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

1. Install Node-GCM Module : npm install node-gcm

2. Install Socket.io : npm install socket.io

3. Create a GCM Enabled application using Google Console.

4. Grabe your GCM Application Id (we will need it later on)

5. Grabe your GCM Application Secret code.

6. Open Your favorite code editor and add the following code :

    '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.
4. I'm simply using the node-gcm module to send my notification which will be handled and shown later on using Service Workers`.

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.