如何用js检测手机是否安装某个app

//html代码中 的 a 标签，以微信为例，默认的是调用weixin scheme，去打开本机的微信，如果没有则跳转到相应连接

　　　　<a href="weixin://" class="btn-download">立即打开</a>

　　

　　　　// 为btn-download 绑定事件，如果在500ms内，没有解析到协议，那么就会跳转到下载链接

　　　　 var appstore, ua = navigator.userAgent;

        if(ua.match(/Android/i)){

            appstore = 'market://search?q=com.singtel.travelbuddy.android';

        }

        if(ua.match(/iphone|ipod|ipad/)){

            appstore = "https://itunes.apple.com/cn/app/wei-xin/id414478124?mt=8&ign-mpt=uo%3D4";

        }

        function applink(fail){ 

            return function(){ 

                var clickedAt = +new Date; 

                // During tests on 3g/3gs this timeout fires immediately if less than 500ms. 

                setTimeout(function(){ 

                     // To avoid failing on return to MobileSafari, ensure freshness! 

                      if (+new Date - clickedAt < 2000){ 

                          window.location = fail; 

                      } 

                }, 500);     

            }; 

        } 

        $('.icon-download, .btn-download')[0].onclick = applink(appstore);