The Chai test library provides a robust set of assertions and helpers to aid the legibility and organization of tests.

Chai's own unit tests for expect (at https://github.com/chaijs/chai/blob/master/test/expect.js) provide a great starting point from which the API can be explored. Chai conveniently uses Mocha for its test framework (configured with the TDD interface). So the entire test suite should feel very familiar.

In this section, we will use a series of assertions to introduce most of the Chai BDD APIs. The assertion examples are accumulated into a single driver file for this chapter—chapters/03/test/test-chai.html.