Chai vs Sinon.js comparison of testing frameworks
What are the differences between Chai and Sinon.js?

	Chai https://www.chaijs.com	Sinon.js https://sinonjs.org/
Programming language	JavaScript	JavaScript
Category	Unit Testing	Unit Testing
General info	Chai is a BDD / TDD assertion library for Node.js and the browser that can be paired with any JavaScript testing framework. Chai is an assertion library, similar to Node's built-in assert and makes testing easier by giving you many assertions you can run against your code. It also includes a robust Plugin architecture for extending Chai's assertions and interfaces.	Standalone test stubs and mocks for JavaScript. Works with any unit testing framework. Sinon.js is a great library when you want to unit test your code with supports spies, stubs, and mocks. The library has cross browser support and also can run on the server using Node.js.
xUnit Set of frameworks originating from SUnit (Smalltalk's testing framework). They share similar structure and functionality.	N/A	Yes Compatible
Client-side Allows testing code execution on the client, such as a web browser	Yes Chai can be used in a browser environment to test front-end components and functionality	Yes Sinon.js is a good library for unit testing your front-end code and has cross browser support
Server-side Allows testing the bahovior of a server-side code	Yes Chai is used in a Node.JS environment to test server behaviour and functionality	Yes Sinon.js can run on node.js to test back-end components and server-side behaviour
Fixtures Allows defining a fixed, specific states of data (fixtures) that are test-local. This ensures specific environment for a single test	N/A	N/A
Group fixtures Allows defining a fixed, specific states of data for a group of tests (group-fixtures). This ensures specific environment for a given group of tests.	N/A
Generators Supports data generators for tests. Data generators generate input data for test. The test is then run for each input data produced in this way.	Yes By using chai-spec-generator plugin which will generate chai test specs from a JavaScript object
Licence Licence type governing the use and redistribution of the software	MIT License	New BSD License; Modified BSD License
Mocks Mocks are objects that simulate the behavior of real objects. Using mocks allows testing some part of the code in isolation (with other parts mocked when needed)	Yes By using by using a third party library like chai-jest-mocks which is a chai plugin that adds assertions for jest mock functions. You can also use sinon.js	Yes SinonJS is basically a library that we can use to create object mocks for unit testing
Grouping Allows organizing tests in groups	N/A
Other Other useful information about the testing framework