canvas && htmlToCanvas

值得参考：
https://blog.csdn.net/weixin_43392489/article/details/108368993

利用htmlToCanvas生成缩略图
文档地址：
http://html2canvas.hertzen.com/
没找到中文文档

   const htmlCanvas = await html2canvas(
            document.querySelector('.target')
        );
        const { width, height } = htmlCanvas; 
        // 生成canvas的大小
        let num;
        if (width >= 500 || height >= 500) {
            num = 500 / width;
        } else {
            num = 1.0;
        }
        this.thumbnailBase = htmlCanvas.toDataURL('image/jpeg', num);

canvas
https://developer.mozilla.org/zh-CN/docs/Web/API/Canvas_API