HTML--控制小人自由移动

<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
        <title>键盘控制DIV移动</title>
        <script type="text/javascript">
            function keyEvent(event) {
                var distance = document.getElementById("move");
                  switch(event.keyCode){
                  case 37:{
                    distance.style.left = distance.offsetLeft-50+"px";
                      break;
                  }
                  case 38:{
                    distance.style.top = distance.offsetTop-50+"px";
                      break;
                  }
                  case 39:{
                    distance.style.left = distance.offsetLeft+50+"px";
                      break;
                  }
                  case 40:{
                    distance.style.top = distance.offsetTop+50+"px";
                      break;
                  }
                }
            }
        </script>
        <style type="text/css">
            #move {
                width: 100px;
                height: 100px;
                border: 1pxsolid #000;
                background: #EAEAEA;
                position: absolute;
            }
        </style>
    </head>
    <body onkeydown="keyEvent(event)">
        <div id="move"></div>
    </body>
</html>