//导出Excel
  public async btnExport() {
   let that = this as any;
    this.loading = true;
    let a = document.createElement(‘a‘);//创建a标签
    a.href =  "";//文件url
    document.body.appendChild(a);
    a.click();//触发下载
    document.body.removeChild(a);
    setTimeout(async () => {
      that.loading = false;
    }, 4000);//4s后关闭loading，可以根据条数增减
  }

js创建A标签实现下载