function downloadApp(){
 var u = navigator.userAgent; 
 var ua = navigator.userAgent.toLowerCase(); 
 var isAndroid = u.indexOf('Android') > -1 || u.indexOf('Adr') > -1; //android终端 
  var isiOS = !!u.match(/\(i[^;]+;( U;)? CPU.+Mac OS X/); //ios终端 
 if(ua.match(/MicroMessenger/i)=="micromessenger") { //微信内置浏览器
  $(".download a").click(function(){
   window.location.href='http://a.app.qq.com/o/simple.jsp?pkgname=应用名 '
  });

 }else{
  if(isiOS){

   

   $(".download a").click(function(){
    window.location.href='https://itunes.apple.com/cn/app/应用名'
   });
  }else if(isAndroid){
   $(".download a").click(function(){
    window.location.href='http://a.app.qq.com/o/simple.jsp?pkgname=应用名'
   });
  }else{

   $(".download a").click(function(){
    window.location.href='应用链接'

   });

  }

 }

  

 }