canvas绘制多边形

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>canvas绘制多边形</title>
</head>
<body>
<canvas id="canvas" style="border: 1px solid darkcyan;" width="400" height="400"></canvas>

<script>
    var c = document.getElementById("canvas").getContext("2d");
    //定义一个以（x,y）为中心，半径为r的规则n边型；
    //每个顶点都是均匀分布在圆周上
    //将第一个顶点放置在最上面，或者指定一定角度
    //除非最后一个参数是true，否则顺时针旋转
    function polygon(c,n,x,y,r,angle,counterClockwise) {
        angle = angle || 0;
        counterClockwise = counterClockwise || false;
        c.moveTo(x+r*Math.sin(angle),y-r*Math.cos(angle));
        var delta = 2*Math.PI/n;
        for(var i=1;i<n;i++){
            angle+=counterClockwise?-delta:delta;
            c.lineTo(x+r*Math.sin(angle),y-r*Math.cos(angle));
        }
        c.closePath();
    }
    c.beginPath();
    polygon(c,3,50,70,50);
    polygon(c,4,150,60,50,Math.PI/4);
    polygon(c,5,255,55,50);

    c.fillStyle = "#ccc";
    c.strokeStyle = "#008";
    c.lineWidth = 5;
    c.fill();
    c.stroke();
</script>
</body>
</html>