完整代码如下：
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
canvas{
background:#f2f2f2;
}
</style>
<script></script>
</head>
<body>
<canvas width="500" height="400" id="canvas"></canvas>
<script>
var ctx=canvas.getContext('2d');
var x=0;
var y=0;
var s=10;
setInterval(function(){
ctx.clearRect(0,0,500,400);
ctx.fillRect(x,y,50,40);
x+=s;
if(x>=450){s=-10}
else if(x<=0){s=10}
},100);
</script>
</body>
</html>