Jest

Testing is a critical part of software development that ensures your code works as expected and continues to work as you make changes. Jest is a popular, zero-configuration testing framework developed by Facebook. It's widely used for testing JavaScript applications, especially those built with React. The core idea of testing is to write small, isolated tests for individual units of your code, such as functions. This is called unit testing. In Jest, you write test files (often named `*.test.js`). Inside these files, you use functions like `describe` to group related tests and `test` (or `it`) to define an individual test case. Within a test case, you typically call the function you want to test and then make assertions about the result using Jest's `expect` function combined with 'matcher' functions. For example, you might write `expect(add(2, 2)).toBe(4);`. This assertion checks if the result of `add(2, 2)` is strictly equal to `4`. Jest comes with a rich set of matchers for all sorts of conditions. It also includes features like 'mocking' to isolate your code from external dependencies (like APIs or other modules) and tools to measure code coverage, which shows what percentage of your code is covered by tests. Writing tests gives you confidence in your code and makes refactoring safer.