Below i highlight every single step you need in order to deploy your Node, Mongodb app to heroku:

1. Assumming your code works on localhost
2. Sign Up to www.heroku.com and setup your billing information (it won't charge anything but we need it to create a MongoDB add-on to our app on heroku. It won't accept without billing info)
3. On root create a .env file. Inside write MONGO_DB_URI=mongodb+srv://<YourUsername>:<yourPassword>@cluster1.qcpvo.mongodb.net/<dbname>?retryWrites=true&w=majority (Your own uri without any string " ")
4. Then change in index.js (or server.js based on your setup): const uri = 'mongodb+srv://YourUsername:<yourPassword>@cluster1.qcpvo.mongodb.net/Students?retryWrites=true&w=majority'; TO const uri = process.env.MONGO_DB_URI || 'mongodb+srv://Ulan:123@cluster1.qcpvo.mongodb.net/Students?retryWrites=true&w=majority';
5. On root directory create Procfile file.
   • On terminal run:
   • touch Procfile
   • Inside Profile write web: npm start
6. On terminal run these commmands by order:
   • git init
   • git add --all
   • git commit -m" deployment"
   • heroku login (it will open a browser to confirm login. Hit login and get back to terminal)
   • heroku create your-app-name (this will output something like this: Creating ⬢ your-app-name... done)
   • heroku addons:create mongolab:sandbox (It installs mLab MongoDB add-on with the "Sandbox" plan which is free)
   • heroku config:set MONGO_DB_URI='mongodb+srv://Ulan:<yourPassword>@cluster1.qcpvo.mongodb.net/Students?retryWrites=true&w=majority'
   • git push heroku master (push it to heroku master)
   • heroku open (or open from heroku app dashboard)
7. If you created routes try CRUD operations with your new deployed link.

P.S: mLab is shutting down its Heroku add-on on November 10, 2020. You will lose access to your data unless you detach from Heroku or migrate to MongoDB Atlas before then.