I was sure most of this existed so I looked it up... Mostly centers around adding row_to_json() to the query. Using the example from the github repo:

    $ psql -P pager -nqtc "select row_to_json(pg_database) from pg_database where datname = 'template0'" | jq
    {
      "datname": "template0",
    ...
      "datacl": [
        "=c/postgres",
        "postgres=CTc/postgres"
      ]
    }

Use --tuples-only / -t and --no-align / -A. Or -tA for short.

For example:

  $ psql dbname -tAc 'SELECT json_agg(some_table) FROM some_table'

That will produce a valid JSON if you ever need to pipe it to another command. And I'm sure, psql will do the job a lot faster than node.

By the way, you should not underestimate the built-in psql power and postgresql's json capability. Postgresql's JSON power rivals that of other JSON based No-SQL databases.