Mocha

Mocha is a feature-rich JavaScript test framework with a BDD/TDD interface, providing flexibility to choose your own assertion library (Chai), mocking framework (Sinon), and coverage tool (Istanbul/nyc). This modular approach appeals to developers who want control over their testing stack. While Jest has overtaken Mocha in popularity, Mocha remains widely used in enterprise Node.js projects and legacy codebases. Its flexibility and mature ecosystem make it reliable for complex testing scenarios.