We'll start it with creating a new file. Inside the spies directory, we'll make a new file called app.test.js, and we can start playing around with spies. Now, spies comes built-in with expect, so all we have to do is load it in:

const expect = require('expect');

From here we can create our very first test. We'll put this in a describe block so it's easier to find over in our test output:

const expect = require('expect');

describe('')

We'll call this describe block App and we'll add my callback function:

describe('App', () => {

});

Now we can add individual test cases. First up, we'll call it and make a new test where we can just play around with spies:

describe('App', () => {

  it('')
  
});

We won't be calling the function in our app.js file just yet. We'll add in the it object a string say, Should call the spy correctly:

describe('App', () => {

  it('should call the spy correctly', () => {
  });
  
});

In order to visualize how spies work, we'll go through the most basic example we can. First up, creating a spy.