With socket.io we can execute a callback function over WebSockets (or a relevant fallback). The function is defined client side, yet called server side (and vice versa). This can be a very powerful way to share processing resources and functionality between clients and servers.
In this recipe, we'll create a way for the server to call a client-side function that squares a number, and for the client to call a server-side function that sends Base64 encoding (http://en.wikipedia.org/wiki/Base64) of a sentence back to the client.
On our server, as before, we load our http module and the client.html file, create our HTTP server, attach socket.io, and set the origins policy.
var http = require('http');
var clientHtml = require('fs').readFileSync('client.html');
var plainHttpServer = http.createServer(function (request, response) {
response.writeHead(200, ...