Async via EventEmitter

parallel.js

// parallel
// 3 requests happen in parallel
var urls = ['google.com', 'yahoo.com']
  , emitter = new EventEmitter()
  , done = []
;

function increment(res) {
  done.push(res);
  if(done.length === urls.length) {
      emitter.emit('done');
  }
}

urls.forEach(function(url) {
  req.get(url, increment);
});

emitter.on('done', function() {
  // do something will all responses
});

serial.js

// serial
// 3 requests happen in order
var urls = ['google.com', 'yahoo.com']
  , emitter = new EventEmitter()
  , done = []
;

function increment(res) {
  done.push(res);
  if(done.length < urls) {
    emitter.emit('continue');
  } else {
    emitter.emit('done');
  }
}

emitter.on('continue', function() {
  req.get(urls[done.length], increment);
});

emitter.on('done', function() {
  // do something will all responses
});

// start
emitter.emit('continue');