You should have an extra local .env file like below.
And then you run: npm run watch and to see your deploying
on localhost:3000

Remember to set config vars.
Ref:

CONNECTIONSTRING=<connection string, no any quote>
PORT=<port value>
JWTSECRET=<secret string.>
SENDGRIDAPIKEY=<send grid key>

Note that I have moved from heroku to onrender for free deploying services.