可以通過document.hidden屬性判斷當前頁面是否是激活狀態。
兼容性：IE10+，Firefox10+,Chrome14+,Opera12.1+,Safari7.1+
兼容性寫法示例：

    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=‘被發現啦(*′?｀*)‘;
        }else{
            document.title=‘藏好啦(つд?)  ‘;
        }
    }
    document.addEventListener(visibilityChangeEvent, onVisibilityChange);
 

32.js 判斷當前頁面是否被瀏覽