Jest

Jest is a delightful JavaScript Testing Framework with a focus on simplicity. It was created by Facebook and is the most popular choice for testing JavaScript applications, especially those built with React. One of Jest's main selling points is that it's a 'zero-configuration' framework. It works out of the box for most JavaScript projects, providing you with a test runner, an assertion library, and mocking capabilities all in one package. The core of writing a Jest test involves using global functions like `describe`, `test` (or its alias `it`), and `expect`. You use `describe` to group related tests together into a test suite. The `test` function defines an individual test case. Inside a test, you use the `expect` function along with 'matcher' functions to make assertions about your code. For example, `expect(sum(1, 2)).toBe(3);` asserts that the result of `sum(1, 2)` is equal to 3. Jest has powerful mocking capabilities, which are essential for unit testing. You can easily create mock functions (`jest.fn()`) to track calls and return specific values, and you can mock entire modules to isolate the code you're testing from its dependencies. Jest also includes features like snapshot testing, which is useful for testing UI components by saving a 'snapshot' of the rendered output and comparing it on subsequent test runs to detect any unexpected changes. Its speed, powerful features, and great developer experience make it an excellent choice for any JavaScript project.