[portlander12345]

Surprised no one here has mentioned Transit. It's an extensible typed data format with both JSON and binary representations. In other words, you can configure custom types, such as Immutable data structures, and they'll be automatically serialized and restored for you.

Good intro: http://cognitect.github.io/transit-tour/

GitHub: https://github.com/cognitect/transit-js

Note that in the introduction they provide a simple benchmark where Transit is both more compact and faster to parse than JSON with custom hydration.