resize.js

var q = []

function (req, res) {
  q.push(function (cb) {
    resize(req.body, function(error, complete) {
      res.send(complete) 
      cb()
    })
  })  
}

function next() {

  process.nextTick(function () {
    if (q.length > 0) {
      var work = q.pop()
      work(next)
    }
  })

}

next()
next()
next()
next()


///

var q = async.queue(function (fn, callback) {
  fn(callback)
}, 6)

// assign a callback
q.drain = function() {
  console.log('all items have been processed');
}

function (req, res) {
  q.push(function (cb) {
    resize(req.body, function(error, complete) {
      res.send(complete) 
      cb()
    })
  })  
}

q.push({name: 'foo'}, function (err) {
    console.log('finished processing foo');
})