实现兼容IE11的文件下载功能

思路是通过iframe进行下载

const downloadFile = {};
// 入参是下载地址
downloadByUrl(url) {
  // 判断是否已经创建过iframe
  if(downloadFile.iframe) {
    const domFrame = document.getElementById('iframeDom');
    domFrame && document.body.removeChild(domFrame)
    delete downloadFile.iframe
  }
  if (downloadFile.iframe  === undefined) {
    const domFrame = document.createElement('iframe');
    domFrame.setAttribute('id', 'iframeDom');
    domFrame.setAttribute('display', 'none');
    domFrame.src = url;
    downloadFile.iframe = domFrame;
    document.body.appendChild(downloadFile.iframe);
  }
},