‘Self signed certificate’ error during query the Heroku hosted Postgres database from the Node.js application

Check you pg config. It sounds like you are using pg 8 which deprecates
implicit disabling of certificate verification (as you have in your config where ssl is set to true but no ssl configuration is provided). Specify rejectUnauthorized: true to require a valid CA or rejectUnauthorized: false to explicitly opt out of MITM protection.

You can do so where you set up your pg config as follows

const client = new Client({
  connectionString: connectionString,
  ssl: { rejectUnauthorized: false }
})