Gmail script to warn when emailing certain domains

// ==UserScript==
// @name         Gmail Protected Recipients
// @namespace    local.gmail.protected-recipients
// @version      1.3
// @description  Warn before emailing protected domains in Gmail
// @match        https://mail.google.com/*
// @run-at       document-start
// ==/UserScript==

(() => {
  'use strict';

  const PROTECTED_DOMAINS = [
    'example.com'
  ];

  const LOG_PREFIX = '[Gmail Protected Recipients]';

  function log(...args) {
    console.log(LOG_PREFIX, ...args);
  }

  function isProtectedEmail(email) {
    const normalized = email.trim().toLowerCase();

    return PROTECTED_DOMAINS.some(domain =>
      normalized.endsWith(`@${domain.toLowerCase()}`)
    );
  }

  function extractEmails(root) {
    const found = new Set();

    if (!root) return [];

    const emailRegex =
      /[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,}/gi;

    root.querySelectorAll('*').forEach(el => {
      const attrs = [
        'email',
        'data-hovercard-id',
        'data-email',
        'aria-label',
        'title',
        'name',
      ];

      for (const attr of attrs) {
        const value = el.getAttribute?.(attr);

        if (!value) continue;

        value.match(emailRegex)?.forEach(email => {
          found.add(email.toLowerCase());
        });
      }

      if (
        el instanceof HTMLInputElement ||
        el instanceof HTMLTextAreaElement
      ) {
        el.value.match(emailRegex)?.forEach(email => {
          found.add(email.toLowerCase());
        });
      }
    });

    return [...found];
  }

  function findComposeArea(sendButton) {
    let node = sendButton;

    while (node && node !== document.body) {
      const emails = extractEmails(node);

      log(
        'Checking ancestor:',
        node,
        'emails:',
        emails
      );

      if (emails.length > 0) {
        log('Using compose area:', node);
        return node;
      }

      node = node.parentElement;
    }

    log('Could not find recipient-containing ancestor');
    return null;
  }

  function getProtectedRecipients(root) {
    const emails = extractEmails(root);

    log('Detected emails:', emails);

    const protectedRecipients =
      emails.filter(isProtectedEmail);

    log(
      'Protected recipients:',
      protectedRecipients
    );

    return protectedRecipients;
  }

  function confirmProtectedSend(root) {
    const recipients =
      getProtectedRecipients(root);

    if (!recipients.length) {
      return true;
    }

    return window.confirm(
      '⚠️ WARNING: PROTECTED RECIPIENT ⚠️\n\n' +
      'You are about to send email to:\n\n' +
      recipients.map(x => `    ${x}`).join('\n') +
      '\n\nAre you ABSOLUTELY SURE?'
    );
  }

  function getSendButton(element) {
    const button = element.closest(
      '[role="button"], button, [data-tooltip], [aria-label]'
    );

    if (!button) return null;

    const labels = [
      button.textContent,
      button.getAttribute('data-tooltip'),
      button.getAttribute('aria-label'),
      button.getAttribute('title'),
    ].filter(Boolean);

    const isSend = labels.some(value =>
      /^\s*send(?:\s|$)/i.test(value)
    );

    return isSend ? button : null;
  }

  document.addEventListener(
    'click',
    event => {
      if (!(event.target instanceof Element)) return;

      const sendButton =
        getSendButton(event.target);

      if (!sendButton) return;

      log('Send clicked:', sendButton);

      const composeArea =
        findComposeArea(sendButton);

      if (!composeArea) {
        log(
          'No compose area found — allowing send'
        );
        return;
      }

      if (!confirmProtectedSend(composeArea)) {
        log('Send cancelled');

        event.preventDefault();
        event.stopPropagation();
        event.stopImmediatePropagation();
      }
    },
    true
  );

  document.addEventListener(
    'keydown',
    event => {
      if (
        event.key !== 'Enter' ||
        (!event.metaKey && !event.ctrlKey)
      ) {
        return;
      }

      if (!(event.target instanceof Element)) {
        return;
      }

      log('Cmd/Ctrl+Enter detected');

      // For keyboard sends, start walking upward
      // from the currently focused compose element.
      let node = event.target;
      let composeArea = null;

      while (node && node !== document.body) {
        const emails = extractEmails(node);

        if (emails.length > 0) {
          composeArea = node;
          break;
        }

        node = node.parentElement;
      }

      if (!composeArea) {
        log(
          'Could not find compose area for keyboard send'
        );
        return;
      }

      if (!confirmProtectedSend(composeArea)) {
        log('Keyboard send cancelled');

        event.preventDefault();
        event.stopPropagation();
        event.stopImmediatePropagation();
      }
    },
    true
  );

  log('Userscript loaded');
})();