.js

var EventEmitter = require('events').EventEmitter,
    inherits = require('util').inherits;

var ID = 0;

function Fetcher(url) {
  EventEmitter.call(this);
  this.id = ID++;
  this.url = url;
}
inherits(Fetcher, EventEmitter);

Fetcher.prototype.start = function () {
  // do your thing
  // when you download a thing:
  this.emit('fetched', {
    id: this.id,
    thing: thingYouDownloaded
  });
  
  // when you're done:
  this.emit('complete', { id: this.id });
};

io.on('connection', function (socket) { 
  socket.fetchers = { };
  
  socket.on('fetch', function (url) {
    var fetcher = new Fetcher(url);
    
    socket.fetchers[fetcher.id] = fetcher;
    
    socket.emit('fetcher', {
        id: fetcher.id,
        url: url
    });
    
    // these relay the event from the fetcher to the websocket
    fetcher.on('fetched', function (thing) {
      socket.emit('fetched', thing);
    });
    fetcher.on('complete', function (thing) {
      // remove this fetcher from the active list
      delete socket.fetchers[fetcher.id];
      socket.emit('complete', fetcher.id);
    });
  });
  
  // note: i don't remember this event and the docs aren't making it clear to me
  socket.on('closed', function () {
    Object.keys(socket.fetchers).forEach(function (key) {
      var fetcher = socket.fetchers[key];
      
      // stop doing things
      fetcher.removeAllListeners();
      
      // or:
      fetcher.stop();
    });
    
    socket.fetchers = { };
  });
});