Mocha

Mocha is another highly popular and mature JavaScript test framework that runs on Node.js and in the browser. Unlike Jest, which is an all-in-one solution, Mocha is more modular and flexible. Mocha itself is just a test runner. It provides the functions for structuring your tests, like `describe()` and `it()`, but it does not include its own assertion library or mocking capabilities. This allows you to choose your own libraries for these tasks. A very common combination is to use Mocha with the Chai assertion library and the Sinon.js library for mocks, spies, and stubs. This flexibility is Mocha's greatest strength and also its main difference from Jest. It allows you to tailor your testing stack to your specific needs. Mocha is highly extensible and has a large ecosystem of plugins and extensions. It supports a wide range of features, including asynchronous testing (with callbacks, promises, and async/await), test hooks (`before`, `after`, `beforeEach`, `afterEach`) for setting up and tearing down test conditions, and various reporters for customizing the test output. While Jest's simplicity has made it more popular for many new projects, Mocha remains a powerful and widely used tool, especially in the Node.js backend ecosystem, where its flexibility and maturity are highly valued.