A Google Apps Script to monitor RSS and Atom feeds, automatically detect new entries, and send email notifications with the new post.

FeedWatcher.gs

function monitor(feedName, url, author, filter = null) {
  console.log(feedName);
  let startTime = Date.now();
  let scriptProperties = PropertiesService.getScriptProperties();
  let lastUpdate = scriptProperties.getProperty(`lastUpdate${feedName}`);
  let errorCount = parseInt(scriptProperties.getProperty(`errorCount${feedName}`) || "0");
  let root;
  try {
    let res = UrlFetchApp.fetch(url);
    root = XmlService.parse(res.getContentText().replace(/content:encoded/g, "content").replace(/&#(\w.+?);/g, (_, p) => {return String.fromCharCode(p)})).getRootElement();
    scriptProperties.deleteProperty(`errorCount${feedName}`);
  } catch (e) {
    errorCount++;
    scriptProperties.setProperty(`errorCount${feedName}`, errorCount.toString());
    console.error(`${feedName} failed`, errorCount, lastUpdate);
    if (errorCount < 24)
      return;
    throw e;
  }
  let ns = root.getNamespace();
  let entries = root.getChild("channel", ns)?.getChildren("item", ns) || root.getChildren("entry", ns);
  entries.reverse().forEach(entry => {
    let published = entry.getChild("updated", ns)?.getText() || entry.getChild("pubDate", ns)?.getText();
    if (new Date(published) <= new Date(lastUpdate))
      return;
    scriptProperties.setProperty(`lastUpdate${feedName}`, published);    
    let title = entry.getChild("title", ns).getText().replace(/&amp;#(\w.+?);/g, (_, p) => {return String.fromCharCode(p)});
    let content = entry.getChild("content", ns)?.getText() || entry.getChild("description", ns)?.getText() || entry.getChild("summary", ns)?.getText() || "";
    let link = entry.getChild("link", ns).getAttribute("href")?.getValue() || entry.getChild("link", ns).getText().replace(/\?.*/, "");
    let body = `<b>${title}</b> von ${author}<br /><a href="${link}">${link}</a><br /><div style="width:700px">${content}</div>`;
    if (!filter || link.match(filter))
      MailApp.sendEmail({to: Session.getActiveUser().getEmail(), subject: title, htmlBody: body, name: author});
  });
  let duration = (Date.now() - startTime) / 1000;
  console.log(`${duration.toFixed(3)} s`);
}