HarmonyOS 从相册读取的图片如何转换成base64

请参考以下关键代码：

/**
 * 图片转换为base64
 * @param {Object} img 图片对象
 */
function getBase64Image(img) {
  var canvas = document.createElement("canvas");
  canvas.width = img.width;
  canvas.height = img.height;
  var ctx = canvas.getContext("2d");
  ctx.drawImage(img, 0, 0, img.width, img.height);
  var ext = img.src.substring(img.src.lastIndexOf(".") + 1).toLowerCase();
  var dataURL = canvas.toDataURL("image/" + ext);
  return dataURL;
}

//调用示例：
//Plain
let img = new Image();
img.setAttribute('crossOrigin', 'anonymous');
// 图片地址
img.src = 'xxxxx';
img.onload = () => {
  // 获得Base64
  let base64Str = getBase64Image(img);
  console.log(base64Str)
}