javascript: detect whether this web page is visible/activated or not

javascript: detect activity of web page

(function() {
  var hidden = "hidden";
  // Standards:
  if (hidden in document)
    document.addEventListener("visibilitychange", onchange);
  else if ((hidden = "mozHidden") in document)
    document.addEventListener("mozvisibilitychange", onchange);
  else if ((hidden = "webkitHidden") in document)
    document.addEventListener("webkitvisibilitychange", onchange);
  else if ((hidden = "msHidden") in document)
    document.addEventListener("msvisibilitychange", onchange);
  // IE 9 and lower:
  else if ("onfocusin" in document)
    document.onfocusin = document.onfocusout = onchange;
  // All others:
  else
    window.onpageshow = window.onpagehide
    = window.onfocus = window.onblur = onchange;

  function onchange (evt) {
    var v = "visible", h = "hidden",
      evtMap = {
        focus:v, focusin:v, pageshow:v, blur:h, focusout:h, pagehide:h
      };

    evt = evt || window.event;
    console.log('evt', evt);
    if (evt.type in evtMap) {
      console.log('set body class to ', evtMap[evt.type]);
      document.body.className = evtMap[evt.type];
    } else {
      console.log('set body class to ', this[hidden] ? 'hidden' : 'visible');
      document.body.className = this[hidden] ? "hidden" : "visible";
    }
  }

  // set the initial state (but only if browser supports the Page Visibility API)
  if( document[hidden] !== undefined )
    onchange({type: document[hidden] ? "blur" : "focus"});
})();