Asynchronous + event emitter

main.js

E = require('events').EventEmitter

function fn(ready, eventArg) {
 var result =  // do some async stuff with to compute result
  if(/* error encountered */) {
    ready(new Error)
  }
  
  ready(null, result)
}

function onready(err, result) {
   // whatever you need when async operation is finished
}

E.on('event',  fn.bind(fn, onready)