pg_notify combine updated and previous values in json payload

client.js

import pg from 'pg'
import EventEmitter from 'events'
import util from 'util'

/*
initialize postgres client...
*/
function PgEmitter() {
  EventEmitter.call(this)
}
util.inherits(PgEmitter, EventEmitter)
const pgEmitter = new PgEmitter;

pgclient.on('notification', (msg) => {
  let payload = JSON.parse(msg.payload)
  pgEmitter.emit(msg.channel, payload)
})

pgclient.query('LISTEN notify_foo')

pgEmitter.on('notify_foo', payload => {
  // handle event...
})

notify.sql

-- combines old and new values into a single json stringified payload
CREATE OR REPLACE FUNCTION notify_foo()
  RETURNS trigger AS
$BODY$
  DECLARE
    payload jsonb;
  BEGIN
    payload := json_build_object(
      'oldRow', OLD,
      'newRow', NEW
    );
    PERFORM pg_notify('notify_foo', payload::text);
    RETURN NULL;
  END; 
$BODY$
LANGUAGE plpgsql VOLATILE;

CREATE TRIGGER notify_foo
  AFTER UPDATE
  ON "bar"
  FOR EACH ROW
  EXECUTE PROCEDURE notify_foo();