js network site test speed

var isLoadingJob = false;
var startTime = {};
var endTime = {};
function pingByImage(host, img_path, pong) {
  if (!isLoadingJob) {
    isLoadingJob = true;
    var img = host+img_path;
    var hashId = hashCode(host);
    var t = InitiateSpeedDetection(host, img);
    
    t.then(function(message){
      pong(message.host, message.duration);
    }).catch(function(reason){
      console.log(reason)
    });
    ;
  } else {
    window.setTimeout(pingByImage.bind(this,host, img_path, pong), 2000);
  }
}

function InitiateSpeedDetection(host, src) {
    return new Promise(function(resolve, reject) { 
      var download = new Image();
          download.onload = function () {
              endTime[host] = (new Date()).getTime();
              var result = {
                host: host,
                startTime: startTime[host],
                endTime: endTime[host],
                duration:(endTime[host] - startTime[host]) / 1000,
              };
              isLoadingJob = false;
              resolve(result);
          }
          download.onerror = function (err, msg) {
            reject(err);
          }
          startTime[host] = (new Date()).getTime();
          download.src = src+'?'+startTime[host] ;
    });
};
/* var pong = function (host, m) {
  console.log(host +" took "+m+" miliseconds.");
}
 used :  pingByImage('url', 'img_path', pong);
 */