Chai Assertion Library

Chai is a BDD and TDD assertion library for Node.js and browsers that provides expressive, readable syntax for writing test expectations. The library offers three interface styles that developers can choose based on preference: the should style extends objects with assertion methods through prototype modification, the expect style uses chainable property accessors that read like natural language sentences, and the assert style offers traditional function-based assertions similar to Node.js built-in assert module. The chainable language in expect and should styles includes connector words like to, be, been, is, that, which, and have that are purely syntactic and do not affect the assertion logic, allowing developers to write statements such as expect(value).to.be.a string that reads naturally. Built-in assertions cover types, equality with deep comparison, truthiness, property existence, length, containment, string matching, error throwing, response status checking, and numeric comparison with delta tolerance. The plugin architecture is central to the library design, enabling third-party packages to add domain-specific assertions. Over three hundred plugins are published on npm, covering areas like HTTP response validation with chai-http, DOM testing with chai-dom, sinon integration with sinon-chai for spy and stub assertions, and array subset comparison with chai-subset. Plugin development is supported through a utility belt API that provides methods for adding properties, methods, chainable methods, and overriding assertion behavior with custom error messages. The library works with any JavaScript test runner, though it is most commonly paired with Mocha, and is used across the Node.js ecosystem in unit tests, integration tests, and end-to-end test suites. The current version ships as an ES module with TypeScript type definitions included.