Slonik is a wrapper around another Node.js postgres driver (pg). I guess slonik could be built on top of Postgres.js, but I'm not sure what the benefit would be since you get the same and more from Postgres.js. If you have an existing project using slonik it might benefit from the better performance if replacing pg with Postgres.js underneath (https://github.com/porsager/postgres-benchmarks#results)
Also what do the error messages look like?