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 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.