Usage of app.use() in express/connect

I saw sample code on express website:

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

// simple logger
app.use(function(req, res, next){
  console.log('%s %s', req.method, req.url);
  next();
});

// respond
app.use(function(req, res, next){
  res.send('Hello World');
});

app.listen(3000);

Here app.use() called functions of exactly the same signature and yet is able to run them in sequence. How is this done in javascript?

Here's definition for app.use(): https://github.com/senchalabs/connect/blob/master/lib/proto.js