Sending e-mails on a schedule with Firebase Functions and Sendgrid

index.js

// index.js

// inititalize firebase
const functions = require('firebase-functions');
const admin = require('firebase-admin');

// install by running "yarn add date-fns"
const { isPast, addHours } = require('date-fns');

admin.initializeApp();

// db is a global constant so that it can be re-used efficiently for each firebase function
// see https://firebase.google.com/docs/functions/tips#use_global_variables_to_reuse_objects_in_future_invocations
const db = admin.firestore();

exports.onUserUpdate = admin.firestore.document('users/{uid}').onUpdate(handleCreateScheduledEmails);
exports.sendScheduledEmails = functions.pubsub.schedule('every 60 minutes').onRun(handleSendScheduledEmails);

async function handleCreateScheduledEmails(snap, context) {
    const { confirmed, emailAddress } = snap.data().after;
    const wasPreviouslyConfirmed = snap.data().before.confirmed;
    const isUserConfirming = confirmed && !wasPreviouslyConfirmed

    // 1. Check if the user is confirming their e-mail address
    if (!isUserConfirming) return null;

    // 2. Create welcomeEmail
    const welcomeEmail = createWelcomeEmail(emailAddress);
    // 3. Create howTo email
    const howToEmail = createHowToEmail(emailAddress);

    // 4. Return a single promise that resolves if all provided promises also resolve
    return Promise.all([welcomeEmail, howToEmail]);
}

async function handleSendScheduledEmails(snap, context) {
    const scheduledEmailsResult = await getScheduledEmails();
    const promises = [];

    scheduledEmailsResult.forEach((emailDoc) => {
        const email = emailDoc.data();
        const { scheduledDate, sendgridEmail, status } = email;

        // .toDate() is needed because this is a Firebase Timestamp
        if (isPast(scheduledDate.toDate()) && status === 'PENDING') {
            promises.push(sendEmail(sendgridEmail, emailDoc.ref));
        }
    });

    // We're using .allSettled() because the results are not dependent on each other
    // i.e: if one fails, it should continue executing other promises
    // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/allSettled
    const promiseResults = await Promise.allSettled(promises);
    for (const result of promiseResults) {
        if (result.status === 'rejected') {
            functions.logger.error(`Could not send scheduled email: `, result.reason);
        }
    }
    return null;
}

function createWelcomeEmail(emailAddress) {
    const today = admin.firestore.Timestamp.now();
    const scheduledDate = addHours(today.toDate(), 4);

    const emailConfig = {
        to: emailAddress,
        type: 'welcomeEmail',
        scheduledDate
    }

    return createScheduledEmail(emailConfig);
}

function createHowToEmail(emailAddress) {
    const today = admin.firestore.Timestamp.now();
    const scheduledDate = addHours(today.toDate(), 24);
    const data = { firstName: "Bob" }
    const emailConfig = {
        to: emailAddress,
        type: 'howToEmail',
        data,
        scheduledDate
    }

    return createScheduledEmail(emailConfig);
}

async function createScheduledEmail({ to, type, data, scheduledDate }) {
    const scheduledEmail = {
        scheduledDate,
        status: 'PENDING', // PENDING, COMPLETED, ERROR
        sendgridEmail: {
            to,
            template: { name: type, data }
        }
    };

    return db.collection('scheduledEmails').add(scheduledEmail);
}

async function getScheduledEmails() {
    return db.collection('scheduledEmails')
        .where('scheduledDate', '<=', admin.firestore.Timestamp.now())
        .where('status', '==', 'PENDING')
        .orderBy('scheduledDate')
        .get();
}

async function sendEmail(email, scheduledEmailRef) {
    try {
        await db.collection('emails').add(email);
        return scheduledEmailRef.update({ status: 'COMPLETED' });
    } catch (error) {
        await scheduledEmailRef.update({ status: 'ERROR', error: error.message });
        return Promise.reject(error.message);
    }
}

I love that approach! Thanks for providing this example!