不知道出了什么错

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>userAgent</title>
    </head>
    <body>
       <input type="button" value="点击获取浏览器信息" id="browser">
       <script type="text/javascript">
          var button=document.getElementById("browser");
          var b=Navigator.userAgent.toLowerCase(),browser;
          button.onclick=function(){ 
            if(b.indexOf("msie")>-1){
                broswer="浏览器是IE";
            }else if(b.indexOf("firefox")>-1){
                broswer="浏览器是FireFox";
            }else if(b.indexOf("safari")>-1){
                broswer="浏览器是SaFari";
            }else if (b.indexOf("chrome")>-1){
               broswer="浏览器是Chrome";
            }else{
                broswer=b;
            }
            alert(broswer);
          }
       </script>
    </body>
</html>

慕粉1473679491 2017-06-14

源自：JavaScript BOM基础 5-4 编程练习