With local Postgresql:
Bash: $ npm run watch-heroku
With Heroku Postgresql:
First we gather parameters.
In Heroku:
Navigate to App -> Resources Click "Heroku Postgres" to get to the Heroku Postgresql service page.
From Heroku Postgresql service page Navigate to Settings -> View Credentials... to see URI (aka connection string, aka DATABASE_URL)
Then we run the command line:
Bash: $ DATABASE_URL=postgres://blahblah npm run watch-heroku
The first part "DATABASE_URL=postgres://blahblah" sets the DATABASE_URL environment variable for the process started with "npm run watch-heroku".
Windows CMD: set DATABASE_URL=postgres://blahblah npm run watch-heroku
Paper on web app setup: https://12factor.net/ -- it will make you better --