Chai is a BDD and TDD assertion library for Node.js and the browser that works with any JavaScript testing framework.
Chai solves the problem of writing verbose, hard-to-read test assertions by providing a rich set of chainable assertion methods. It offers three distinct assertion styles—assert, expect, and should—allowing developers to choose the syntax that best fits their testing philosophy and code style. This flexibility means teams can adopt the library regardless of their preferred testing approach, and individual developers can pick the style most natural to them.
Chai suits any JavaScript project that needs readable, expressive test code. It works equally well in Node.js and browser environments, making it suitable for testing both backend and frontend code. The library's plugin architecture allows extension with custom assertions, so teams building domain-specific test suites can add specialized matchers without forking the core library. Chai pairs seamlessly with any testing framework—Mocha, Jest, Jasmine, or others—rather than forcing a particular testing runner choice.
The project maintains active community engagement through multiple chat channels and accepts contributions from outside developers. Development activity shows consistent attention to the codebase with regular updates and maintenance of the core library and its ecosystem of related utilities for deep equality testing, error handling, and object inspection.
