connection.js

function offline() {
  document.body.innerHTML = 'You are offline :(';
}

function online() {
  document.body.innerHTML = 'You are online :)';
}

window.addEventListener('load', function(e) {
  if (navigator.onLine) {
    online();
  } else {
    offline();
  }
}, false);

if (window.addEventListener) {
  window.addEventListener('offline', offline);
  window.addEventListener('online', online);
} else {
  document.body.attachEvent('onoffline', offline);
  document.body.attachEvent('ononline', online);
}