How to Build Twitter’s Real-time Likes Feature with Node.js and Pusher

If you use Twitter, you’ve probably noticed a nifty little feature that’s been around for a while: real-time tweet statistics. This basically means that you get to see the number of Likes or Retweets of a tweet increase (or decrease) as people around the world like or retweet it, without having to refresh the page. Cool, eh? In this article, I’ll walk you through implementing your own real-time post statistics (we’ll limit ourselves to Likes) in a simple Node.js app. Here’s a demo of the app in action (please forgive my bland UI): Liking a post in one window immediately reflects in the other Set up the project This tutorial assumes you have Node.js (version 6 or higher) and MongoDB (version 3.2 or higher) installed. We’ll be using Express, a popular lightweight Node.js framework. Let’s get our app set up quickly by using the : express application generator # if you don't already have it installednpm install express-generator -g # create a new express app with view engine set to Handlebars (hbs)express --view=hbs postercd poster && npm install Then we’ll add the dependencies we’ll need: npm install --save dotenv faker mongoose pusher Here’s a breakdown of what each module is for. We’re using MongoDB for our data store, so we’ll use to map our MongoDB documents to models (JavaScript objects). Mongoose will help us generate fake data for our app, since we just want to demonstrate the likes feature. Faker We need to talk to Pusher’s APIs. pusher Lastly, is a small package that helps us load our private configuration variables (like our Pusher app credentials) from a file. dotenv .env Basic Data and View First, let’s define our data structures. We’ll limit the scope of this demo to two entities: and . For users. we’ll store only their names. For posts, we’ll store: users posts the text the date it was posted the user who posted it (the author), and the number of likes it has received Since the only detail we need about our users is their names, we won’t set up a User model; we’ll reference the user’s name directly from our Post model. So, let’s create a file, : models/post.js Now, we’ll write a small script to get some fake data into our database. Create a file called in the directory, with the following content: seed.js bin Run the seed using node (remember to start your MongoDB server by running first): sudo mongod node bin/seed.js Let’s set up the route and view for our home page. The first thing we’ll do is add our MongoDB connection setup, so the connection gets created when our app gets booted. Add this line somewhere near the top of your : app.js require('mongoose').connect('mongodb://localhost/poster'); Next up, the route where we retrieve all posts from the db and pass them to the view. Replace the code in with this: routes/index.js let router = require('express').Router();let Post = require('./../models/post'); router.get('/', (req, res, next) => { Post.find().exec((err, posts) => { res.render('index', { posts: posts }); });}); module.exports = router; Lastly, the view where we render the posts ( ). We’ll use Bootstrap for some quick styling. views/index.hbs A few notes: We attach a attribute to each Like button so we can easily identify which post it points to. This will come in useful soon. data-post-id We give each field an which includes the post ID, so we can directly reference the correct with just the post ID. likes_count id likes_count We have a click handler on the Like button ( ) . This is where we’ll toggle the button text (Like → Unlike) and increment the . (And the reverse for when it’s an Unlike button). We’ll implement that in a bit. **actOnPost** likes_count Liking and Unliking Logic When a user clicks on ‘Like’, here’s what we want to happen: The text on the button changes from “Like” to “Unlike”. The likes count displayed next to the post increases by 1. An AJAX request is made to the server to increment the in the database by 1. likes_count The likes count displayed next to the post increases by 1 in all other tabs/windows where the page is open. (This is where Pusher comes in.) For unliking: The text on the button changes from “Unlike” to “Like”. The likes count displayed next to the post decreases by 1. An AJAX request is made to the server to decrement the in the database by 1. likes_count The likes count displayed next to the post decreases by 1 in all other tabs/windows where the page is open. (Once again, via Pusher.) We’ll classify both Likes and Unlikes as that can be carried out on a post, so we can handle them together. actions Let’s add some JavaScript to our home page for the method. We’ll pull in for easy HTTP requests. Add the following to your : actOnPost Axios views/index.hbs var updatePostStats = { Like: function (postId) { document.querySelector('#likes-count-' + postId).textContent++; }, Unlike: function(postId) { document.querySelector('#likes-count-' + postId).textContent--; } }; var toggleButtonText = { Like: function(button) { button.textContent = "Unlike"; }, Unlike: function(button) { button.textContent = "Like"; } }; var actOnPost = function (event) { var postId = event.target.dataset.postId; var action = event.target.textContent.trim(); toggleButtonText[action](event.target); updatePostStats[action](postId); axios.post('/posts/' + postId + '/act', { action: action }); }; Then we define the route in our act routes/index.js: router.post('/posts/:id/act', (req, res, next) => { const action = req.body.action; const counter = action === 'Like' ? 1 : -1; Post.update({_id: req.params.id}, {$inc: {likes_count: counter}}, {}, (err, numberAffected) => { res.send(''); });}); Here, we change the likes_count using MongoDB’s built-in operator for update operations. $inc Notifying Other Clients with Pusher At this point, we’ve got our regular Liking and Unliking feature in place. Now it’s time to notify other clients when such an action happens. Let’s get our Pusher integration set up. if you don’t have one already. Then visit and create a new app and take note of your app’s credentials. Since we’re using the dotenv package, we can put our Pusher credentials in a .env file in the root directory of our project: Create a free Pusher account your dashboard PUSHER_APP_ID=WWWWWWWWWPUSHER_APP_KEY=XXXXXXXXXPUSHER_APP_SECRET=YYYYYYYYPUSHER_APP_CLUSTER=ZZZZZZZZ Replace the stubs above with your app credentials from your Pusher dashboard. Then add the following line to the top of your app.js: require('dotenv').config(); Next we’ll modify our route handler to trigger a Pusher message whenever an action updates the in the database. We’ll initialise an instance of the Pusher client and use it to send a message by calling . likes_count pusher.trigger The trigger method takes four parameters: the name of the to send this message on channel the name of the message the (any data you wish to send with the message) payload the . If this is supplied, Pusher will send this message to every client except the client with this ID. This is useful so we can exclude the client who caused the action from being notified of it again. socket ID Here’s what we want our payload to look like in the case of a Like action: { "action": "Like", "postId": 1234} So let’s add this logic to our route handler: let Pusher = require('pusher'); let pusher = new Pusher({ appId: process.env.PUSHER_APP_ID, key: process.env.PUSHER_APP_KEY, secret: process.env.PUSHER_APP_SECRET, cluster: process.env.PUSHER_APP_CLUSTER}); router.post('/posts/:id/act', (req, res, next) => { const action = req.body.action; const counter = action === 'Like' ? 1 : -1; Post.update({_id: req.params.id}, {$inc: {likes_count: counter}}, {}, (err, numberAffected) => { pusher.trigger('post-events', 'postAction', { action: action, postId: req.params.id }, req.body.socketId); res.send(''); });}); On the client side ( ) we need to handle two things: index.hbs subscribe each client to the channel post-events .add the client’s socket ID to our API request, so the server can use it to exclude the client act We’ll pull in the Pusher SDK: var pusher = new Pusher('your-app-id', { cluster: 'your-app-cluster' }); var socketId; // retrieve the socket ID on successful connection pusher.connection.bind('connected', function() { socketId = pusher.connection.socket_id; }); var channel = pusher.subscribe('post-events'); channel.bind('postAction', function(data) { // log message data to console - for debugging purposes console.log(data); var action = data.action; updatePostStats[action](data.postId); }); All done! Start your app by running: npm start Now, if you open up in two (or more) tabs in your browser, you should see that liking a post in one instantly reflects in the other. Also, because of our console.log statement placed earlier, you’ll see the event is logged: http://localhost:3000 Conclusion In this article, we’ve seen how Pusher’s publish-subscribe messaging system makes it straightforward to implement a real-time view of activity on a particular post. Of course, this is just a starting point; I look forward to seeing all the great things you’ll build. You can check out the source code of the completed application . on Github This article was originally published by the author on Pusher’s blog .