heroku: deploy next.js

# set up nextjs app
git clone https://.../your-nextjs-app.git

# add a new remote branch named as heroku which connects to the heroku servercd your-nextjs-app
heroku create

# create procfile (set command for heroku server to run app)
vim Procfile

# YOU MUST SET PORT as $PORT, NOT a NUMBER LIKE 3000 or 8080. because heroku randomly assigns port nubmer to the deployed app 
npx next start -p$PORT

# deploy your app to heroku server
git push heroku 

# open page
heroku open