isMaster returns only partial information when the MongoDB is not a member of a replica set. In particular, it will return isMaster: true and no primary field (at least on some servers I've tried on). As a result, pick-mongo-primary mongodb://localhost/db returns undefined/db whereas it would ideally return localhost/db.

I know your software is not particularly relevant to such simple databases, but some software such as Kadira uses pick-mongo-primary and it'd be nice to easily use such software on a simple MongoDB without replica sets. Currently I have to bypass pick-mongo-primary.

I got a replica set on cloud.mongodb.com and therefor have to use ?ssl=true. This script is detecting the right master of the replica, but is not forwarding the request to SSL to the primary.