I will introduce an implementation of rollback netcode (aka replay consistency or server reconciliation) in JavaScript. We'll look at the relevant bits of WebRTC Data Channels (a UDP-based bidirectional transport available in all modern browsers) for messaging, do a quick ELI5 of The Redux Pattern™ for deterministic state, and bring it together into an approach that is performant, easy-ish to reason about, server authoritative, optimistic, and convergent. Applicable to web games, XR experiences, collaborative drawing and document editing, and more! There will be demos (plural) and source code!