Simplest possible Express application

simple-express.js

// simplest possible express app
var http = require('http');
var express = require('express');

var app = express();

// while your program is running, requests to "/test" will trigger this
// callback function.
app.get("/test", function(req, res) {
    // the "req" parameter is an object that gives you access to data in the
    // request.
    var message = req.query.message;

    // the "res" parameter lets you manipulate the response.
    res.end(message.toUpperCase());

});

// 4000 is the port. if you're running on your own computer, type
//    http://localhost:4000/test?message=lowercase
// in the location bar. if you're on your digitalocean droplet
// (or whatever), go to
//    http://SERVER-IP-ADDRESS/test?message=lowercase
// instead (replacing "SERVER-IP-ADDRESS" with your server's IP
// address or hostname.
http.createServer(app).listen(4000, function() {
    console.log("Express server listening on port 4000");
});