asyncqueue.js

var async = require('async');

// Send email
var sendEmail = function(email,callback) {
  console.log("Sending email to "+email);
  callback(null);
}

// create a queue object with concurrency 2
var q = async.queue(sendEmail,2);

// Called when every processing is done
q.drain = function() {
    console.log('all emails sent');
}

// add some emails to the queue
q.push(["rwtc66@gmail.com","shahid@codeforgeek.com"]);

// add email to the front of the queue
q.unshift("abc@gmail.com");

//output
/*
Sending email to abc@gmail.com
Sending email to rwtc66@gmail.com
Sending email to shahid@codeforgeek.com
all emails sent
*/