page_visibility_api.js

var pollCount = 1;

var poll = function() {
  if (document.hidden) // if the interval has not been cleared right
      return;
  var p = document.createElement('p');
  p.innerText = 'Polled ' + pollCount + ' times.';
  document.body.insertBefore(p, document.body.firstChild);
  pollCount++;
};

var intervalId = setInterval(poll, 1000);

document.addEventListener('visibilitychange', function() {
  if (document.visibilityState === 'hidden')
    clearInterval(intervalId);
  else
    intervalId = setInterval(poll, 1000);
});