mud.js

// mkdir mud
// cd mud
// npm install --save sprintf
var net = require('net');
var sprintf = require('sprintf').sprintf;
var vsprintf = require('sprintf').vsprintf;

// A convenient combination of console.log + sprintf formatting
// Example: log('[Client %s] Hello', 42);  -> '[Client 42] Hello'
function log(msg) {
  var args = Array.prototype.slice.call(arguments, 1);
  if (args.length === 0)  {
    console.log(msg);
  } else {
    var str = vsprintf(msg, args);
    console.log(str);
  }
}

// Generates a unique client id to represent each connected client (socket conn)
var genClientId = (function() {
  var _id = 0;
  return function() {
    return _id += 1;
  }
})();

var server = net.createServer(function() {
  console.log('Server connected');
});

server.listen(3000, function() {
  console.log('Server listening on 3000');
});

server.on('connection', function(csock) {
  var clientId = genClientId();
  log('[Client %s] Connected', clientId);
  handleClientSocket(csock, clientId);
  csock.write('> ');
});

function handleClientSocket(csock, clientId) {
  csock.on('data', function(buf) {
    var line = buf.toString();
    log('[Client %s] Sent line: %s', clientId, line);
    handleCommand(csock, line);
    csock.write('> ');
  });
  csock.on('end', function() {
    log('[Client %s] Disconnected', clientId);
  });
}

/* //// Handle client commands 
   Dispatch on the first word a client sends us
*/

// If `line` is "say What is your name?"
// Then `firstWord` will be "say"
// And `restOfLine` will be "What is your name?"
// If firstWord is in our `commandFns` lookup map, then dispatch command
function handleCommand(csock, line) {
  var firstWord = line.split(/[ ]+/)[0].trim();
  var restOfLine = (function() {
    var match = line.match(/\W(.+)/);
    if (match) {
      return match[1];
    } else {
      return null;
    }
  })();
  var commandFn = commandFns[firstWord];
  if (commandFn) {
    commandFn(csock, restOfLine);
  } else {
    csock.write('What?\r\n');
  }
}

function goNorth(csock, line) {
  csock.write('You went north\r\n');
}
function goSouth(csock, line) {
  csock.write('You went south\r\n');
}
function goEast(csock, line) {
  csock.write('You went east\r\n');
}
function goWest(csock, line) {
  csock.write('You went west\r\n');
}
function say(csock, line) {
  csock.write(sprintf('You say "%s"\r\n', line));
}

var commandFns = {
  /* Movement */
  n: goNorth,
  north: goNorth,
  s: goSouth,
  south: goSouth,
  e: goEast,
  east: goEast,
  w: goWest,
  west: goWest,
  /* Communication */
  say: say
};

In one terminal (server):

danneu ~/Code/Node/mud
$ nodemon server.js
Server listening on 3000
Server connected
[Client 1] Connected
[Client 1] Sent line: n
[Client 1] Sent line: s
[Client 1] Sent line: e
[Client 1] Sent line: w
[Client 1] Sent line: say Hello world!
[Client 1] Disconnected
$

In another terminal (client):

danneu ~/Code/Node/mud
$ telnet localhost 3000
Trying ::1...
Connected to localhost.
Escape character is '^]'.
> n
You went north
> s
You went south
> e
You went east
> w
You went west
> say Hello world!
You say "Hello world!"
> ^]