process n number of async functions in order. used when promises aren't viable e.g. cross file or service calls.

taskQueue.js

//process n number of async functions in order. used when promises aren't viable e.g. cross file or service calls.

var TaskQueue = function(){
  var queue = [],
    isProcessingQueue = false,
    commonState = { task: 0 };


  function add(fn){
    queue.push(fn);

    if(!isProcessingQueue)
      next();
  }

  function next(err){
    if(err)
      throw new Error(err);
    
    var fn = queue.shift();

    if(fn){
      isProcessingQueue = true;
      fn(commonState, next);
    }
    else
      isProcessingQueue = false;
  }

  return {
    add: add
  };
}();



function doWork(commonState, next){
  commonState.task++;
  console.log('Starging something async:' + commonState.task);

  setTimeout(function(){
    console.log('Completed async task: ' + commonState.task);
    next();
  }, 2000);  
}

TaskQueue.add(doWork);
TaskQueue.add(doWork);
TaskQueue.add(doWork);

/* OUTPUT

Starging something async:1
Completed async task: 1
Starging something async:2
Completed async task: 2
Starging something async:3
Completed async task: 3

*/