What did Andrew mean by the use of closures?

What Andrew wanted to explain was how the (req, res, next) parameters of the middleware functions, are passed down into predefined, 3rd party middleware methods such as cookieParser or bodyParser.

First lets have a look at how to include cookieParser into our express app:

        const cookieParser = require('cookie-parser');
        app.use(cookieParser()); 

As you can can see, we even though we pass cookieParser() as middleware, we don't explicity pass req, res, next parameters when invoking the method. So how do those arguments end up being used inside the cookieParser() method?

The answer is by the method being a closure.

function closure() {
  return function (req, res, next) {
    //Doing the actual modification of the request/response object and calling next() inside of here
    console.log(req); //logs 'Here we pass our request object'
    console.log(res); //logs 'Here we pass our response object'
    console.log(next);   //logs 'Here we pass the next() method'
  }
}

let cookieParser = closure();

cookieParser('Here we pass our request object', 'Here we pass our response object', 'Here we pass the next() method') 



module.exports.cookieParser = cookieParser; //export our module

In this oversimplified example, you can can see how a cookieParser() module could be developed, granted that there would be actually a lot of work to be done first, before this would parse anything!

So you might be asking yourself, why using a closure? While there are many possible reasons for this, one that everybody can directly see is that is cleans up the interface of our express application. cookieParser() just looks much cleaner than having to manually pass in the three parameters, while achieving the same result, having the req, res, next parameters downpassed automatically when being omitted in the app.use() method.

For further reading on why closures are being used so widely I refer to [https://softwareengineering.stackexchange.com/questions/203507/whats-so-useful-about-closures-in-js]

I hope that my explanation was understandable, closures are a really simple construct, whose use-cases for some weird reason tend to be really hard to grasp.