On SIGCHLD do waitpid(-1). use the package 'waitpid'.

Not sure if there is a better way to handle this but this works.
Related PR on pty.js : https://github.com/chjj/pty.js/pull/37

app.js

@@ -5,6 +5,11 @@ var path = require('path');
 var ws = require('websocket').server;
 var pty = require('pty.js');
 var fs = require('fs');
+var waitpid = require('waitpid');
+
+process.on('SIGCHLD', function(args){
+    waitpid(-1);
+});
 
 var opts = require('optimist')
     .options({

package.json

@@ -5,7 +5,8 @@
     "express": "3.5.1",
     "websocket": "",
     "pty.js": "",
-    "optimist": ""
+    "optimist": "",
+    "waitpid": ""
   },
   "description": "Wetty = Web + tty. Terminal access in browser over http/https ",
   "main": "app.js",