如何触发javascript请求从后台下载文件

<a href="path/to/file.pdf" download="filename.pdf">点击下载文件</a>

var xhr = new XMLHttpRequest();
xhr.open('GET', 'path/to/file.pdf', true);
xhr.responseType = 'blob';

xhr.onload = function(e) {
  if (this.status == 200) {
    var blob = new Blob([this.response], {type: 'application/pdf'});
    var downloadUrl = URL.createObjectURL(blob);
    var a = document.createElement('a');
    a.href = downloadUrl;
    a.download = 'filename.pdf';
    document.body.appendChild(a);
    a.click();
    document.body.removeChild(a);
  }
};

xhr.send();

fetch('path/to/file.pdf')
  .then(response => response.blob())
  .then(blob => {
    var downloadUrl = URL.createObjectURL(blob);
    var a = document.createElement('a');
    a.href = downloadUrl;
    a.download = 'filename.pdf';
    document.body.appendChild(a);
    a.click();
    document.body.removeChild(a);
  });