Parallel url-scanner with concurrency limit

scan.js

var http = require('http');

function async(url, next) {
  // Log for demo purposes - remove
  console.log('url: ', url);
  http.get(url, function (res) {
    // setTimeout for demo purposes - remove
    setTimeout(function () {
      next(res.statusCode);
    }, 1000);
  });
}

function done() {
  console.log('Done', results);
  process.exit(0);
}

var urls = [
  'http://google.com',
  'http://yahoo.com',
  'http://google.com',
  'http://yahoo.com',
  'http://google.com',
  'http://yahoo.com',
  'http://google.com',
  'http://yahoo.com',
];
var results = [];
var running = 0;

// Set your concurrency limit here
var limit = 2;

function start() {
  while(running < limit && urls.length > 0) {
    var url = urls.shift();
    async(url, function(result) {
      results.push(result);
      running--;
      if(urls.length > 0) {
        start();
      } else if (running === 0) {
        done();
      }
    });
    running++;
  }
}

start();