outstanding.js

var EventEmitter = require( "events" ).EventEmitter
  , oldOn = EventEmitter.prototype.addListener
  , id = 0
module.exports = { }
EventEmitter.prototype.addListener = EventEmitter.prototype.on = function on(evt,cb) {
  var capsule = {
      source:this
      , callback: cb
      , event: evt
    }
    , myId = id++
  outstanding[ myId ] = capsule
  return oldOn( evt,function callback( ) {
    cb.apply( this, arguments )
    delete outstanding[ myId ]
  } )
}