javascript表单验证使用try{}catch(err){}的调用，失败后阻止默认提交行为

<script type="text/javascript">
      function LoginValidate()
      {
        var username=document.forms["loginform"]["username"].value;
        var password=document.forms["loginform"]["password"].value;
        var email=document.forms["loginform"]["email"].value;

        var atpos=email.indexOf("@");
        var dotpos=email.lastIndexOf(".");
        var charcode,euser;
       
        try{
          if(!/^[0-9a-zA-Z]*$/.test(username))
          {
            
            throw "账号只能是数字和字母组合";
          }else if(username.lenght<8)
          {
            throw "账号最小长度为8位";
          }else if(password.length <6)
          {
            throw "密码最少为6位";
          }else if(atpos<1 || dotpos<atpos+2 ||dotpos+2 >=email.length)
          {
            throw "不是一个有效的Email";
          }else
          {
            document.getElementById("msg").innerHTML="登陆成功！";
            return true;
          }
        }catch(err)
        {
        //获取错误信息，然后传给元素显示
          document.getElementById("msg").innerHTML=err;
          return false;
        }
      }
        EventUtil.addHandler(document.forms[0],"submit",function(event){
          if(LoginValidate()==true)
          {
            alert("登陆成功");
          }else
          {
          //获取事件，阻止提交
            var event=EventUtil.getEvent(event);
            EventUtil.preventDefault(event);
          }
        });
          username=null;
          password=null;
          email=null;
     
    </script>