Jest

Summary

A widely used JavaScript testing framework with a built-in test runner, assertion library and mocking support — long the default choice for testing JS/React apps.

• 1Write a few simple unit tests for a pure utility function.
• 2Practice mocking a module or function with `jest.fn()`/`jest.mock()`.
• 3Learn `describe`/`it`/`expect` — the core structure of every Jest test.