When hosting your application on another service (like Heroku, Nodejitsu, and AWS), your host may independently configure the process.env.PORT variable for you; after all, your script runs in their environment.
Amazon's Elastic Beanstalk does this. If you try to set a static port value like 3000 instead of process.env.PORT || 3000 where 3000 is your static setting, then your application will result in a 500 gateway error because Amazon is configuring the port for you.
This is a minimal Express application that will deploy on Amazon's Elastic Beanstalk:
var express = require('express');
var app = express();
app.get('/', function (req, res) {
res.send('Hello World!');
});
// use port 3000 unless there exists a preconfigured port
var port = process.env.PORT || 3000;
app.listen(port);