Before we get started making the HTTPS request, let's talk about callback functions, and we have already used them. Refer to the following code (we used it in the previous section):

console.log('Starting app');

setTimeout(() => {
  console.log('Inside of callback');
}, 2000);

setTimeout(() => {
  console.log('Second setTimeout');
}, 0);

console.log('Finishing up');

Inside the setTimeout function we used a callback function. In general, a callback function is defined as a function that gets passed as an argument to another function and is executed after some event happens. Now this is a general definition, there is no strict definition in JavaScript, but it does satisfy the function in this case:

setTimeout(() => {
  console.log('Inside of callback');
}, 2000);

Here we have a function and we pass it as an argument to another function, setTimeout, and it does get executed after some event—two-second pass. Now the event could be other things, it could be a database query finishes, it could be an HTTP request comes back. In those cases, you will want a callback function, like the one in our case, to do something with that data. In the case of setTimeout, we don't get any data back because we're not requesting any; we're just creating an arbitrary delay.