I have already written a post about mocking moment.js for tests. Today let's talk about how to use mocks in a specific tests framework - Jest. Jest was developed by Facebook and was based on another tests library called Jasmine. It meant that Jest used widely popular test syntax and could be adapted very easily.

I'm going to talk about unit tests here, not any other kind of tests. And unit tests have, in my opinion, the following list of criteria that they have to meet. This list makes it hard to write tests for code that use external libraries.