The syntactic sugar

Most of the getting started examples of ExpressJs include this piece of code

var express = require('express');
var app = express();
...
app.listen(1337);

Well, app.listen is just a shortcut for:

var express = require('express');
var app = express();
var http = require('http');
http.createServer(app).listen(1337);

The Express App

The famous http.createServer accept a function which is known as the handler. The handler takes 2 parameters request and response as inputs, then manipulating them inside it's scope to do various things.

So basically app = express() is a function, taking place as the handler and dealing with request, response through a set of special components referred as middlewares.

A basic middleware is a function that takes 3 arguments request, response and next.

Then by app.use, a middleware is mounted to the Express App Middlewares Stack. Request and response are manipulated in each middleware then piped to the next one through the call of next().

For example, the below code:

var express = require('express');
var app = express();

app.use((request, response, next) => {        
    request.propA = "blah blah"; 
    next(); 
});

app.use('/special-path', (request, response, next) => {
    request.propB = request.propA + " blah";
    if (request.propB === "blah blah blah")
        next();
    else
        response.end('invalid');
});

app.use((request, response, next) => {
    response.end(request.propB);
});

app.listen(1337);

Can roughly be translated to:

var http = require('http');
http.createServer((request, response) => {
    
    //Middleware 1
    if (isMatch(request.url, '*')) {
        request.propA = "blah blah";
    }

    //Middleware 2
    if (isMatch(request.url, "/special-path")) {
        request.propB = request.propA + " blah";
        if (request.propB !== "blah blah blah")
            return response.end('invalid');
    }

    //Middleware 3
    if (isMatch(request.url, "*")) {
        return response.end(request.propB);
    }
});

server.listen(1337);