Window/Tab Focus events

HTML

<script src="https://code.jquery.com/jquery-3.2.1.min.js"></script>
<p>
Click off an on this output, switch tabs etc...
</p>
<div id="output"></div>

JavaScript

(function () {
  var hasFocus = document.hasFocus();
  setInterval(function () {
    var hasFocusNow = document.hasFocus();
    if (hasFocus !== hasFocusNow && hasFocus) {
      // lost focus
      $("#output").append("<p>focus lost</p>");
    } else if (hasFocus !== hasFocusNow) {
      // gained focus
      $("#output").append("<p>focus gained</p>");
    }
    hasFocus = hasFocusNow;
  }, 500);
})();