填充和描边均为圆弧

// Usage:
var arc={
    cx:150, cy:150,
    innerRadius:75, outerRadius:100,
    startAngle:-Math.PI/4, endAngle:Math.PI
}

drawArc(arc,'skyblue','gray',4);

function drawArc(a,fill,stroke,strokewidth){
    ctx.beginPath();
    ctx.arc(a.cx,a.cy,a.innerRadius,a.startAngle,a.endAngle);
    ctx.arc(a.cx,a.cy,a.outerRadius,a.endAngle,a.startAngle,true);
    ctx.closePath();
    ctx.fillStyle=fill;
    ctx.strokeStyle=stroke;
    ctx.lineWidth=strokewidth
    ctx.fill();
    ctx.stroke();
}