An example of a server configuration for using node and postgres on heroku

database.js

// Since It's not a good idea to have everything on the server.js I moved the pool creation to this file

// Require the pg module
const { Pool } = require('pg');

// You can either do this or check if we are in production like in server.js
// In this case if the DATABASE_URL is declared we use it to the connection
const { DATABASE_URL } = process.env;
if (DATABASE_URL){
    console.log("Using database url");
    var pool = new Pool({
        connectionString: DATABASE_URL
    });
}else{
    // if the database url is not set use a local database for dev.
    console.log("Using database info");
    var pool = new Pool({
      user: '<USER>',
      host: 'localhost',
      database: '<DATABASE_NAME>',
      password: '<DATABASE_PASSWORD>',
      port: 5432 // default port for postgres
    });
}

module.exports = pool;

// To use this pool on another file, use something like this:
// const pool = require('path/to/file/database');

server.js

// A simple example to run node in heroku

//I use helmet to secure headers on production
// see https://www.npmjs.com/package/helmet for more info
const helmet = require('helmet'),
      { Pool } = require('pg');
// ...
// Your other require or config

// When deploying, heroku will set the env port to whatever port is use, if you set it static your app will not run
const PORT = process.env.PORT || 8080;

// By default heroku will set NODE_ENV to production, I stored it in a variable to set various configuration 
// like pg and helmet
const PROD = process.env.NODE_ENV === "production" || false;

// If in production this will be true
if (PROD){
  // Apply helmet 
  app.use(helmet());
  console.log("Added helmet headers");
  
  // An example of connecting with the postgres db on heroku
  // You can obtain the url of your database with the following command:
  // heroku pg:credentials:url DATABASE
  // then create a new config var DATABASE_URL in the Settings tab on heroku and paste the url.
  // For an example of how to create the pool in another file, see database.js
  const { DATABASE_URL } = process.env;
  var pool = new Pool({
    connectionString: DATABASE_URL
  });
}else{
 // Set stuff you use in dev like logs     
}

// Your routes and other stuff

app.listen(PORT);
console.log(`Staring app on ${PORT}`);