这通过在window的onfocus和onblur事件上注册处理程序来实现。
jQuery风格：

$(window).on('blur', function() { ... });

不使用jQuery：

window.onblur = function() { ... }

如果不清楚：页面标题可以通过document.title读取/写入

$(function() {
  var message = "Don't forget us";
  var original;

  $(window).focus(function() {
    if (original) {
      document.title = original;
    }
  }).blur(function() {
    var title = $('title').text();
    if (title != message) {
      original = title;
    }
    document.title = message;
  });
});

原始编码人：https://flintobox.com/blog/

$(function () {
    var message = "Hey, come back!";
    var original;

    $(window).focus(function() {
      if(original) {
        document.title = original;
      }
    }).blur(function() {
      var title = $('title').text();
      if(title != message) {
        original = title;
      }
      document.title = message;
    });
  });

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

$(function () {
    var message = "Hey, come back!";
    var original;

    $(window).focus(function() {
      if(original) {
        document.title = original;
      }
    }).blur(function() {
      var title = $('title').text();
      if(title != message) {
        original = title;
      }
      document.title = message;
    });
  });

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>