Detect HTML Window Focus

gistfile1.js

var focused = false,
	focusedTimeout,
	lastDate;

function anim() {
  clearTimeout(focusedTimeout);
  focused=true;
  
  if (!lastDate) {
    lastDate = new Date();
  } else if (new Date() - lastDate > 1000) {
    lastDate = new Date();
    console.log(lastDate);
  }

  focusedInterval = setTimeout(function () { focused = false; }, 100);
  requestAnimationFrame(anim);
}

setInterval(function () {
	if (focused) {
		console.log('Focused!');
	} else {
		console.log('Not focused!');
	}
}, 1000);

anim()