图片转换为base64

const getBase64 = (url: string) => {
    return new Promise((resolve, reject) => {
      // 通过构造函数来创建的 img 实例，在赋予 src 值后就会立刻下载图片，避免了文档冗余和污染
      let Img = new Image();
      Img.src = url;
      Img.crossOrigin = 'Anonymous';
      Img.onload = () => { // 要先确保图片完整获取到，这是个异步事件
        let dataURL = '';
        let canvas = document.createElement('canvas'); // 创建canvas元素
        let width = Img.width; // 确保canvas的尺寸和图片一样
        let height = Img.height;
        canvas.width = width || 0;
        canvas.height = height || 0;
        let ctx = canvas.getContext('2d');
        if (ctx) ctx.drawImage(Img, 0, 0, width, height); // 将图片绘制到canvas中
        dataURL = canvas.toDataURL('image/jpeg'); // 转换图片为dataURL
        resolve(dataURL);
      };
    });
  }

let res = await getBase64('图片地址');
或者
getBase64('图片地址').then((thumbnail: any) => {
  //TODO
});