index.js

/**
 * Env Config
 */

require("dotenv").config();
const Mailchimp = require("mailchimp-api-v3");
const moment = require("moment");
const nodemailer = require("nodemailer");
const { google } = require("googleapis");
const OAuth2 = google.auth.OAuth2;

const {
  API_KEY,
  RESET_DAY,
  EMAIL_LIMIT,
  GOOGLE_CLIENT_ID,
  GOOGLE_CLIENT_SECRET,
  GOOGLE_REFRESH_TOKEN,
  TO_EMAIL,
  FROM_EMAIL,
} = process.env;

/**
 * Finds the number of campaigns left to send, and sends off an email!
 * @return {integer} Campaigns left
 */
exports.mailchimpCampaignsLeft = async (req, res) => {
  try {
    const subscriberCount = await getSubscriberCount();

    const emailSent = await getEmailsSent();

    const campaignsLeft = getCampaignsLeft(subscriberCount, emailSent);
    const nextResetDate = getNextResetDate();

    sendEmail(
      `There are ${campaignsLeft} MailChimp Campaigns left to send!`,
      `Hi! You can send <strong>${campaignsLeft}</strong> more MailChimp Campaigns before ${nextResetDate}!<br><br><br>
      <strong>Statistics:</strong>
      <ul>
          <li><strong>${emailSent}</strong> emails sent of your <strong>${EMAIL_LIMIT}</strong> email limit</li>
          <li><strong>${subscriberCount}</strong> subscribers</li>
          <li><strong>${
            EMAIL_LIMIT - emailSent
          }</strong> more emails you can send</li>
          <li><strong>${
            (EMAIL_LIMIT - emailSent) / subscriberCount
          }</strong> campaigns left.</li>
      </ul>`
    );

    res.send("OK");
  } catch (error) {
    console.log(error);
  }
};

/**
 * Using the REST_DAY env variable, get the reset date
 * @return {string} Reset Date
 */
function getLastResetDate() {
  // Get Date right Now
  const now = moment();

  /* If the current Day is less than the RESET_DAY, then we want last month's month, 
  so decrement month by one. Otherwise, stay with the current month. */
  if (now.date() < RESET_DAY) {
    now.month(now.month() - 1);
  }

  // Set the day to the RESET_DAY
  now.date(RESET_DAY);

  return now.format("YYYY-MM-DD");
}

/**
 * Using getLastResetDate(), gets the next reset date
 * @return {string} Next Reset Date
 */
function getNextResetDate() {
  const now = moment(getLastResetDate());

  // Zero indexed
  now.month(now.month() + 1);

  return now.format("YYYY-MM-DD");
}

/**
 * Returns the number current subscribers
 * @return {integer} Current subscribers
 */
async function getSubscriberCount() {
  const mailchimp = new Mailchimp(API_KEY);

  try {
    const result = await mailchimp.request({
      method: "get",
      path: "/",
    });

    const subcribers = await result.total_subscribers;

    return subcribers;
  } catch (error) {
    console.log(error);
  }
}

/**
 * Returns the number of emails sent so far since the reset date
 * @return {integer} Emails sent
 */
async function getEmailsSent() {
  const mailchimp = new Mailchimp(API_KEY);

  const lastResetDate = getLastResetDate();

  try {
    const result = await mailchimp.request({
      method: "get",
      path: `/campaigns?since_send_time=${lastResetDate}`,
    });

    const emailsSent = result.campaigns.reduce(
      (total, current, i, campaign) => {
        const emailSent = campaign[i].emails_sent || 0;

        return total + emailSent;
      },
      0
    );

    return emailsSent;
  } catch (error) {
    console.log(error);
  }
}

/**
 * Returns the number of campaigns left to send
 * @return {integer} Campaigns left
 */
function getCampaignsLeft(subscriberCount, emailSent) {
  // Avoid devide by zero
  if (!subscriberCount) return;

  return Math.floor((EMAIL_LIMIT - emailSent) / subscriberCount);
}

/**
 * Sends an email!
 *
 * https://medium.com/@nickroach_50526/sending-emails-with-node-js-using-smtp-gmail-and-oauth2-316fe9c790a1
 * @param {string} Email subject
 * @param {string} Email content
 * @return {null} Email is sent off!
 */

async function sendEmail(subject, content) {
  const oauth2Client = new OAuth2(
    GOOGLE_CLIENT_ID,
    GOOGLE_CLIENT_SECRET,
    "https://developers.google.com/oauthplayground" // Redirect URL
  );

  oauth2Client.setCredentials({
    refresh_token: GOOGLE_REFRESH_TOKEN,
  });

  const accessToken = oauth2Client.getAccessToken();

  const smtpTransport = nodemailer.createTransport({
    service: "gmail",
    auth: {
      type: "OAuth2",
      user: FROM_EMAIL,
      clientId: GOOGLE_CLIENT_ID,
      clientSecret: GOOGLE_CLIENT_SECRET,
      refreshToken: GOOGLE_REFRESH_TOKEN,
      accessToken: accessToken,
    },
  });

  const mailOptions = {
    from: FROM_EMAIL,
    to: TO_EMAIL,
    subject,
    generateTextFromHTML: true,
    html: content,
  };

  smtpTransport.sendMail(mailOptions, (error, response) => {
    error ? console.log(error) : console.log(response);
    smtpTransport.close();
  });
}

exports.mailchimpCampaignsLeft();