Chai vs Mocha: What are the differences?

Chai and Mocha are both popular JavaScript testing frameworks. Here are the key differences between the two frameworks.

1. Assertion Styles: Chai provides multiple assertion styles, including should, expect, and assert, allowing developers to choose their preferred style. In contrast, Mocha does not have built-in assertion libraries and requires developers to use external libraries like Chai or the Node.js built-in assert module for assertions.

2. Test Runner vs Assertion Library: Mocha acts as a test runner and provides a testing framework with features like test suites, hooks, and reporters, while Chai is primarily an assertion library and focuses on providing expressive syntax for assertions. Chai can be used with any test runner, including Mocha, to enhance the assertion capabilities.

3. Synchronous vs Asynchronous Testing: Mocha supports both synchronous and asynchronous testing out of the box. It allows the usage of promises, callbacks, and async/await syntax for handling asynchronous code. Chai, on the other hand, is agnostic to asynchronous testing and can be used in both synchronous and asynchronous contexts.

4. Extendability and Customizability: Mocha offers a wide range of plugins and customizations, allowing developers to extend its functionality and customize the test runner behavior. Chai, being an assertion library, focuses more on providing flexible and customizable assertion syntax rather than extensive customization of the testing framework itself.

5. Chaining and Readability: Chai provides chaining methods like to, be, and have, which enable developers to create readable, expressive assertions. These chains can be nested to create complex assertions. Mocha, being a test runner, does not provide this level of chaining and readability, as its main focus is on managing the test suites and generating test reports.

6. Community and Documentation: Both Chai and Mocha have active communities, but Mocha has a larger user base due to its versatility as a test runner. Mocha also has comprehensive documentation, including examples and guides, making it relatively easier for developers to get started and find support.

In summary, Chai provides different assertion styles, while Mocha acts as a versatile test runner with support for synchronous and asynchronous testing. Chai focuses on providing expressive syntax and customizable assertions, whereas Mocha offers extensive customization options for the testing framework itself.