No REST for Cap'n Web

Node Congress

Node Congress 2026

Kenton Varda

<p>Cap'n Web is a new open source library that lets you expose JavaScript and TypeScript APIs across the Web, including over HTTP or WebSocket. Unlike almost all other RPC systems before it, Cap'n Web supports passing functions and objects over RPC "by reference". This simple-sounding feature has deep implications that completely change how APIs are designed. In this talk I will present a series of examples showing how to use Cap'n Web's first-class functions and objects to implement a variety of common API design patterns, such as authorization, pub-sub, and streaming. I will show how Cap'n Web can integrate nicely with reactive UI frameworks, and even touch briefly on how it can be a powerful tool for sandboxing AI agents.</p>