html5-canvas画空心圆

<canvas width="500" height="800"    id="canvas">
</canvas>
<script>
//获取画布DOM  还不可以操作
var canvas=document.getElementById('canvas');
//alert(canvas);
//设置绘图环境
var cxt=canvas.getContext('2d');
//alert(cxt);

//画一个空心圆形  凡是路径图形必须先开始路径，画完图之后必须结束路径
//开始新路径
cxt.beginPath();
//重新设置画笔
cxt.lineWidth=3;
cxt.strokeStyle="green";
cxt.arc(200,200,50,0,360,false);
cxt.stroke();
//封闭新路径
cxt.closePath();