Use node-canvas to draw verification code

const Canvas = require('canvas');
const canvas = new Canvas(100, 30),
    ctx = canvas.getContext('2d');

  ctx.font = '24px "Microsoft YaHei"';

  // 绘制文本
  let drawText = (text, x) => {
    ctx.save();
    // 旋转角度
    const angle = Math.random() / 10;
    // y 坐标
    const y = 22;
    ctx.rotate(angle);
    ctx.fillText(text, x, y);
    ctx.restore();
  }

  // 随机画线
  let drawLine = () => {
    const num = Math.floor(Math.random() * 2 + 3);
    // 随机画几条彩色线条
    for (let i = 0; i < num; i++) {
      const color = '#' + (Math.random() * 0xffffff << 0).toString(16);
      const y1 = Math.random() * height;
      const y2 = Math.random() * height;
      // 画线
      ctx.strokeStyle = color;
      ctx.beginPath();
      ctx.lineTo(0, y1);
      ctx.lineTo(width, y2);
      ctx.stroke();
    }
  }

  // 数字的文本随机从小写汉字、大写汉字、数字里选择
  const numArr = [
    '〇一二三四五六七八九',
    '0123456789',
    '零壹贰叁肆伍陆柒捌玖'  
  ];
  // 第一个数字
  const fir = Math.floor(Math.random() * 10);
  // 第二个数字
  const sec = Math.floor(Math.random() * 10);
  // 随机选取运算
  const operArr = ['加', '减', '乘'];
  const oper = Math.floor(Math.random() * operArr.length);
  
  drawLine();
  drawText(numArr[Math.floor(Math.random() * numArr.length)][fir], 10);
  drawText(operArr[oper], 40);
  drawText(numArr[Math.floor(Math.random() * numArr.length)][sec], 70);
  drawText('=', 100);
  drawText('?', 130);
  
  // 验证码值的计算
  let captcha;
  switch(oper) {
    case 0: 
      captcha = fir + sec;
      break;
    case 1:
      captcha = fir - sec;
      break;
    case 2:
      captcha = fir * sec;
      break;
  }

  // 存入 session
  req.session.captcha = captcha;

res.send({
  status: 200,
  data: canvas.toDataURL()
})