Export a factory function that takes the configuration object and returns an instance of the client. Client isn't a default export in pg so it needs to be destructured.

const { Client } = require('pg');

const createPgClient = (conf) => {
    const pgClient = new Client(conf)({
        host: conf['postgres'].host,
        port: conf['postgres'].port,
        dbname: conf['postgres'].dbname,
        username: conf['postgres'].username,
        password: conf['postgres'].password,
        dbconn: null,
    });

    pgClient.prototype.connect = function (callback) {

        client.connect()
        .then(() => {
            console.log(new Date() + " | Postgres Server Authenticated...");
        })
        .catch((err) => {
            console.log(new Date() + " | Postgres Server Error in connection...");
            console.log(err);
        })
        .finally(()=> {
            self.dbconn = this;
            callback(this);
        });

    };
    return pgClient;
};
module.exports = createPgClient;

Use this factory function in your app.js to create a client.

const createPgClient = require('./adapters/postgres')
cont pgClient = createPgClient(conf);

pg.connect(function (dbconn) {
    app.dbconn = dbconn;
    app.conf = conf;

    console.log("************************************************************");
    console.log(new Date() + ' | CRUD Server Listening on ' + conf['web']['port']);
    console.log("************************************************************");

    server.listen(conf['web']['port']);

    const Routes = require('./routes/http-routes');
    new Routes(app);
});