function networkChanged(status){
  networkStatus = status ? "online":"offline";
  document.write("network status = " + networkStatus + " " + new Date() + "<br>")
}

window.addEventListener("load", function(){
  networkChanged(navigator.onLine);

  window.addEventListener("online", function(){
    networkChanged(true);
  });

  window.addEventListener("offline", function(){
    networkChanged(false);
  });
});