二进制转base64

有时候需要将二进制图片文件 转base64，方法如下：

    _arrayBufferToBase64(buffer) {
      let binary = "";
      let bytes = new Uint8Array(buffer);
      let len = bytes.byteLength;
      for (let i = 0; i < len; i++) {
        binary += String.fromCharCode(bytes[i]);
      }
      return "data:image/jpeg;base64," + window.btoa(binary);
    }