Such Example is knowing wide spreading among PWAs (Progressive Web Applications) and in this example we're going to send a simple Backend like notification using NodeJS and ES6

 'use strict';

 const express = require('express');
 const app = express();
 const gcm = require('node-gcm');
 app.io = require('socket.io')();
 
 // [*] Configuring our GCM Channel.
 const sender = new gcm.Sender('Project Secret');
 const regTokens = [];
 let message = new gcm.Message({
     data: {
         key1: 'msg1'
     }
 });
 
 // [*] Configuring our static files.
 app.use(express.static('public/'));
 
 // [*] Configuring Routes.
 app.get('/', (req, res) => {
     res.sendFile(__dirname + '/public/index.html');
 });
 
 // [*] Configuring our Socket Connection.
 app.io.on('connection', socket => {
     console.log('we have a new connection ...');
     socket.on('new_user', (reg_id) => {
         // [*] Adding our user notification registration token to our list typically hided in a secret place.
         if (regTokens.indexOf(reg_id) === -1) {
             regTokens.push(reg_id);
 
             // [*] Sending our push messages
             sender.send(message, {
                 registrationTokens: regTokens
             }, (err, response) => {
                 if (err) console.error('err', err);
                 else console.log(response);
             });
         }
     })
 });
  
 module.exports = app

PS : I'm using here a special hack in order to make Socket.io works with Express because simply it doesn't work outside of the box.

Now Create a .json file and name it : Manifest.json, open it and past the following :

{
    "name": "Application Name",
    "gcm_sender_id": "GCM Project ID"
}

Close it and save in your application ROOT directory.

PS : the Manifest.json file needs to be in root directory or it won't work.

In the code above I'm doing the following :

1. I seted up and sent a normal index.html page that will use socket.io also.
2. I'm listening on a connection event fired from the front-end aka my index.html page (it will be fired once a new client successfully connected to our pre-defined link)
3. I'm sending a special token know's as the registration token from my index.html via socket.io new_user event, such token will be our user unique passcode and each code is generated usually from a supporting browser for the Web notification API (read more here. (opens new window)
4. I'm simply using the node-gcm module to send my notification which will be handled and shown later on using Service Workers (opens new window)`.

This is from NodeJS point of view. in other examples I will show how we can send custom data, icons ..etc in our push message.

PS : you can find the full working demo over here. (opens new window)