Detect internet active connection: influenced by https://gist.github.com/jpsilvashy/5725579, https://github.com/HubSpot/offline/blob/master/js/offline.js

checkConnection.js

function checkConnection(callback, timeout) {
  var checkStatus = function() {
    report((xhr.status && xhr.status < 12000) ? 'up' : 'down');
  };
  var last = '';
  var report = function(status){
    if (status !== last){
      last = status;
      callback(status === 'up');
    }
  };
  
  var url = "//" + window.location.hostname + "/?_=" + (new Date()).getTime();
  var xhr = new XMLHttpRequest();
  xhr.timeout = timeout || 2000;

  if (xhr.onprogress === null) {
    xhr.onload = checkStatus;
    xhr.onerror = checkStatus;
    xhr.ontimeout = checkStatus;
  } else {
    xhr.onreadystatechange = function() {
      if (xhr.readyState === 4) {
        checkStatus();
      } else if (xhr.readyState === 0) {
        report('down');
      }
    };
  }

  try {
    xhr.open("HEAD", url);
    xhr.send();
  } catch (error) {
    report('down');
  }
}