Push notifications on JavaScript with NodeJs Express

I found the solution. I don't know if is optimal, but it workks for me:

at first many configuration steps are required:

//Installl web push module
npm install web-push --save

//Generate VAPID keys by console
cd ./node_modules/.bin/web-push generate-vapid-keys

//Store VAPID keys on environment
publicVapidKey: 'generated key',
privateVapidKey: 'generated key'
process.env.PUBLIC_VAPID_KEY = config.publicVapidKey;
process.env.PRIVATE_VAPID_KEY = config.privateVapidKey;

here is the nodejs controller code that receive the subscription from client and store it in DB:

//Subscriptions
app.post('/subscribe', (req, res) => {
    const subscription = req.body.subscription;
    const userId = req.body.userId;
    console.dir(subscription);
    //TODO: Store subscription keys and userId in DB
    webpush.setVapidDetails(
        process.env.DOMAIN, 
        process.env.PUBLIC_VAPID_KEY, 
        process.env.PRIVATE_VAPID_KEY
    );
    res.sendStatus(200);
    const payload = JSON.stringify({
        title: model.lang.pushTitle,
        body: model.lang.pushBody
    });
    webpush.sendNotification(subscription, payload);
});

Here is the method I found to push a message from server to client or clients:

//Send push message
//TODO: Recover subscription keys from DB
var subscription = { 
    endpoint: 'recover from DB',
    expirationTime: null,
    keys: { 
        p256dh: 'recover from DB',
        auth: 'recover from DB' 
    } 
};
const payload = JSON.stringify({
    title: 'Notification Title',
    body: 'Notification message'
});
webpush.setVapidDetails(
    process.env.DOMAIN, 
    process.env.PUBLIC_VAPID_KEY, 
    process.env.PRIVATE_VAPID_KEY
);
webpush.sendNotification(subscription, payload)
.catch(function(err) {
    console.log(err);
});

here there are the methods in client view script to make a subscription wirth the necesary data:

//Start subscription
const publicVapidKey = 'public key of server';
if (window.Notification) {
    if (Notification.permission != 'granted') {
        Notification.requestPermission(() => {
            if (Notification.permission === 'granted') {
                getSubscriptionObject().then(subscribe)
            }
        }).catch(function(err) {
            console.log(err);
        });
    }
}

//Generate subscription object
function getSubscriptionObject() {
    return navigator.serviceWorker.register('/layout/service-worker-push.js')
    .then((worker) => {
        return worker.pushManager.subscribe({
            userVisibleOnly: true,
            applicationServerKey: urlBase64ToUint8Array(publicVapidKey)
        }).catch(function(err) {
            console.log(err);
        });
    }).catch(function(err) {
        console.log(err);
    });
}

//Send subscription to server
function subscribe(subscription) {
    return fetch(window.location.origin + '/subscribe', {
        method: 'POST',
        body: JSON.stringify({
            subscription: subscription,
            userId: mv.user_id != undefined ? mv.user_id : ''
        }),
        headers: {
            'content-type': 'application/json'
        }
    }).catch(function(err) {
        console.log(err);
    });
}

//Decoder base64 to uint8
function urlBase64ToUint8Array(base64String) {
    const padding = '='.repeat((4 - base64String.length % 4) % 4);
    const base64 = (base64String + padding)
        .replace(/-/g, '+')
        .replace(/_/g, '/');
    const rawData = window.atob(base64);
    const outputArray = new Uint8Array(rawData.length);
    for (let i = 0; i < rawData.length; ++i) {
        outputArray[i] = rawData.charCodeAt(i);
    }
    return outputArray;
}

Here is the woker (must be imported in view) who do the magic in client

//Event that shows a notification when is received by push
self.addEventListener('push', event => {
    const data = event.data.json();
    self.registration.showNotification(data.title, {
      body: data.body,
      icon: "/layout/src/android-chrome-192x192.png"
    });
});

//Event on notification click (have problems almost in Chrome)
self.addEventListener('notificationclick', () => {
    console.log('Notificación pulsada!');
});

and, well, the import of the worker in view

<script type="text/javascript" src="/layout/service-worker-push.js"></script>

note: I only tested it on localhost so I don't know if an SSL certificate is required.