onkeydown事件

<img src="images/hot.jpg" alt="" 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>