onkeydown事件

<img src="images/hot.jpg"  id="imgId" class="img1"/></body><script>    var imgObj=document.getElementById("imgId");    var xs=0;    var xv=10;    var ys=0;    var yv=30;    document.onkeydown=function(event){      //实现图片的移动        kc=event.keyCode;        console.log(kc);        switch(kc){            case 37:                xs-=xv;imgObj.style.left=xs+"px";                document.body.style.background="#567";                break;            case 38:ys-=yv;imgObj.style.top=ys+"px";break;            case 39:xs+=xv;imgObj.style.left=xs+"px";break;            case 40:ys+=yv;imgObj.style.top=ys+"px";break;        }    }</script>