A little while ago I did this awesome course on udemy.com called: "The complete node.js developer course". While not everything in the course was new to me it is very well structured and I was able to gain a solid understanding of how node.js actually works.
One of the greatest things of the course was the focus on a real "production line". Meaning a clean, production ready set up of your coding environment including git and deploying to Heroku with the Heroku CLI.
The chat app
One of the nicest projects in the course was the chat app. It (obviously) uses node.js and sockets.io - so it's a super fast, realtime app. You can choose your username and your chatroom and get chatting. Actually much like slack but - yeah - much simpler.
You can check it out on heroku.