Javascript 键盘事件移动图片

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    <div style="width:600px;height:300px;border:1px solid green">

        <img >>

    </div>

    <script>

        var pic = document.getElementById("pic");

        var step = 10;        

        var x = pic.style.pixelLeft;

        var y = pic.style.pixelTop;

        function movePic(){

            var code = event.keyCode;

            switch(code) {

                case 37:

                    x-=step;

                    pic.style.left = x; 

                    break;

                case 38:

                    y-=step;

                    pic.style.top = y;

                    break;

                case 39:

                    x+=step;

                    pic.style.left = x;

                    break;

                case 40:

                    y+=step;

                    pic.style.top = y;

                    break;            

            }

        }    

    </script>

</body>