Throttle Ajax Requests

file.js

(function () {
  
  // ajax function from https://gist.github.com/Xeoncross/7663273
  function ajax(url, callback, data, x) {
      try {
          x = new (this.XMLHttpRequest || ActiveXObject)("MSXML2.XMLHTTP.3.0");
          x.open(data ? "POST" : "GET", url, 1);
          x.setRequestHeader("X-Requested-With", "XMLHttpRequest");
          x.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
          x.onreadystatechange = function () {
              x.readyState > 3 && callback && callback(x.responseText, x);
          };
          x.send(data);
      } catch (e) {
          window.console && console.log(e);
      }
  }
  
  /**
   * Throttles ajax calls rather than calling all at once
   * @param {string[]} listOfUrls - a list of URLS
   * @param {function} callback - callback function once all URLS have been called
   * @param {object[]} [results] - optional list of previous results. Initially null.
   */
  function ajaxCallThrottle(listOfUrls, callback, results) {
    var url;
    
    // if results not set, should be an empty list.
    results = results || [];
    
    // if list of urls is null or an empty list (or not a list), run the callback and stop;
    if (!listOfUrls || !listOfUrls.length || typeof listOfUrls.shift !== 'function') {
      callback(results);
      return;
    }
    
    // pull the url off the top of the list of URLS
    url = listOfUrls.shift();
    
    ajax(url, function (response) {
      results.push(response);
      // call the call throttler on the rest of the list.
      ajaxCallThrottle(listOfUrls, callback, results);
    });
  }
  
  // an example
  
  var sites = [
    'http://www.yahoo.com/',
    'http://www.myspace.com/',
    'http://www.digg.com/'
    ];
  ajaxCallThrottle(sites, function (results) {
    console.log('Testing if these sites ever go away.');
    results.forEach(function (result, i) {
      console.log(sites[i], result);
    });
  });
  
}());