<a href="/images/liang.jpg" rel="external nofollow" download="文件名稱">

/**
 * 獲取 blob
 * url 目標文件地址
 */
function getBlob(url) {
  return new Promise(resolve => {
    const xhr = new XMLHttpRequest();

    xhr.open('GET', url, true);
    xhr.responseType = 'blob';
    xhr.onload = () => {
      if (xhr.status === 200) {
        resolve(xhr.response);
      }
    };

    xhr.send();
  });
}

/**
 * 保存 blob   
 * filename 想要保存的文件名稱
 */
function saveAs(blob, filename) {
  if (window.navigator.msSaveOrOpenBlob) {
    navigator.msSaveBlob(blob, filename);
  } else {
    const link = document.createElement('a');
    const body = document.querySelector('body');
　　　
    link.href = window.URL.createObjectURL(blob);
    link.download = filename;

    // fix Firefox
    link.style.display = 'none';
    body.appendChild(link);
    
    link.click();
    body.removeChild(link);

    window.URL.revokeObjectURL(link.href);
  }
}

/**
 * 下載
 * @param {String} url 目標文件地址
 * @param {String} filename 想要保存的文件名稱
 */
function download(url, filename) {
  getBlob(url).then(blob => {
    saveAs(blob, filename);
  });
}