text: window.onload = () => {
      main();
  }
  
  let main = () => {
      const canvas = document.getElementById('app');
      if(!canvas) {
          console.log('Failed to retrieve the <canvas> element.');
          return;
      }
      // 获取上下文
      let ctx = canvas.getContext('2d');
      // 设置填充色
      ctx.fillStyle = 'rgba(0, 0, 255, 1.0)';
      // 绘制矩形：x, y, w, h
      ctx.fillRect(100, 50, 200, 200);
  }