app.listen()

app.listen(port, [hostname], [backlog], [callback])

Binds and listens for connections on the specified host and port. This method is identical to Node’s http.Server.listen().

var express = require('express');
var app = express();
app.listen(3000);

The app returned by express() is in fact a JavaScript Function, designed to be passed to Node’s HTTP servers as a callback to handle requests. This makes it easy to provide both HTTP and HTTPS versions of your app with the same code base, as the app does not inherit from these (it is simply a callback):

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

http.createServer(app).listen(80);
https.createServer(options, app).listen(443);

The app.listen() method returns an http.Server object and (for HTTP) is a convenience method for the following:

app.listen = function() {
  var server = http.createServer(this);
  return server.listen.apply(server, arguments);
};