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room_pack_load_file runs room_pack_load_map and returns whether succeeded (as per documentation).

If you have packed all of your rooms into a file, you’d want to load it up like

var _buf = buffer_load("rooms.json");
if (_buf != -1) {
    var json_rooms = json_decode(buffer_read(_buf, buffer_string));
    // rest of the code you have
    buffer_delete(_buf);
}

If you only have a single room in that file, then just calling room_pack_load_file will suffice.