Mocking a component's dependencies can make the component easier to test. However, if we mock too many things, is the test really verifying that the component will work in our real app?

Establishing what to mock is one of the hardest tasks when writing unit tests. There are some things that make a lot of sense to mock, though, such as REST APIs. A REST API is a pretty fixed contract between the frontend and backend. Mocking a REST API also allows our tests to run nice and fast.

In this section, we'll eventually learn how to mock REST API calls made with axios. First, though, we'll learn about Jest's function mocking feature.