While I've not dug into ForerunnerDB, it sounds like it is playing in the same space as PouchDB[1]. PouchDB is a JS database that works in the browser or in Node with various back-ends for data storage, syncs with CouchDB (or compatible, such as PouchDB Server) on the back-end, and currently uses map-reduce queries, though there is work in-progress to implement a MongoDB-like query language[2]. (This will match the Mango query language soon to be seen in CouchDB [3].)

Plus I've been really impressed with the development team—responsive to issues, consistent releases, high code-quality. It's a great project!

[1] http://pouchdb.com/ [2] https://github.com/nolanlawson/pouchdb-find [3] https://github.com/cloudant/mango