Detect Window / Tab lost focus

lost-focus.js

(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);
  } else if ('onfocusin' in document) {
    // IE 9 and lower:
    document.onfocusin = document.onfocusout = onchange;
  } else {
    // All others:
    window.onpageshow = window.onpagehide = window.onfocus = window.onblur = onchange;
  }

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

    evt = evt || window.event;
    if (evt.type in evtMap) {
      document.body.className = evtMap[evt.type];
    } else {
      if(this[hidden]) {
        // lost focus
      } else {
        // gained focus
      }
    }
  }
})();