We currently use utility functions like resolveReadPreference, in order to determine the correct read preference/write concern/read concern for operations. These should become properties of the potential sources for inheritance (e.g. MongoClient.readPreference, Db.readConcern), and those getters should look for inherited values in the absence of a set value, e.g.

class Db {
  get readPreference() {
    if (this.s.readPreference == null) {
      return this.s.client.readPreference;
    }

    return this.s.readPreference;
  }
}