<canvas id="canvas" width="300" height="300"
  style="background-color: orange;"></canvas>

<script>
    var canvas = document.getElementById("canvas");
    var cxt = canvas.getContext('2d');

    cxt.lineWidth = 5;
    cxt.beginPath();
    cxt.moveTo(20, 20);
    cxt.lineTo(180, 20)
    cxt.stroke();
    cxt.closePath();
    //设置异次元空间
    cxt.save();
    //异次元空间 重置原点,默认是画布的(0,0)点
    cxt.translate(20, 20);
    //设置旋转角度 参数时弧度 角度0--360 弧度=角度*Math.PI/180
    cxt.rotate(-30 * Math.PI / 180);

    //旋转一个线段
    cxt.lineWidth = 5;
    cxt.beginPath();
    cxt.moveTo(0, 0);
    cxt.lineTo(20, 180);
    cxt.stroke();
    cxt.closePath();
    //将旋转之后的元素返回原画布
    cxt.restore();
</script>