Function fetchUrl

lesson5/app.js:4–12

(url, callback)

var concurrencyCount = 0;
var fetchUrl = function (url, callback) {
  var delay = parseInt((Math.random() * 10000000) % 2000, 10);
  concurrencyCount++;
  console.log('现在的并发数是', concurrencyCount, '，正在抓取的是', url, '，耗时' + delay + '毫秒');
  setTimeout(function () {
    concurrencyCount--;
    callback(null, url + ' html content');
  }, delay);
};

var urls = [];
for(var i = 0; i < 30; i++) {