simple demonstration. this pattern is used by connect to stack middlewares

MiddlewarePattern.js

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

var MiddlewareService = function(){
  var stack = [],
    isProcessingStack = false,
    commonState = { task: 0 };


  function push(fn){
    stack.push(fn);

    if(!isProcessingStack)
      next();
  }

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

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

  return {
    push: push
  };
}();



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

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

MiddlewareService.push(doWork);
MiddlewareService.push(doWork);
MiddlewareService.push(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

*/