How does one write a test for JavaScript functions? You need a testing framework, and luckily, Facebook has built its own unit test framework for JavaScript called Jest. It is built on top of Jasmine; another well-known JavaScript test framework. Those of you who are familiar with Jasmine will find Jest's approach to testing very similar. However, I'll make no assumptions about your prior experience with testing frameworks and discuss the basics first.

The fundamental idea of unit testing is that you test only one piece of functionality in your application that usually is implemented by one function. You test it in isolation, which means that all the other parts of your application that the function depends on are not used by your tests. Instead, they are imitated by your tests. To imitate a JavaScript object is to create a fake one that simulates the behavior of the real object. In unit testing, the fake object is called mock and the process of...