js判断回车，判断焦点控件

  document.onkeydown=function(event){
        e = event ? event :(window.event ? window.event : null);
        if(e.keyCode==13){ //判断是否回车
          
            if(document.activeElement.){ //焦点所在ID
              
                return false;
            }
            return false;
        }
    }