db.js

var pg = require('pg');

var c = require('./config');

var db = (function db() {
  'use strict';

  var cnxStr = `postgres://${c.db.user}:${c.db.password}@${c.db.host}/${c.db.database}`

  function query(sql, params) {
    params = params || [];

    return new Promise(function (resolve, reject) {
      pg.connect(cnxStr, function(err, client, done) {
        if (err) {
          return reject(err);
        }

        client.query(sql, params, function (err, result) {
          // Release client back to pool.
          done(client);

          if (err) {
            return reject(err);
          }

          return resolve(result);
        });
      });
    });
  }

  return {
    query: query,
  };
})();


module.exports = db;

test.js

var db = require('./db');

db.query('select * from users').then(function (d){
  console.log('hi', d);
}, function (data) {
  console.log('a', data);
});