onkeydown小练习

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>键盘事件小练习</title>
    <style>
        #mydiv{
            width: 30px;
            height: 30px;
            background-color: red;
            position: absolute;
            left: 0;
            top: 0;
        }
    </style>
</head>
<body>
<div id="mydiv"></div>
<script>
    var myDiv = document.getElementById("mydiv");
    var xs = 0;
    var xv = 30;
    var ys = 0;
    var yv = 30;
    document.onkeydown = function(event){
        var kc = event.keyCode;
        switch (kc){
            case 37:
                xs -= xv;
                myDiv.style.left = xs+"px";
                myDiv.style.backgroundColor = "red";
                break;
            case 38:
                ys -= yv;
                myDiv.style.top = ys + "px";
                myDiv.style.backgroundColor = "blue";
                break;
            case 39:
                xs+=xv;
                myDiv.style.left = xs+"px";
                myDiv.style.backgroundColor = "green";
                break;
            case 40:
                ys += yv;
                myDiv.style.top = ys + "px";
                myDiv.style.backgroundColor = "yellow";
                break;
        }
    }
</script>
</body>
</html>