Vulcan/packages/vulcan-email/lib/server/routes.js

import { Picker } from 'meteor/meteorhacks:picker';
import { getSetting } from 'meteor/vulcan:lib';
import VulcanEmail from '../namespace.js';

Meteor.startup(function() {
  _.forEach(VulcanEmail.emails, (email, key) => {
    // template live preview routes
    Picker.route(email.path, async (params, req, res) => {
      let html;
      // if email has a custom way of generating test HTML, use it
      if (typeof email.getTestHTML !== 'undefined') {
        html = email.getTestHTML.bind(email)(params);
      } else {
        const locale = params.query.locale || getSetting('locale');

        // else get test object (sample post, comment, user, etc.)
        const testVariables =
          (typeof email.testVariables === 'function' ? email.testVariables(params) : email.testVariables) || {};
        // delete params.query so we don't pass it to GraphQL query
        delete params.query;
        // merge test variables with params from URL
        const variables = { ...testVariables, ...params };

        const { data: emailTestData, subject, html: builtHtml } = await VulcanEmail.build({
          emailName: key,
          variables,
          locale
        });

        // remove Strings object to avoid echoing it out
        delete emailTestData.__;

        html = `
          ${builtHtml}
          <h4 style="margin: 20px;"><code>Subject: ${subject}</code></h4>
          <h5 style="margin: 20px;">Variables:</h5>
          <div style="border: 1px solid #999; padding: 10px 20px; margin: 20px;">
            <pre><code>${JSON.stringify(variables, null, 2)}</code></pre>
          </div>
          <h5 style="margin: 20px;">Data:</h5>
          <div style="border: 1px solid #999; padding: 10px 20px; margin: 20px;">
            <pre><code>${JSON.stringify(emailTestData, null, 2)}</code></pre>
          </div>
        `;
      }

      // return html
      res.end(html);
    });

    // raw template
    Picker.route('/email/template/:template', (params, req, res) => {
      res.end(VulcanEmail.templates[params.template]);
    });
  });
});