server-side-web-push.js

// Service Worker Notifications
const publicVapidKey = '<<<Public Vapid Key>>>';
const privateVapidKey = '<<<Private Vapid Key>>>';

webpush.setVapidDetails('mailto:someEmail@emailSite.com', publicVapidKey, privateVapidKey);

app.post('/subscribe', jsonParser, async function(req, res) {
    try {
        let hash = objectHash(req.body);
        let subscription = req.body;
        let checkSubscription = await Subscription.Subscription.find({ 'hash' : hash });
        
        let theMessage = JSON.stringify({ title: 'You have already subscribed', body: 'Some body text here.' });
        if(checkSubscription.length == 0) {
            const newSubscription = new Subscription.Subscription({
                hash: hash,
                subscriptionEl: subscription
            });
            newSubscription.save(function (err) {
                if (err) {
                    theMessage = JSON.stringify({ title: 'We ran into an error', body: 'Please try again later' });
                    webpush.sendNotification(subscription, payload).catch(error => {
                        console.error(error.stack);
                    });
                    res.status(400);
                } else {
                    theMessage = JSON.stringify({ title: 'Thank you for Subscribing!', body: 'Some body text here' });
                    webpush.sendNotification(subscription, payload).catch(error => {
                        console.error(error.stack);
                    });                    
                    res.status(201);
                }
            });
        } else {
            webpush.sendNotification(subscription, theMessage).catch(error => {
                console.error(error.stack);
            });
            res.status(400);
        }
    } catch(e) {
        console.log(e);
    }
});