The basic structure of a NATS message modeled in the client libraries includes the subject the message was published to, the application-defined payload, and an optional set of headers (for requests, there is also a reply-to subject). The payload is a sequence of bytes, so it is up to the application to define how to serialize and deserialize the payload.
JSON is ubiquitous and simple data-interchange format that is supported in virtually all programming languages. This example demonstrates how to serialize and deserialize a message payload using a JSON library.