text: function main() {
    const canvas = document.getElementById('example');
  
    if (!canvas) {
      return;
    }
  
    // 获取上下文类型，区分大小写
    const ctx = canvas.getContext('2d');
  
    ctx.fillStyle = 'rgba(0, 0, 255, 1.0)';
    ctx.fillRect(120, 30, 150, 150);
  }