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Cypress relies on many best-of-breed open source testing libraries to lend stability and familiarity to the platform from the get-go. If you’ve been testing in JavaScript, you’ll recognize many old friends in this list. Understand how we exploit them and hit the ground running with Cypress!



Cypress has adopted Mocha’s bdd syntax, which fits perfectly with both integration and unit testing. All of the tests you’ll be writing sit on the fundamental harness Mocha provides, namely:

Additionally, Mocha gives us excellent async support. Cypress has extended Mocha, sanding off the rough edges, weird edge cases, bugs, and error messages. These fixes are all completely transparent.

Check out our guide to writing and organizing tests.



While Mocha provides us a framework to structure our tests, Chai gives us the ability to easily write assertions. Chai gives us readable assertions with excellent error messages. Cypress extends this, fixes several common pitfalls, and wraps Chai’s DSL using subjects and the .should() command.

List of available Chai Assertions



When writing integration tests, you will likely work a lot with the DOM. Cypress brings in Chai-jQuery, which automatically extends Chai with specific jQuery chainer methods.

List of available Chai-jQuery Assertions



When writing unit tests, or even in integration-like tests, you often need to ability to stub and spy methods. Cypress includes two methods, cy.stub() and cy.spy() that return Sinon stubs and spies, respectively.

Cypress also exposes a utility so that sinon can be called anywhere inside of your tests using Cypress.sinon.

Check out our guide for working with spies, stubs, and clocks.



When working with stubs or spies you’ll regularly want to use those when writing Chai assertions. Cypress bundles in Sinon-Chai which extends Chai allowing you to write assertions about stubs and spies.

List of available Sinon-Chai Assertions

Other Library Utilities

Cypress also bundles the following tools on the Cypress object. These can be used anywhere inside of your tests.