示例10：绘制时钟

<canvas id="canvas"
            width="600"
            height="600"></canvas>

    <script>
        let canvas = document.querySelector('#canvas');
        let ctx = canvas.getContext('2d');
        let width = canvas.width
        let height = canvas.height

        // 绘制内容区域相对画布的大小 80%
        let scale = 0.8;

        // 计算半径
        let radius = Math.floor(Math.min(width, height) * scale * 0.5);
        console.log('radius', radius);

        // 绘制外边框
        // ctx.beginPath();
        // ctx.strokeRect(0, 0, width, height);
        // ctx.closePath()


        // 绘制时钟刻度
        function drawClockScale({
            number = 12,
            strokeStyle = 'black',
            lineWidth = 3,
            lineLength = 20,
        }) {
            ctx.save();

            ctx.strokeStyle = strokeStyle;
            ctx.lineWidth = lineWidth;
            let rotateAngle = (360 / number);

            for (let i = 0; i < number; i++) {
                ctx.rotate((Math.PI / 180) * rotateAngle);
                ctx.beginPath();
                ctx.moveTo(radius - lineLength, 0);
                ctx.lineTo(radius, 0);
                ctx.stroke();
                ctx.closePath()
            }

            ctx.restore();
        }

        function drawClock() {
            ctx.clearRect(0, 0, width, height);

            ctx.save();
            // 将坐标轴原点移动到画布中心
            ctx.translate(width / 2, height / 2);
            // 旋转坐标轴x指向画布正上方
            ctx.rotate(-Math.PI / 180 * 90);



            // 分针和秒针的刻度
            drawClockScale({
                number: 60,
                strokeStyle: 'green',
                lineWidth: 4,
                lineLength: 14
            });

            // 时针刻度
            drawClockScale({
                number: 12,
                strokeStyle: 'red',
                lineWidth: 8,
                lineLength: 20
            });

            // 绘制指针
            let now = new Date();
            let hour = now.getHours();
            let minute = now.getMinutes();
            let second = now.getSeconds();

            console.log(`${hour}: ${minute}: ${second}`);

            // 绘制秒针
            ctx.save();
            ctx.beginPath();
            ctx.rotate((Math.PI / 180) * (360 / 60) * second);
            ctx.strokeStyle = "deepskyblue";
            ctx.lineWidth = 2;
            ctx.moveTo(-20, 0);
            ctx.lineTo(radius - 30, 0);
            ctx.stroke();
            ctx.closePath()
            ctx.restore();

            // 绘制分针
            ctx.save();
            ctx.rotate((Math.PI / 180) * ((360 / 60) * minute + (360 / 60 / 60) * second));
            ctx.beginPath();

            ctx.strokeStyle = "green";
            ctx.lineWidth = 4;
            ctx.moveTo(-20, 0);
            ctx.lineTo(radius - 40, 0);
            ctx.stroke();

            ctx.closePath()
            ctx.restore();

            // 处理成12小时制
            if (hour > 12) {
                hour = hour - 12
            }

            // 绘制时针
            ctx.save();
            ctx.beginPath();
            ctx.rotate((Math.PI / 180) * ((360 / 12) * hour + (360 / 12 / 60) * minute + (360 / 12 / 60 / 60) *
                second));
            ctx.strokeStyle = "red";
            ctx.lineWidth = 8;
            ctx.moveTo(-20, 0);
            ctx.lineTo(radius - 50, 0);
            ctx.stroke();
            ctx.closePath()
            ctx.restore();

            // 绘制圆心
            ctx.beginPath();
            ctx.fillStyle = "grey";
            ctx.lineWidth = 10;
            ctx.arc(0, 0, 10, 0, Math.PI / 180 * 360);
            ctx.fill();
            ctx.closePath()

            // 绘制圆
            ctx.beginPath();
            ctx.strokeStyle = "grey";
            ctx.lineWidth = 10;
            ctx.arc(0, 0, radius, 0, Math.PI / 180 * 360);
            ctx.stroke();
            ctx.closePath()
            
            ctx.restore();
        }

        // 每隔1秒重绘一次
        setInterval(() => {
            drawClock();
        }, 1000)
    </script>

需要修改坐标轴之前，最好把当前状态保存，绘制完成后再恢复