Some notes:

Dev

1. create .env file with contents like these

 NODE_ENV=local
 DB_CONNECTION_STRING=mongodb://localhost  
 PORT=5000
 SLACK_WEBHOOK=https://hooks.slack.com/services/....
 DARTS_CHANNEL=darts

2. run heroku local dev
3. cd client
4. npm start

Prod

1. Heroku - configure env variables
2. Deploy (eg. from heroku dashboard, github integration)
3. Done (heroku will automatically call npm start and npm run heroku-postbuild which will build the API and Client and start listening on PORT)