不支持 ie

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//网页当前状态判断
let hidden, state, visibilityChange;
if (typeof document.hidden !== "undefined") {
  hidden = "hidden";
  visibilityChange = "visibilitychange";
  state = "visibilityState";
} else if (typeof document.mozHidden !== "undefined") {
  hidden = "mozHidden";
  visibilityChange = "mozvisibilitychange";
  state = "mozVisibilityState";
} else if (typeof document.msHidden !== "undefined") {
  hidden = "msHidden";
  visibilityChange = "msvisibilitychange";
  state = "msVisibilityState";
} else if (typeof document.webkitHidden !== "undefined") {
  hidden = "webkitHidden";
  visibilityChange = "webkitvisibilitychange";
  state = "webkitVisibilityState";
}
// 添加监听器，在title里显示状态变化
document.addEventListener(
  visibilityChange,
  function() {
    if (document[state] === "visible") {
      document.title = "回来啦！";
    } else {
      document.title = "去哪了？";
    }
  },
  false
);
//初始化页面状态
document.title = "回来啦！";