Subscribe a web page by GoogleAppScript

main.js

const URL = 'https://example.com';
const CELL = 'A1'
const MAIL_TO = 'mail@example.com';

// Create a trigger to run `main` periodically
function main() {
  const sheet = SpreadsheetApp.getActiveSheet();
  const lastDate = readLastDate(sheet);
  const today = Utilities.formatDate(new Date(), 'UTC', 'yyyy-MM-dd');
  if (lastDate === null || lastDate == today) {
    return; // skip
  }
  const html = getHtmlContent(URL);
  const date = extractLatestNewsDate(html);
  if (lastDate == date) {
    return;
  }
  writeLatestDate(sheet, date);
  sendMail(date);
}

const getHtmlContent = (url) => {
  var html = UrlFetchApp.fetch(url).getContentText('UTF-8');
  return html;
};

const extractLatestNewsDate = (html) => {
  const pattern = /<small>(\d{4}-\d{2}-\d{2})</small>/;
  return html.match(pattern)[0];
};

const readLastDate = (sheet) => {
  return sheet.getRange(CELL).getValue();
}

const writeLatestDate = (sheet, value) => {
  sheet.getRange(CELL).setValue(value);
}

const sendMail = (date) => {
    var subject = `Update from ${URL}`;
    var name = "GAS robot";
    var body = `<a href=${URL}>${URL}</a> updated at ${date}`;

    MailApp.sendEmail({
      to: MAIL_TO, 
      subject: subject,
      name: name, 
      htmlBody: body,
      body: body
    });
}