To get started, we do need to install rewire in the Terminal. It's a fantastic test utility. It's pretty essential for testing functions with side effects, like the one we have seen in this section. Let's run npm install. The module name itself is called rewire, and we'll be grabbing the most recent version as of this filming, version @3.0.2. This is a test-specific module. We'll not need it for our application to run regularly, so we will be using the --save-dev flag to add it to our package.json dependencies list:

npm install rewire@3.0.2 --save-dev

Once the module is installed we can get started using it, and it's pretty simple to set up. Inside app.test.js we can start by loading it in. Up at the very top, we'll create a new constant. This one will be called rewire, and we'll set it equal to the returned result from requiring rewire:

const expect = require('expect');
const rewire = require('rewire');