In the Express part of your MEAN application, your business logic is mostly encapsulated inside controllers; however, you also have Mongoose models that obfuscate many tasks, including data manipulation and validations. So, to properly cover your Express application code, you will need to write tests that cover both models and controllers. In order to do so, you will use Mocha as your test framework, the Should.js assertion library for your models, and the SuperTest HTTP assertion library for your controllers. You will also need to create a new test environment configuration file that will provide you with special configuration options for testing purposes, for example, a dedicated MongoDB connection string. By the end of this section, you will learn to use the Mocha command-line tool to run and evaluate your test results. We'll begin with presenting the Mocha test framework.