// 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);
...