messages are buffered and printed together with test results. Sandboxed test files and automatic global state resets for every test so no two tests conflict with each other.
• Built-in code coverage reports: Easily create code coverage reports using --coverage. No additional setup or libraries needed! Jest can collect code coverage information from entire projects, including untested files.
• Powerful mocking library: Jest has powerful mocking library for functions and modules. Mock React Native components using jest-react-native.
• Works with TypeScript: Jest works with any compile-to-JavaScript language and integrates seamlessly with Babel and with TypeScript through ts-jest.