js 判断当前页面是否被浏览

var hiddenProperty =
  "hidden" in document
    ? "hidden"
    : "webkitHidden" in document
    ? "webkitHidden"
    : "mozHidden" in document
    ? "mozHidden"
    : null;
var visibilityChangeEvent = hiddenProperty.replace(
  /hidden/i,
  "visibilitychange"
);
var onVisibilityChange = function() {
  if (!document[hiddenProperty]) {
    document.title = "被发现啦(*´∇｀*)";
  } else {
    document.title = "藏好啦(つд⊂)  ";
  }
};
document.addEventListener(visibilityChangeEvent, onVisibilityChange);