npm install async --save

var async = require('async')

async.map(['file1','file2','file3'], fs.stat, function(err, results) {
  // results is now an array of stats for each file
  });
  async.filter(['file1','file2','file3'], function(filePath, callback) {
   fs.access(filePath, function(err) {
    callback(null, !err)
   });
  }, function(err, results) {
    // results now equals an array of the existing files
  });
  async.parallel([
    function(callback) { ... },
    function(callback) { ... }
  ], function(err, results) {
    // optional callback
  });
  async.series([
    function(callback) { ... },
    function(callback) { ... }
  ]);

async.series([
 function(callback) {
  setTimeout(function() {
   callback(null, 1)
    }, 2000);
  },
 function(callback) {
  callback(null, 2);
 }],
 function(err, results) {
  console.log(results);
 });

async.series({
  one: function(callback){
    callback(null, 1);
  },
  two: function(callback){
    callback(null, 2);
  }
},function(err, results) {
  console.log(results);
});

async.waterfall([
  function(callback) {
    callback(null, 'one', 'two');
  },
  function(arg1, arg2, callback) {
    // arg1 now equals 'one' and arg2 now equals 'two'
    console.log('function 2')
    console.log('arg1: ' + arg1)
    console.log('arg2: ' + arg2)
    callback(null, 'three');
  },
  function(arg1, callback) {
    console.log('function 3')
    console.log('arg1: ' + arg1)
      // arg1 now equals 'three'
    callback(null, 'done');
  }
], function(err, result) {
  // result now equals 'done'
  console.log(result);
});

parallel(tasks, [callback])

async.parallel([
  function(callback){
    callback(null, 'one');
  },
  function(callback){
    callback(null, 'two');
  }
],
function(err, results){

});

var async=require("async");
async.parallel([
  function(callback){
    setTimeout(function(){
      callback(null, 'one')
    },1000);
  },
  function(callback){
    setTimeout(function(){
      callback(null, 'two')
    },1000);
  }
],
function(err, results){
  console.log(results);
});
parallelLimit(tasks, limit, [callback])

async.parallelLimit([
  function(callback){
    callback(null, 'one');
  },
  function(callback){
    callback(null, 'two');
  }
],
2,
function(err, results){
});

var async=require("async");
async.parallelLimit([
  function(callback){
    setTimeout(function(){
      callback(null, 'one');
    }, 1000);
  },
  function(callback){
    setTimeout(function(){
      callback(null, 'two');
    }, 1000);
  }
],
1,
function(err, results){
  console.log(results);
});
map(coll, iteratee, callbackopt)

async.map([1, 2, 3, 4, 5],
  function(item, callback) {
    callback(null, item + 5)
  },
  function(err, result) {
    console.log(result)
  })
[ 6, 7, 8, 9, 10 ]