This is a simple wrapper around mailgun.send that adds easy templating and forces tags to be used.

MailgunSender.js

//
// MailgunSender
//   Copyright 2017 Joe Noon <[email protected]>
//   MIT License
//
// This is a simple wrapper around mailgun.send that adds easy templating and forces
// tags to be used.
//
// Initialize a MailgunSender with the path to your templates, and instance of mailgun:
//
// const mailgunSender = new MailgunSender({
//   path: require('path').join(__dirname, './templates/'),
//   mailgun: require('mailgun-js')({ apiKey: "key-xyz", domain: 'mydomain.local' }),
// });
//
// The template path should be structured like this:
// ./
//  - news.html
//  - news.txt
//  - welcome.html
//  - bye.txt
//
// If both an html and txt are given for the same name, both parts will be sent to mailgun.
// Otherwise only the part you provide will be sent.
//
// Send mail
//
// const res = await mailgunSender.send({
//   from: 'Me <[email protected]>',
//   to: '[email protected]',
//   subject: 'Hello',
//   tags: ['tag1', 'tag2'],
//   template: 'news',
// });
//
// Required: `from`, `to`, `subject`, `tags`
//           `template` OR one or both of `html`, `text`
// Optional: `template_locals` - passed to ejs template
//

const ejs = require('ejs');
const _path = require('path');
const fs = require('fs');
const util = require('util');

const readFile = util.promisify(fs.readFile);
const readdir = util.promisify(fs.readdir);

function MailgunSender ({path,mailgun}) {
  
  const templates = {};

  let loaded = false;

  const load_templates = async () => {
    if (loaded) return;
    loaded = true;
    const files = await readdir(path, 'utf8');
    for (const file of files) {
      const content = await readFile(_path.join(path, file), 'utf8');
      const template = ejs.compile(content);
      const parts = _path.parse(file);
      const {name,ext} = parts;
      templates[name] = {
        ...templates[name],
        [ext]: template,
      }
    }
  };

  this.send = async ({from,to,subject,tags,template,template_locals,...rest}) => {

    if (!from) throw new Error('from required');
    if (!to) throw new Error('to required');
    if (!subject) throw new Error('subject required');
    if (!tags) throw new Error('tags required');

    const opts = {
      from,
      to,
      subject,
      'o:tag': tags,
      ...rest
    };

    if (template) {
      await load_templates();
      const tparts = templates[template] || {};
      const html_t = tparts['.html'];
      if (html_t) opts.html = html_t(template_locals);
      const text_t = tparts['.txt'];
      if (text_t) opts.text = text_t(template_locals);
    }
    
    if (!opts.html && !opts.text) throw new Error(`no html or text rendered.  check that template '${template}' is defined.`);
    
    const res = await mailgun.messages().send(opts);
    return res;
  }

}

module.exports = MailgunSender;