function initCanvas (w, h) {
        var canvas = document.createElement('canvas'),
            ctx = canvas.getContext('2d');

        canvas.width = w;
        canvas.height = h;

        ctx.lineWidth = 1;
        ctx.lineJoin = 'round';

        document.body.appendChild(canvas);

        return ctx;
    }

    function drawPath(ctx, x, y, r, radianStart, radianEnd, anticlockwise) {
        ctx.beginPath();
        // ctx.moveTo(50, 50);
        ctx.arc(x, y, r, radianStart, radianEnd, anticlockwise);
        // ctx.closePath();
        ctx.stroke();
    }

    var ctx = initCanvas(100, 100);

    drawPath(ctx, 50, 50, 20, -1/2 * Math.PI, 1/2 * Math.PI, false);

    setTimeout(function () {
        ctx.clearRect(0,0,100,100);
        drawPath(ctx, 50, 50, 20, -1/2 * Math.PI, 2/3 * Math.PI, false);
    }, 1000);

    setTimeout(function () {
        ctx.clearRect(0,0,100,100);
        drawPath(ctx, 50, 50, 20, -1/2 * Math.PI, 2 * Math.PI, false);
    }, 2000);

</script>