for what it's worth, ST_AsGeoJSON lets you specify decimal precision as an optional argument, so you at least don't have to write a truncating loop.

Another good option with many polygon geometries is Base64 encoding the output of ST_AsGeobuf -- it is many times smaller than GeoJSON and still pretty easy to handle from Javascript. (Needs libprotobuf-c on the PostGIS side and geobuf.js+pbf.js on the JS side)