Technically, https://github.com/msgpack/msgpack-javascript is the official MessagePack implementation of msgpack for JavaScript. A developer who goes by the handle "uupaa" wrote the implementation of it. He hasn't touched the code in some time.
I have an outstanding pull request that fixes a map handling issue that I hope he implements at some point. Barring that, though, I have had no problems using this library.
The reason I decided not to go with creationix's solution is because it adds JS-specific extensions to the msgpack specifiction. The extensions did not work well with other msgpack implementations.