url图片转base64

const base64Str: string = await new Promise(resolve => {
      const img = new Image()
      img.crossOrigin = 'anonymous';
      img.src = url; // 图片链接
      img.onload = function () {
        const canvas = document.createElement('canvas')
        canvas.width = img.width;
        canvas.height = img.height;
        const ctx = canvas.getContext('2d');
        ctx?.drawImage(img, 0, 0, img.width, img.height);
        const ext = img.src.substring(img.src.lastIndexOf('.') + 1).toLowerCase();
        const dataURL = canvas.toDataURL('image/' + ext);
        resolve(dataURL);
      }
    });