Demonstration nodeJS JSON Application Server, enough to learn client JavaScript, JSON and some server programming.
NodeJS (tested with 0.8.11) and commander module (npm install commander)
Copy all (two) files and invoke aritmetica.js.
Port 80 is used by default. If you do not have the proper privileges, you can change port with -p / --port argument option.
Client invokes server provided aritmetic provides. Use client --help for further instructions. It can start several concurrent connections.