Protomux is a Node .js library that multiplexes several message‑oriented protocols over a single framed stream (e.g., a length‑prefixed or secret‑stream connection). It lets developers define protocol channels, register typed messages with compact‑encoding, and handle opening, closing, and handshakes in a lightweight, event‑driven API—making it useful for building peer‑to‑peer or client‑server applications that need to run multiple protocols simultaneously over one transport.