Send email with template

Code.gs

/**
 * This function generates a custom menu on the Spreadsheet Toolbar
 */
const onOpen  = () => {
  const ui = SpreadsheetApp.getUi();
  ui.createMenu('Custom Menu')
  .addItem('Send E-mails', 'startProcess')
  .addToUi();
}

/**
 * This function is triggered by the user when he runs the custom menu 'Send E-mails'
 */
const startProcess = () => {
  const ss = SpreadsheetApp.getActiveSpreadsheet(); // Get the active Spreadsheet

  const emailsSheet = ss.getSheetByName('Email List'); // Get the sheet 'Email List'
  const listEmails = emailsSheet.getRange('A:A').getValues().flat(); // Get list of emails in simple Array
  
  const recipients = emailsSheet.getRange(2,1,emailsSheet.getLastRow() - 1, emailsSheet.getLastColumn())
                     .getValues().filter(item => item[1]); // Get only rows where checkboxes are checked

  const paramSheet = ss.getSheetByName('Params'); // Get the sheet 'Params'
  const docIdTemplate = DocumentApp.openByUrl(paramSheet.getRange('B1').getValue()).getId(); // Get Docs ID from the url in cell 'B1'
  const subject = paramSheet.getRange('B2').getValue(); // Get the subject for email in cell 'B2'
  const date = Utilities.formatDate(new Date(), Session.getScriptTimeZone(), 'dd/MM/yyyy') // Get the current date

  DriveApp.getStorageUsed(); // unlock Drive Scope for using UrlFetchApp
  
  const url = "https://docs.google.com/feeds/download/documents/export/Export?id=" + docIdTemplate + "&exportFormat=html";
  
  const param = {
    method: "get",
    headers: {
      "Authorization": "Bearer " + ScriptApp.getOAuthToken()
    },
    muteHttpExceptions: true,
  };

  const html = UrlFetchApp.fetch(url, param).getContentText(); //Get the html from the Docs template

  recipients.forEach(recipient => {
    const email = recipient[0];
    const body = formatBody_(html, email);
    sendEmail_(email, subject, body);
    const row = listEmails.indexOf(email);
    emailsSheet.getRange(row + 1, 3, 1, 1).setValue(date);
  });

  ss.toast('All emails have been sent','✅ DONE');
}

/**
 * This function sends a e-mail to the recipients selected 
 * @param {String} e-mail adress
 * @param {String} e-mail subject
 * @param {HTML} html body for e-mail
 */
const sendEmail_ = (email, subject, body)  => {  
  MailApp.sendEmail(email, subject, null, {          
      htmlBody: body,
      //noReply: true //this option is only allowed for Google Workspace accounts, not Gmail users
    });
}

/**
 * This function formats HTML by replacing placeholders 
 * @param {HTML} html body for e-mail
 * @param {String} e-mail adress 
 * @return {HTML} html where placeholders has been replaced 
 */
const formatBody_ = (html, email) => {
  const fullName = formatFullName_(email); // get the full name from an email in the following format: firstName.lastName@mail.com
  html =  html.replace('#FULL_NAME', fullName); // replace placeholder
  //html =  html.replace('#OTHER_PLACEHOLDER', otherThing); // replace placeholder
  //html =  html.replace('#ANOTHER_PLACEHOLDER', anotherThing); // replace placeholder
  return html;
}

/**
 * This function extracts full name from an email 
 * @param {String} e-mail adress
 * @return {String} full name
 */
const formatFullName_ = (email) => {
  if (email.split('@')[0].indexOf('.') === -1) return '';

  const fullName = email.split('@')[0].split('.');
  return fullName.map(name => capitalize_(name)).join(' ');
}

/**
 * This function convert the first letter of the word to uppercase 
 * @param {String} full name
 */
const capitalize_ = (name) => {
  return name.charAt(0).toUpperCase() + name.slice(1);
}