Find whether a tab is visible with Page Visibility API

visibility.js

var hidden, visibilityState, visibilityChange;
if (typeof document.hidden !== "undefined") {
  hidden = "hidden", visibilityChange = "visibilitychange", visibilityState = "visibilityState";
}
else if (typeof document.mozHidden !== "undefined") {
  hidden = "mozHidden", visibilityChange = "mozvisibilitychange", visibilityState = "mozVisibilityState";
}
else if (typeof document.msHidden !== "undefined") {
  hidden = "msHidden", visibilityChange = "msvisibilitychange", visibilityState = "msVisibilityState";
}
else if (typeof document.webkitHidden !== "undefined") {
  hidden = "webkitHidden", visibilityChange = "webkitvisibilitychange", visibilityState = "webkitVisibilityState";
}
document.addEventListener(visibilityChange, function() {
  switch (document[visibilityState]) {
  case "visible":
    console.log('visible');
    amivisible = 'visible';
    break;
  case "hidden":
    console.log('hidden');
    amivisible = 'hidden';
    break;
  }
});
amivisible = document[visibilityState];
console.log(amivisible);